Benzaouia Mohammed
email: m.benzaouia@ump.ac.ma

Hajji Bekkay

Anne Migan Dubois

Mellit Adel

Abdel- Hamid Rabhi

An intelligent irrigation system based on fuzzy logic control: A case study for Moroccan oriental climate region

Keywords: Smart irrigation, Irrigation strategy, Drip irrigation, Fuzzy logic, Water saving. Temperature Solar Radiation Dark Medium Light Cold

Existing traditional agricultural systems consume an enormous amount of water and energy, with more than 69% of the world's freshwater volume attributed to agriculture. This fact leads to a degradation of water levels in several aquifers, as well as water quality. In Morocco, agriculture consumes about 88% of water resources, and the management of these resources is a significant issue, especially after the decline in rainfall rates in recent years. Designing an irrigation system that reduces water losses and evapotranspiration losses is the key to sustainable precision irrigation. This paper proposes an irrigation system based on an intelligent and efficient strategy for the automatic watering of agricultural areas and more adapted to the climatic conditions of Oriental Morocco. The system uses fuzzy logic to decide the duration of watering in an adaptive and precise way. Weather conditions (air temperature and solar radiation) and soil moisture (water content) are the inputs to the fuzzy system. The acquisition of these data is made by analog and digital sensors. The variables are fuzzified using trapezoidal and triangular membership functions. In the Fuzzification step, the Max-Min inference engine and the Mamdani rule base are used to decide the best decision for each given situation. The control strategy was implemented in the Arduino Uno R3 board. Typical three-day data from the summer and fall seasons were used to validate the proposed system. The simulation results showed that the proposed irrigation system maintains the soil moisture above the user-defined value and eliminates the under-irrigation risk.

Introduction

The development of the agricultural sector is one of the most powerful levers to ensure economic prosperity and social stability. Morocco has placed this area in the center of its development priorities with regard to the essential economic, social, and territorial issues raised by this sector. This sector generates about 14% of Morocco's gross domestic product (GDP). Climatic conditions (drought season) and rainfall contribute significantly to the increase or decrease of GDP [START_REF] Abioye | A review on monitoring and advanced control strategies for precision irrigation[END_REF][START_REF] Jaiswal | Fuzzy inference based irrigation controller for agricultural demand side management[END_REF].

An efficient and appropriate provision of irrigation water leads to a significant improvement in agricultural productivity and water conservation in view of its scarcity, which has become a major global preoccupation. Irrigation and rainfall are the two major sources of water for agriculture. However, precipitation is insufficient to satisfy the plant's water needs. On the contrary, excessive moisture limits the assimilation of nutrients by plants and increases disease development risk [START_REF] Li | A real-time fuzzy decision support system for alfalfa irrigation[END_REF]. The different irrigation control techniques that can be used are illustrated in Fig. 1 and can be classified into two types: open loop and closed-loop control [START_REF] Touati | A fuzzy logic based irrigation system enhanced with wireless data logging applied to the state of Qatar[END_REF]. In an open-loop control system, irrigation decisions are manually made by the farmer. The operator, based on his knowledge of the perceived crop response, sets irrigation time, the volume of water to be dispensed and irrigation speed. This technique is widely used thanks to its simplicity of implementation and the absence of the necessity to use sensors. However, it can lead to over-irrigation in some areas while other areas are under-irrigated, which may cause undesirable water stress [START_REF] Benzaouia | Design and Performance Analysis of a Photovoltaic Water Pumping System based on DC-DC Boost Converter and BLDC Motor[END_REF][START_REF] Benzaouia | Energy Management Strategy for an Optimum Control of a Standalone Photovoltaic-Batteries Water Pumping System for Agriculture Applications[END_REF]. In addition, the losses of water and energy are considerable, which is incompatible with the general policy of the world to conserve and rationalize water resources.

Therefore, the design of an intelligent irrigation system (closed-loop control) represents an alternative and efficient solution to traditional irrigation methods. The intelligent system applies to the soil the amount of water necessary for plant growth and development while monitoring several parameters that influence irrigation performance. The control is done by feedback. The control action depends on the result, allowing the preservation of the desired output condition while comparing it to the input conditions to make the best decisions. The decisions made are generally guided by the data acquired by sensors and compared to the desired set points [START_REF] Ma'mun | Robust irrigation system institutions: A global comparison[END_REF]. The main task of these systems is to accurately determine the crop irrigation needs. Table 1 summarizes recent intelligent irrigation strategies and techniques.

In this paper, an intelligent irrigation system adapted to the climate of the oriental region of Morocco has been developed. It uses feedback control based on fuzzy logic, which can also be easily integrated into drip irrigation systems without further modifying the overall system design. The system is designed to efficiently program irrigation times to maintain soil moisture above 30%. Irrigation periods are programmed in a way that evapotranspiration times are minimal. The fuzzy controller provides the irrigation duration required for the crop according to the recorded conditions of the soil moisture, ambient temperature and solar radiation sensors. This data is then transmitted to the fuzzy controller to apply well-designed fuzzy rules to control irrigation. The structure of this paper is as follows: The first section presents the introduction of the paper. Section 2 explains the design of the studied system, section 3 deals with the mathematical modeling of the fuzzy logic controller, section 4 describes the steps of the operation of the system, section 5 discusses the obtained results and their analysis, and the last section concludes the paper.

Presentation of the irrigation system

The structure of the irrigation system is given as follow (Fig. 2). The irrigation process depends on the data received from the weather station and the soil moisture sensor placed beside the plant, which is then processed by the Arduino controller. Based on the values acquired by the sensors, the fuzzy controller applies the well-designed rules to obtain the optimal irrigation decision. Wind speed and air humidity are not taken into account because their influence on the irrigation process in the oriental region is minor.

Mathematical modeling

The proposed fuzzy logic system consists of three modules: a Fuzzification module, an Inference engine and a Defuzzification module (Fig. 3) [START_REF] Zerouali | Improvement of Conventional MPPT Techniques P&O and INC by Integration of Fuzzy Logic[END_REF]. In the fuzzy logic system, a universe of discourse (𝑈) is developed (a membership function), and an interval of (𝑈) is defined by (𝑚𝑆). 𝑚𝑆(𝑒) is defined as a membership function that is based on a set of ordered pairs of elements e and the probability that e belongs to (𝑆). The system takes three (3) input parameters: soil moisture (𝑆𝑀), temperature (𝑇) and solar radiation (𝑆𝑅), each parameter consists of three membership functions, which are designated by a fuzzy subset of (𝑆). Soil moisture is designated by a variable 𝐴, temperature is given by a variable 𝐵 and solar radiation is designated by a variable 𝐶. For all three variables, 𝑚𝐴(𝑥), 𝑚𝐵(𝑥), 𝑚𝐶(𝑥), indicate the degree to which 𝑥 belongs to variables 𝐴, 𝐵, and 𝐶. The fuzzy intersection and the union set are given by equations ( 1) and (2) [START_REF] Zerouali | Fuzzy logic MPPT and battery charging control for photovoltaic system under real weather conditions[END_REF].

𝐴 ∩ 𝐵 ∩ 𝐶 = {𝑥, min(𝑚𝐴(𝑥), 𝑚𝐵(𝑥), 𝑚𝐶(𝑥)) ⋮ 𝑥 ∈ 𝑆}

(1) 𝐴 ∪ 𝐵 ∪ 𝐶 = {𝑥, max(𝑚𝐴(𝑥), 𝑚𝐵(𝑥), 𝑚𝐶(𝑥)) ⋮ 𝑥 ∈ 𝑆}

(2)

Fuzzification Module

The Fuzzification step is designed to convert a digital input measured by a sensor into a linguistic variable based on membership functions. A membership function is a function that defines the belonging degree of a numerical input to a linguistic variable. The membership function used controls several parameters such as variations of soil moisture, temperature and solar radiation. In order to obtain the right decision, the triangular and trapezoidal membership functions represent a typical choice in the proposed irrigation system. Equation ( 3) and (4) shows the structure of a triangular and trapezoidal adhesion for a three-value fuzzification system. Fig. 4 and 5 shows the membership functions of the fuzzy logic controller inputs and outputs.

Fuzzy Inference Module

Fuzzy inference allows interpreting the input values obtained from the sensors and, based on sets of rules, assigns values to the output. To achieve this, a set of if-then rules has been designed to operate with the membership functions of each input. In literature, two types of fuzzy inference systems can be implemented: Mamdani type and Sugano type. The Mamdani concept has been chosen for the proposed irrigation system since it is well suited for expert system applications. For each given scenario, the controller's output will be the irrigation duration, which varies according to the fuzzy rules grouped in Tables (2-4).

Defuzzification Module

Defuzzification is the last step in the process of implementing the intelligent irrigation strategy. The rules are then applied to the fuzzified values and a fuzzy output is generated, which is then converted to a scalar quantity. The centroid defuzzification method has been chosen due to its ability to provide accurate results. The defuzzified output (𝑥 ′ ) is expressed by Equation ( 5

System Operation Steps

The operation of the irrigation system is illustrated in Fig. 6. In the first step, temperature, solar radiation and soil moisture are measured in real time through the metrological station and the soil moisture sensor integrated with the plant. The second step consists of comparing the measured soil moisture value with the desired value. In case the soil moisture content is higher than 30%, the system is turned "off"; otherwise, the process of determining the irrigation duration is started and calculated by the fuzzy controller integrated into the Arduino board. The process of aspiration of water through the soil is estimated to take about 60 minutes, so the system is programmed to wait for the same amount of time before checking the sensor values and deciding on irrigation times.

Results and Discussion

To examine and evaluate the proposed system's behavior and the irrigation strategy, an apple plant was selected. According to experts in the field of agriculture, the apple tree requires a soil moisture content above 30%. For this purpose, typical three day data from two different seasons: summer (August 2020) and autumn (November 2020) has been used. The variations of temperature and solar radiation are illustrated in Fig. 7 and Fig. 8 respectively.

In the summer season the temperature varies between a maximum and a minimum of 38 °C / 19 °C, while the solar radiation reaches its maximum value around 1 Pm and 2 Pm of each day. In the fall season, weather variations are average. The temperature range is about 5°C / 20°C and solar radiation reaches 60% as a maximum value. Fig. 9 and Fig. 10 show that the proposed fuzzy controller avoids irrigating the plant during periods when temperatures are high (corresponds to high solar radiation), which allows reducing losses due to the phenomenon of evapotranspiration along the day, by referring to the well-designed fuzzy rules (Tables 2, 3 and4). The system selects the appropriate times and durations for irrigation to satisfy the plants' water needs, which also allows saving water. Soil moisture is therefore maintained at the desired value (30%) (Fig. 11).

During the fall season

During the fall season, since temperature and solar radiation don't reach high values, the irrigation times and periods are programmed when the plant requires water, with determined durations (Fig. 12 and13). The soil moisture is also maintained at the desired value (Fig. 14).

Conclusion

In this paper, closed-loop feedback control and fuzzy logic based irrigation strategy were presented to develop an optimal and sustainable irrigation approach that aims at conserving water in the oriental region of Morocco. Mamdani's Fuzzification, trapezoidal and triangular membership functions set irrigation duration for a specific type of plants. The use of the fuzzy control maintains the soil moisture above the user-defined value and eliminates the risk of under-irrigation. The obtained results show the efficiency and reliability of the system during both summer and fall seasons.

Fig. 1 .

 1 Fig. 1. Classification of different irrigation control techniques.

Fig. 2 .

 2 Fig. 2. Design of the proposed irrigation system.

Fig. 3 .

 3 Fig. 3. Fuzzy logic controller structure.

Fig. 4 .

 4 Fig. 4. Membership function of: a) Soil moisture, b) Temperature and c) Solar radiation (%).

Fig. 5 .

 5 Fig. 5. Membership function of: Duration (min).

Fig. 6 .

 6 Fig. 6. Flowchart software of the proposed irrigation system.

Fig. 7 .

 7 Fig. 7. Variation of temperature and solar radiation during the summer season.

Fig. 8 .

 8 Fig. 8. Variation of temperature and solar radiation during the fall season.

Fig. 9 .

 9 Fig. 9. Irrigation duration depending on solar radiation variation.

Fig. 10 .

 10 Fig. 10. Irrigation duration depending on temperature variation.

Fig. 11 .

 11 Fig. 11. Evolution of soil moisture content.

Fig. 12 .

 12 Fig. 12. Irrigation duration depending on solar radiation variation.

Fig. 13 .

 13 Fig. 13. Irrigation duration depending on temperature variation.

Fig. 14 .

 14 Fig. 14. Evolution of soil moisture content.

Table 1 .

 1 Summary of irrigation techniques discussed in the literature.

	Techniques	Reference	Controller	Measurement Parame-	Drawbacks
				ters
		[8]		Soil moisture	PID controllers are easy to im-
	Linear control	[9]	PID		plement, but are unable to con-trol multivariable and moving
					processes with time delays.
		[10]
		[11]	ANN	Soil moisture	Training data are necessary to estimate the irrigation action.
		[12]	FL	Soil moisture, Tempera-
				ture, Air humidity
	Intelligent
	control
		[13]	FL	Soil moisture, 𝐸𝑇 𝑐 , Rain forecast, Rain forecast probability, DAP	Requires an evaluation of ref-erence evapotranspiration (𝐸𝑇 𝑐 ), and the knowledge of stress and cultural coefficients
					(𝑘 𝑠 , 𝑘 𝑐 ).
		[14]	FL	Soil moisture, Crop water Stress Index (CWSI), Ambient temperature, So-lar Radiation, Humidity,	Multiple sensors are used which can be costly for large applications.
				Infrared Temperature

The measured parameters are not sufficient to decide the irrigation time precisely. Furthermore, the decision is made based on the pump's state, whether on or off, leading to over-or under-irrigation.

Table 2 .

 2 Rules of duration (in min) at wet soil moisture.

	Temperature	Dark	Solar Radiation Medium	Light
	Cold	Zero	Zero	Zero
	Medium	Zero	Zero	Zero
	Hot	Zero	Zero	Zero

Table 3 .

 3 Rules of duration (in min) at medium soil moisture.

Table 4 .

 4 Rules of duration (in min) at dry soil moisture.

	Temperature	Dark	Solar Radiation Medium	Light
	Cold	Very long	Very long	Very long
	Medium	Long	Long	Short
	Hot	Very long	Very Short	Zero


Khadim Ullah
email: khadim.jan@geeps.centralesupelec.fr

Aurore Brézard Oudot
email: aurore.brezardoudot@centralesupelec.fr

Anne Migan Dubois
email: anne.dubois@centralesupelec.fr

Demba Diallo
email: demba.diallo@centralesupelec.fr

Experimental Evaluation of the True Remaining Capacity of Legacy Lead-Acid Batteries

Keywords: Peukert's law, second-life battery, battery capacity, microgrid, capacity measurement

One of the common challenges in remote microgrid installations is inexpensive, reliable, and less laborintensive battery tests to measure the capacity of used batteries and link it to their remaining life. These capacity tests are usually misinterpreted, which results in a healthy battery being junked or may arise economic issues due to oversized battery banks in microgrids. In this work, we conducted several discharge experiments on 12V 100Ah lead-acid batteries in a controlled manner using an electronic load. The battery is subsequently discharged to 10.5V at C2.5, C3, C5, C10, C20, and C40 rates. When summed up using altered C-rate discharge sequences, the individual capacities resulted in 100.71Ah, 100.23Ah and 78.96Ah, 89.43Ah for the fresh and weak batteries, respectively. Irrespective of the discharge sequence, the cumulative net capacities are found quite close to the battery's actual capacity for fresh but much different for weak batteries. Results show that the remaining battery's true capacity does not disappear at a high rate and may further be utilized at lower discharge rates. The outcome of these experiments provides the basis to identify fresh and weak batteries and to size the battery banks for the low-cost rural microgrids more efficiently and economically.

I. INTRODUCTION

The lead-acid batteries hold a significant leading position in the microgrid market. Energy storage in batteries had remained an increasingly critical asset in many systems, including the starting/lighting/ignition (SLI) in vehicles [START_REF] Lencwe | Performance studies of lead acid batteries for transport vehicles[END_REF][START_REF] Ceraolo | Lithium-ion startinglighting-ignition batteries: Examining the feasibility[END_REF], renewable systems, electric traction, and the most trending one-off-grid electrification, as reported in the Global Energy Storage Database (DOE) report in 2018 [3,[START_REF] Farhadi | Energy Storage Technologies for Highpower Applications[END_REF]. The proven versatility and low cost enabled this technology to use over a hundred years for storing electrical energy [START_REF] Plett | Method for Monitoring Remaining Battery Capacity[END_REF]. The basic chemistry of the lead-acid batteries is fairly simple [START_REF] Salkind | Lead-Acid Batteries[END_REF], but they are complicated, especially when it comes to the capacity response to various loads and different operational situations [START_REF]Sealed Lead Acid Batteries[END_REF][START_REF] Pinson | Theory of SEI formation in rechargeable batteries: capacity fade, accelerated aging and lifetime prediction[END_REF][START_REF] Linden | Handbook of Batteries[END_REF]. The degradation mechanisms in batteries are complex, and a detailed analytical description of electrochemical processes involved for the determination of the residual capacity is not explicitly available [START_REF] Hughes | The Residual Capacity Estimation of Fully Sealed 25Ah Lead/Acid Cells[END_REF]. Due to the complicated nature of the batteries, most often, the batteries are replaced before their diminished capacities, or on the other hand, the battery bank is incorrectly designed that eventually result in failure of the target application of which the most affected is the off-grid renewable or hybrid renewable electrification systems [START_REF] Nasir | Solar PV-Based Scalable DC Microgrid for Rural Electrification in Developing Regions[END_REF].

In applications like off-grid electrification systems, apart from the popular flooded lead-acid batteries, another attractive option is valve-regulated lead-acid (VRLA) batteries, which are studied in this work-they require low maintenance and are classified as absorbent glass mat (AGM) batteries and gel electrolyte batteries [START_REF] Jie | Design of Energy Storage System using Retired Valve Regulated Lead Acid (VRLA) Batteries in Substations[END_REF]. In addition, VRLA batteries use much less electrolyte than traditional lead-acid batteries, they are also occasionally referred to as an acid-starved design. VRLA batteries can be mounted in any position. These batteries are designed to be recombinant to eliminate the emission of gases during overcharge, thereby reducing room ventilation requirements. Furthermore, little or no acid fumes are emitted during the normal operation of VRLA batteries. In the event of damage to a VRLA battery, the volume of free electrolyte that could be released is quite small. Finally, there is no need to check electrolyte levels or add water which is lost due to electrolysis. Thus, VRLA batteries are well suited in rural electrification as they are safer, versatile, environment friendly, and require less maintenance [START_REF] May | Lead batteries for utility energy storage: a review[END_REF].

In modern applications, where the remaining battery capacity is crucial, i.e., not only the SLI, but the most critical one-off-grid electrification services; battery users frequently overcharge, inefficiently cycle, and prematurely replace their batteries because accurate and affordable measurement systems for the test and recovery of these batteries do not exist. As a battery gets older, its usable capacity does not fall off in a cliff-like manner, but instead tends to slowly decrease, which is also recoverable. In areas with an increased power cut or no access to electricity, such poor management of batteries and early replacement increases financial stress on the poor and, in parallel, reduces their trust in such stand-alone community microgrids.

Generally, battery state of charge (SOC) is measured by various battery testing equipment, battery management systems (BMS), and software tools to determine the health of a battery [START_REF]IEEE Approved Draft Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic (PV) Systems[END_REF][START_REF] Visser | State of Charge Characteristics of the Lead Acid Battery[END_REF]. However, testing SOC does not give an accurate measure of a battery's health or remaining capacity [START_REF] Roscher | Detection of Utilizable Capacity Deterioration in Battery Systems[END_REF]. The key measurement of remaining battery life is the percentage of original design capacity or the residual capacity determined by a series of discharge tests at different C-rates. The better way to test capacity is using a constant DC current sink such as an electronic load for a certain amount of time. However, the constant current sink method is misunderstood and the obtained results are never realistic to reflect the true residual capacity. More than that, most often, the capacity measurement tests are carried out without considering the recovery and loss of battery capacity due to self-discharge, off-service status, and battery relaxation and thermal equilibrium of electrolyte [START_REF] Shi | Nondestructive forensic pathology of Lead-Acid batteries[END_REF].

Paper contribution

In almost all applications, as stated before-the battery bank stays off-service due to one or other reason and the battery itself is left discharged for a couple of days or an indefinite time. Therefore, our proposed repeated discharge tests at different C-rates and altered sequences enable us to accurately calculate the actual remaining capacity in a supervised manner. Also, it helps to mark/ sort the fresh and weak batteries based on the accumulated individual capacity measurements.

The remaining paper is organized as follows. Section II covers the capacity response and the use of Peukert's equation. In Section III, experimental setup and procedures have been carried out to calculate the battery's actual residual capacity and clarify the confusion created by misinterpretations of Peukert's equation. The measurements and results are discussed with more details in Section IV. Finally, in Section V, conclusions are drawn and some perspectives are given for the possible future experimental outcomes.

II. PEUKERT'S EQUATION AND CAPACITY RESPONSE OF A BATTERY AT HIGH DISCHARGE

The misinterpreted Peukert's law was initially developed for lead-acid batteries over a hundred years ago [START_REF] Peukert | Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren[END_REF]. This law states that the delivered charge increases when the discharge current decreases or conversely, the battery capacity decreases with an increase in discharge current. In fact, the battery's capacity does not depend on how quickly it is being discharged or charged. The battery datasheet provided by the manufacturers is a piece of useful information. At some point, the datasheets are used incorrectly and mainly misunderstood, especially for the battery capacity.

The concept behind Peukert's equation is widely available in the prior art [START_REF] Doerel | A critical review of using the peukert equation for determining the remaining capacity of lead-acid and li-ion batteries[END_REF]. It came about over a millennium back in 1897 from a German scientist named-Wilhelm Peukert. Peukert realized that the behavior of storage batteries is complicated to understand. There are no good models even today to understand how a battery behaves in all. In fact, there is no general model that is correct for capacity determination. With many complications, even today, the battery models that exist; most of these are the computer models, as they are specifically tailored for one specific application and technology.

Peukert's law expresses the capacity of a battery in terms of the rate at which it is discharged. It is described as the following equation:

= * ( > 1) (1)
where, is the nominal capacity of the battery, is the discharge current, is the Peukert's constant, which is determined by the internal characteristics of the batteries and its value is greater than one, and is the discharge time. The equation indicates that the discharge time and battery capacity decrease exponentially at any given nominal capacity as the discharge current is increased. In addition, the discharge capacity is described in (2) by using the same notation as in (1):

= * = ( ) * (k > 1) (2)
Equation ( 2) indicates that excessive causes a reduction in the discharge capacity. However, it is shown experimentally that the capacity does not disappear with a high discharge current for fresh but significantly lost for used batteries.

Peukert was able to go through some empirical testing with battery technology and computing tools of that time. He demonstrated that if a battery is charged very quickly, much useful capacity cannot be obtained during discharge. If the same battery is discharged very slowly, far more useful energy out of the battery can be extracted. This complex discharge behavior of batteries is usually misinterpreted for sizing storage batteries in microgrids. This has many practical implications and, in parallel, a lot more confusion. Interestingly, Peukert did a very good job. His data set had remained quite useful, but sometimes not used in its true sense for the capacity response of a battery.

The battery datasheet is misunderstood when the discharge capacity is confused with the battery's true capacity. The datasheet of the battery used in this work [START_REF]Ultracell batteries[END_REF] claims that the amp-hour (Ah) capacity depends on the discharge rate, but as we demonstrate, this capacity is just the useful capacity. If the battery is continuously discharged at the ten-hour rate (C10) with a constant DC load current of 10A, the battery will drop to a voltage, i.e., 10.5V, which is deemed not useful for further discharge. If the 100Ah battery is discharged very quickly, say at a five-hour rate (C5) with 20A, it will drop to that minimum acceptable voltage much quicker. However, this does not mean that the capacity went away at C5, it means that the remaining capacity is no more useful at C5. The remaining capacity can still be used at a lower discharge rate and more out of the Ah can be extracted from the battery.

III. EXPERIMENTAL SETUP AND METHODS

The batteries used in this demonstration are 12V 100Ah Ultracell ® UCG 100-12 lead-gel technology. The system setup is as shown in Fig. 1. It consists of a ZS-1812 H&H DC electronic load, lead-acid battery, and a supervisor PC to discharge batteries in a controlled manner. The interface NI-6009 is used between the supervisor PC and electronic load. All the batteries under test are 100Ah each, weigh 67 pounds, and are of AGM gel electrolyte F10 terminal construction [START_REF] Peukert | Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren[END_REF]. Before starting the discharge tests, batteries are fully charged using H-Tronic HTDC 5000 (5A) smart charger. In the first test, a single 12V battery is randomly selected among the bank of 16 batteries. The lower discharge threshold is set to 10.50V (1.75V/cell).

A. Experimental Procedure

The whole experimental bench is depicted in Fig. 2. In this figure, the electronic load is connected to the supervisor PC via NI6009 to Analog I/O interface to run the experiment in a controlled manner. The cables are of sufficient gauge and minimal length to minimize the voltage drop during high discharge currents. It was observed that these cables survived during the high discharge current test, and they did not heat up abnormally. However, the cable selection could be different for other equipment and may better be adapted to as per application.

The electronic load is initialized and configured to the external mode using LabVIEW ® to observe the battery's behavior at different C-rates. This interface is also used to send control signals and measure the battery voltage and current. As shown in the flow diagram of Fig. 3, the discharge process is repeated for other C-rates until C40 (2.5A). Onehour relaxation time is used for the open-circuit voltage (OCV) recovery and thermal equilibrium of electrolyte. The total discharge time is recorded each time and multiplied with the discharge current to get the extracted capacity in Ah. After the test is finished for a sequence, the battery is fully recharged to pass through another discharge using a different sequence. Important control parameters and different thresholds for the experiment are detailed in Table I.

B. Discharge Sequences

The following C-rates are used in a shuffled sequence to extract the residual capacity of a fully charged battery:

C2.5 C5 C10 C20 C40 (fresh battery) C3 C5 C20 C10 C40 (weak battery)
where C is the rated capacity of the battery under test, which is 100Ah at C10. The discharge currents are configured in LabVIEW ® and sent to ZS-1812 electronic load using NI-6009. At the R2 resolution settings (configured via port 0) with a bit sequence of 00000100, the following voltage ranges are fed to the control of different discharge currents:

C2.5 C3 C5 C10 C20 C40 (2V) (1.67V) (1V) (0.5V) (0.25V) (0.125V)
The electronic load is easily disconnected from the battery through a TTL signal at P1.0 (disconnect HIGH). Altering the discharge sequence enabled the authors to identify fresh and weak batteries based on the net extracted capacity. The fresh battery has no significant capacity loss with a high discharge current. However, the capacity of a weak battery is affected by a high discharge current.

IV. MEASUREMENTS AND DATA RECORDING

In our case, a fully charged 12V lead-acid battery is drained to the cut-off threshold of 10.5V, and still, the remaining capacity turned useful on successive low discharge currents. It is an indication that this capacity calculation method using repeated discharges and different C-rates could help calculate the overall remaining capacity of the battery. The discharge process was carefully controlled to avoid the battery getting sulfated or being damaged from deep discharge using set threshold of 10.5V at the ambient room temperature. As depicted in Table II, we got 100.7Ah and 100.2Ah during the two different discharge sequences for fresh batteries. These results are quite close to the battery's rated capacity. It is observed that altering the discharge sequence did not affect the fresh battery but affected the weak battery.

Out of the 16 batteries on-site, few were identified as weak batteries based on the obtained results (89.4Ah to 78.9Ah) by altering the discharge sequence. The high discharge current roughly dropped 10Ah of the useful capacity for the weak battery. It is a result of internal capacity loss due to high plate resistance and degradation. The obtained results for the different discharge sequences and different batteries are compared in Table II and graphically represented using Fig. 5. It is interesting to note that the extracted capacities for the weak batteries have a significant deviation due to the altered discharge sequence. It indicates aging and capacity loss due to an increase in the internal plate resistance. We have demonstrated with a real practical battery that the battery does not lose its residual capacity at high C-rates as is being misinterpreted. The response in Fig. 6, when summed up, the individual capacities resulted quite nearly to the rated Ah capacity of the fresh battery as the internal loss is quite low and the impact of altered sequence is negligible. The adequate experimental validation in this work shows that a battery does not lose much capacity by discharging it at a high current. It is obvious from the obtained results that the batteries could still be utilized in different groups with different discharge current sequences to take out the maximum of their true capacities and serve longer. These experiments could be of particular interest for the selection of batteries in low-cost stand-alone microgrids.

V. CONCLUSION

The experimental validations on real batteries using successive discharge had reliably determined the remaining true capacity of lead-acid batteries. It was practically demonstrated that a certain battery, e.g., 100Ah AGM in our case, was discharged five times to the threshold of 10.5V with an hour relaxation for voltage recovery and thermal equilibrium of electrolyte. It is observed that irrespective of the C-rates, the net extracted capacities 100.71Ah, 100.23Ah matched exactly the datasheet values for fresh batteries but found a quite large deviation of 78.96Ah, 89.43Ah for weak batteries. The overall analysis also enabled the authors to quickly sort the second-life batteries in fresh and weak groups for use as a battery pack of different health status in reducing the storage cost of off-grid electrification systems for the rural areas. For future work, an extension of these experiments could be used for the parameter extraction of real batteries. This could be helpful in adaptive tuning of the energy management system to evolve with the degraded performance of aged batteries, and also health monitoring of the batteries.

Fig. 1 .

 1 Fig. 1. Schematic representation of the experimental platform for the capacity determination using successive discharges

Fig. 3 .

 3 Fig. 3. Flow diagram of the experimental process

Fig. 4 .Fig. 5 .

 45 Fig. 4. ZS1812 electronic load settings: Data recording and linear conversion (from ZS1812 to LabVIEW ® )

Fig. 6 .

 6 Fig. 6. Net extracted capacity from a fresh battery through five successive discharges

TABLE I :

 I ZS1812 CONTROL PARAMETERS AND SETTINGS

	Load type	Range/	Interface	Mode	Accuracy
		Resolution
	ZS-1812	75A DC /	NI-6009 to	CC const.	of
	current sink	R2 settings	Analog I/O	current	measured value
	Supervisor	Control	Battery	Cut-off	Relaxation/
		mode	type	threshold	Recovery time
	LabVIEW ®	External	UCG-100/	10.50V	60 mins
	2013	mode	12 lead-gel
	Charger	Fully	Temp.	Capacity	Discharge
		charge volt		C=C10	Sequences
	H-Tronic	14.30V	Ambient	100Ah	2
	HTDC5000

Fig. 2. Experimental bench for the net capacity calculation in a controlled manner

TABLE II :

 II IDENTIFICATION OF FRESH AND WEAK BATTERIES

		Battery A (fresh)			Battery B (weak)
			Discharge sequence (Test-1)
				C=C10=100Ah
	C3	C5	C10 C20 C40	C3	C5	C20 C10 C40
	83.2	6.6	5.0	2.9	2.4	66.6	9.3	8.9	0.8	3.7
			100.2Ah				89.4Ah
	(Total extracted capacity)	(Total extracted capacity)
			Altered discharge sequence (Test-2)
				C=C10=100Ah
	C2.5	C5	C10 C20 C40 C2.5	C2.5 C10 C20 C40
	74.6 11.3 6.0	4.1	4.6	64.6	4.6	5.5	2.2	1.8
			100.7Ah				78.9Ah
	(Total extracted capacity)	(Total extracted capacity)
	Negligible capacity loss			High capacity loss
						(due to internal plate resistance)

ACKNOWLEDGMENT

Discussion with several colleagues was valuable in clarifying some of the thoughts given here, for which we are grateful. The authors would also like to thank Eric Labouré, Eric, B. and Richard, A. for the experimental bench setup, Olivier, H. and Anthony, G. for the IT support, and Islamic Development Bank (IsDB) for the funding.

This research is fully funded by the Islamic Development Bank (IsDB) under Agreement No. 600036563 Sustainable Energy & Power. *As a faculty member of the Department of Electrical Engineering, full support is received from the


Guillaume Noirbent

Damien Brunel

Thanh-Tuân Bui

Sébastien Péralta

Pierre-Henry Aubert

Didier Gigmes

Frédéric Dumur

D-A Dyads and A-D-A Triads based on Ferrocene: Push-pull Dyes of Unusual Behaviours in Solution

Keywords: 28.5, Found: C, 50.9, O, 12.9, Found: C, 48.9

Ferrocene has been extensively used for the design of chromophores with reversible electrochemical properties. In this work, a series of twelve D-A dyads and twelve symmetrically substituted A-D-A triads have been developed based on twelve different electron acceptors. Photophysical characteristics of the different dyes have been studied by various techniques including UV-visible absorption and photoluminescence spectroscopy. Their electrochemical behaviours have also been determined. To evidence the specificities of the triads relative to that of their related donor-acceptor (D-A) dyads analogues, comparisons between these two families of dyes (D-A and A-D-A) have been established. Solvatochromic behaviour of the twenty-four dyes has been examined in 22 solvents of different polarities. Parallel to the solvatochromism, suppression of the push-pull effect by oxidation of the ferrocene unit with a strong oxidizing agent, i.e. nitrosonium tetrafluoroborate, drastically modified the optical properties of the different dyes. To support the experimental results, theoretical calculations were carried out. Interestingly, during this work, an unprecedented modification of the UV-visible absorption spectra in solution was demonstrated over time for all ferrocene-based push-pull dyes in various solvents. Evidence of such a behaviour for ferrocene-based push-pull dyes is unprecedented in the literature.

Introduction

During the past decades, push-pull dyes comprising an electron donor connected to an electron acceptor by mean of a conjugated or a none-conjugated spacer have been extensively studied, due to the wide range of applications in which these different chromophores are involved. [START_REF] Bures | [END_REF] Among the most popular applications, organic photovoltaics (OPVs), [2][3][4][5][6][7] organic field effects transistors (OFETs) 8 or non-linear optics (NLO) [9][10][11][12][13] can be cited as the most representative ones. However, emerging research fields such as visible light photopolymerization under low light intensity [14][15][16][17][18][19][20][21][22][23] or the blooming field of 3D-printing requiring dyes absorbing at 405 nm can also be both mentioned. [24][25] Compared to their inorganic counterparts i.e. the pigments that suffer from an incredible insolubility in common organic solvents, organic dyes offer several advantages such as an easy tunability of their absorption maxima, [26][27][28] a facile processability due to their remarkable solubility in most of the common organic solvents, and an easy synthetic access, especially, if the Knoevenagel reaction is selected as the reaction used to form the conjugated spacer between the donor and the acceptor. For sub-disciplines [29][30] derived from NLO such as bioimaging, non-linear frequency conversion of laser sources or electro-optic modulators, a fast response time is also required, what can be obtained with organic dyes and metal complexes such as ferrocene. [START_REF] Prasad | Introduction to Nonlinear Optical Effects in Molecules and Polymers[END_REF][START_REF] Nalwa | Nonlinear Optics of Organic Molecules and Polymers[END_REF][START_REF] Dalton | Organic Thin Films for Photonic Applications[END_REF][START_REF] Verbiest | [END_REF][START_REF] Nalwa | Nonlinear Optics of Organic Molecules and Polymers[END_REF] In numerous applications, organic dyes are often preferred over metal complexes due to the toxicity concerns raised by the use of metal-based dyes. However, non-toxic metals such as iron can however be used to create push-pull dyes and one of the most representative complexes in this field is ferrocene. [START_REF] Brunel | [END_REF] This metallocene comprising an iron (II) cation sandwiched between two coplanar cyclopentadienyl cycles can be reversibility oxidized at a moderate potential so that this metallocene is internationally used as a reference for electrochemistry. 37 However, its scope of application is not limited to electrochemistry and ferrocene is now extensively used in medicine, 38 (photo)polymerization, [39][40] enantioselective catalysis, 41 for the design of molecular machines, 42 metal organic frameworks 43 or molecular rectifiers. 44 Due to its facile oxidation, ferrocene is also a good candidate for the design of donor-acceptor (D-A) conjugates and numerous dyads have been reported over the years. [45][46][47][48][49][50][51][52][53][54][55] Conversely, acceptor-donoracceptor (A-D-A) triads based on ferrocene have only been scarcely examined in the literature (See Figure 1). [56][57][58][59][60][61] Especially, photophysical properties of A-D-A triads based on ferrocene have only been examined in details in the case of Meldrum's derivatives (UV-visible absorption, fluorescence spectroscopy and spectroelectrochemistry).

Ferrocene possesses a low oxidation potential (0.52, 0.44 and 0.43 V vs SCE in dichloromethane, acetonitrile, or DMF) 62 and can be thus oxidized as ferrocenium by a one-electron oxidation process. 63 This oxidation can be done electrochemically [64][65] but also chemically by use of oxidizing agents such as nitrosonium a. Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France. b. CY Cergy Paris Université, LPPI, F-95000 Cergy, France. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins tetrafluoroborate 66 or iron (III) sulfate. 67 One of the main advantages of nitrosonium tetrafluoroborate arises from its excellent solubility in common organic solvents, enabling to carry out the discoloration experiments in dichloromethane. Indeed, when ferrocene is used as an electron donor in pushpull structures, conversion of the electron-donating ferrocene to the electron-deficient ferrocenium can drastically affect the optical properties, as exemplified with A-D-A triads previously reported in the literature and comprising BODIPY and ferrocenes. 68 More generally, colour change is observed for all electrochromic devices comprising ferrocene as the electrondonating moiety. [69][70] Figure 1. Chemical structures of A-D-A ferrocene triads reported in the literature.

In this work, a series of twelve A-D-A triads comprising ferrocene-1,1'-dicarbaldehyde ED2 as the electron donor have been synthesized and their photophysical properties determined (See Figure 2). To evidence the main differences existing between the twelve A-D-A triads (A-D-Ax, x = 0-11) with their twelve corresponding D-A dyads (D-Ax, x=0-11), these latter have been synthesized with EA0-EA11 as the electron acceptors and ferrocenecarboxaldehyde ED1 as the donor for comparison (See Figure 2). Parallel to the different investigations concerning their optical properties and their solvatochromic behaviour in solution, electrochemical measurements were also carried out to characterize the dyads and triads. Finally, by chemical oxidation of ferrocene as ferrocenium, suppression of the electron-donating part in the dyads and triads, and conversion of the electron-donating ferrocene as an electron acceptor (ferrocenium), resulted in a dramatic modification of their absorption spectra and colour changes of the solutions. Modulation of the spectra obtained on oxidation has notably been preciously examined in the literature, especially to demonstrate "switchable" NLO properties of ferrocene-based push-pull dyes. 71 In this work, for several compounds, use of an excess of chemical oxidant resulted in successive oxidation processes that will be presented in this work. Finally, as one of the most interesting finding of this work, a modification of the optical properties of the ferrocene-based push-pull dyes in solution was demonstrated upon standing for the 24 dyes. This usual behaviour has never been reported in the literature for ferrocene-based dyes, even if numerous dyads and triads comprising ferrocene as the electron donor have been reported in the literature. [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] This unusual behaviour was investigated by theoretical calculations, UV-visible absorption spectroscopy and cyclic voltammetry. , D-A5 and A-D-A5 48 were synthesized as previously reported in the literature, without modifications and in similar yields. Computational details: All quantum mechanical calculations were computed using Gaussian Package. 74 All geometry optimizations were performed using density functional theory (DFT) with the global hybrid exchangecorrelation functional B3LYP 75 and all minima on the potential energy surface were verified via a calculation of vibrational frequencies, ensuring no imaginary frequencies were present. The Pople double-zeta basis set with a double set of polarization functions on non-hydrogen atoms (6-3111G(d,p)) 76 was used throughout. This computational approach was chosen in consistency with previous works, as it provides good agreement with experimental data. Excited states were probed using time dependent density functional theory (TD-DFT) using the same functional. All transitions (singlet-singlet) were calculated vertically with respect to the singlet ground state geometry. Solvent effects were taken into account by using the implicit polarizable continuum model (PCM). 77,78 DCM was chosen in analogy with the experiments. Computed spectra were simulated by convoluting each transition with Gaussians functions-centered on each absorption maximum-using a constant full width at half maximum (FWHM) value of 0.2 eV. The assignment of electronic transitions for max has been determined with GaussSum 3.0 software. 79

Experimental details

Synthesis of the dyes

General procedure used for the synthesis of D-A0-D-A11, except for D-A3 and D-A5.

Electron acceptors EA0-EA3, EA4-EA11 (5.79 mmol) and ferrocenecarboxaldehyde (1.24 g, 5.79 mmol, 1. eq.) were dissolved in absolute ethanol (50 mL) and a few drops of piperidine were added. Immediately, the solution colour changed. The solution was introduced in a preheated bath at 100°C and progress of the reaction was monitored by TLC. Typically, the reaction was stopped after 10 minutes. After cooling, a precipitate formed. It was filtered off, washed several times with ethanol and dried under vacuum.

General procedure used for the synthesis of D-A3. EA3 (5.79 mmol) and ferrocenecarboxaldehyde (1.24 g, 5.79 mmol, 1. eq.) were dissolved in acetic anhydride (50 mL) and the solution was refluxed overnight. After cooling, a precipitate formed. It was filtered off, washed several times with ethanol and dried under vacuum.

General procedure used for the synthesis of D-A5. EA5 (5.79 mmol) and ferrocenecarboxaldehyde (1.24 g, 5.79 mmol, 1. eq.) were dissolved in DMF (50 mL) and the solution was stirred at room temperature overnight. During that time, a precipitate formed. It was filtered off, washed several times with ethanol and dried under vacuum. D-A0: EA0 (0.38 g, 5.79 mmol), mexp = 1.49 g, 98% yield. [START_REF] Bures | [END_REF]

Results and discussion

Synthesis of the different dyes

The twelve D-A dyads D-Ax, x = 0-11 and the twelve triads A-D-Ax, x = 0-11 have been synthesized using a Knoevenagel condensation between the twelve electron acceptors EAx, x = 0-11 and the two ferrocene donors ED1 and ED2. All reactions were carried out using ethanol as the solvent and a catalytic amount of piperidine, excepted for all dyes comprising EA3 and EA5 as the electron acceptors (see Scheme 1). In the case of EA3, EA3 anion being highly stable and unreactive in basic conditions, D-A3 and A-D-A3 were thus prepared by refluxing ED1 or ED2 and EA3 in acetic anhydride. The two dyes were obtained in 54 and 39% yields respectively. Conversely, the methylene group of 2,4,5,7-tetranitro-9H-fluorene (TNF) is highly acidic so that this electron acceptor spontaneously deprotonates in highly polar solvents such as DMF. Consequently, D-A5 and A-D-A5 could be prepared in DMF, at room temperature without using any base. D-A5 and A-D-A5 could be obtained in 88 and 82% yields respectively. Overall, the different dyes were obtained with reaction yields ranging from 39% for A-D-A3 to 99% for A-D-A6 (see Table 1). Interestingly, for dyes prepared in ethanol, dyes were obtained as precipitates upon cooling of the ethanolic solutions so that the workup could be reduced to a simple filtration to recover the targeted products in pure form. Similarly, D-A5 and A-D-A5 could be obtained by precipitation from DMF whereas D-A3 and A-D-A3 precipitated from the acetic anhydride solutions.

Optical properties

Optical properties in chloroform

Optical data of dyads D-Ax, x = 0-11 and triads A-D-Ax, x = 0-11 are summarized in Table 2 and the UV-visible absorption spectra of all dyes recorded in chloroform are presented in Figure 3, except for A-D-A6 and A-D-A7 for which their low solubilities in chloroform did not allow to determine their molar extinction coefficients with accuracy. Absorption spectra of all dyes were quite similar. As shown in Figure 3, all dyes exhibited an intense absorption band centered in the visible range. This absorption band corresponds to the intramolecular charge transfer (ICT) band between the electron-donating ferrocene and the different electron-accepting groups. 80 However, recent works done on ferrocene-based push-pull dyes and especially theoretical calculations done on such structures by Barlow et al. have revealed this transition to be more complex than a simple HOMO -> LUMO transition. Indeed, this optical transition detected at low energy corresponds to a transition originating between a set of degenerate HOMOs and the LUMO energy level. 81 The contributions of the different transitions in this absorption band is discussed later in this work. A stronger absorption band can also be detected in the near-UV-visible range. By finely tuning the electron-accepting ability and by examining twelve electron acceptors (EA0-EA11), positions of the ICT bands could vary from 531 nm for D-A0 exhibiting the weakest electron acceptor to 630 nm for D-A3 comprising the strongest electron acceptor EA3. Interestingly, lower variations of the absorption maxima could be found for the absorption bands located in the near-UV-visible range than for the absorption band at low energy. Based on previous works reported in the literature, this transition was assigned to a transition between an occupied π-orbital consisting in a combination of highest occupied cyclopentadienyl orbital and highest occupied π bridge orbital and the LUMO energy level. 82 Considering the specificity of this transition detected at high energy, increase of the electron donating ability of the electron acceptor both lowers the energy levels of the LUMO as well as the energy level of the degenerated π-orbitals. Consequently, the similar variations of these two orbitals let the position of this transition almost unchanged. Conversely, the HOMO level remains unaffected while increase the acceptor strength so that a greater variation of the ICT band can be observed while increasing the electron-accepting ability of the acceptors. Concerning the symmetrically substituted A-D-A triads, positions of the ICT bands varied from 533 nm for A-D-A0 to 632 nm for A-D-A3. These results are consistent with the electron- Interestingly, a decrease of the intensity of the charge transfer band was observed while going from D-A dyads to A-D-A triads. Reduction of the molar extinction coefficients of the A-D-A triads relative to that of their dyad counterparts can be assigned to the fact that, in the case of A-D-A triads, the electrondonating ability of ferrocene is splitted between the two electron acceptors. On the opposite, the electronic delocalization is unidirectional for the D-A dyads so that higher molar extinction coefficients can be measured due to the enhanced oscillating strength. However, geometry of D-A and A-D-A complexes have also to be considered to support these differences of molar extinction coefficients. Indeed, if only one cyclopentadienyl ring is functionalized in the case of D-A dyads, the free rotation existing between the two cyclopentadienyl rings of A-D-A triads can position the two electron-accepting groups in parallel, antiparallel positions, or an intermediate situation that can be hardly anticipated. 60 As a result of this, the conformational preferences in solution for the different dyes can also contribute to modify their molar extinction coefficients as well as the total dipole moment of the molecules. Noticeably, ICT band with the highest molar extinction coefficient for D-A dyads was determined for D-A4 bearing 2-(3-cyano-4,5,5trimethylfuran-2(5H)-ylidene)malononitrile (EA4, TCF) (ε = 9750 L.mol -1 .cm -1 ) as the acceptor whereas the highest molar extinction coefficient for the ICT band of A-D-A triads was measured for A-D-A3 (ε = 6300 L.mol -1 .cm -1 ) possessing 2,2'-(1H-indene-1,3(2H)-diylidene)dimalononitrile EA3 as the acceptor. Therefore, it can be concluded that the highest molar extinction coefficients were obtained with the strongest electron acceptors, inducing the highest electronic delocalization. Surprisingly, no clear trend could be established while comparing the maximum absorption of dyads and triads. Indeed, as observed for D-A0, D-A1, D-A5, D-A8 and D-A10, almost no modification of the absorption maxima between dyads and triads was found. This result can be assigned to the weakness of the different electron acceptors used in these dyes so that the presence of one or two electron acceptors doesn't drastically affect the optical properties. Conversely, a hypsochromic shift was detected for the ICT bands of A-D-A2, A-D-A4, A-D-A9 and A-D-A11 compared to that of their diad analogues. Only A-D-A3 (λmax = 632 nm) showed a red-shifted absorption compared to that of D-A3 (λmax = 623 nm). These unexpected behaviors detected for A-D-Ax, x = 0-11 can be assigned to the presence of two broad absorption bands in the visible range with positions varying differently from a dye to another one. ) As shown in Figure 4, three different situations could be found, affecting the determination of the absorption maxima for the ICT band. Thus, to illustrate this, for A-D-A0, A-D-A9 and A-D-A10, two distinct absorption bands could be detected in the visible range (400-600 nm) so that positions of the two bands could be easily determined. This situation was notably found for dyes prepared with malononitrile EA0, 1,3-diethyl-2thioxodihydropyrimidine-4,6(1H,5H)-dione EA9 and 1,3dimethylpyrimidine-2,4,6(1H,3H,5H)-trione EA10. Conversely, for A-D-A2, a broad band extending from 500 to 700 nm could be evidenced, with no real presence of peaks so that the positions of the two bands could not be determined with accuracy. Such absorption spectra were nonetheless found for

Wavelength (nm) A-D-A0 A-D-A1 A-D-A2 A-D-A3 A-D-A4

A-D-A2 comprising 2-(3-oxo-2,3-dihydro-1H-inden-1-

ylidene) malononitrile EA2 but also for A-D-A3, A-D-A4, A-D-A6, A-D-A8, A-D-A9

and A-D-A11. In terms of electron-accepting groups,

2,2'-(1H-indene-1,3(2H)-diylidene)dimalononitrile EA3, 2-(3- cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile EA4, 2- thioxodihydropyrimidine-4,6(1H,5H)-dione EA6, 3-ethyl- thiazolidine-2,4-dione EA8, 1,3-diethyl-2-thioxo- dihydropyrimidine-4,6(1H,5H
)-dione EA9 and 3-allyl-2-thioxothiazolidin-4-one EA11 drastically differs by their electronwithdrawing abilities. Except for EA8 and EA11, EA2 and EA3, EA6 and EA9 that are of similar structures, EA4 is the only representant of its category. Especially, if two ICT bands are found with indane-1,3-dione EA2, an overlap of the two bands can be found for EA3 and EA4-based dyes (A-D- A3 andA-D-A4), what can be related to the improvement of the electronwithdrawing ability in EA3 and EA4 compared to EA2. However, as counter-examples, EA8 and EA10 that are rhodanine-based acceptors are weaker acceptors than EA2-EA4. Therefore, no trend between electron-withdrawing ability and coalescence of the two ICT bands can be established. Finally, as observed for A-D-A-1, a coalescence of the bands within a unique absorption band was observed. This situation was also found for A-D-A5 and A-D-A7. Here again, A-D-A5 comprises the strongest electron-acceptor of the series whereas EA7 is of comparable electron-withdrawing ability than EA10. Considering that EA7 and EA10 differs by the alkylation of the barbituric acceptor, once again, no trend between coalescence of the ICT bands or not and electron-withdrawing ability can be established. Considering the difficulty to determine with accuracy the positions of the ICT band of lowest energy for A-D-A triads, the diversity of situations found for A-D-A0-A-D-A11, only the solvatochromism of the dyads D-Ax, x = 0-11 could be analyzed, due to the presence of a unique ICT band. Optical properties of the twenty-four dyes were also examined theoretically by performing density functional theory (DFT) calculations at the B3LYP / 6-311G (d, p) level using the Gaussian09 program. For the different calculations, dichloromethane was selected as the solvent and a polarizable continuum model (PCM) was used to simulate the solvent. A summary of the optical properties is provided in the Table 3. As shown in Figure 5, a redshift of the absorption spectra of all triads compared to that of dyads was predicted. A severe mismatch between the positions of the ICT bands determined experimentally and theoretically can also be evidenced. Thus, comparison of the absorption maxima for D-A0 showed a difference as high as 100 nm between the experimental and the theoretical values (λexp = 531 nm vs. λth = 632 nm). A higher disagreement between the theoretical and the experimental values was found for the triads, resulting from the presence of two ICT bands. While examining the main contributions involved in the optical transitions detected at low energy for all dyes (See contributions in SI), these absorption bands clearly correspond to an admixture of numerous transitions and transitions occurring between HOMO-2 -> LUMO, HOMO-2 -> LUMO+1, HOMO-1 -> LUMO+1, HOMO -> LUMO, HOMO -> LUMO+1 can notably be cited. Additionally, contributions of each transition varying from a dye to another one, it can thus support the mismatch existing between the theoretical and the experimental absorption spectra. Finally, examination of the photoluminescence properties of the different dyes revealed these compounds not to be emissive at all.

8 | J. Name., 2012, 00, 1-3
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Please do not adjust margins Please do not adjust margins This result is not surprising considering that ferrocene is commonly used to quench the photoluminescence of fluorescence dyes. In this field, a relevant example of this has recently been reported with a ferrocene-BODIPY dyad so that the photoluminescence of the highly emissive BODIPY could be efficiently quenched by ferrocene, by photoinduced electron transfer. 83 In the present case, presumably, non-radiative pathways win out because (i) oscillator strengths are smaller than in organic dyes, (ii) Fe leads to greater ISC rates than in typical organics, and perhaps (iii) because of extra internal conversion pathways associated with rotation around Cp-Fe-Cp bond.

Modification of the UV-visible absorption spectra of dyads and triads over time.

Stability of dyes in solution is an important parameter that is required for numerous applications such as photopolymerization, non-linear optics or water waste treatment. In the literature, stability of dyes in solution under dark and light conditions has been extensively studied, especially for azo dyes 84 , polymethine dyes [START_REF] Henary | Stability and Reactivity of Polymethine Dyes in Solution[END_REF] and anthocyanins. [START_REF] Oancea | [END_REF]87 To date, this point has never been examined for ferrocene-based dyes, even if numerous reports can be found in the literature concerning dyes comprising ferrocene as the electron donor. [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] Stability of the twenty-four dyes in solution was thus examined by UV-visible absorption spectroscopy, first for A-D-A0 used as a model compound. As shown in the Figure 6, if no modification of the UV-visible absorption spectra of A-D-A0 was found in anisole, 1,2dichloroethane, dichloromethane, diglyme or toluene, a complete change of the UV-visible absorption spectra was found in solvents such as acetone, acetonitrile, ethyl acetate, chloroform, diethyl carbonate, dioxane, dimethylacetamide, DMF, DMSO, diethyl ether, THF or xylene. It has to be noticed that for this study, only solvents of HPLC grade were used and that no purifications (distillation, dehydration, etc…) were

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Please do not adjust margins Please do not adjust margins carried out prior to analyses. All experiments were carried out at ambient temperature i.e. at 18-20°C. Face to these results, investigations were enlarged to all dyads D-Ax, x = 0-11 and triads A-D-Ax, x = 0-11 and DMSO was considered as an appropriate solvent for this study, all dyes being soluble in this solvent. Interestingly, a complete change of the UV-visible absorption spectra could be evidenced for the 24 dyes in DMSO (See Figure 7). Conversely, control experiment done with ferrocene alone in DMSO revealed its UV-visible absorption spectrum to remain unchanged after 24 hours, in the dark, but also at ambient light. Therefore, the crucial role of the push-pull structure and the unavoidable role of the electron acceptors in these unexpected behaviors was clearly evidenced. Based on previous works reported in the literature, the ability of ferrocene to form a charge-transfer-to-solvent (CTTS) interactions with electron accepting solvents has been notably reported for solvents such as carbon tetrachloride, according to the mechanism proposed below. 88 Precisely, appearance of a CTTS band located in the near-UV visible region was demonstrated with ferrocene, resulting from the oxidation of ferrocene to ferrocenium. [89][90][91] By exciting in this absorption band, a photoinduced electron transfer from ferrocene towards the electron accepting solvent could be demonstrated, enabling to initiate an anionic polymerization. In the present case, due to the electron-withdrawing ability of the different electron acceptors, a redshift of the CTTS band can be expected and absorption maxima varying between 400 and 500 nm could be determined for dyads D-Ax, x = 0-11 and triads A-D-Ax, x = 0-11, after evolution of their UV-visible absorption spectra. Besides, monitoring of the cyclic voltammograms of A-D-A0 over time in DMSO and by using tetrabutylammonium hexafluorophosphate as the supporting electrolyte revealed the new product(s) to be easier to oxidize than the initial dye, the oxidation process being detected at lower anodic potentials (See Figure 8). Therefore, formation of a ferrocenium cation in DMSO cannot be excluded. However, the ferrocenium cation should be more difficult to oxidize than ferrocene due to the loss of one electron. To support the electrochemical process detected in solution, the formation of ferrocenium decomposition products can be tentatively proposed. It has to be noticed that the distinction between ICT and CTTS can be easily done for the following reasons. First, even if a solvatochromism is observed, ICT bands are observed at almost the same positions, irrespective of the solvents. This point has been demonstrated with the solvatochromic study. Conversely, CTTS is a less kinetically favorable process since it can be monitored over time and is not observed immediately after dissolution of the compound. Therefore, positions of the ICT bands given in the solvatochromic study are the correct ones and corroborated by the similarity of values observed in the twenty-three solvents. Interestingly, if a clear modification of the UV-visible absorption spectra of the push-pull dyes was determined in DMSO, conversely, for ferrocene, no modification of the absorption spectrum was found in DMSO and even in the twenty-two solvents, evidencing a specific activation process originating from the presence of the electron-withdrawing groups in dyads and triads. In light of these results and considering that an interaction between the solvent and the push-pull dyes exists, several hypotheses were examined, notably with A-D-A0 used as a model compound. Thus, interactions between ferrocene in the neutral form and DMSO was examined by positioning the DMSO molecule on top or in lateral position with regards to the ferrocene unit. Calculations done by Density-functional theory (DFT) using ωb97xd/6-311++g(d,p) as the basis set revealed that similar energies could be determined in the two situations (See Figure 9) As shown in the Figure 9, the energy difference between solvent molecules standing on top or in lateral position to the ferrocene group is negligible. Consequently, considering that the energy difference is low, no preferential orientations can be expected. Interestingly, a blue-shift of the theoretical UV-visible absorption spectra compared to that determine for A-D-A0 in vacuum conditions could be determined in these two situations.

12 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins

Possibility of bimolecular interactions between A-D-A0

molecules was also envisioned. Besides, in this case, a red-shift of the absorption spectra should be observed for the dimers, what is in contradiction with what is experimentally observed (See Figure 10). Indeed, by theoretical calculations, a maximum absorption around 600 nm is expected, what is not experimentally observed. Besides, from an energy viewpoint, such interactions can theoretically exist. Therefore, based on the different investigations, the formation of a CTTS or the formation of dimers in solutions cannot be proposed as plausible explanations supporting the experimental results. Evolution of the UV-visible absorption spectra over time is also not the result of photoinduced reactions, similar modifications of the UV-visible absorption spectra over time being detected in the dark and under ambient light.

Solvatochromism

Solvatochromic behaviors of dyads D-Ax, x = 0-11 and triads A-D-Ax, x = 0-11 were measured at room temperature in 22 common organic solvents ranging from nonpolar to highly polar solvents. A summary of the optical properties is provided in Tables 4 and5. Intramolecular nature of the charge transfer detected for the dyads D-Ax, x = 0-11 was demonstrated by performing successive dilutions of the solutions and linear correlations between the intensity of the ICT bands and the concentrations could be established. Over the years, numerous empirical polarity scales have been developed to interpret the solvent-solute interaction and the Kamlet-Taft's, 92 Dimroth-Reichardt's, 93 Lippert-Mataga's, 94 Catalan's, 95 Kawski-Chamma Viallet's, 96 McRae Suppan's 97 and Bakhshiev's 98 scales are the most widely used. Among the different empirical polarity scales examined, the solvent polarity/polarizability Catalan (SPP) scale and the Kamlet-Taft polarity scale were determined as being highly useful for analyzing the solvatochromic behaviors of dyads D-Ax, x = 0-11. More precisely, in the case of the Kamlet-Taft empirical scale, better linear correlations were obtained while using a multiparameter approach instead of plotting ΔE = f (π*) (See Table S2 and S3 in SI). However, it has to be noticed that the multiparameter approach better interpret the solvatochromic behaviors of D-Ax, x = 0-7 than for D-Ax, x = 8-11. Indeed, for these four dyes, the linear correlations ΔE = f (π*) were more adapted with regards of their larger values for the square of the correlation coefficient (R 2 ). In fact, when using the multiparameter approach, two parameters are considered in addition to the dipolarity/polarizability π*, namely, the hydrogen bond-donating ability α and the hydrogen bondaccepting ability β. These two parameters are of crucial importance in the case of dyes with electron acceptors comprising NH groups such as D-A6 and D-A7. The same hold true for dyes bearing cyano groups such as D- A0, D-A2, D-A3, D-A4, carbonyl groups such as D-A1, D-A8 and D-A11 or polar groups such as nitro groups. In these different cases, hydrogen bonds can be formed with the precited groups. Conversely, for D-A9 and D-A10, alkylation of the NH groups drastically reduces the sensitivity to the hydrogen-bonding ability of the solvents. Among all dyes, the best correlations were obtained for D-A0, D-A1, D-A8 and D-A11 respectively, with R² > 0.88 (See Figure 11). While comparing the results obtained with the linear correlations based on the Catalan and the Kamlet-Taft empirical scales, higher R² values were obtained with the Catalan SPP parameters, indicating that the polarity/polarizability of the solvents governs the solvatochromism of these dyes. These results were confirmed with the multiparameter approach of the Kamlet-Taft polarity scale for which R² similar to that obtained with the Catalan SPP parameter could be determined. Surprisingly, a positive solvatochromism was found for all dyes except for D-A5 for which a negative solvatochromism was determined (See Figure 12). This behavior is relatively unusual but is sometimes observed for dyes such as merocyanines. [99][100] The pronounced negative solvatochromism observed for D-A5 can be assigned to a highly dipolar electronic ground state and a considerably less dipolar first excited state. Upon increase of the polarity of the medium, the dipolar ground state was more stabilized than the first excited state so that a hypsochromic shift of the ICT band can be observed. 101 The highly polar ground state of D-A5 can be assigned to the exceptional electronwithdrawing ability of TNF, this acceptor being the strongest electron-acceptor reported to date. 102 It has to be noticed that D-A5 has previously been examined for its solvatochromic properties in the literature. 103 However, in this work reported in 2001 by Perepichka and coworkers, the authors already mentioned the complexity of the solvatochromism of this TNFbased dye. Indeed, the Kamlet-Taft or the Reichardt parameter were determined not to be satisfactory to investigate the Please do not adjust margins Please do not adjust margins solvatochromic behavior. Notably, the authors evidenced that the four-parameter Koppel-Palm equation was even unable to rationalize the solvatochromic behavior of D-A5. [104][105][106] In the present case and differing from the previous study of Perepichka and coworkers, an acceptable linear correlation was obtained while using the Catalan SPP parameters since a R² of 0.73 was obtained with this solvent polarity scale. Therefore, it can be concluded that this dye is exceptionally sensitive to the polarity/polarizability of the solvents (taken into account with the Catalan SPP parameter), more than the solvent dipolarity which is primarily considered by the Catalan SdP parameter. Complexity of the solvatochromism for D-A9 can also arise from the unequal contribution of the nitro groups to the mesomeric forms. Indeed, as shown in the Figure 13, the difference of the conjugation length between the electron-donating ferrocene and the nitro groups can support the presence of several ICT bands for D-A9, but also the unexpected solvatochromic behavior.

Finally, sensitivity of the ICT bands of dyads D-Ax, x = 0-11 to the solvent polarity was confirmed by examining the slopes obtained by plotting the HOMO-LUMO gaps vs. the Taft parameters. Indeed, a great slope is indicative of a significant charge redistribution upon excitation. Especially, the zwitterionic form is stabilized in highly polar solvents. As shown in the Figure 14, three dyes were more sensitive than the others to the solvent polarity, namely, D-A4, D-A8 and D-A11, comprising TCF or rhodanines as the electron acceptors.

A-D-A2 2

A-D-A 3 2

A-D-A 4 2

A-D-A 5 2

A-D-A 6 2

A-D-A 7 2

A-D-A 8 2

A-D-A 9 2

A-D-A

Electrochemical properties

The two series of compounds examined in this work were analyzed by cyclic voltammetry (CV) in order to determine their electrochemical properties in a dilute solution of dichloromethane. A set of four CV curves is provided in Figure 15 and all CV curves are given in SI. The redox potentials of all dyes relative to the half-wave oxidation potential of the ferrocene/ferrocenium couple are presented in the Table 6.

In this study, the D-Ax, x = 0-11 series has exactly the same electron donor than the A-D-Ax, x = 0-11 series but differs by the number of electron-accepting groups. As anticipated, major differences could be found concerning their electrochemical properties. Notably, oxidation potentials ranging from + 0.84 V Please do not adjust margins

Please do not adjust margins to + 0.60 V could be determined for the D-Ax, x = 0-11 series, proving that the oxidation is located on the ferrocene moiety of the push-pull dyes. Indeed, the one-electron oxidation process is detected at low oxidation potentials, comparable to that reported for other push-pull dyes comprising ferrocene as the electron donor. 50,[105][106] For D-A8-D-A11, an additional oxidation and reduction peak was also detected in the region of -0.5 V to 0 V. These additional peaks were specifically observed for the thio(barbituric) and the rhodanine-based dyes. Considering that the different electron acceptors possess amino group, an oxidation process occurring on the amino group with formation of a radical cation can be tentatively proposed. As other explanation, an oxidation of the vinylic spacer followed by a dimerization reaction can also be proposed. Indeed, such oxidation/dimerization reaction has previously been reported in the literature for organic dyes comprising good electron donating groups. 107 For the A-D-Ax, x = 0-11 series, a first oneelectron process could be determined at higher oxidation potential than that detected for the dyads. Notably, an oxidation located at + 0.51 V for A-D-A8 to + 1.16 V for A-D-A0 could be thus determined. In some cases, presence of a second oxidation process could be detected at anodic potentials, notably, at + 0.90 V for A-D-A5 and 1.03 V for A-D-A2. Likewise, presence of two or even three reduction potentials could be detected for several dyes, especially for the triads as a result of the presence of two electron-accepting groups. From their redox potentials, energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) could be estimated from formulas previously reported by Pommerehne et al. 108 Values of all HOMO and LUMO energy levels of all molecules are summarized in the Table 6. A comparison between the experimental and theoretical values for the HOMO and LUMO energy levels are presented in Figure 16. In this Figure, a good agreement between the experimental and theoretical results could be evidenced, especially for the D-Ax, x = 0-11 series. Conversely, a greater mismatch was found for the A-D-Ax, x = 0-11 series, especially for the HOMO energy levels. This can be assigned to the fact that the oxidation process is affected by the solvent used to perform electrochemistry but also by the fact that the oxidation process can occur on a specific side of the dye Please do not adjust margins Please do not adjust margins rather than on the ferrocene moiety, as exemplified for the thio(barbituric) or the rhodanine derivatives.

Redoxchromic Properties

Redoxchromism of the two series of compounds were analyzed by UV-visible spectroscopy at a concentration of 8.0 x 10 -5 M for all compounds. For all experiments, five equivalents of nitrosonium tetrafluoroborate (NOBF4) were added to the solutions to oxidize ferrocene while maintaining a reasonable acquisition time. Indeed, while using only one equivalent of oxidant, the reaction proved to be relatively slow. Nitrosonium cation is a strong oxidant used in numerous chemical reactions and extensively used in Organic Chemistry to oxidize many organic compounds. Indeed, the oxidation potential E° of the nitrosonium cation is lower than 1.7 V (versus the normal hydrogen electrode). 109 By oxidizing ferrocene, suppression of the electron donating moiety was clearly evidenced by a color modification of the different solutions. Thus, conversion of the electron-donating ferrocene as an electron acceptor (ferrocenium) induced the disappearance of the intramolecular charge transfer (ICT) band on the UV-visible absorption spectra of the difference dyes. 17. Interestingly, depending on the electron acceptor, different situations could be clearly identified during the chemical oxidation. As anticipated, a decrease of the intensity of the ICT bands could be detected upon oxidation of ferrocene as can be observed, as exemplified with compound D-A2 (See Figure 17a). For compounds D-A0, D-A1, D-A3, D-A5, A-D-A0, A-D-A1, A-D-A2, A-D-A3, A-D-A5 and A-D-A9, a similar behavior was observed, with the loss of the ICT band as well as the absorption peaks around 400 nm and 450 nm due to the oxidation of ferrocene to ferrocenium. For the other compounds, a different behavior could be observed, with the appearance during the oxidation process of a new absorption band which eventually decreases after complete disappearance of the ICT band. This specific behavior can be easily evidenced for D-A4 (See Figure 17b), with a decrease of the intensity of the charge transfer band at 640 nm during 10 minutes followed by the appearance of a new absorption band at 560 nm starting to emerge 7 minutes after the suppression of the ICT band and reaching a maximum of intensity after 14 minutes (See Figure 18c). Finally, a decrease of the band intensity is then observed and a reduction of half of its initial intensity is observed after one hour of reaction (See Figure 17d). This behavior is detected for several dyes including D-A8, D-A11, A-D-A4, A-D-A8 and A-D-A11. To a lesser extent, this behavior can also be demonstrated for D-A6, D-A7, D-A9, D-A10 and A-D-A10, except that the appearance of the new absorption band is extremely weak. This behavior is found for the same acceptors irrespective of the series (D-A or A-D-A series), which is certainly indicative of the presence of side-reactions on the electron acceptor, subsequent to the oxidation of ferrocene to ferrocenium. Thus, for A-D-A11, this reaction led to the precipitation of a solid in the solution, resulting in the acquisition of noisy absorption spectra during precipitation (See Figure 17e). For this compound, one possible explanation supporting the formation of this precipitate is the polymerization of A-D-A11 due to the presence of an allyl chain onto the electron acceptor. This precipitation is not detected for D-A11, also bearing allyl rhodanine as the acceptor. This can be supported by the fact in the case of ADA11, the presence of the two allyl groups allow to form extended chains favoring the precipitation. Conversely, in the case of D-A11, the presence of a unique polymerizable group per dye only allows the formation of dimers that remain soluble in dichloromethane. Please do not adjust margins Please do not adjust margins

Theoretical investigations

In light of the unexpected optical properties of the triads, electronic distribution of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) were determined theoretically. DFT calculations were carried out at the B3LYP/6-311G(d,p) level of theory using Gaussian 09 programs. Dichloromethane was used as the solvent and the polarizable continuum model (PCM) was used as the solvent model for the TD-DFT calculations. [74][75][76][77][78][79] More details about the theoretical calculations are given in SI. As shown in Figure 18, a classical distribution of the molecular orbitals was found for the different dyads, with the HOMO energy level located on the ferrocene moiety and the LUMO energy level on the electron-accepting part. Conversely, an unusual electronic distribution was found for the triads (See Figure 19). Thus, for A-D-Ax, x = 0-7 and A-D-A10, a classical electronic distribution could be evidenced. On the opposite, for A-D-A8 and A-D-A10, a HOMO-1 and a HOMO level extending on the ferrocene and the rhodanine moiety was determined. More surprisingly, for A-D-A9 which possesses the strongest electron-withdrawing group, no electronic distribution of the ferrocene moiety was found nor for the HOMO neither the HOMO-1 energy levels. In fact, HOMO and HOMO-1 levels located onto the TCF moiety were determined. Only an electronic distribution on HOMO-2 can be found on the ferrocene moiety. By theoretical calculations, the energetic transitions involved in the optical transitions could be determined. Thus, as shown in the Table 7, all ICT bands are mainly issued from an admixture of a HOMO → LUMO and a HOMO-1→LUMO transition. These results are consistent with the aforementioned distributions of the electronic density detected for ferrocene. Indeed, HOMO and HOMO-1 are mainly located on the electron donor. These two orbitals are also the highest occupied orbitals, favorable to an electron transfer towards the LUMO level upon excitation. Please do not adjust margins Please do not adjust margins

Conclusions

To conclude, a series of twenty-four dyes comprising ferrocene as the electron donor have been designed and synthesized. All dyes showed a strong absorption extending over the visible range. An efficient tuning of the absorption maxima could be obtained by mean of the number of electron acceptors per dyes but also by mean of the electron-withdrawing ability. Among the most interesting findings, a positive solvatochromism was found for all dyes except for the triad comprising the tetranitrofluorene acceptor for which the ground state is more polarized than the excited state. Parallel to this, chemical oxidation of ferrocene in the different dyes suppressed the electron donating ability of ferrocene so that a complete color change of the different solutions could be demonstrated. Use of an excess of nitrosonium cation during the chemical oxidation of ferrocene to ferrocenium induced additional reactions onto the electron acceptors subsequent to the oxidation of ferrocene. Among the most surprising findings, a Please do not adjust margins Please do not adjust margins drastic modification of the UV-visible absorption spectra of the 24 dyes could be observed in numerous solvents and this modification of the UV-visible absorption spectra has never been reported in the literature, even if some of these dyes have been synthesized prior to this work. If several hypotheses have been examined to support the modification of the absorption spectra over time, isolation and identification of the different reaction products deserve to be investigated in future works. Indeed, if the phenomenon could be clearly evidenced, no clear conclusions could be established concerning the exact origin of these modifications.

Figure 2 .

 2 Figure 2. Chemical structures of electron acceptors EAx, x = 0-11 and A-D-A triads A-D-Ax, x = 0-11.

Scheme 1 .

 1 Scheme 1. Synthetic route to D-A dyads D-Ax, x = 0-11 and A-D-A triads A-D-Ax, x = 0-11

Table 1 .

 1 Reaction yields obtained for the synthesis of dyads D-Ax, x = 0-11 and triads Adone in acetic anhydride. b : reaction done in DMF

7 Please

 7 This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 |

Figure 3 .

 3 Figure 3. UV-visible absorption spectra of D-A dyads D-Ax, x = 0-11 (a and b) and A-D-A triads A-D-Ax, x = 0-11 (c and d) in chloroform.

Figure 4 .

 4 Figure 4. Normalized UV-visible absorption spectra of A-D-A triadsA-D-A0, A-D-A1, A-D-A2 and A-D-A10 in chloroform.

Figure 5 . 11 PleaseFigure 6 .

 5116 Figure 5. Simulated absorption spectra of D-A dyads D-Ax, x = 0-11 and A-D-A triads A-D-Ax, x = 0-11 in dichloromethane.

Figure 7 .

 7 Figure 7. Evolution of the UV-visible absorption spectra of D-A1 and A-D-A1 in DMSO over time

Figure 8 .

 8 Figure 8. Cyclic voltammograms of A-D-A0 over time in DMSO solutions at room temperature.

Figure 9 . 13 PleaseFigure 10 .

 91310 Figure 9. The two possible interactions between A-D-A0 and DMSO with a) DMSO on top of the ferrocene unit (model 1) and b) DMSO in lateral position (model 2)

14 |

 14 J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Figure 11 .

 11 Figure 11. Linear correlations obtained for D-A0 and D-A1 using the Kamlet-Taft and the Catalan empirical scales.

Figure 12 .

 12 Figure 12. Linear correlations obtained for D-A5 using the Kamlet-Taft and the Catalan empirical scales.

Figure 13 .

 13 Figure 13. The different mesomeric forms supporting the presence of two ICT bands for the TNF-based dye D-A9.

1Figure 14 . 17 Please do not adjust margins Please do not adjust margins Table 5 .

 14175 Figure 14. Variations of the HOMO-LUMO gaps with the Taft parameters.

Figure 15 .

 15 Figure 15. Comparisons between the cyclic voltammograms of D-A5/A-D-A5 (up) and D-A11/A-D-A11 (down) measured in dichloromethane, with tetrabutylammonium perchlorate (0.1 M) as the supporting electrolyte

Figure 16 .

 16 Figure 16. Comparison of frontier orbitals' energy levels obtained from cyclic voltammetry and DFT calculation.

  From a mechanism viewpoint, several mechanisms can be proposed. Notably, polymerization of A-D-A11 can be induced by an electron transfer reaction between the nitrosonium cation and the allyl function present onto D-A11 and A-D-A11 (See Scheme 2a), inducing the formation of radicals and initiating the polymerization or the dimerization process.[110][111] Parallel to this, for D-A8, D-A11, A-D-A8 and A-D-A11, other reactions are possible on the rhodanine acceptors such as the addition of the nitrosonium cation to the thioether function (See Scheme 2b). For these rhodanine-based compounds but also for compounds having an amine function such as D-A6, D-A7, D-A9, D-A10, A-D-A6, A-D-A7, A-D-A9 and A-D-A10, the nitrosation of the amines is also possible, with the addition of the nitrosonium cation leading to a dealkylation of the tertiary amine (See Scheme 2c). Concerning D-A4 and A-D-A4, the two dyes comprise 2-(3cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (TCF) as the electron acceptor with numerous cyano groups. In this last case, the nitrosonium cation can also add onto the cyano groups of TCF, modifying its electron-withdrawing ability (See Scheme 2d). Finally, another plausible hypothesis, the nitrosonium cation can also add onto the ether function in a similar way to that observed for the thioether functions of This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 21 Please do not adjust margins Please do not adjust margins rhodanines (See Scheme 2e). In light of the numerous reactions which can occur between the nitrosonium cation and the electron acceptors, it clearly supports the multiple changes detected onto the different absorption spectra during the kinetics of discoloration.

Figure 17 .

 17 Figure 17. UV-Visible spectrum of (a)D-A2, (b) full spectra for D-A4 (c) during the first minute for D-A4, (d) during the last minute for D-A4 and (e) A-D-A11 in dichloromethane.

Scheme 2 .

 2 Scheme 2. Proposed reaction occurring with the use of nitrosonium cation on the different functions of the electron acceptor part

Figure 18 .

 18 Figure 18. Contour plots of the electronic distribution of the HOMO and LUMO orbitals of D-A0 and D-A2.

Figure 19 . 24 |

 1924 Figure 19. Contour plots of the electronic distribution of the HOMO and LUMO orbitals of A-D-A0 and A-D-A8 and A-D-A9

ARTICLE Journal Name 28 |

 28 J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

  ED2

Table 2 .

 2 Summary of the UV-visible absorption characteristics of all dyads and triads in dilute chloroform.

	Compounds	max (nm)	Epsilon (L.mol -1 .cm -1 )	max (ICT1) (nm)	Epsilon (ICT) (L.mol -1 .cm -1 )	ΔE (eV)
	D-A0	326	13980	531	2490	2.33
	D-A1	361	25070	571	6000	2.17
	D-A2	319	24180	629	6720	1.97
		413	12200
	D-A3	318	14000	623	3000	1.98
	D-A4	424	24890	630	9750	1.97
	D-A5	366	43700	607	7964	2.00
		427	20300
	D-A6	377	24900	583	5650	2.13
	D-A7	356	20300	565	4850	2.20
	D-A8	382	26920	542	6200	2.29
	D-A9	377	31900	583	7550	2.13
	D-A10	353	21700	558	5100	2.22
	D-A11	382	26150	542	6200	2.29
	A-D-A0	341	17700	445	2400	2.78
				533	2500	2.33
	A-D-A1	375	18500	572	6300	2.17
	A-D-A2	314	30200	580	4800	2.14
	A-D-A3	-a	-a	632	6300	1.97
	A-D-A4	311	12300	603	1600	2.05
		412	4800
	A-D-A5	362	7500	616	1350	2.01
		429	3400
	A-D-A6	n.s.	n.s.	n.s.	n.s.	n.d.
	A-D-A7	n.s.	n.s.	n.s.	n.s.	n.d.
	A-D-A8	364	29600	474	4750	2.62
		393	20400	536	4300	2.31
	A-D-A9	309	17800	568	3100	2.18
		395	18000
	A-D-A10	311	18400	463	2900	2.67
		365	10700	564	2450	2.20
	A-D-A11	363	34800	529	4843	2.34

n.s.: not soluble, a : peak detected below 300 nm This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9

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Table 3 .

 3 Summary of the theoretical absorption characteristics of dyads and triads in dichloromethane.

	Compounds	max (nm)	Epsilon (L.mol -1 .cm -1 )	max (ICT1) (nm)	Epsilon (ICT) (L.mol -1 .cm -1 )
	D-A0	323	37500	632	1350
	D-A1	358	50400	650	2400
	D-A2	417	25600	687	4000
	D-A3	438	27500	672	5000
	D-A4	429	80500	683	6200
	D-A5	401	28100	546	7700
				831	3300
	D-A6	305 (sh)	4300	669	2500
		371	55800
	D-A7	295 (sh)	6300	650	1800
		339	39250
	D-A8	293	12200	625	2400
		388	42000
	D-A9	300	5450	659	2800
		374	56700
	D-A10	250	7600	643	1900
		289	6400
		338	42800
	D-A11	294	10900	623	2400
		388	41900
	A-D-A0	351	59000	712	1300
	A-D-A1	394	43500	745	2200
	A-D-A2	450	52000	520 (sh)	23700
	A-D-A3	492	16200	751	3600
	A-D-A4	477	96300	765	5400
	A-D-A5	458(sh)	8800	796	6600
		539	25400
	A-D-A6	439	15500	790	1700
	A-D-A7	370	68250	759	1150
	A-D-A8	429	69500	680	3200
	A-D-A9	421	30300	777	1700
	A-D-A10	369	73100	745	1300
	A-D-A11	428	69700	683	3100

Table 4 .

 4 Summary of the optical properties of D-Ax, x = 0-11 in twenty-two solvents and Kamlet and Taft parameters π*

	compounds	π* 1	D-A0 2	D-A1 2	D-A2 2	D-A3 2	D-A4 2	D-A5 2	D-A6 2	D-A7 2	D-A8 2	D-A9 2	D-A10 2	D-A11 2
	acetone	0.71	524	565	624	612	611	606	565	545	534	571	544	535
	acetonitrile	0.75	528	566	617	614	618	598	573	537	536	572	545	538
	AcOEt	0.54	523	563	615	607	602	609	566	543	529	567	544	531
	anisole	0.73	530	567	617	616	619	619	575	552	536	575	548	539
	chloroform	0.78	533	573	631	619	629	607	585	564	543	581	558	543
	cyclohexane	0.00	516	554	613	617	591	643	n.s. 3	n.s. 3	514	560	538	514
	1,2-dichloroethane	0.81	531	569	627	611	627	624	582	558	537	574	550	540
	dichloromethane	0.82	530	569	628	620	628	609	584	561	541	578	551	542
	diethyl carbonate	0.45	523	560	626	608	594	602	565	542	529	567	542	529
	diethyl ether	0.27	523	559	617	617	603	613	560	542	524	567	540	526
	1,4-dioxane	0.55	519	562	616	607	596	596	564	544	532	567	543	530
	dimethylacetamide	0.88	530	572	626	627	619	595	567	544	526	579	547	530
	DMF	0.87	529	571	624	616	622	584	566	545	526	576	546	529
	DMSO	1.00	531	575	626	620	628	591	572	548	524	579	550	526
	ethanol	0.54	528	569	624	616	625	591	570	553	538	576	n.s. 3	536
	heptane	-0.08	515	554	612	613	585	n.s. 3	n.s. 3	n.s. 3	511	557	538	513
	nitrobenzene	1.01	534	573	639	625	636	605	581	558	543	580	557	545
	pentane	-0.09	512	551	611	609	n.s. 3	605	n.s. 3	n.s. 3	512	556	534	515
	THF	0.58	525	565	620	622	607	609	567	543	531	572	545	533
	toluene	0.54	525	565	620	616	602	616	572	551	530	572	545	529
	triethylamine	0.14	519	558	n.s. 3	n.s. 3	597	n.s. 3	567	n.s. 3	519	n.s. 3	564	522
	p-xylene	0.43	522	562	617	618	593	614	570	550	527	574	543	529

  Kamlet and Taft parameters 2 Position of the ICT bands are given in nm 3 n.s.: Not soluble

														A-D-A
													10 2	11 2
	acetone	0.71	437, 525	562	539	631	597	617	608	545	467, 537	487, 572	451, 546	478, 526
	acetonitrile	0.75	392, 522	566	429, 532	628	603	614	596	537	473, 538	482, 576	457, 547	492, 516
		0.54	438,	562	br.	626	402,	602	n.s. 3	543	541	479,	454,	468,
	AcOEt		525		band		426,					571	558	539
							583
	anisole	0.73	442, 534	568	557, 603	637	n.s. 3	609	584, 613	552	537	480, 585	458, 556	473, 536
	chloroform	0.78	445, 530	573	580	632	600	n.s. 3	n.s. 3	564	474, 536	480, 568	459, 557	475, 529
	cyclohexane	0.00	521 429,	555	n.s. 3	624	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	583 471,	n.s. 3	522
	1,2-dichloroethane	0.81	443, 531	569	557, 624	637	593, 614	620	n.s. 3	558	470, 536	484, 565	462, 550	475, 536
	dichloromethane	0.82	445, 527	n.s. 3	574	637	606	620	n.s. 3	561	470, 536	484, 580	460, 554	478, 538
	diethyl carbonate	0.45	436, 526	562	574	626	580	602	n.s. 3	542	530	472, 576	453, 559	463, 536
	diethyl ether	0.27	434, 526	559	n.s. 3	626	n.s. 3	602	n.s. 3	542	524	n.s. 3	n.s. 3	467, 520
	1,4-dioxane	0.55	437, 523	564	br. band	632	577	596	473, 562	544	537	474, 572	452, 547	475, 536
		0.88		573	544,	699	401,	624	575	543	493,	480,	467,	439,
	dimethylacetamide				650		468,				522	582	561	545
							617
		0.87	528	571	441,	608	399,	n.s. 3	570	545	502,	br.	461,	504,
	DMF				521		459,				521	band	557	520
							618
		1.00	399,	574	440,	599	401,	n.s. 3	580	548	502,	491,	467,	501,
	DMSO		533		535		466,				521	580	553	521
							611
		0.54	439,	575	424,	588	398,	593	563	553	542	478,	474,	474,
	ethanol		522		498		449,					578	553	532
							611
	heptane	-0.08	520 428,	553	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	516
	nitrobenzene	1.01	529	574	br. band	633	592, 633	642	n.s. 3	558	478, 547	486, 586	591, 638	484, 561
	pentane	-0.09	559	552	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	507
	THF	0.58	435, 524	564	555	633	594	608	470, 581	543	538	474, 595	452, 544	468, 542
	triethylamine	0.14	503	559	n.s. 3	621	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	n.s. 3	527
	toluene	0.54	441, 529	564	587	630	413, 578	594	n.s. 3	551	528	477, 581	454, 558	467, 538
	p-xylene	0.43	440, 524	563	587	630	418, 598	600	n.s. 3	550	516	471, 577	453, 562	471, 536
	1

Table 6 .

 6 Electrochemical This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins During these analyzes, monitoring of the discoloration was obtained by recording a spectrum each minute. All spectra are provided in SI. A panel of three representative kinetics are given in Figure

	characteristics of studied compounds D-Ax, x = 0-11 and A-D-Ax, x = 0-11
	Ered	Ered	Ered	EOx	EOx	EHOMO	ELUMO	ΔEET	ΔEopt
	V/Fc	V/Fc	-	-	V/Fc	eV		eV	eV	eV
	D-A0	-	-1.36	-	-	0.84	-5.64	-3.43	2.21	2.33
	D-A1	-	-1.27	-	-	0.72	-5.52	-3.52	2.00	2.17
	D-A2	-	-0.88	-	-	0.71	-5.51	-3.92	1.60	1.97
	D-A3	-0.91	-0.71	-	-	0.60	-5.40	-4.09	1.31	1.98
	D-A4	-	-0.86	-	-	0.62	-5.42	-3.94	1.49	1.97
	D-A5	-0.68	-0.49	-	-	0.69	-5.49	-4.30	1.19	2.00
	D-A6	-	-1.00	-	-	0.80	-5.60	-3.79	1.81	2.13
	D-A7	-	-1.17	-	-	0.74	-5.54	-3.63	1.91	2.20
	D-A8	-	-1.33	-0.62	-0.30	0.74	-5.55	-3.46	2.08	2.29
	D-A9	-	-1.30	-0.63	-0.04	0.71	-5.52	-3.49	2.03	2.13
	D-A10	-	-1.39	-0.70	-0.08	0.76	-5.56	-3.40	2.16	2.22
	D-A11	-	-1.29	-0.56	-0.23	0.75	-5.55	-3.50	2.05	2.29
		Ered	Ered	Ered	EOx	EOx	EHOMO	ELUMO	ΔEET	ΔEopt
		V/Fc	V/Fc	V/Fc	V/Fc	V/Fc	eV	eV	eV	eV
	A-D-A0	-	-	-1.08	1.16	-	-5.96	-3.72	2.25	2.33
	A-D-A1	-	-1.13	-0.78	0.85	-	-5.65	-4.02	1.62	2.17
	A-D-A2	-	-0.93	-0.63	0.74	1.03	-5.54	-4.17	1.37	2.14
	A-D-A3	-1.28	-0.91	-0.60	0.82	-	-5.62	-4.20	1.42	1.97
	A-D-A4	-	-	-0.72	0.80	-	-5.60	-4.08	1.52	2.05
	A-D-A5	-1.02	-0.69	-0.50	0.70	0.90	-5.50	-4.30	1.20	2.01
	A-D-A6	n.s	n.s	n.s	n.s	n.s	n.s	n.s	n.s	n.d
	A-D-A7	n.s	n.s	n.s	n.s	n.s	n.s	n.s	n.s	n.d
	A-D-A8	-	-	-1.16	0.51	0.97	-5.31	-3.64	1.67	2.31
	A-D-A9	n.s	n.s	n.s	n.s	n.s	n.s	n.s	n.s	2.18
	A-D-A10	-	-	-1.10	0.83	-	-5.63	-3.70	1.93	2.20
	A-D-A11	-	-1.35	-1.11	0.54	0.97	-5.34	-3.68	1.65	2.34

All potentials recorded in 0.1M TBAClO4/CH2Cl2. EHOMO (eV) = -4.8 -Eox and ELUMO (eV) = -4.8 -Ered 20 | J. Name., 2012, 00, 1-3

Table 7 .

 7 Summary of simulated absorption characteristics in dilute dichloromethane of synthetized compounds. Data were obtained in dichloromethane solution

	Compounds	EHOMO (eV)	ELUMO (eV)	max (nm)	Transitions
	D-A0	-6,326	-2,773	669	HOMO-1->LUMO (64%)
				632	HOMO->LUMO (51%)
				536	HOMO-2->LUMO (24%)
				500	HOMO->LUMO+1 (62%)
	D-A1	-5,887	-2,657	689	HOMO-1->LUMO (64%)
				653	HOMO->LUMO (54%)
				506	HOMO->LUMO+3 (63%)
	D-A2	-6,071	-3,291	744	HOMO-1->LUMO (57%)
				688	HOMO->LUMO (53%)
				515	HOMO->LUMO (37%)
				513	HOMO->LUMO+3 (45%)
	D-A3	-6,360	-3,613	746	HOMO-1->LUMO+1 (76%)
				683	HOMO->LUMO+1 (67%)
				597	HOMO->LUMO (97%)
				563	HOMO-1->LUMO (85%)
				507	HOMO->LUMO+3 (48%)
	D-A4	-6,336	-3,459	736	HOMO-1->LUMO (74%)
				687	HOMO->LUMO (62%)
				553	HOMO-3->LUMO (36%)
				512	HOMO->LUMO+2 (48%)

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 25 This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 27

Acknowledgements

The authors thank Aix Marseille University and The Centre National de la Recherche Scientifique (CNRS) for financial supports. The Agence Nationale de la Recherche (ANR agency) is acknowledged for its financial support through the PhD grant of Guillaume Noirbent (ANR-17-CE08-0054) VISICAT project. The Direction Générale de l'Armement (DGA)/Agence Innovation Defense (AID) is acknowledged for its financial support through the PhD grant of Dr. Damien Brunel.

Conflicts of interest

There are no conflicts to declare.