Yves Mahéo

Frédéric Guidec

Luc Courtrai

Towards Resource-Aware Parallel Components

Keywords: Parallel components, resource-awareness, distributed applications, monitoring, Java

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Introduction

Building distributed applications is known to be a difficult task, more especially as the targeted architectures become more and more heterogeneous. Indeed, there is a growing demand for distributed applications that can be ported on a wide variety of platforms. Such a platform is for example a cluster obtained by assembling regular workstations in a lab, and may include devices linked through wireless connections. The component-oriented approach can help significantly in developing complex applications. So-called "software components" can serve as deployment units, and further be assembled while developing other components, or full-featured applications. Although the advantages of this approach are now widely admitted, little effort has been invested so far on the development of components specifically designed for distributed platforms.

We present in this paper a middleware platform for "parallel components", that is, components that are meant to be deployed on a cluster 1 and that each encapsulate a parallel and distributed code. Moreover, we provide these components with means to perceive their execution environment, so they can adapt dynamically to the changes they can observe in runtime conditions. By doing so, we try to foster the development of adaptive parallel components.

Although component adaptability is our prime objective, this paper does not focus on this topic per se. Instead it presents a Java-based middleware platform we designed in order to provide support for such components. The platform Concerto thus implements a number of facilities for deploying, launching, and controlling the execution of parallel components on a distributed system. Concerto relies on facilities we have implemented in D-RAJE (Distributed Resource-Aware Java Environment), an open and extensible framework that makes it possible to monitor the state of the many resources offered in a cluster.

Support for resource modeling and monitoring

One of the main objectives of project Concerto is to provide parallel software components with means to perceive their runtime environment, so that they can adapt to variations observed in its characteristics. Environment modeling and monitoring in distributed systems has already been addressed in numerous projects pertaining for example to Grid computing or to network computing (such as [START_REF] Czajkowski | Grid Information Services for Distributed Resource Sharing[END_REF][START_REF] Krauter | A Taxonomy and Survey of Grid Resource Management Systems for Distributed Computing[END_REF][START_REF] Kon | 2K: A Distributed Operating System for Dynamic Heterogeneous Environments[END_REF][START_REF] Sacerdoti | Wide Area Cluster Monitoring with Ganglia[END_REF]). In many of these projects, though, resource modeling is performed at a rather coarse grain (eg computing node, global free memory, network link), and information about resources is often limited to system resources, and collected using ad hoc methods. An important part of Concerto is made of a software framework called D-RAJE. This platform was designed so as to offer an extensible model in which any kind of resource in a distributed environment can be reified as a Java object. It also provides various facilities for monitoring these resources in a homogeneous way. A brief description of D-RAJE is given below (see [START_REF] Guidec | A Java Middleware Platform for Resource-Aware Distributed Applications[END_REF] for a more detailed presentation).

Resource modeling

With D-RAJE a distributed system can be modeled and monitored using Java objects that reify the various resources offered in this system. As a general rule, we qualify as "resource" any hardware or software entity a software component is liable to use during its execution. To date the types of resources considered in D-RAJE -and thus in the Concerto platform-include system resources (CPU, system memory, swap, network interfaces, etc.) that chiefly characterize the underlying hardware platform, as well as so-called "conceptual" resources (sockets, processes, threads, directories, files, RMI servers, etc.) that rather pertain to the applicative environment in which a component is running. An extensible hierarchy of classes models these resources. Any resource object (that is, any instance of one of these classes) is capable of producing on demand an observation report, which provides a summary of its state.

From the viewpoint of an application programmer, resource objects can be created explicitely, or one can rely on a resource discovery monitor that is able to automatically instantiate (and destroy) resource objects pertaining to some classes of resources.

At creation time, resource objects are added to a local resource register. A fully distributed resource manager, accessible on each node of the system, offers the view of a global register and thus makes it possible to access to information about resources in the same way, regardless of where the resources considered are actually located in the system. D-RAJE somehow compares to the Common Information Model [START_REF][END_REF] as far as resource modeling is concerned. However, D-RAJE goes beyond pure modeling as it additionally implements facilities for collecting and exploiting information about the state of any system or conceptual resource.

Resource selection and observation

The resource manager can be consulted when looking for a specific kind of resource in the system, or in order to request information about the status of any resource in the system. Since the population of resource objects can be quite large, a selection can be applied based on their location (e.g. on a specific node, globally or in the neighbourhood), their types, or any other criterion. Resource filtering is obtained by using so-called resource patterns. Any programmer can define new patterns, although many patterns are already available as predefined patterns in D-Raje. With these patterns, one can for example identify all objects modeling CPUs, network interfaces or TCP sockets. These objects can additionally be searched for on a specific cluster node, or in the whole cluster. If required one can even perform a more selective search, and look for example for TCP or UDP sockets that have been bound to a local port whose number is in a specific range (say, values between 0 than 1024), and that maintain connections with remote hosts whose IP addresses conform to a specific pattern (say, all adresses in domain 195.83.160/22). Resource identification can thus be achieved at quite a fine grain if needed.

Once a resource has been identified, information on its status can be obtained through any one of two observation mechanisms. The first of these mechanisms allows direct observation of the resource considered: the resource manager can be asked to collect and return an observation report concerning a specific resource, wherever this resource is located in the system. The second obervation scheme relies on an event-based model: local or distant monitors are automatically created when needed. These monitors can observe the resource considered periodically, and notify the user when user-specified events occur. [START_REF] Guidec | A Java Middleware Platform for Resource-Aware Distributed Applications[END_REF] provides a thorough description of these topics.

Parallel Components

The Concerto platform is dedicated to supporting adaptive parallel components. Although the concept of parallel component is central in our project, our aim is not to propose a new component model, but to provide an infrastructure that enforces the adaptability of components. We propose below a definition of what we mean by parallel component in Concerto.

A programmer who wishes to develop a parallel component for Concerto must design his component as a set of cooperating threads. He must also define the business part of the component (name, interface, implementation). The platform offers facilities for managing non-functional aspects of the component.

Component interface

Any parallel component hosted by the Concerto platform must defined the following interfaces.

• Business interface: No constraint is put on the type of the business interface. The programmer of a component may for example propose an interface based on Java RMI. The component is then an object implementing the Remote interface, whose methods will be called remotely by the clients of the component. The component may also behave as a server listening to a specific TCP or UDP port, with which a client program can communicate. Alternatively, one may design a distributed interface (that is to say, an interface associated with several objects that each implement a part of the interface) in order to be connected in parallel with another parallel component.

• Life cycle interface: Through this interface, the different steps of the life of the component can be controlled. To date, this covers deploying the component on the cluster, launching all the activities (ie starting the threads) of the components and stopping these activities. In the future we may include persistence services in this interface.

• Resource interface: The component exhibits a resource interface through which the user can access information related to the resources used by the component. Actually, the component itself is considered as a resource. Hence any component in Concerto must implement a method observe() that returns an observation report. By default, the observation report generated by a component simply aggregates the observation reports of all the resources it itself uses. For security reasons, the component's programmer may change this default implementation, for example by restricting the amount of information disclosed to the component's client.

Internal Structure

When building a parallel component, the programmer develops a set of Java threads that cooperate to perform the functions specified in the component's business interface.Threads are gathered into placement (or distribution) entities called fragments. A fragment is a set of threads that belong to a single component, and that must run within the same virtual machine on the same cluster node. The threads of a fragment will thus be able to share a common object space. Communication and synchronization between such threads are performed just like in any other multi-threaded Java program. On the other hand, threads that belong to different fragments must rely on external communication and synchronization mechanisms, such as sockets and RMI.

It must be noticed that information on the component's structure is available through facilities implemented in D-Raje, since threads, fragments and the component itself are modeled as resources in D-Raje. As already mentioned, some of this information is also available to other components through each component's resource interface.

Deployment

The deployment of a distributed application on a parallel platform is known to be a tedious and error-prone task. In order to alleviate the deployment of parallel components on a cluster of workstations, we have developed an XML dialect that permits a component programmer to describe the component's structure, placement directives for its fragments, and constraints imposed for the deployment to be feasible (availability of a specific version of the JVM, of a RMI registry, etc.)

The Concerto platform includes an administration tool with which a cluster of workstations can be managed, using either a command line or a graphical user interface. With this administration tool workstations can be added to or removed from the cluster at any time. A parallel component can be loaded in the platform by providing a .jar file that aggregates Java code for the component's threads, an XML deployment descriptor, and possibly additional documentation and data associated with to the component. During this loading operation, a new Component resource object is created in D-RAJE, and automatically registered with the resource manager. Once a component has been loaded, it can be considered as a new resource in the cluster. Another component can therefore trigger its deployment and activation on the cluster. Alternatively, these operations can be triggered by any user of the platform through the abovementioned administration tool.

Implementation and experimentation

The Concerto platform is still under development. The overall architecture of the Concerto middleware on one host is depicted in figure 1. The main part of Concerto consists in portable Java code. A small part of D-Raje requires platform-specific native code to implement system resource monitoring. Presently, only Linux is supported but thanks to the design of D-Raje, porting to other systems should not be a difficult task. In the current prototype, the support for monitoring of some conceptual resources (eg threads, sockets) relies on a modified version of the Kaffe virtual machine and instrumented standard Java classes. Several experiments on distributed applications are currently carried out. We are developing for example a Concerto component that implements a parallel active router for wide area networks. This component can dynamically apply an appropriate service (filtering, compression, encryption,...) to each of the streams that passes through the router. The parallel router-component is deployed on a high-performance cluster in order to accept several concurrent streams and to sustain acceptable bandwidth. In this case, the use of Concerto eases developing multi-criteria adaptation strategies to perform load-balancing according to resource availability.

Related Work

The originality of the Concerto approach lies in the fact that it combines a parallel component model together with some support for distributed resource observation. Componentbased application development is already possible through the use of "standard" component technologies, such as Microsoft COM [START_REF]The Component Object Model Specification[END_REF] or Sun's Enterprise Java Beans. The OMG (Object Management Group) also developed its own solution with the CORBA Component Model [START_REF] Omg | CORBA Components[END_REF]. However these technologies were not specifically designed to support parallel components, that is, components that involve parallel activities. Few works have been carried out on models or platforms that target parallel and distributed components. One of them is the Common Component Architecture [START_REF] Amstrong | Towards a Common Component Architecture for High-Performance Scientific Computing[END_REF], which defines a component model suited for parallel scientific applications. The main objective in CCA is to allow the efficient interoperability of pre-existing scientific codes. Like in Concerto, few constraints are put on the way the functional part of a parallel component is implemented. However, no support is provided for component adaptation. Project Padico [START_REF] Denis | Padico: A Component-Based Software Infrastructure for Grid Computing[END_REF] bears similarities with CCA as it is dedicated to coupling scientific codes. It extends the Corba Component Model, and relies on a specific communication toolkit in order to provide efficient communication between several SPMD parallel codes encapsulated in components. Some form of adaptation is provided in the communication layer that allows for several commonly used communication libraries.

The work described in [2] is also focused on parallel and distributed components: an implementation of the Fractal [START_REF] Bruneton | Recursive and dynamic software composition with sharing[END_REF] hierarchical component model is proposed in order to provide so-called Grid components. This implementation is based on the Pro-Active library [START_REF] Caromel | Towards Seamless Computing and Metacomputing in Java[END_REF]. It provides facilities for asynchronous communication, migration of activities, deployment and debugging. Like in Concerto, a parallel component may be implemented according to the MIMD model. Introspection on both the components structure and placement is possible, though both kinds of introspection rely on disctinct mechanisms.

None of the abovementioned works proposes a homogeneous way to get precise information about the environment of a component (including the system), and about its subcomponents.

Conclusion

In this paper we have presented the Concerto platform, which aims at allowing the deployment and the support of parallel components on clusters of workstations. Ongoing work focuses on proposing a basic parallel component model, as well as tools for the deployment of such components. In a first stage, our objective is to adopt as simple and flexible a model as possible.

The clusters targeted by the Concerto platform are primarily those composed by enlisting non-dedicated, heterogeneous, and possibly unstable workstations. The runtime environment offered to parallel components is therefore liable to exhibit much heterogeneity and dynamicity. For this reason, the Concerto platform implements facilities for discovering how resources are distributed in the cluster, and for monitoring these resources at runtime.

The development of the Concerto platform is still in progress. Moreover a number of parallel components are also being developed. In the near future we plan to use the platform for deploying instances of these components on different kinds of cluster platforms.

Aknowledgments

This work is supported by the French Ministry of Research in the framework of the ACI GRID programme.

Figure 1 :

 1 Figure 1: Overview of the Concerto platform (on one host)

In this paper, we use the term cluster to refer to the distributed platform targeted by the deployment of a component. It is not restricted to a high-performance LAN but can be any heterogeneous network of computers.


Prof Zakaria F Ziani

Frédérique Loiseau

Dr Elise Lognon

Martial Boggio-Pasqua
email: martial.boggio@irsamc.ups-tlse.fr

Christian Philouze

Saioa Cobo

Guy Royal
email: guy.royal@univ-grenoble-alpes.fr

Dr C Loiseau

Dr S Philouze

Prof G Cobo

Synthesis of a Negative Photochrome with High Switching Quantum Yields and Capable of Singlet Oxygen Production and Storage

Keywords: photochromism, dimethyldihydropyrene, singlet oxygen, endoperoxide, spin-flip time-dependent density functional theory

A dimethyldihydropyrene (DHP) photochromic unit has been functionalized by donor (triphenylamine group) and acceptor (methyl-pyridinium) substituents. This compound was characterized by NMR, absorption and emission spectroscopies as well as cyclic voltammetry and its properties were rationalized by theoretical calculations. The incorporation of both electron donor and withdrawing groups on the photochromic center allows i) an efficient photo-isomerization of the system when illuminated at low energy (quantum yield: Фc-o = 13.3% at lex.= 660 nm), ii) the reversible and quantitative formation of two endoperoxyde isomers when illuminated under aerobic conditions at room temperature and iii) the storage and production of singlet oxygen. The photo-isomerization mechanism was also investigated by spin-flip TD-DFT (SF-TD-DFT) calculations.

Introduction

Organic photochromic molecules, [1][2][3][START_REF] Balzani | Molecular Devices and Machines[END_REF][START_REF] Feringa | Second, Completely Revised and Enlarged Edition[END_REF] such as the well-known spiropyranes, dithienylethenes, azobenzenes or the more recently developed DASA (donor-acceptor Stenhouse adducts) derivatives can be reversibly converted between (at least) two isomeric forms on exposure to light of different wavelengths and these isomers may exhibit distinct properties such as polarisability, luminescence, solubility, shape or conductivity. These photochroms can thus play the role of active components in numerous applications [START_REF] Velema | [END_REF] such as optical imaging, materials science, energy (photovoltaic cells, light emitting diodes…), or medical purposes (diagnostic, therapy…). [7][8][9] However, for most of these domains, and in particular in the field of biology, the use of ultraviolet (UV) light as input must be avoided due to its high energy that can cause damages and because of its short penetration length in tissues. It is thus essential to develop photoswitches that can be triggered by illumination within the biological window (mainly l=650-1000 nm). [10] In this context, the development of the dimethyldihydropyrene (trans-10b,10c-dimethyl-10b,10c-dihydropyrene, DHP, closed isomer, see Scheme 1) photochromic unit is of particular interest. Indeed, this colored and aromatic compound (14 delocalized p electrons) [11,12] belongs to the uncommon class of negative photochroms, i.e. the conversion between the colored and thermodynamically stable DHP to the colorless cyclophanediene (CPD, open form) isomer is triggered by visible light. In addition, the reversion of the metastable CPD form to the stable DHP can be quantitatively realized either by illumination with UV light or thermally (T-type).

Scheme 1. Schematics of the dimethyldihydropyrene (left) / cyclophanediene (right) photochromic couple.

Another major asset of the DHP core is that its structure can be regarded as a real molecular platform that can be readily modified chemically in order to adapt and tune its properties towards the targeted applications. For example, functionalization of the DHP unit by metal complexes may lead to all-visible light switches [10] and the incorporation of both donor and acceptor substituents (push-pull systems) was recently shown to promote efficient DHP to CPD photoconversions triggered by NIR light (l >800 nm). [13,14] In addition, we have previously reported that the suitable functionalization of the DHP moiety with pyridinium substituent(s) has two effects: i) the quantum yield of the ring-opening conversion (DHP to CDP) may be significantly increased (by ~2 orders) [15,16] and ii) the system may display an unusual reactivity towards dioxygen: the closed-ring isomer can play the role of O2 photosensitizer (photo-production of singlet oxygen) and may also quantitatively and reversibly generate endoperoxide derivatives when illuminated by red light under aerobic conditions. [17,18] Following these results, we describe herein the preparation and characterization of a new donor-acceptor DHP derivative (DA-DHP, Chart 1) bearing both an electro-acceptor pyridinium group and an electron-donor triphenylamine unit. The physico-chemical and photochromic properties of this compound are investigated as well as its reactivity under aerobic conditions. Related compounds (DHP, A-DHP, D-DHP, Chart 1) are also studied for comparison.

Results and Discussion

Synthesis

The investigated DA-DHP system was prepared following the synthetic route represented in Scheme 2. The procedure starts with the 2,7-di-t-butyl-trans-10b,10cdimethyl-10b,10c-dihydropyrene (DHP) which was first brominated in positions 4 and 9 by reaction with 2 molar equivalents of N-bromosuccinimide (NBS) in DMF and CH2Cl2 as solvents (the tert-butyl groups were incorporated for solubility and synthetic reasons). [11] In order to introduce different substituents on the DHP core, the bis-brominated compound 1 was subjected to two consecutive Suzuki-Miyaura coupling reactions in the presence of tetrakis-triphenylphosphine palladium as catalyst. The first coupling was done with 1 molar equivalent of 4pyridylboronic acid and compound 2 was isolated with 54% yield. Then, coupling between 2 and 1.3 equivalent of triphenylamine-4-boronic acid yielded 3 which was isolated as an orange powder (yield: 78%). This latter was then reacted with an excess of methyl iodide and the crude reaction product was subjected to an anion exchange procedure to afford the targeted red DA-DHP which was isolated as its PF6 -salt. Reference molecules containing only one acceptor (A-DHP) or donor (D-DHP) group (see Chart 1) were prepared following similar experimental protocols by coupling the mono-brominated DHP with the corresponding pyridine or triphenylamine boronic derivatives, respectively (see experimental section).

Characterization of DA-DHP and related compounds

The synthetized compounds investigated in this work were analyzed by UV/vis absorption and 1 H and 13 C NMR spectroscopies as well as mass spectrometry measurements (see experimental section and S.I.). In particular, the 1 H NMR spectrum of DA-DHP, recorded in CD3CN, is shown in Figure 1 and confirms the formation of the targeted compound. Signals observed at low field (7-9 ppm) were attributed to the resonance of the hydrogen atoms of the DHP core and the pyridine and triphenylamine units. The singlet signals corresponding to the N + -CH3 groups and to the t-Bu units appear at +4.40 and ~+1.6 (2 singlets) ppm, respectively. In contrast, peaks of the internal methylenic protons are found in the negative region of the spectrum, at -3.65 and -3.67 ppm. The unusual position of these signals is due to the ring current flowing through the delocalized p-electrons of the DHP core and constitutes thus a real signature indicating that the photochromic unit is in its closed state. [11] The structures of the other compounds investigated in this work were similarly confirmed by NMR studies (see S.I. and experimental section). The structure of D-DHP has also been confirmed via single crystal X-ray diffraction analyses (see Figure S19). This compound crystallizes in the space group P-1 (triclinic system). The sp 2 carbon atoms of the DHP core are planar and the distance measured between the two central sp 3 carbon atoms is 1.52 Å, which is similar to values found for other DHP derivatives [16] and in agreement with the gas phase optimized structure using DFT at 1.54 Å. The phenyl group of the triphenylamine unit directly connected to the DHP adopts a dihedral angle of 47.1° with respect to the main DHP plane, close to the gas phase optimized structure which gives a slightly larger dihedral angle of 52.9°. The investigated compounds were analyzed by UV/vis absorption spectroscopy associated to theoretical calculations. Experimental absorption spectra are given in Figure 2 and corresponding data are collected in Table 1. Vertical transition energies computed with TD-DFT are reported in Table 2 for comparison and Table 3 illustrates the nature of the lowest two electronic transitions involved for each system. DHP [19] 641 (0.09), 477 (1.1), 379 (3.9), 341 (10.3) 0.00% (660) 0.08% (470)

A-DHP [16] 666 (0. In the 300-800 nm range, the absorption spectrum of DHP exhibits main bands attributed to p-p* transitions that involve singlet electronic states of the chromophore core. [20] In D-DHP, these absorption bands appear red-shifted, in accordance with an extension of the p-system and this bathochromic shift is nicely reproduced by the TD-DFT calculations (Table 2). Note that the two bands observed experimentally at 484 nm and 470 nm result from the same electronic transition according to the TD-DFT calculations, and are thus the results of vibronic structures. In the case of A-DHP, the shifts of the absorption bands towards lower energies are even more pronounced and it is important to note that the extinction coefficient of the band at lowest energy is also significantly increased. These observations are supported by the TD-DFT results (Table 2). As explained in a previous study, [16] these effects are partially due to the charge transfer character between the DHP moiety and the methyl-pyridinium unit, which is illustrated by the pair of NTOs associated with these electronic transitions (Table 3). In the case of DA-DHP, which contains both pyridinium and the donor amino group, no intramolecular chargetransfer (ICT) excitation from the electron donor group to the electron acceptor moiety has been observed (see NTOs for S1 and S2 states of DA-DHP in Table 3). However, the absorption spectrum is clearly the most red-shifted among the investigated compounds (see Figure 2). In particular, the absorption peak appearing at lowest energy (S0®S1 transition) is located at 641 nm in the simple DHP and is found at 676 nm in DA-DHP, which corresponds to a shift of -0.106 eV in energy. This experimental value is thus in reasonable agreement with an energy shift of -0.131 eV obtained from TD-DFT data. The redox properties of the derivatives were characterized by cyclic voltammetry (CV) experiments conducted in CH3CN (charged compounds) or CH2Cl2 (neutral molecules) solutions containing tetra-n-butylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. Electrochemical data are summarized in Table 4. For all CV curves, in addition to oxidation (DHP + /DHP and DHP 2+ /DHP + couples) and reduction (DHP/DHP -) processes centered on the DHP unit, reduction of the pyridinium centers (Py/Py -) and/or oxidation waves of the triphenylamine group (TPA + /TPA) were also logically observed with the substituted compounds. Potential shifts measured for the redox systems centered on the DHP moiety were in full accordance with the electronic effects of the donor and acceptor groups. However, when comparing the effects of the methyl-pyridinium (A) and TPA (D) substituents, it should be outlined that the major impacts are due to the presence of the pyridinium group. In particular, a decay of +140 mV was observed for E1/2 (DHP + /DHP) upon introduction of the pyridinium group to the DHP core (in A-DHP), whereas a shift of only -20 mV was seen in D-DHP (Table 4). In the same manner, the potential corresponding to the first oxidation of the DHP core in DA-DHP is positively shifted compared to the signal of the unsubstituted photochrome, showing that the acceptor group plays a larger electronic effect than the donor group. Such minor influence of the donor unit may be explained by the noncoplanarity between the triphenylamine unit and the main plane of the DHP moiety (as shown above, a dihedral angle of 47.1 was observed at the solid state with D-DHP; see Figure S19). A-DHP [d] Epc= -1.98 D-DHP [c] [f] ─ 0.17 (0.08) 0.52 (0.08)

1.00 (0.10) [d] DA-DHP [d] -1.97 (0.07) -1.45 (0.07) 0.27 (0.07) 0.52 (0.07) [d] 0.75 (0.08)

Study of the photo-ring opening process by illumination under inert atmosphere (DHP®CPD)

Conversions of the closed-ring (DHP) isomers to their corresponding open-ring (CPD) forms were realized by irradiation with visible light and were followed by 1 H NMR and absorption spectroscopies. For these experiments, deaerated solutions of the different compounds (~10 -3 and 10 -6 M for NMR and UV/vis absorption spectroscopy, respectively) were cooled at 4°C during the entire irradiation period in order to avoid a possible thermal return reaction. Solutions were analysed at regular time intervals until no more changes in the spectra were observed. In the case of DA-DHP, the photochemical conversion to DA-CPD was found to be effective when the solution was irradiated at lex ≥ 630 nm (using a high-pass filter) which corresponds to absorption on the lowest energy band of the compound (S0 to S1 transition). Figure 3 shows the evolution of the UV/vis spectra during the photo-conversion process. During illumination, a rapid colour change from red to light yellow was observed, in agreement with the DA-DHP to DA-CPD conversion. This result is reflected by a disappearance of the visible absorption bands accompanied by the growth of the signal in the UV part of the spectrum and the presence of an isosbestic point at 313 nm. In the same manner, 1 H NMR peaks of DA-DHP were progressively replaced by new sets of resonance of the corresponding open-ring isomer DA-CPD during illumination (see Figure 1B). In particular, the two singlets of the internal methyl groups were considerably shifted from -3.6 ppm in the closed isomer to +1.5 ppm in the open form. At the end of the experiment, no peak of the closed-ring isomer could be detected, showing a quantitative photo-conversion, at the NMR scale. In contrast, no conversion was observed when DHP or D-DHP were irradiated under the same conditions (at lex ≥ 630 nm) and only a slow conversion was seen when these compounds were illuminated at lex = 470 nm. In accordance with our previous results, [15,16] this difference is attributed to the fact that, when a pyridinium group is introduced, excitation of the system to its lowest excited electronic state (S1 level) may lead to the photoring opening reaction, whereas higher excitation levels must be reached The efficiency of the photo-controlled reaction from closed to open-ring isomers was quantified by the determination of their quantum yields (Фc-o) which represent the number of CPD molecules produced per photon absorbed. For this, 10 -6 to 10 -5 M solutions of closed isomers were irradiated at specific wavelengths and the conversion was followed by absorption spectroscopy. Quantum yields were then determined by fitting the results using the kinetic model reported by Maafi and Brown. [21,22] The Фc-o values are collected in Table 1. Whereas the DHP and D-DHP compounds exhibit low quantum yields (<0.1%) when irradiated at 470 nm and could not be isomerized at 660 nm, values of 9.3% and 13.3% were found in A-DHP and DA-DHP, respectively when illuminated at low energy (lex = 660 nm). Such increase of the quantum yield between DA-DHP and A-DHP (+43%) confirms the major positive effect played by the introduction of both the electron-withdrawing pyridinium and the electron-donor triphenylamine groups on the dimethyldihydropyrene skeleton. In order to investigate the photo-isomerization mechanism on the lowest S1 excited state of the DA-DHP system, we report for the first time the excited-state relaxation pathway from the Franck-Condon region to the photochemical funnel, that is the S0/S1 conical intersection, responsible for the nonradiative decay down to S0 on the way to the CPD photoproduct formation. The photoisomerization pathway is illustrated in Figure 5 based on SF-TD-DFT calculations. [23][24][25][26] Our results show that upon initial relaxation from the Franck-Condon region, the system reaches a minimum, DA-DHP*, on the S1 potential energy surface. The associated relaxation coordinate involves mainly some C-C skeletal deformation of the DHP core (see Figure S20). To reach the minimum energy conical intersection (i.e., the lowest energy crossing between S1 and S0), S0/S1 MECI, the system needs to undergo a loss of planarity associated with an increased transannular C-C bond length and a reorientation of the pyridinium group (see Figure S20). This process is activated and the S1 transition state optimized (at the TD-DFT level; see Computational Details) along this relaxation path provides an estimated S1 potential energy barrier of 6.3 kcal/mol (0.272 eV). The S0/S1 MECI is energetically accessible, lying 4.1 kcal/mol (0.180 eV) above DA-DHP* and only 2.0 kcal/mol (0.087 eV) above the Franck-Condon energy. The main difference with the reference DHP system is that the S1 state of DA-DHP can lead to photoisomerization, whereas this is not the case in DHP. In the parent DHP, the system needs to be excited in S2 and the low photoisomerization quantum yield is due to an efficient depopulation of S2 towards S1, which is unreactive. [24,25] The present mechanism will thus lead to a much more efficient photoisomerization in DA-DHP compared to the reference compound DHP.

Investigation of the optically and thermally triggered ringclosing reaction (CPD®DHP)

As seen previously, the ring-closure reaction, i.e. the CPD to DHP conversion, can generally be achieved by UV illumination or by heating. When a solution of DA-CPD (freshly prepared by visible irradiation of a solution of DA-DHP) was exposed to UV light (254 nm or 366 nm, 7W), a change of colour was rapidly observed and the reaction could be followed by 1 H NMR and UV/vis absorption spectroscopies. Spectra of the initial DA-DHP could be regenerated upon few seconds of illumination without any observable degradation, showing the efficient optically-controlled ring-closing process. In the same manner and in order to evaluate the thermal return reaction, CH3CN solutions of DA-CPD were heated under dark conditions and the closed-ring isomer could be progressively and quantitatively generated (see Figures S6, S10 andS11). Different temperatures were used and, assuming a first order kinetic, an activation energy Ea of 22.4 ± 2.1 kcal.mol -1 was measured. This value is in accordance with DFT calculations for which an activation energy of 19.4 kcal/mol was found and is similar to most CPD derivatives reported in the literature. [27] In addition, a half lifetime (t1/2) of 69 ± 2 min was found for DA-DHP at 37°C.

Fluorescence properties

The fluorescence properties of CH3CN solutions of DA-DHP and DA-CPD were investigated. DA-DHP displays a narrow fluorescence signal at 687 nm upon excitation at 470 nm (see Figure S7), attributed to the emission from the first singlet excited state, with a lifetime of 4.3 ns and a fluorescence quantum yield (Ф em ) of 2.6.10 -3 . These data are close to values found for other DHP derivatives bearing pyridinium substituents and the low value of Stokes shift (+11 nm / 0.029 eV) indicates that the DA-DHP adopts similar geometries in its ground state and in its lowest excited state. The emission wavelength can be theoretically estimated from the S1®S0 vertical transition energy computed at DA-DHP*. The TD-DFT result gives an emission wavelength of 698 nm in acetonitrile, and the computed Stokes shift is 70 nm (0.198 eV), in good agreement with that observed experimentally. When the solution of DA-DHP was irradiated by visible light (lex ≥ 630 nm) at room temperature, the photo-opening of the central C-C bond was accompanied by a decrease in fluorescence, according to the formation of the non-emissive open-ring isomer when excitation is realized at 470 nm.

Reactivity of DA-DHP with dioxygen

As indicated above, upon suitable functionalization, DHP derivatives may exhibit a remarkable reactivity with O2. [16] Indeed, during irradiation under aerobic conditions, they may act as photosensitizers allowing the direct production of singlet oxygen ( 1 O2, first excited state of dioxygen) and they may also form endoperoxide derivatives by reaction between the photogenerated open-ring isomers and 1 O2 (i.e. CPD + 1 O2 ® CPDO2) In order to evaluate the ability of DA-DHP to react with O2, a CH3CN solution of this compound was illuminated by red light (lex ≥ 630 nm) under aerobic atmosphere (air, 1 atm.). During irradiation, the initial DA-DHP was rapidly and completely consumed and the solution became light yellow. Evolution of the solution was followed by UV/vis and 1 H NMR spectroscopies. The final 1 H-NMR spectrum (when no more evolution was observed) obtained upon illumination is given in Figure 1C. Clearly, the two DA-DHP and DA-CPD isomers were not detected and new sets of signals were obtained at the end of the experiment. Analysis showed that two products were formed with molar proportions equal to 2:1. The starting DA-DHP product being unsymmetrical because of the presence of two different substituents, the photogenerated compounds were identified as two endoperoxide derivatives (named DA-CPDO2, forms A and B, Scheme 3) in which the position of the peroxide group can be located on the two different benzene rings of the CPD structure. In particular, the two singlets observed at ca. 0 ppm are attributed to proton signals of the internal -CH3 bound to the peroxide group and constitute a real signature corroborating the presence of two endoperoxide compounds. NMR analysis also shows that the donor part of the compound, i.e. the tris-phenylamino group, is not oxidized during the reaction. In addition, it should be noted that the solution containing the two DA-CPDO2 isomers was analyzed by several 1D and 2D NMR techniques, but the major endoperoxide form that was obtained could not be unambiguously attributed. The formation of DA-CPDO2 was also corroborated by mass spectrometry experiments, with a signal corresponding to [M+O2] at m/z = 711.34. DFT calculations could confirm the existence of the two endoperoxide isomers A and B of DA-CPDO2 (Figure 6). To investigate the cycloaddition pathways of 1 O2 with CPD, two electrostatic complexes denoted DA-CPD••• 1 O2 (A) and DA-CPD••• 1 O2 (B) were first optimized with 1 O2 approaching CPD on its two different benzene rings. These complexes provide stable structures allowing the two reactants to interact in order to proceed to the endoperoxide formation. According to a previous DFT study of the cycloaddition pathways of 1 O2 with CPD for a bis-pyridinium appended system, the concerted cycloaddition pathway is more favorable than the stepwise mechanism. [28] This is also the case for the cycloaddition of 1 O2 with benzene. [29] Thus, we only computed the concerted pathway herein. The results are provided in Figure 6. The two starting complexes are only separated by 0.1 kcal/mol and can be considered degenerate. The concerted transition states are associated with potential energy barriers of 8.6 and 9.5 kcal/mol for A and B isomers, respectively. The formation of the two corresponding endoperoxides is energetically favorable. They are lying about 12 kcal/mol below the reactant DA-CPD••• 1 O2 complexes. The two endoperoxides are quasi-isoenergetic with the A isomer lower than the B one by only 0.5 kcal/mol. The results of these two concerted cycloaddition pathways thus suggest that the formation of the A isomer would be favored over the formation of the B isomer. One can also note that only mono-endoperoxyde forms were obtained and that the formation of the bis-endoperoxide, certainly energetically unfavored, was not observed. The formation of these two endoperoxide forms is explained by the following simplified mechanism (eq. 1-5): During illumination, DA-DHP which is excited (eq. 1) can then be opened to form DA-CPD (eq. 2) and may also act as O2 sensitizer to generate 1 O2 (eq. 4) after intersystem crossing to its triplet excited state (eq. 3). DA-CPD can then react with 1 O2 to generate the endoperoxide derivatives DA-CPDO2, forms A and B (eq. 5).

The formation of DA-CPD as an intermediate is clearly seen when the reaction is followed by NMR. The reaction describes by eq. 5, was demonstrated by an additional NMR experiment: to a solution of freshly prepared DA-CPD (obtained by illumination of DA-DHP under inert atmosphere), were added 10 mol% of tetraphenylporphyrin (TPP), which is a well-known O2, photosensitizer. [30] Upon irradiation of the solution by visible light (l≥ 490 nm, where only TPP absorbs), the quantitative formation of DA-CPDO2 (forms A and B) was rapidly observed (see Figure S15). This demonstrates the efficiency of the reaction between the DA-CPD isomer and singlet oxygen. Interestingly, the molar proportion of the two generated endoperoxide forms A and B was 2:1, which is identical to what has been observed previously when DA-DHP was illuminated in the presence of air.

The photo-production of singlet oxygen (eq. 4) was then realized by continuous irradiation of DA-DHP with visible light under air (1 atm.) in the presence of 15 molar eq. of 2,3-dimethyl-2-butene (90 mM), which is known to react with O2 to give a hydroperoxide. The reaction was followed by proton NMR and the data show that the hydroperoxide was gradually produced during illumination and the starting alkene was almost consumed (yield: 89%) upon 74 hours of irradiation. It is important to note that the production of endoperoxide from the simple dimethyldihydropyrene has been reported, but it proceeds with low yields and with higher energy of illumination. [31] Importantly, the above mechanism is only possible if DA-DHP can be populated in its lowest triplet excited state by intersystem crossing (ISC) as shown in eq. ( 3). Our theoretical calculations of the triplet states along the initial S1 relaxation pathway (Figure S21) show that the energy gap between S1 and the triplet states T2 and T3 is getting smaller, reaching only 3.5 and 1.8 kcal/mol at the S1 minimum geometry. The spin-orbit couplings (SOCs) between S1 and these two triplet states are computed at 1.5 and 2.5 cm -1 , respectively, at this geometry. In addition, the minimum energy crossing point between S1 and T3 has been located at only 0.05 kcal/mol above the S1 minimum, making this funnel for S1®T3 ISC easily accessible energetically. The SOC values are unchanged within 0.1 cm -1 at this crossing geometry. All these results thus point to a possible S1®T3 ISC, reminiscent to what was found for a bispyridinium-appended DHP for which an S1®T2 ISC was proposed. [28] Thermal releasing of 1 O2 from DA-CPDO2 It is recognized that aromatic endoperoxides can acts as chemical sources of 1 O2 following a clean decomposition reaction upon heating. [32][33][34] The thermal stability of DA-CPDO2 was investigated using UV/vis absorption spectroscopy. Upon heating at 37°C, the yellowish solution of DA-CPDO2 (forms A and B) in CH3CN gradually turned red and the UV/vis spectrum of the parent closed state DA-DHP was fully recovered within a few hours, indicating the cleavage of the peroxidic unit and thus, the complete reversibility of O2 binding. This result was corroborated by 1 H NMR spectroscopy that shows that DA-CPDO2 cycloreverts to DA-CPD upon heating, which then thermally isomerizes to yield the initial DA-DHP, as described by equations 6 and 7 (Figure S16, S17):

1 DA-CPDO2 (forms A and B) → 1 DA-CPD + 1 O2 eq. 6 1 DA-CPD → 1 DA-DHP eq. 7
The amount of singlet oxygen thermally released was quantified using NMR trapping experiments. A freshly prepared solution of DA-CPDO2 (3.9 mM) was left at 37°C in the dark in the presence of 2,3-dimethyl-2-butene (117 mM). Upon 48 hours, the starting DA-DHP was fully recovered and the yield of produced hydroperoxide was 60 ± 10 % (measured by integration of the 1 H-NMR signals). Considering that the reaction between 1 O2 and 2,3dimethyl-2-butene is usually not quantitative due to the short lifetime of singlet dioxygen, the amount of 1 O2 delivered by DA-CPDO2 is certainly underestimated.

Conclusion

We have reported the synthesis and characterization of a photochromic system (DA-DHP) based on the dimethyldihydropyrene (DHP) unit. Using consecutive Suzukireaction couplings, both donor and acceptor units have been incorporated onto the DHP core. Spectroscopic and electrochemical characterizations associated to theoretical calculations showed that this functionalization allows a modification of the redox properties and a significant red-shift of the absorption spectrum of the photochromic DHP center. As a consequence, the photoisomerization of DA-DHP to its corresponding cyclophanediene form (DA-CPD) proceeds by illumination at low energy with very high quantum yield (Фc-o = 13.3% at lex = 660 nm for DA-DHP vs. Фc-o = 0.08% at lex = 470 nm for the non-functionalized DHP). In addition, when illuminated with red light under aerobic conditions at room temperature, DA-DHP was shown to quantitatively trap molecular oxygen and two endoperoxyde isomers (DA-CPDO2, forms A and B) are produced. This reaction is reversible: DA-DHP and singlet oxygen can then be produced by heating a solution of DA-CPDO2. The efficient photochromic behavior and the particular reactivity towards dioxygen of the DA-DHP system can be explored for several domains such as photochromic materials and devices, photodynamic therapy or (photo-)oxidation reactions.

Experimental Section

Materials and instrumentation. All solvents were purchased and used as received except THF that was distillated over sodium/ benzophenone under argon. Water was purified by reverse osmometry with an Elgastat purification system (5 MΩ.cm). Organic and inorganic reagents used in the procedures described below were purchased on Aldrich, Acros, Fluorochem or TCI Europe and used without further purification. All evaporations were carried out under reduced pressure with a rotatory evaporator, and all organic extract were washed with water and dried with MgSO4. Silica gel used for column chromatography refers to Merck silica gel, 40-63 μm.Melting points were measured thanks to capillary tubes on a Büchi B-545 device. Infrared spectra (IR) were recorded on a Perkin Elmer Spectrum Two Fourier transform infrared spectrometer using ATR (Attenuated Total Reflexion) modes. 1 H NMR and 13 C NMR spectra were recorded on a Brucker Avanced 500 or 400 MHz spectrometer in CD2Cl2 or CD3CN. Chemicals shifts are calibrated to residual solvent peaks. Coupling constant values (J) are given in hertz and chemical shifts (δ) in ppm. High-resolution mass spectrometry analyses were conducted using the HRMS, Bruker maXis mass spectrometer and were performed in positive Electrospray Ionisation (ESI + ) at the DCM mass facility. Data for single crystal X-Ray diffraction were collected at room temperature on a Bruker AXS Enraf-Nonius Kappa CCD diffractometer using the Cu-Ka graphite monochromated radiation. Intensity data were collected for Lorentz and polarization effects with EVAL 14 and for absorption using SADABS. Structures solutions and refinements were performed with the SHELX softwares implemented by Olex2. All nonhydrogen were refined by full matrix least-squares with anisotropic thermal parameters. Hydrogen atoms were introduced at calculated positions as riding atoms. Crystallographic structures were drawn with the Mercury 2020.3.0 software. A summary of the data collection and structure refinements is provided in the supplementary information. CCDC 2093111 (D-DHP) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Electrochemical measurements (cyclic voltammetry, CV) were conducted under an argon atmosphere with a standard one-compartment, threeelectrode electrochemical cell using a CH-Instrument 660B potentiostat. Anhydrous CH3CN or CH2Cl2 were used as solvents and tetra-nbutylammonium hexafluorophosphate (TBAPF6, 0.1M) was used as supporting electrolyte. CH-Instrument vitreous carbon (3 mm in diameter) working electrodes were used for CV experiments. Electrodes were polished with 1 µm diamond paste (Mecaprex Presi) prior to each recording. Counter-electrode was a platinum wire immerged directly in the solution. A CH-Instrument AgNO3/Ag (10 -2 M + tetra-n-butylammonium perchlorate 10 -1 M in CH3CN) electrode was used as a reference electrode and the potential of the regular ferrocenium / ferrocene (Fc + /Fc) redox couple in acetonitrile is 0.07 V under our experimental conditions. Cyclic voltammetry (CV) curves were recorded at a scan rate of 0.1 V s -1 . An automatic ohmic drop compensation procedure was systematically implemented prior to recording CV data.

Spectroscopy, irradiation procedures and quantum yields measurements. UV/vis absorption spectra were recorded using a Varian Cary 60 spectrophotometer with temperature controller. The measurements over the spectral range from 250 to 800 nm were carried out in Hellma quartz cells (QS) having an optical path length of 1 cm. Emission spectra were recorded in CH3CN at room temperature on a Horiba Fluoromax fluorescence spectrophotometer. Samples were placed in 1 cm optical path length quartz cells. Luminescence lifetimes measurements were performed using a 400 nm irradiation wavelength obtained by the second harmonic of a titanium: sapphire laser (picosecond Tsunami laser spectra physics 3950-M1BB and 39868-03 pulse picker doubler) at 800 kHz repetition rate. Fluotime 200 from AMS technologies was used for the decay acquisition. It is composed of a GaAs microchannel plate photo-multiplier tube (Hamamatsu model R3809U-50) followed by a time-correlated single-photon counting system from Picoquant (PicoHarp300). Luminescence decays were analyzed with Fluofit software available from Picoquant. Emission quantum yields Φ Em were determined at room temperature in CH3CN solutions (λex ≈ 480 nm). The [Ru(bpy)3] 2+ complex in aqueous solution was used as quantum yield standard (Φ Em = 0.028) according to the following equation:

Φ ! "# = Φ $%('())! "# × 𝐴𝑏𝑠 $%('())! 𝐴𝑏𝑠 ! × 𝐼 ! 𝐼 $%('()) ! × 𝑛 +,!+-² 𝑛 ,". ²
Where s and Ru(bpy)3 denote the sample and the standard respectively. I is the integrated emission intensity, Abs the absorption at the excitation wavelength and n the refractive index of the solvent.

All the irradiation experiments were performed in solution, using different cells (classical UV/vis, quartz cells, NMR tubes). The visible irradiations were carried out with a Xe-Hg lamp (500W) equipped with cut-off filters or with Thorlabs mounted LEDs of specific wavelengths combined with an adjustable Thorlab collimation adapter. The samples were kept at 4°C during irradiation in order to limit the thermal back reaction. The open to closed isomerization was investigated by irradiation at 254 nm or 366 nm using a hand-held chromatography lamp (7W). Samples were placed at 15 cm from the visible light Xe-Hg lamp and 4 cm from the UV lamp or LEDs.

For the experiments under anaerobic conditions, all the samples were prepared in a Jacomex glove box with carefully degassed deuterated and non-deuterated solvent.

The conversion between the closed (DHPs) and open (CDPs) isomers was investigated with UV/vis and NMR experiments at 4°C. It was considered that the photo-stationary state was reached when no evolution was observed in three consecutive intermediate spectra or after a long period of irradiation. The ratio between DHP and CPD forms was determined by 1 H NMR from the relative integration of their characteristic resonance peaks of the internal methyl groups. This ratio was also determined with UV/vis spectroscopy by comparing the intensity of the DHP absorption band (visible region) before and after irradiation. Quantum yields for the photo-induced ring-opening process (DHPs®CPDs, Φc-o) were determined by illumination of solutions of DHP derivatives at 4°C using LEDs at specific wavelengths. The light power was measured with a Newport (818-SL) photodetector and the photoconversion was followed by UV/vis absorption spectroscopy. Illuminations and absorption measurements were performed at wavelengths for which only the closedring isomers absorb. Data were extracted by fitting of the experimental curves using the kinetic model reported by Maafi and Brown. [21,22] Computations. Density functional theory (DFT) and its time-dependent version (TD-DFT) were used to perform calculations on the ground and excited states, respectively, of the four studied DHPs shown in Chart 1 (i.e., DA-DHP, A-DHP, D-DHP and DHP). Ground-state geometry optimizations were carried out with the hybrid B3LYP functional, [35] while excited states were computed and optimized with the long-range corrected hybrid wB97X-D functional [36] within linear-response TD-DFT. Pople's 6-311G(d,p) basis set was used throughout. [37] All calculations were performed with the corresponding solvent used in the experiment (i.e., cyclohexane for D-DHP and DHP, and acetonitrile for DA-DHP and A-DHP) within the integral equation formalism polarizable continuum model (IEFPCM). [38] Vertical absorption transition energies were computed using linear-response non-equilibrium solvation. Note that within the crude vertical approximation, deviations of about 0.25 eV are rather common for valence excited states of organic molecules computed with TD-DFT. [39] Calculation of the emission wavelength was carried out using the more accurate external iteration state-specific non-equilibrium approach. [40] Natural transition orbitals (NTOs) [41] for relevant electronic transitions were generated in order to compare the nature of the excited states involved between the various studied DHPs.

Transition state for the thermal DA-CPD to DA-DHP conversion was located using broken-symmetry DFT due to the biradical (open-shell singlet) nature of this species. To account for the spin contamination, spinprojected energies were computed with the approximate spin-correction procedure proposed by Yamaguchi and coworkers. [42] , [43] The associated activation energy was estimated from the spin-projected energy at the transition state taking also into account thermochemical corrections. The cycloaddition of DA-CPD with 1 O2 was also investigated with brokensymmetry DFT. For this mechanism, the two tert-butyl groups were replaced by hydrogen atoms to reduce the computational cost. All DFT and TD-DFT calculations were performed with Gaussian 16. [44] The photoisomerization pathway of DA-DHP was computed with spin-flip TD-DFT (SF-TD-DFT) [45] in the gas phase. This quantum chemical method allows the correct physical description of S0/S1 conical intersection, unlike TD-DFT. [46][47][48][49] It uses a triplet reference state to generate the singlet ground state and electronic excited states applying one-electron spin-flip excitations. Thus, ground and excited states are described on an equal footing, in contrast to TD-DFT. SF-TD-DFT calculations were performed within the Tamm-Dancoff approximation using a spin-restricted triplet reference. For these calculations, the recommended half-and-half BHHLYP functional [50] was used and the tert-butyl groups were also replaced by hydrogen atoms. GAMESS [51] was used to carry out the SF-TD-DFT calculations. Note that because analytic hessian is not implemented at the SF-TD-DFT in GAMESS, the excited-state transition state was optimized with TD-DFT to evaluate the excited-state barrier on S1 along the DA-DHP to DA-CPD photoisomerization.

Triplet excited states were also computed along the S1 initial photoisomerization pathway at the TD-DFT level using wB97X-D/6-311G(d,p). The Tamm-Dancoff approximation was used for a more reliable description of the singlet/triplet energy gaps. [52] Calculations of spin-orbit couplings and optimization of the minimum energy crossing point between S1 and T3 were performed using Q-Chem 5.4. [53] All the optimized Cartesian coordinates are collected in Supporting Information.

Synthetic procedures

2,7-di-tertbutyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene, 4-bromo-2,7-di-tertbutyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene and 4,9-dibromo-2,7-di-tertbutyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene, 4-(2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyren-4-yl)-1methylpyridin-1-ium hexafluorophosphate (A-DHP) were synthetized following reported procedure. [16] 4 ) and freshly distilled THF (6 mL). A degassed aqueous solution (2 mL) of potassium carbonate (36 mg, 0.26 mmol, 4.8 equiv.) and tetrakistriphenylphosphine palladium (0) (12 mg, 0.010 mmol) were added into the flask and the resulting mixture was heated to reflux under stirring for 72 h. After cooling down the mixture to room temperature, the solvent was evaporated under reduced pressure until dryness. The solid residue was dissolved in CH2Cl2 (50 mL) and the solution is filtered over celite. The resulting solution was washed with water (3 x 50 mL), brine (50 mL), dried over anhydrous MgSO4 and evaporated under reduce pressure. The crude product was purified by column chromatography over silica gel using cyclohexane:ethyl acetate gradient Methyl iodide (1 mL) was added to the previous solution and the resulting mixture was allowed to stir for 4 h under inert atmosphere. After the required time, the solvent was evaporated under reduced pressure and the residue was then dissolved in MeOH (1 mL). The product was subjected to an anion exchange using a saturated aqueous solution of KPF6 (2 mL). The solid was filtered under vacuum and washed several times with water and diethyl ether. The solid was then dried under reduced pressure to give the targeted product as a red powder (32 mg, 0.039) in 93% yield. If needed, the compound can be further purified either by column chromatography over silica gel using a mixture of Acetonitrile:MeOH:H2O:NaBr (4:1:1:1) (v:v:v:v) followed by a an anion exchange using a saturated aqueous solution of KPF6 (2 mL) in methanol (

Chart 1 .

 1 Investigated Donor-Acceptor dimethyldihydropyrene photochromic compound (DA-DHP) and reference molecules investigated in this work.

Scheme 2 .

 2 Scheme 2. Synthetic procedure for the preparation of DA-DHP.

Figure 1 .

 1 Figure 1. 1 H-NMR of (A) DA-DHP, (B) DA-CPD and (C) DA-CPDO2 in CD3CN. • and * indicate the signals of the internal methyl and N + -CH3 groups, respectively. s: solvent peak

Figure 2 .

 2 Figure 2. UV/vis spectra of DHP (green line), D-DHP (black line), A-DHP (blue line) and DA-DHP (red line). Solvents used: cyclohexane for DHP and D-DHP; acetonitrile for A-DHP and DA-DHP.

  For solubility reasons, DHP and D-DHP were studied in cyclohexane and A-DHP and DA-DHP were investigated in acetonitrile. [b] Values indicated in parentheses correspond to wavelengths used for both excitation and detection during quantum yields measurements.

  Main pair of natural transition orbitals (with k>0.5) reflecting the dominant particle-hole excitation character obtained at TD-wB97X-D/6-311G(d,p) level. [b] Largest natural transition orbital eigenvalue (k) for each state.

  [a] E (V) vs. E1/2(ferrocenium/ferrocene); [b] E1/2= (Ep a +Ep c )/2 at 0.1V s -1 ; DEp= Ep a -Ep c ; [c] in CH2Cl2 + 0.2 M TBAPF6 ; [d] in CH3CN + 0.1M TBAPF6; [d] nonfully reversible system ; [e] peak potential / irreversible system; [f] not observed in the measured potential range.

Figure 3 .

 3 Figure 3. Top: isomerization of DA-DHP to DA-CPD by irradiation. Below: UV/vis absorption spectra evolution during irradiation (lex ≥ 630 nm, 4°C) of DA-DHP in CH3CN under inert atmosphere. Time between two consecutive spectra: 2 min.

  to isomerize DHP or D-DHP into their open forms. In order to illustrate the effects of the chemical substitutions of the DHP core, solutions of DHP, D-DHP, A-DHP and DA-DHP were illuminated at lex ≥ 700 nm and the consumptions of the closed form were followed by UV/vis spectroscopy. As shown in Figure 4, under these experimental conditions, only A-DHP and DA-DHP could be logically converted to their open-ring isomers and the higher rate of conversion was clearly obtained with DA-DHP. For example, upon 1 hour of irradiation, more than 95 % of DA-CPD were formed whereas 25% of A-DHP were still remaining and no CPD or D-CPD were detected.

Figure 4 .

 4 Figure 4. Percentage of DHP, D-DHP, A-DHP and DA-DHP closed-ring isomers measured during illumination at lex ≥ 700 nm as function of time. The amount of the closed form is determined by the absorbance at the maximum wavelengths. [C]~ 10 -5 M in CH3CN (A-DHP and DA-DHP) or cyclohexane (DHP, D-DHP).

Figure 5 .

 5 Figure 5. Photo-isomerization pathway from ground-state DA-DHP to DA-CPD: (a) excited-state relaxation on S1 to DA-DHP*, (b) activated process to reach the photochemical funnel (S0/S1 MECI), and (c) nonradiative deactivation to S0 and ground-state relaxation to DA-CPD. All the molecular structures and Cartesian coordinates are provided in SI.

Scheme 3 .

 3 Scheme 3. Reversible formation of two endoperoxide isomers (DA-CPDO2, forms A and B) by illumination of DA-DHP with red light under aerobic conditions.

Figure 6 .

 6 Figure 6. DFT cycloaddition concerted pathways between DA-CPD and 1 O2 forming the two endoperoxides DA-CPDO2 (A) and (B). Energies are given in kcal/mol relative to the lowest electrostatic complex DA-CPD••• 1 O2 (A). Black and grey lines show the two distinct paths leading to the two endoperoxides DA-CPDO2 (A and B, respectively) via concerted transition states (TSconc). The 1 O2 molecule and the interacting methylbenzene ring of DA-CPD are shown in ball and sticks. Hydrogen atoms are not shown for clarity reasons.

1 DA 1 DA-DHP* → 3 3 3 4 1

 11334 DA-DHP* (intersystem crossing) eq. DA-DHP* + 3 O2 → 1 DA-DHP + 1 O2 eq.DA-CPD + 1 O2 → 1 DA-CPDO2 (forms A and B) eq. 5

-( 2 , 7 -

 27 di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyren-4yl)-N,N-diphenylaniline (D-DHP): A round bottom flask was filled with 4bromo-2,7-di-tertbutyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene (50 mg, 0.10 mmol), 4-diphenylamino)phenylboronic acid (38 mg, 0.13 mmol, 1.3 equiv.) and freshly distilled THF (6 mL). A degassed aqueous solution (3 mL) of potassium carbonate (65 mg, 0.48 mmol, 4.8 equiv.) and tetrakistriphenylphosphine palladium (0) (21 mg, 0.022 mmol) were added into the flask and the resulting mixture was heated to reflux under stirring for 48 h. After cooling down the mixture to room temperature, the solvent was evaporated under reduced pressure until dryness. The solid residue was dissolved in CH2Cl2 (50 mL) and the solution was filtered over celite. The resulting solution was washed with water (3 x 50 mL), brine (50 mL), dried over anhydrous MgSO4 and evaporated under reduce pressure. The crude product was purified by column chromatography on silica gel using cyclohexane as eluent.

4-( 2 , 7 -

 27 di-tert-butyl-trans-10b,10c-dimethyl-9-(pyridine-4-yl)-10b,10cdihydropyren-4-yl)-N,N-diphenylaniline (3): A round bottom flask was filled under an argon atmosphere with the compound (2) (27 mg, 0.054 mmol), (4-(diphenylamino)phenyl)boronic acid (20.2 mg, 0.07 mmol, 1.3 equiv.

  (1:0 to 95:5) (v:v). The desired product (3) was obtained as an orange solid (28 mg, 0.042 mmol) with 78% yield. m.p: 93-96°C. IR: 3057, 3029, 2957, 2924, 2867, 1590, 1491, 1266, 893, 832, 750, 692, 667 cm -1 . 1 H NMR (400 MHz, CD2Cl2) δ : 8.81 (m, 3H, Har), 8.64 (s, 1H, Har), 8.60 (d, J = 8 Hz, 2H, Har), 8.54 (s, 1H, Har), 8.47 (s, 1H, Har), 7.77 (dd, J1 = 18 Hz, J2 = 7 Hz, 4H, Har), 7.34 (m, 6H, Har), 7.26 (d, J = 8 Hz, 4H, Har), 7.09 (t, J = 7 Hz, 2H, Har), 1.63 (s, 9H, HtBu), 1.61 (s, 9H, HtBu), -3.68 (s, 3H, HMe), -3.69 (s, 3H, HMe).

4-( 2 , 7 -

 27 di-tert-butyl-9-(4-(diphenylamino)phenyl)-10b,10c-dimethyl-10b,10c-dihydropyren-4-yl)-1-methylpyridin-1-ium hexafluorophosphate (DA-DHP): A solution of the compound (3) (28 mg, 0.042 mmol) in CH2Cl2 (5 mL) is degassed.

Table 2 .

 2 Vertical transitions computed for the DHP

, A-DHP, D-DHP and DA- DHP

	at the TD-ωB97X-D/6-311G(d,p) level..
	Compounds	Vertical transitions [a] l, nm (f [b] )
	DHP	589 (0.009), 503 (0.090), 378 (0.333), 340 (1.419)
	A-DHP	620 (0.047), 529 (0.069), 413 (0.715), 379 (0.290), 344 (0.471), 309 (0.448)
	D-DHP	600 (0.017), 511 (0.075), 392 (0.660), 349 (1.348)
	DA-DHP	628 (0.073), 547 (0.144), 422 (1.088), 382 (0.266), 352 (0.306), 323 (0.362), 308 (0.174)
	[a] Transitions above 300 nm are reported. [b] Oscillator strength.

Table 3 .

 3 Comparison of the first two electronic transitions in

DHP, A-DHP, D- DHP and DA-DHP. [a]

	Compound	State	Excitation	NTOs [a]	k [b]
			Energy
			(eV)
	DHP	S1	2.103		0.68
		S2	2.467		0.84
	A-DHP	S1	2.000		0.76
		S2	2.343		0.81
	D-DHP	S1	2.067		0.6 8

Table 4 .

 4 Electrochemical data: oxidation and reduction potentials (E1/2 (DEP)[a,

	b] ,V) measured by cyclic voltammetry.
		DHP/	Py + /	DHP + /	TPA + /	DHP 2+ /
		DHP -	Py	DHP	TPA	DHP +
	DHP [c]	(0.07) -2.35	─	(0.07) 0.19	─	Epa= 0.77 [e]

  13 C RMN (100 MHz, CD3CN) δ: 149.5, 147.9, 137.8, 132.5, 132.4, 128.7, 128.6, 126.8, 126.1, 125.1, 123.0, 122.1, 121.2, 32.00, 31.8, 14.9, 14.6. HRMS (ESI) m/z: Calc. for C31H35Br + 500.1953. Found. 500.1957.

	under an
	argon atmosphere with compound (1) (50 mg, 0.1 mmol), 4-
	pyridinylboronic acid (13 mg, 0.1 mmol, 1 equiv.) and freshly distilled THF
	(6 mL). A degassed aqueous solution (3 mL) of potassium carbonate (67
	mg, 0.48 mmol, 4.8 equiv.) and tetrakistriphenylphosphine palladium (0)
	(18 mg, 0.016 mmol) were added into the flask and the resulting mixture
	was heated to reflux under stirring for 72 h. After cooling down the mixture
	to room temperature, the solvent was evaporated under reduced pressure
	until dryness. The solid residue was dissolved in CH2Cl2 (50 mL) and the
	solution is filtered over celite. The resulting solution was washed with water
	(3 x 50 mL), brine (50 mL), dried over anhydrous MgSO4 and evaporated
	under reduce pressure. The crude product was purified by column
	chromatography over silica gel using cyclohexane:ethyl acetate gradient
	(1:0 to 93:7) (v:v). Compound (2) was obtained as a dark green solid (27
	mg, 0.054 mmol) with 54% yield. m.p: 221-223°C. IR: 3059, 3019, 2960,
	2952, 2962, 2904, 2865, 1592, 1238, 904, 897, 800, 671, 533 cm -1 . 1 H
	RMN (400 MHz, CDCl3) δ : 8.88-8.83 (m, 3H, Har), 8.69 (d, 1H, Har), 8.62
	(s, 2H, Har), 8.52 (s, 1H, Har), 8.48 (s, 1H, Har), 7.80 (d, J = 8 Hz, 2H, Har),
	1.75-1.72 (m, 9H, HtBu), 1.61 (s, 9H, HtBu), -3.69 (s, 3H, HMe), -3.73 (s, 3H,
	HMe).

The desired product was obtained as a dark green solid (25 mg, 0.042 mmol) with 42% yield. m.p: 208-210°C. IR: 3034, 2959, 2953, 2924, 2903, 2865, 1589, 1486, 1270, 882, 751, 696, 666, 624, 508 cm -1 . 1 H RMN (400 MHz, CD2Cl2) δ : 8.78 (s, 1H, Har), 8.57 (s, 2H, Har), 8.56 (s, 1H, Har), 8.53 (s, 1H, Har), 8.46 (s, 2H, Har), 7.80-7.72 (m, 2H, Har), 7.37-7.30 (m, 6H, Har), 7.29-7.21 (m, 4H, Har), 1.71 (s, 9H, HtBu), 1.64 (s, 9H, HtBu), -3.84 (s, 3H, HMe), -3.87 (s, 3H, HMe). HRMS (ESI) m/z: Calc. for C44H45N 587.3532. Found. 587.3536. UV/vis (λmax [nm] ε [M -1 .cm -1 ]): 652 (1000), 484 (8000), 470 (8000), 395 (45000), 348 (48000). ( 13 C NMR signal could not be collected due to the low solubility of the compound, but a X-ray structure was obtained, see S.I.) 4-(9-dibromo-2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10cdihydropyren-4-yl)pyridine (2) : A round bottom flask was filled

  13 C NMR (400 MHz, CD2Cl2) δ : 151.19, 150.00, 148.49, 147.52, 147.47, 146.87, 137.78, 137.18, 137.16, 135.73, 133.93, 132.33, 131.86, 129.91, 126.57, 126.53, 125.82, 125.03, 124.83, 124.07, 123.52, 122.52, 121.93, 121.33, 119.47, 36.64, 36.59, 32.11, 32.08, 30.91, 30.75, 15.19, 15.18. HRMS (ESI) m/z: Calc. for C49H49N2 + 665.3897. Found. 665.3887.

  1 mL) or by a recrystallization in methanol. m.p: 241-243°C. IR: 3032, 2959, 2923, 2906, 2867, 1634, 1588, 1503, 1489, 831, 751, 695, 556 cm -1 . 1 H NMR (500 MHz, CD3CN) δ : 8.84 (s, 1H, Har), 8.78 (s, 1H, Har), 8.77 (s, 1H, Har), 8.74 (d, J = 7 Hz, 2H, Har), 8.70 (s, 1H, Har), 8.62 (s, 1H, Har), 8.63 (s, 1H, Har), 8.44 (d, J = 7 Hz, 2H, Har), 7.76 (d, J = 9 Hz, 2H, Har), 7.38 (m, 4H, Har), 7.32 (d, J = 9 Hz, 2H, Har), 7.23 (dd, J1 = 8 Hz, J2 = 1 Hz, 4H, Har), 7.13 (tt, J1 = 8 Hz, J2 = 1 Hz, 2H, Har), 4.40 (s, 3H, HNMe), 1.65 (s, 9H, HtBu), 1.62 (s, 9H, HtBu), -3.65 (s, 3H, HMe), -3.67 (s, 3H, HMe).13 C NMR (125 MHz, CD3CN) δ : 159.73, 150.48, 148.78, 148.40, 148.11, 145.47, 139.70, 137.80, 137.47, 136.69, 134.81, 134.59, 132.80, 130.55, 130.07, 127.88, 126.56, 125.56, 125.52, 124.75, 124.40, 124.38, 124.11, 123.22, 123.10, 48.44, 37.13, 36.83, 31.90, 31.81, 31.67, 30.94, 15.48, 15.44. HRMS (ESI) m/z: DA-DHP: Calc. for C50H51N2 + 679.4047. Found. 679.4042. ESI-MS (ESI) m/z: DA-CPD: Calc. for C50H51N2 + 679.41. Found. 679.35. ESI-MS (ESI) m/z: DA-CPDO2: Calc. for C50H51N2O2 + 711.39. Found. 711.34. UV/vis (λmax [nm] ε [M -1 .cm -1 ]): 676 (2000), 535 (6000), 422 (18000), 341 (27000), 304 (22000). Ф Em = 2.6.10 -3 . t = 4.3 ns.

Acknowledgements

This work was supported by the French Agence National de la Recherche (Grant ANR-18-CE29-0012 PHOTOCHROMICS). The NanoBio ICMG (UAR 2607) is acknowledged for providing facilities for mass spectrometry analyses (A. Durand, L. Fort, R. Gueret) as well as the DCM (Département de Chimie Moléculaire, Plateau Technique de Synthèse) for the synthesis of the DHP precursors (Martine Fayolle, Mathieu Curtil, Pierre-Yves Chavant). This work was granted access to the HPC resources of CALMIP supercomputing center under the allocation 2021-[12158].

Entry for the Table of Contents

A dimethyldihydropyrene photochromic unit bearing electron-donor and -acceptor substituents has been synthetized and investigated by spectroscopy, electrochemistry and theoretical calculations (spin-flip TD-DFT). This system can be reversibly isomerized with very good quantum yields to its corresponding cyclophanediene form when irradiated by red light under inert atmosphere and is able to generate two endoperoxide isomers upon illumination under air. These endoperoxides can thermally release singlet oxygen.

Institute and/or researcher Twitter usernames: @DCMGrenoble ; @GuyRoyal3 ; @ZianiZakaria7