Ziyang Song

Hui Duan

Ling Miao

Laurent Ruhlmann

Yaokang Lu

Wei Xiong

Dazhang Zhu

Liangchun Li

Lihua Gan

Mingxian Liu

Yaokang Lv

Zhiwei Wang

Carbon hydrangeas with typical ionic liquid matched pores for advanced supercapacitors

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Introduction

As one of the most advanced energy storage devices, carbon-based supercapacitors (CSs) have attractedconsiderable attention because of outstanding cycle lifetime, competitive power density and reliable operation safety. [1][2][3][4][5] CSs store energy through potential-driven accumulation of electrolyte ions to counterbalance the surface charge of porous carbon electrodes, identified as electric double layer formation. [6][7][8][9][10] However, only limited energy densities are obtained for such systems, which are about an order of magnitude lower than those of Li-ion batteries. 11,12 Consequently, unlocking the energy potential of CSs without sacrificing the power output is highly required, to extend their utilization into high-energy applications. [13][14][15][16][17] To achieve this goal, considerable efforts have been devoted to design diverse carbon electrode materials with tunable geometry and rational pore size distribution for efficient charge/ion transfer. Doped heteroatoms have been also incorporated for enhanced surface wettability and additional pseudocapacitance, usually associated with the use of high-voltage electrolytes such as ionic liquids (ILs). [18][19][20][21] The capacity of a CSs, or more intuitively, the number of charged ions adsorbed, is closely relevant to the pore architecture of the electroactive materials and the ion sizes of the used electrolyte. 22 Gogotsi's group firstly reported the anomalous capacitance promotion in the case of nanocarbons with pore sizes smaller than 1 nm, defying long-held axiom about inaccessibility of small pores to solvated ions. 23 The abnormal phenomenon is ascribed to a desolvation of the electrolyte ions entering subnanopores, resulting in a sieving effect at pore sizes below the size of the solvated ions. 23 This result gives a better understanding of ionic transport/accumulation into porous media for realizing efficient charge storage in CSs. Further experimental and theoretical studies have established that the pore size of an electrode tailored for achieving the maximal capacitance should be very approximate to the ion dimension of a given electrolyte. 13,24,25 If the pore size is too small, the confined pores are ion-inaccessible. If the pore size is too large, there will be a wider space between ions and pore wall, both leading to a notable decrease of the capacitance value. This means, ingenious regulation and optimization of the pore size to perfectly match the dimensionality of desired electrolyte ions, is a key tactic to help improve ion transport/adsorption dynamics. However, up till now, it is a great challenging to govern the pores of carbons in a the subnanometer region, especially with an electrolyte ion-adapted level.

Herein we report the development of heteroatom-doped hydrangea-like porous carbons (HHPCs) with ion-comparable pore sizes for advanced CSs, using IL/gel-polymer electrolytes. HHPCs are designed based on a facile polymerization of benzoquinone and benzidine, which hold unique geometry and large surface area with improved surface wettability due to high-level N/O doping.

Two ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM.TFSI) and 1ethyl-3-methylimidazolium tetrafluoroborate (EMIM.BF4) with identical cation (EMIM + , 0.76 nm) but with different anions (TFSI -, 0.79 nm; BF4 -, 0.48 nm) are used as electrolytes to investigate the electrochemical energy storage capability. HHPCs exhibit dominant 0.80 nm and subordinate 0.50 nm pores. The dominant pore is perfectly comparable to the cation dimension of both ILs (EMIM + ) while the secondary pores is well-matched to the anion size of tetrafluoroborate, BF4 -.

As a result, HHPCs-based solid-state symmetric supercapacitor using EMIM.BF4 gel electrolyte delivers an extraordinary energy density of 101.2 Wh kg -1 against the substitute of EMIM.TFSI (78.3 Wh kg -1 ). This study opens up a new opportunity for employing pore-ion size matching criterion in the building of high-efficient energy storage devices.

Experimental

Synthesis of HHPCs

Benzoquinone (3.24 g, 0.03 mol) was added into the mixture of benzidine (1.84 g, 0.01 mol) and ethanol (100 mL) at 70 °C, followed by stirring for 2 h to obtain a polymer. After filtrating, washing and drying, the polymer was mixed with KOH (1:1, w/w), and then carbonized/activated at 650-850 °C (3 °C min -1 ) for 2 hours to fabricate HHPCs (labelled as HHPCx where x denotes the carbonization temperature).

Characterization

Thermogravimetric analysis was tested using a synchronous thermal analyzer (NETZSCH STA409 PC) at a heating rate 10 °C min -1 under an N2 atmosphere. Fourier transform infrared spectroscopy (FT-IR) spectrum was recorded on a Thermo Nicolet NEXUS spectrometer. XRD patterns were characterized on a Bruker D8 diffractometer equipped with Cu radiation. The sample morphology was observed by a scanning electron microscopy (SEM, Hitachi S-4800). Scanning transmission electron microscopy (STEM) and corresponding elemental mapping characterization were performed on a JEM-F200 instrument equipped with an energy diffraction system. X-ray photoelectron spectrometer (XPS, AXIS Ultra DLD) was utilized to analyze the surface chemical composition. N2 adsorption/desorption measurement was collected at -196 °C using a micromeritics apparatus (ASAP 2460). The surface area and pore size distribution were investigated by the Brunauer-Emmett-Teller method and nonlocal density functional theory.

Assembly of supercapacitors

The working electrode was prepared by pressing a slurry of HHPCs (80 wt%), polytetrafluoroethylene (10 wt%), and graphite (10 wt%) mixture onto a piece of stainless steel mesh and then dried at 80 °C. The mass loading of active substance on each working electrode is about 10 mg cm -2 . To obtain IL/gel-polymer electrolytes, 5 g of polyvinylidenefluoride-cohexafluoropropylene (PVDF-HFP) and a proper amount of EMIM.BF4 or EMIM.TFSI was dissolved in 50 mL acetone, and then the mixture was magnetically stirred at 50 °C to obtain a homogeneous and transparent solution. The solution was put in a glass petri dish for acetone evaporation and dried in vacuum for 12 hours to form EMIM.BF4/PVDF-HFP and EMIMTFSI/PVDF-HFP gel-polymer electrolytes. Solid-state supercapacitors were assembled in a glovebox by stacking two identical working electrodes with an EMIM.TFSI or EMIM.BF4 gelpolymer electrolyte in the middle. To obtain a flexible device, the configuration was sealed by adhesive tape and compressed with appropriate pressure to get good interfacial contact between electrode/electrolyte.

Electrochemical measurement

All electrochemical measurements including cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS) were conducted with a CHI660E electrochemical workstation. The gravimetric capacitances (Cs, F g -1 ) of a single electrode were calculated from GCD profiles according to form (1):

Cs (F g -1 ) = 4 × I × ∆t m × ∆V (1)
where I (A), ∆t (s), m (g) and ∆V (V) represents charge current, discharge time, the total mass of active material on two electrodes and the voltage window, respectively.

The gravimetric energy (E) and power densities (P) of the devices are denoted as follows:

E (Wh kg -1 ) = 1 7.2 × 4
Cs∆V 2

(2)

Ev (Wh L -1 ) = ρ × E = (Vt + 1/ρcarbon) -1 × E (3) P (W kg -1 ) = E ∆t × 3600 (4) Pv (W L -1 ) = ρ × P (5)
where Ev and Pv represents the volumetric energy and power density of the symmetrical devices, respectively, and Vt denotes the total pore volume of HHPCs, and ρcarbon is the true density of carbon

(2 g cm -3 ).

Results and discussion

HHPCs was synthesized based on a Michael addition reaction between benzidine (electron-pair donicity) and benzoquinone (electron-drawing capability) with a mild condition. This strategy avoids any harsh and/or complicated procedures (e.g., template synthesis, high-temperature hydrothermal treatment, self-assembly, chemical vapor deposition), time-consuming process or multistep manipulation, which provides a highly efficient and competitive route for engineering functionalized porous carbons. Fig. S1 displays FT-IR spectrum of the benzidine-benzoquinone polymer, in which sharp peaks at 1564 and 1483 cm -1 can be assigned to the stretching vibration of C=O and C=C groups. 26,27 The characteristic absorption peaks centered at 1284 and 813 cm -1 are attributed to the stretching vibration of C-N groups and the blending vibration of -NH-groups, respectively. 28 Besides, the peak at 1178 cm -1 is ascribed to the stretching vibration of phenolic -OH groups in the formed hydroquinone resulted from the partial oxidation of benzoquinone. XRD profiles (Fig. S2) reveal two signals at about 26 and 43° indexed to the (002) and (100) planes, suggesting an amorphous carbon framework. 29 HHPCs has a high carbon yield of 71% at 750 °C (Fig. S3), and show hydrangea-like micron-scale carbon spheres with refined surface structure composing of enormous tunnels and furrows (Fig. 1a-c). Such a unique architecture allows rapid ion diffusion kinetics for full access of electrolyte ions into the adsorption sites. The elemental mapping images corresponding to STEM image (Fig. 1d-g) clearly displays the presence of C, N

and O atoms and a homogeneous distribution in the carbon skeleton. ). The highly stable C=O covalent bond can effectively resist the bond breaking and the release of O species during carbonization. 28,30 High-resolution C 1s spectra (Fig. S4) can be deconvoluted into four peaks positioned at about 284.4, 285.4, 286.7 and 288.5

eV, assigned to C=C/C-C, C-N, C-O and C=O components, respectively. 31,32 N 1s species (Fig. 2c) include pyridine pyridinic N (N-6, 398.2 eV), pyrrolic N (N-5, 399.6 eV), quaternary N (N-Q, 400.6 eV), as well as pyridine N-oxide (N-X, 402.9 eV). 33,34 The first three nitrogen species are proposed to provide stable pseudocapacitance in acid and base electrolytes through Faradic redox reactions, while the last one is beneficial to reduce effectively the electronic transfer resistance. O 1s spectra (Fig. 2d) can be fitted to three peaks at 530.9, 532. As depicted in Fig. 3a, HHPCs demonstrate combined type-I and IV isotherms which consist of steep adsorption regions in the low-pressure (P/P0<0.1), and well-defined plateaus with distinctive hysteresis loops (P/P0>0.45), implying the presence of micro-and mesopores. 36,37 HHPCs exhibit micropore sizes concentrated at ~0.50, 0.80 and 1.26 nm, originating from the release of small molecule volatile matter during carbonization/activation, and mesopores of 10-40 nm (Fig. 3b)

arisen by the hydrangea-like surface architecture (tunnels and furrows). A suitable annealing temperature (750 °C) endows the resultant HHPC750 a highest surface area of 2212 m 2 g -1 , as shown in Table 1. Lower temperature does not guarantee an entire carbonization/activation of the polymer, and higher one causes a shrink of carbon framework, both giving an obvious drop in surface area. 38, The exceptional morphology, pore texture and surface feature of HHPCs inspire us to construct symmetrical CS devices. Two ILs incorporated gel-polymer electrolytes are utilized to investigate the influence of geometric sizes of the constituents on the ion diffusion dynamics within the pores.

Solid-state IL/gel-polymer electrolytes can provide the same benefit of the extended voltage window as ILs, and exhibit high competitiveness with conventional liquid-based energy storage systems. Moreover, they are environmentally benign, highly safe, and not easily leaked and flexible. 40,41 CV profiles exhibit rectangle-like shapes (Fig. 4a,b) which is indicative of ideal electric double-layer behaviors with excellent charge/discharge properties. 42 Compared with HHPC650 and HHPC850 electrodes, HHPC750 displays the highest capacitance in EMIMBF4/PVDF-HFP and EMIMTFSI/PVDF-HFP electrolytes, which can be attributed to the highest surface area and suitable N/O species. As shown in Fig. S5, the lower contact angles of 31-73° for EMIMBF4 and 21-66° for EMIMTFSI on HHPC surfaces than those of activated carbon (123/122°) manifest enhanced surface wettability due to N/O heteroatom doping. 43 The carbons with high affinity to electrolytes benefit fast ion transportation into the active surfaces and thereby achieve enhanced ion-accessible surface areas for electric double-lay capacitance. 44 CV profiles of HHPC750//HHPC750 solid-state CS operated in various potential windows (Fig. 4c,d) demonstrate that the device can stably work at 3.5 V with no obvious increase of anodic current in IL/gel electrolytes. 45 Symmetric and linear GCD curves (Fig. 4e,f) reflect the excellent capacitive behavior and superb reversibility. 46,47 HHPC750 electrode in EMIMBF4/PVDF-HFP electrolyte exhibits gravimetric and volumetric capacitances as high as 218 F g -1 and 105 F cm -3 at 1 A g -1 ,

superior to those in EMIMTFSI/PVDF-HFP electrolyte (184 F g -1 and 88 F cm -3 ). Furthermore, the electrode capacity remains 167 (EMIMBF4/PVDF-HFP) and 118 F g -1 (EMIMTFSI/PVDF-HFP) at 10 A g -1 , revealing notable capacitive endurance at large current loads.

Fig. 4g gives Nyquist plots of two HHPC750-based devices, which adopt a fitting equivalent circuit to study the electrolyte ion diffusion and transport kinetics. 48 In EMIMBF4/PVDF-HFP electrolyte, the device shows lower electrical series resistances (Rs, the intersections of the real axis) of 2.5 Ω and charge transfer resistances (Rct, the diameter of semicircle in the high-frequency region) of 4.2 Ω, relative to 3.1 and 6.7 Ω in EMIMTFSI/PVDF-HFP. This result manifests a better ion diffusion and charge transfer behavior of EMIMBF4-based device. 49 As displayed in Fig. 4e, a maximum gravimetric energy density of 101.2 Wh kg -1 is achieved at a power density of 875 W kg -1 (38.7 Wh L -1 at 334 W L -1 ) for the cell assembled with EMIMBF4/PVDF-HFP electrolyte, which is 29.2% higher than that of EMIMTFSI/PVDF-HFP device. To our best knowledge, the solid-state CSs to date. More impressively, the energy outputs still remain 69.8 and 45.9 Wh kg -1 for EMIMBF4/PVDF-HFP and EMIMTFSI/PVDF-HFP based devices at an ultrahigh power density of 17.5 kW kg -1 . For a comparison, we also fabricated liquid-based supercapacitors using these two ILs (Fig. S6) which show basically the same energy outputs as the solid-state devices.

This result illustrates that the introduction of polymer gel into the IL electrolytes does not obviously affect the electrochemical properties of the resultant CSs, but can address their leakage concerns.

The capacitance plots followed by current density increasing and cycle performance within ten cycles of the two devices are exhibited in Fig. S7. After consecutive cycles and high-current tests, the electrode capacitance reversibly recovers to the initial value once the current density goes back to 0.5 A g -1 , demonstrating prominent electrochemical reversibility and structural robustness of the active materials and the IL/gel-polymer electrolytes. In addition, the both devices exhibit high energy reservation of 91.9 and 90.7% after 10,000 successive cycles (Fig. 4i), with high Coulombic efficiencies of 99.3 and 98.7%, validating the superior long-term cyclic stability and outstanding reversibility. of that tested at 30 °C. The remarkable boost in energy density can be mainly attributed to that the high temperature lows the electrolyte viscosity, which gives enhanced ionic conductivity and quicker mobility of ions from electrolyte to electrode interface, 50,51 increasing the surface utilization and hence achieving improved energy storage. Notably, the device energy basically recovers to the initial value when the temperature goes back to -10 °C. A red light-emitting diode (LED) bulb powered by the solid-state device can normally work with a reliable energy supply under different temperatures, indicating wide applicable operation temperature and thus great potential uses of the device for environmentally adaptable energy supply.

The achievement on high-energy HHPC750//HHPC750 solid-state CS can mainly be ascribed to three aspects: well-designed electrode material, high-voltage IL electrolyte, and the most important point of size matching between both. First, high-surface-area HHPCs with ingenious hydrangealike texture enables sufficient interfaces for charge accumulation, and provides efficient ion diffusion kinetics. Meanwhile, N/O species endow the carbon surfaces with good affinity and enhanced wettability, which facilitate electrolyte ions enter and soak into the pore channels and increase ion-accessible surface area. Second, EMIMBF4 and EMIMTFSI ILs with a wide potential window of 3.5 V allows the enhancement of the maximum energy supply. Third, the superior electrochemical performances of EMIMBF4-based cell benefit most from well-matching between pore architecture and ion dimension (Fig. 5).

The electrode capacitance is closely related to the distance between the ion center and pore wall according to the form of C=ɛA/d, 24 where C denotes the capacity, ɛ represents the dielectric constant of electrolyte, A is the surface area of electrode, and d is the distance between pore surface and ions.

If the ionic diameter (dion) is far less than the pore width (D), electrolyte ions positioned around the center of large pores would be at a wide distance (d) to the pore wall, resulting in a capacitance decline. The capacitance has an abnormal promotion when the D/dion ratio is in the range of 1.0< D/dion<1.7. 13,24 The ratio of 1.26 nm micropore in HHPCs to EMIM + (0.76 nm) or TFSI -(0.79 nm) is just within this range. Moreover, the closer this ratio is to 1, the larger the capacity will be, 52 i.e., ion/pore size matching is a key to maximize the electrode capacitance. HHPCs exhibit controlled pore size distribution in which the major subnanopores of 0.80 nm are perfectly adapted to EMIM + cations, and the subordinate pores of 0.5 nm are comparable with BF4 -anions (0.48 nm). This minimizes the free space and guides the oriented arrangement of electrolyte ions along the pore outspread direction, achieving excellent ion transportation pathways and improved ions adsorption number at the pore surface. Nevertheless, the electrolyte ions are difficult to get into the slit pores if the ion size is larger than that of the pores, which prevent the formation of EDLC. Compared with BF4 -anions, TFSI -has a larger size which is inaccessible into the 0.5 nm pores, leading to a lower capacitance and energy density of the fabricated device (101.2 Wh kg -1 for EMIMBF4 and 78.3 Wh kg -1 for EMIMTFSI). Therefore, making full use of the well-designed pores in HHPCs significantly improve the electrochemical energy storage performances. The leakage current and self-discharge behaviors of supercapacitors are a matter of major practical significance in their operation. As shown in Fig. 6a, a small leakage current of 9.7 µA can be achieved after 10 h, indicative of less shuttle reactions caused by the impurities in carbons. 53 Moreover, the self-discharge course undergoes a rapid decline in the beginning several hours and the final output voltage stabilizes at 2.72 V after 30 h with 77.7% reservation of the original charged potential, which is highly desirable for real applications. To satisfy the specific energy and power demands, the operational characteristics of multiple supercapacitors working simultaneously were explored. As displayed in Fig. 6b andc, two devices can be connected to integrate in series to achieve even higher operating voltages and in parallel to obtain larger output currents. The voltage window or discharge time of the device in series or in parallel is doubled compared with a single device. The insets show that two devices in series/parallel can power one green LED, demonstrating superior energy and power supply of the solid-state devices. 54 To investigate the potential use of our supercapacitor in wearable and portable electronics from a practical point of view, the mechanical flexibility characteristic was performed on the device constructed by stacking two symmetric HHPC electrodes with the EMIMBF4/PVDF-HFP gelpolymer electrolyte sandwiching between them. There is no obvious distortion in the CV and GCD profiles (Fig. 6d-f), with high retention of capacitance (90.3%) and Coulombic efficiency (97.5 %) from the initial flat to 180° bending state. This means the ion/charge transport kinetics are not significantly affected, implying highly flexibility and outstanding electrochemical stability of the device. In addition, the device still holds 87.5% capacitance reservation even after being bent for 1000 cycles at 180° (Fig. S9), further confirming the robust mechanical property of the flexible solid-state device.

Conclusion

In conclusion, we exert the pore-ion size matching guideline to engineer HHPCs with exquisitely adapted pore dimension for IL gel electrolytes in the buliding of advanced supercapacitors. The exceptional carbon materials are developed based on a facile benzidinebenzoquinone synthetic strategy, which show superior hydrangea-like surface architecture, large surface area, high-level heteroatoms and multiscaled pore network. There is a dramatic differential charge storage in the CSs using two EMIMBF4 and EMIMTFSI gel electrolytes, which originated from the diameters of electrolyte constituents and the consequent ion diffusion and adsorption behaviors within carbon nanopores. The HHPC750//HHPC750 solid-state supercapacitor using a more matched EMIMBF4/PVDF-HFP gel electrolyte delivers a remarkable energy density of 101.2

Wh kg -1 against the replacement of EMIMTFSI (78.

Fig. 1 .

 1 Fig. 1. (a, b) SEM images, (c) a TEM image, (d) energy-dispersive X-ray images and corresponding

  5, and 533.6 eV, reflecting the C=O, C-OH/C-O-C, and COO-functionalities.35

Fig. 2 .

 2 Fig. 2. (a) XPS survey spectra and high-resolution (b) N 1s and (c) O 1s spectra of HHPCs.

Fig. 3 .

 3 Fig. 3. (a) N2 sorption isotherms and (b) pore size distributions of HHPCs.

Fig. 4 .

 4 Fig. 4. CV curves at 10 mV s -1 of HHPCs-based devices employing (a) EMIMBF4/PVDF-HFP and

Fig. 5 .

 5 Fig. 5. Schematic illustration of the ion matching in pores with various sizes.

Fig. 6 .

 6 Fig. 6. (a) Leakage current measurement and self-discharge curve of the EMIMBF4/PVDF-HFP

Table 1

 1 Surface areas (S), micropore surface areas (Smicro), pore volumes (Vt), and chemical compositions of HHPCs.

	Samples	S (m 2 g -1 ) Smicro (m 2 g -1 ) Vt (cm 3 g -1 )	C (wt%) N (wt%) O (wt%)
	HHPC650	1167	972	0.75	76.01	8.70	15.29
	HHPC750	3185	2731	2.12	85.65	5.14	9.21
	HHPC850	1913	1680	1.12	87.89	2.84	9.27

  3 Wh kg-1 ). Besides, the constructed solidstate device shows outstanding temperature robustness, excellent series/parallel features, and notably mechanical flexibility. High-capacitance electrode, high-voltage electrolyte, and a key pore-ion size matching between both are crucial to unlock the energy potential of solid-state supercapacitors to a new level. This work demonstrates a break-through in the bottleneck to design functionalized carbons with brilliant narrow ion-comparable pore structure, opening a new door for the development of high-energy solid-state CSs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21875165, 51772216, 21501135), the Science and Technology of Shanghai Municipality, China (Grant No.14DZ2261100), the Fundamental Research Funds for the Central Universities.

Conflicts of interest

The authors declare no conflict of interest.


Govindasamy Jayamurugan

Vijayendran Gowri

David Hernández

Santiago Martin

Alejandro González-Orive

Cagatay Dengiz

Oliver Dumele

Francesc Pérez-Murano

Jean Paul Gisselbrecht

Prof. Dr Corinne Boudon

Dr J.-P Gisselbrecht

Prof. Dr L Ruhlmann

Prof. Dr G Jeschke

Prof. Dr P Cea

Design and Synthesis of Aviram-Ratner-Type Dyads and Rectification Studies in Langmuir-Blodgett (LB) Films

Keywords: Aviram-Ratner dyad, Langmuir-Blodgett films, rectification, chargetransfer, [2+2] cycloaddition-retroelectrocyclization (CA-RE), molecular electronics materials

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In 1974, Aviram and Ratner developed the concept of a "molecular rectifier", a single organic molecule showing unidirectional electron flow in an applied field. [1] Their molecular design criteria for rectification behavior are simple: a dyad consisting of an electron-rich "π-donor" (D) and an electron-poor "π-acceptor" (A) which are separated by a rigid, insulating "spacer" () The originally proposed dyad 1 (Figure 1) has been theoretically proven to be an effective candidate for an organic molecular rectifier, [1] but in view of synthetic complexity, has not been reported to date. [2] However, 1 is not alone in its potential to display rectification behavior and several groups have since followed the Aviram-Ratner design criteria to find new leads for molecular rectifiers with D--A architectures. [3] The synthetic challenges are considerable. The donor and acceptor need to be sufficiently strong to have high-lying HOMOs and low-lying LUMOs, respectively, but without undergoing spontaneous redox chemistry. The insulating spacer needs to be rigid and long enough to efficiently inhibit intramolecular charge-transfer (CT). Only a few dyads have been reported to overcome these challenges and to display rectification behavior. [3a,3e,3r] While dyads featuring tetrathiafulvalene (TTF) as donor and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) or other strong acceptors have been prepared, the rigid σ-spacer required by the Aviram-Ratner proposal containing strong donor and acceptor moieties has so far not been realized, due to the reasons mentioned above and inherent donor-acceptor interactions. Although the same is realized for dyads featured with strong donor and relatively weak acceptors. [4] Flexible spacers have prevented rectification behavior by leading to thermal electron transfer in the ground state [5] or intramolecular CT. [6] Here, we report the synthesis and properties of the two new dyads 2 and 3 (Figure 1) featuring for the first time all three main components of the original Aviram-Ratner design, with a TTF donor separated by a bicyclo[2.2.2]octane (BCO) spacer from a donor-substituted extended TCNQ (exTCNQ) acceptor moiety. We show rectification behavior for 3 in Langmuir-Blodgett films. Aviram and Ratner. [1] Bottom: D--A dyads 2 and 3 featuring all three key elements of the original proposal.

Results and Discussion

Synthesis and characterization of D--A dyads 2 and 3. The preparation of dyad 3 is shown in Scheme 1 whereas the synthesis of dyad 2 is described in Scheme S1 in the Supporting Information (SI). We chose a synthetic strategy that allows for late-stage generation of the anilino-substituted exTCNQ acceptor moiety, using the [2+2] cycloaddition-retroelectrocyclization (CA-RE) cascade reaction between aniline donor-activated alkyne and TCNQ. [7,8] Scheme 1: Synthesis of dyads 2 and 3. The synthesis of 3 started from 1,4-diethynylbicyclo[2.2.2]octane (4) [9] which was mono-silyl protected and transformed by Sonogashira cross-coupling with N,N-dioctyl-4-iodoaniline [10] into compound 5. Silyl-deprotection afforded 6 which was cross- coupled with 2-iodo-TTF [11] to give dyad precursor 7. Precursor 8 to dyad 2 was obtained by a similar reaction sequence (Scheme 1; see Scheme S1 in the SI).

The generation of the exTCNQ moiety by the CA-RE reaction was first attempted by treating TCNQ with precursor 8 at room temperature. However, TCNQ did not undergo the CA-RE reaction with 8 at 25 °C; instead, an intense color change was observed from brownish yellow to green, due to the formation of an 1:1 intermolecular charge-transfer complex, presumably with the TTF moiety. To overcome this, we adopted a TTF oxidation-masking approach developed by Stoddart and co-workers. [12] Oxidation of the TTF moiety of 8 with Fe(ClO4)2•2 H2O in MeCN allowed TCNQ to react with the anilino donor-activated alkyne moiety at room temperature; subsequent reduction of the oxidized TTF with sodium L-ascorbate in MeOH gave dyad 2 in 40% isolated yield. [START_REF]A solution of 8 (40 mg, 71 μmol) was treated with Fe(ClO4)3 H2O (53 mg, 142 μmol) in MeCN (5 mL) and stirred for 2 h at 25 °C. Then 7,7,8,8 tetracyanoquinodimethane (TCNQ; 159 mg, 78 μmol) was added and continued the stirring for additional 15 h. Then sodium L-ascorbate (28 mg, 142 μmol) was added in MeOH (5 mL) and stirred for 0.5 h. Evaporation and column chromatography (pentane/CH2Cl2 4:1→1:2) gave 2 (22 mg[END_REF] Later, we found that this approach was unnecessary; simply heating 8

or 7 with TCNQ to 120 °C in (CHCl2)2 for 2.5 h gave dyads 2 and 3, respectively, in 70-75% yield (Scheme 1). Notably, the TTF donor-activated alkyne moiety did not react with TCNQ [START_REF] Kato | [END_REF] and only the anilino-activated alkyne moiety underwent the CA-RE reaction, as this regiochemistry was confirmed by single crystal X-ray diffraction of dyad 2 (Figure 2, also see the SI, Section B). The two dyads are stable, marooncolored metallic solids, giving green solutions in CH2Cl2. They melt at 226-230 °C (2)

and at 109-110 °C (3). Figure 3 shows important control compounds 9 and 10 containing only the anilinesubstituted exTCNQ acceptor moiety and compound 11 as a reference for the donor part. Their synthesis is shown in Scheme S2 in the SI. X-ray analysis of 9 revealed that the conformation around the single bond located between the dicyanovinyl group and the 2-(4-methylene-2,5-dien-1-ylidene)malononitrile group is similar to that observed for dyad 2 (torsional angle C2-C3-C4-C25 of 71.6(4) for 9 and C10-C9-C13-C14 of 74.1(6) for 2; Figure 2).

Electronic absorption spectra

The UV/Vis absorption spectra of dyads 2, 3 and reference compounds 9-11 are shown in Figure 4, in order to obtain information about the nature and origin of the CT interactions, which may occur between the donor (dialkylanilino (DAA) and TTF moieties) and the acceptor (exTCNQ) moieties in either inter-or intramolecular fashion. spectral data indicate that the TTF donor and the exTCNQ acceptor moieties are not interacting in the ground state. The proposed intramolecular CT character of the longest-wavelength absorption bands of dyads 2, 3 and reference compounds 9, 10 was confirmed by protonation of the aniline donor moiety. Treatment of these compounds with CF3COOH in CH2Cl2 led to disappearance of the CT band (see Figures S3 andS4), with reappearance of the band upon re-neutralization with NEt3. However, it should be noted that under acidic conditions, a new weaker band appears at around 640 nm in both dyads 2 and 3 (Figure S3), but not in the case of the reference compounds 9 and 10 indicating that this band is possibly due to partial protonation/oxidation of the TTF moiety. [15] Dyads 2, 3 and reference compounds 9, 10 display strong solvatochromic behavior.

A remarkably large hypsochromic shift is observed from ~660 nm (1.89 eV) in CH2Cl2, to ~575 nm (2.16 eV) nm in hexane, owing to high electric transition dipole moments (see Figures S5-S6 in the SI). This pronounced solvatochromism is also reflected in the change of the color of 2 from green in CH2Cl2 to purple in hexane (see Figure S7 in the SI).

Electrochemistry

The redox properties of dyads 2,3 and reference compounds 9-11 were studied by cyclic voltammetry (CV) and rotating-disc voltammetry (RDV) in CH2Cl2 (+0.1 M nBu4NPF6, values reported vs. Fc + /Fc) (see Table S1 in the SI).

Dyads 2 and 3 display one irreversible one-electron oxidation step of the DAA moiety at +0.42 and +0.45 V, respectively, and two reversible one-electron oxidation steps of the TTF moiety at +0.00 and ~+0.50 V. However, for dyad 2 the cathodic peak associated to the second oxidations typical to the redissolution peak of one deposit at the glassy carbon electrode (important cathodic current and symmetrical peak). Thus, the second oxidation is reversible but induces one electrodeposition. The deposit is then dissolved in the cathodic potential sweep. This behavior is probably due to the poor solubility of the bi-oxidized species. Dyad 3 does not show such deposition because of the presence of n-octyl instead n-butyl pendant subunits. The dyads also exhibit reversible two-electron reduction steps at -0.90 V, centered on the exTCNQ acceptor.

One two-electron reduction was also observed for references 9 and 10 at potentials similar to that of 2 and 3 (-0.90 V), suggesting that the reduction waves are not affected by the presence of the TTF donor.

The UV/Vis and electrochemical data are clear evidence that intramolecular CT of 2, 3, 9, and 10 occurs intramolecularly from the DAA donor to the directly connected exTCNQ acceptor moiety. As predicted, the CT from the TTF moiety to the exTCNQ acceptor part is suppressed by the insulating BCO moiety. This finding was further corroborated by the electron paramagnetic resonance (EPR) spectral investigations. A negligible amount of EPR signal is observed for dyad 2, with the intensity measured to be 0.02% against (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO) as reference (see The spatial separation of the donor and acceptor orbitals in dyads 2 and 3 was further supported by computational methods. Geometry optimization and orbital visualization of 2 was performed by DFT calculations (B3LYP/6-31G (d), Gaussian09; see the SI, Section F). [START_REF] Frisch | Gaussian 09[END_REF] As expected, the HOMO is localized on the TTF donor, the LUMO and LUMO+1 on the exTCNQ acceptor, and the HOMO-1 on the aniline moiety. The electron affinity (EA) and ionization potential (IP) of 2 are 2. 35 V and 5.15 V, respectively which may be favorable for achieving an asymmetric current flow. [3f,17] The optical gaps of 2, 3, 9, and 10 were also calculated from the end-absorptionend of the longest-wavelength UV/Vis band. The deviation between the optical and electrochemical gaps are negligibly small (range 0.01 to 0.07 eV) evidencing that the similar orbitals are involved in both optical and electrochemical gaps (Table 1). Compound max

[nm (eV)] end [nm (eV)] Eox-Ered) [V] E1/2 [V] 2 650 (1.91) 890 (1.39) 1.32 +0.
42, [a] -0.90

3 660 (1.88) 910 (1.36) 1.35 +0.
45, [a] -0.90 9 660 (1.88) 900 (1.38) 1.35 +0.43, [a] -0.92 10 660 (1.88) 900 (1.38) 1.39 +0.43, [a] -0.96 [a] Irreversible peak potential. The electrochemical gaps were listed for the DAA donor oxidation and the exTCNQ acceptor reduction values.

The estimated electrochemical gaps of 2 and 3 (0.90 V; calculated from the TTF donor oxidation and the exTCNQ acceptor reduction potentials) and their electron affinities (∆ELUMO; 2.35 V) (Figure 5) encouraged us to apply the TTF-BCO-exTCNQ dyads to monolayer and rectification studies. Dyads 2 and 3 were assembled by the LB technique. [18] Figure 6 shows the representative surface pressure (π-A) isotherms of 2 and 3. The π-A isotherm of 3 is significantly more expanded suggesting a lower molecular aggregation during the compression process. Changes in the slope upon the compression process reveal a progressive orientation and/or reorientation of the molecules at the interface. Brewster angle microscopy (BAM) investigations [19] were made upon the compression of the Langmuir (L) film and gave further insight into the formation of the monolayers (see Section G, Figure S11 in the SI). BAM images indicate significant differences between the two monolayers. Non-homogeneous domains were observed for 2, while a very homogeneous and compact L film was detected for 3 in the compression process even at low surface pressures. The differences in the wavelengths (both L and LB films show a ~5 nm bathochromic shift of the ~320 nm band as well as a ~20 nm bathochromic shift of the ~650 nm band) between the solution in chloroform, the L film, and the LB film spectra are indicative of a different chemical environment of 3, in agreement with the solvatochromism mentioned above. In addition, the UV/Vis spectrum of the dyad 3 in solution shows mainly two broad low energy absorption bands at ~650 nm and ~420 nm with the corresponding molar absorptivity ratio of 3:1, respectively (Figure S13). Whereas the UV/Vis spectra of the L and LB films of 3 showed significant change in the molar absorptivity ratios of 1:1 and 1:1.5, respectively. These results reveal molecular reorganizations during the transference process of the L film onto the substrate and a different relative orientation of these moieties in the L and LB films (see Figure S13 in the SI). The different molar absorptivities of the films with respect to the chloroform solution and cast films are also attributed to orientation effects.

XPS experiments were carried out in LB films of dyad 3 deposited onto gold substrates to obtain more information about the molecular orientation. Figure 7 shows the XPS scans of the S2p and N1s region in LB films transferred at a surface pressure of 10 mN•m -1 ; the XPS of dyad 3 powder and cast films of 3 are also included for comparison purposes. The XPS spectrum of a solid sample of 3 powder in the S2p region shows doublet peaks due to the spin-orbit splitting effect [10][11][12] at 164.0 and 165.2 eV, with an area ratio of 2:1 and a peak separation of 1.2 eV, corresponding to the (2p3/2) and (2p1/2) peaks. In contrast, the XPS spectrum of the LB film of 3 shows four peaks.

Two of them appear practically at the same bonding energy to the ones observed for the powder and keep the same area ratio, 2:1, and peak separation of 1.2 eV revealing that part of the sulfur species do not interact with the gold substrate. However, a new doublet peak, also with a 2:1 area ratio and a peak separation of 1.2 eV, appears at 161.7 and 162.9 eV. This decrease in the binding energy has generally been interpreted as sulfur being adsorbed onto the gold surface. [START_REF] Kato | [END_REF] These results indicate that part of the sulfur atoms are chemisorbed on the gold substrate. In contrast, the XPS spectrum of cast films (prepared from a diluted solution) exhibits only two peaks centered at 161.7 and 162.9 eV. This result indicates that in cast films molecules tend to adopt a planar position with all the sulfur atoms interacting with the gold surface.

The XPS spectrum of the LB film in the N1s region does not show any significant changes in the binding energy of the peaks (402.3 and 399.8 eV attributable to the amine and the cyano nitrogens, respectively) compared to the powder XPS spectrum indicating that there is no chemical interaction between the nitrogen species of dyad 3 and the gold substrate. However, there is a significant shift of these peaks towards lower binding energies in cast films. This result suggests that in cast films the nitrogen atoms of dyad 3 are chemisorbed on the gold surface.

Fabrication and electrical studies of junction devices with 3.

Rectification studies for LB films have been reported before. [3c,3h,3r,17b,20] Here, rectification studies of LB films incorporating dyad 3 were performed by making use of current-voltage (I-V) curves recorded with a conductive-AFM instrument. Further details about the experimental conditions and advantages of using the PF-TUNA™ operation mode for the AFM [21] are detailed in the Experimental Section. The electrical behavior of LB films of reference compound 10 was also studied (details for the LB fabrication and electrical characterization of 10 are in Section G in the SI). Figure 8 shows an averaged I-V curve, from 180 I-V curves, for dyad 3 recorded using a peak force set-point of 65 nN. A compromise was made in order to select the most suitable contact force to be applied during the measurement. Thus, 65 nN was the minimum force needed for an adequate electrical probing and did not result in damage to the monolayer. Figure 8 shows a typical rectification behavior, since above a threshold voltage the current significantly decreases for negative voltages while at positive voltages the current hardly changes. Importantly, such a rectification behavior was not observed for reference compound 10, that exhibits symmetric I-V curves (see Figure S15 in the SI). These results indicate that the rigid -spacer, not present between the aniline donor and exTCNQ acceptor moieties in 10, is essential in dyad 3 to act as a rectifier. Importantly, they also unambiguously document that rectification in 3 is not due to current flow between the (weakened) DAA donor and the directly attached exTCNQ acceptor moieties.

The monolayer of 3 behaves as a rectifier, since above a threshold voltage the current significantly decreases for negative voltages while at positive voltages the current hardly changes. The rectification ratio RR in dyad 3 (negative current/positive current, RR) is 9 (inset of Figure 8 shows a statistical distribution of the rectification ratio obtained 180 I-V curves), which suggests the alignment of dyad 3 to form a well-ordered monolayer [3r] (see BAM and AFM images at 10 mN•m -1 , Figures S11 andS12, respectively, in the SI). In addition, the RR practically does not decrease after several scans as it has been shown previously in the literature for similar compounds. [3r] The direction of the observed rectification would indicate that the preferred electron current is from the exTCNQ acceptor to the TTF donor.

Conclusions

Two closely related Aviram-Ratner-type molecular dyads comprising TTF as strong donor and DAA-substituted exTCNQ as strong acceptor, separated by the rigid spacer bicyclo[2.2.2]octane (BCO), have been prepared. By installing the acceptor in the final step using the CA-RE cascade, problems such as spontaneous redox chemistry or intramolecular CT from TTF to the acceptor, which had been encountered in previous approaches to molecular rectifying dyads, could be avoided. All electrochemical, optical, and EPR studies in comparison with appropriate reference compounds revealed that there is no significant CT between the TTF moiety and the acceptor moiety exTCNQ in the ground state. Only intramolecular CT between the DAA moiety and the directly connected exTCNQ acceptor part exists in the ground state. Langmuir-Blodgett monolayer formation was tested for the n-butyl-(2) and the n-octyl-(3) substituted TTF-exTCNQ dyads as well as n-octyl reference compound 10.

Dyad 3 and reference 10 indeed both formed homogeneous LB films. Gratifyingly, rectification behavior with a rectification ratio (RR) of 9 was observed for dyad 3 due to its proposed charge-separated state under applied voltage. On the other hand, reference 10 did not exhibit an asymmetric I-V curve but instead showed a symmetric curve, demonstrating the crucial role of the insulated TTF donor moiety in 3, besides the exTCNQ acceptor moiety for rectification. This result fully validates the rectification proposal by Aviram and Ratner and demonstrates the essential nature of the rigid -spacer in 3. Practical applicability of this system may be possible given the fact that 3 has high thermal and reversible electrochemical stability, which are necessary for an organic molecular rectifier. Synthesis of dyads with even lower HOMO-LUMO gaps based on strong acceptor such as F4-TCNQ (Ered,1 = +0.16 V in CH2Cl2 vs. Fc + /Fc; TCNQ: Ered,1 = -0.25 V in CH2Cl2 vs. Fc + /Fc) is in progress.

Experimental Section

Materials and methods: Chemicals were purchased from Acros, Aldrich, Fluka, and TCI, and used as received. CH2Cl2 was freshly distilled from CaH2 under nitrogen atmosphere. Column chromatography (CC) and plug filtrations were carried out with The solid substrates used to support the LB films were quartz, mica, and gold. Gold substrates were purchased from Arrandee®, Schroeer, Germany and were flameannealed at approximately 800-1000 ºC with a Bunsen burner immediately prior to use to prepare atomically flat Au(111) terraces. [22] Cast films were prepared by spreading a drop of a very diluted solution (5•10 -6 M ) onto quartz and gold substrates. The solvent was then allowed to evaporate. These films were prepared with the purpose of comparison with LB films. UV/Vis spectra were recorded using a normal incident angle with respect to the film plane. X-ray photoelectron spectroscopy (XPS) spectra were acquired on a Kratos AXIS ultra DLD spectrometer with a monochromatic Al K Xray source (1486.6 eV) using a pass energy of 20 eV. To provide a precise energy calibration, the XPS binding energies were referenced to the C1s peak at 284.6 eV.

Atomic force microscopy (AFM) experiments were performed by means of a Multimode 8 AFM system from Veeco, using the tapping mode in ambient air conditions. The data were collected with a silicon cantilever provided by Bruker, with a force constant of 40 mN and operating at a resonant frequency of 300 kHz. Thus, this is a method for the conductivity mapping of soft or fragile samples and as such it has been chosen for rectification probing of our LB films, rather than using STM or conducting AFM in conventional contact mode. The peak force tunneling AFM used here combines "tapping" mode AFM with a conducting AFM tip and low-noise current amplifier to probe current flow through the LB film. To be sure that a reasonable contact between the tip and the LB film is obtained without damaging the monolayer a peak force set-point of 65 nN for dyad 3 and 22 nN for compound 10 was applied. Using this peak force set-point, current-voltage (I-V) curves were then recorded with the AFM probe placed on top of the LB film and a bias between the substrate and the tip was applied. I-V characteristics were recorded by sweeping the tip voltage (±1.6 V) with the LB-coated Au substrate held at ground. To ensure reproducibility and reliability of the results, the I-V curves were averaged from multiple scans.

Electrical properties of the

Synthesis:

Only the synthesis of dyad 3 is described in this Experimental Section. All other preparations and characterizations are in the Supporting Information.

N,N-Dioctyl-4-(2-{4-[2-(trimethylsilyl)ethynyl]bicyclo[2.2.2]oct-1-yl}ethynyl)aniline (5).

A solution of 4 [9] (50 mg, 0.32 mmol) in Et2O (3 mL) under nitrogen was cooled to -78 °C, treated with 1.6 M n-BuLi in hexane (0.25 mL, 0.39 mmol), stirred for 1 h, treated with Me3SiCl (0.06 mL, 0.47 mmol), and stirred for 1 h at -78 °C and for 1 h at 25 °C. The mixture was diluted with saturated NH4Cl solution and extracted with Et2O. Evaporation afforded a colorless solid, which was directly used for the Sonogashira coupling reaction. A solution of the solid residue in Et3N ( 5mL) was purged with nitrogen for 15 min and added to a mixture of N,N-dioctyl-4iodoaniline [10] (2S, 350 mg, 0.79 mmol), [PdCl2(PPh3)2] (7 mg, 9.6 mol), and CuI (2.5 mg, 13 mol) in C6H6 (4 mL). The mixture was stirred for 15 h at 25 °C, diluted with Et2O (20 mL), and filtered through a plug of SiO2. Evaporation and CC (SiO2; hexane/CH2Cl2 1:0  9:1) afforded 5 (60 mg, 35%) [23] colorless solid (17 mg, 15%); see Scheme S1 and Section A in the SI).

Data of 5: Rf = 0.48 (SiO2; hexane/CH2Cl2 8:2); 1 H NMR (400 MHz,CDCl3,298 K; assignment based on DQF-COSY, HSQC, and HMBC spectra):  = 0.13 (s, 9 H;

Figure 1 :

 1 Figure1: Top: Original molecular rectifier 1 proposed by Aviram and Ratner.[1]

  Figure 1

  Scheme 1

Figure 2 :

 2 Figure 2: Crystal structures of 2 (a,b) and 9 (c) at 100 K. H-atoms for 2 and 9 and

Figure 3 :

 3 Figure 3: Control compounds 9-11.

Figure 4 :

 4 Figure 4: UV/Vis absorption spectra of dyads 2, 3 and reference compounds 9-11 in

Figure

  Figure S10 in the SI). The vanishing amount of radical contamination is also reflected by the sharp signals in the 1 H and 13 C NMR spectra of 2 (Figures S17 and S18 in the

Figure 5 :

 5 Figure 5: Top: HOMO of the TTF donor and LUMO of the exTCNQ acceptor parts of

Figure 6 :

 6 Figure 6: Surface pressure vs. area per molecule isotherms of dyads 2 and 3 on a water

Figure 7 .

 7 Figure 7. XPS spectra of (a) S2p and (b) N1s photoelectrons of dyad 3 in powder, in cast films, and in LB films transferred at 10 mN•m -1 . In all cases the supporting

Figure 8 :

 8 Figure 8: I-V curve of a monomolecular LB film of dyad 3 transferred onto Au(111) at 10 mN•m -1 . Inset figure: statistical distribution of the rectification ratio obtained

SiO2 60 (

 60 particle size 0.040-0.063 mm, 230-400 mesh; Aldrich) or with neutral Al2O3 (Brockmann activity I). Thin layer chromatography (TLC) was performed using aluminum sheets coated with 0.2 mm Merck silica gel or < 60 mm Aldrich neutral Al2O3. The compounds were visualized by UV light (254 nm) or by visual color change without staining with a colorizing agent. Melting points(m.p.) were measured in open capillaries with a Büchi melting point B540 apparatus and are uncorrected. "Decomp" refers to decomposition.1 H NMR and[START_REF]A solution of 8 (40 mg, 71 μmol) was treated with Fe(ClO4)3 H2O (53 mg, 142 μmol) in MeCN (5 mL) and stirred for 2 h at 25 °C. Then 7,7,8,8 tetracyanoquinodimethane (TCNQ; 159 mg, 78 μmol) was added and continued the stirring for additional 15 h. Then sodium L-ascorbate (28 mg, 142 μmol) was added in MeOH (5 mL) and stirred for 0.5 h. Evaporation and column chromatography (pentane/CH2Cl2 4:1→1:2) gave 2 (22 mg[END_REF] C NMR spectra were measured on a Bruker DRX 400 MHz, a Bruker AV 400 MHz or Avance II 600 MHz at 25 °C. Chemical shifts (δ) are reported in ppm downfield from SiMe4, with the residual solvent signal as an internal reference. Coupling constants (J) are given in Hz. The apparent resonance multiplicity is described as s (singlet), d (doublet), dd (doublet of doublet), quint.(quintet), hex. (hextet), and m (multiplet). Infrared spectra (IR) were recorded on a Perkin-Elmer BX FT-IR spectrometer; signal designations: s (strong), m (medium), w (weak). Selected absorption bands are reported in wavenumbers (cm -1 ). UV/Vis spectra were recorded on a Varian CARY-500 spectrophotometer. The spectra were measured in a quartz cuvette of 1 cm at 298 K. The absorption maxima (λmax) are reported in nm with the extinction coefficient (ε) in M -1 cm -1 in brackets. Shoulders are indicated as sh. High-resolution (HR) ESI-TOF spectra were measured on a Bruker maXis ESI-Q-TOF spectrometer. HR FT-ICR-MALDI spectra were measured on IonSpecUltima Fourier transform (FT) instrument with 3-hydroxypicolinic acid (3-HPA) and 2-[(2E)-3-(4-tertbutylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) in THF. The most important signals are reported in m/z units with M + as the molecular ion. Size exclusion chromatography (GPC) was performed with BIO-RAD Beads S-X3 as stationary phase and distilled technical solvents at ambient pressure and temperature.Continuous wave (CW) EPR:The experiment was performed at a frequency of 9.8756 GHz with a Bruker ElexSys E500 spectrometer equipped with a Bruker super high Q resonator ER4122SHQ at ambient temperature. The sample was placed into a thin glass capillary (~0.4 mm inner diameter, BLAUBRAND® micropipettes), and the EPR spectrum was recorded with 100 kHz field modulation, 20 dB attenuation (2 mW incident microwave power), 0.2 mT modulation amplitude, 5.12 ms time constant, and 20.48 ms conversion time.Preparation and characterization of cast films and LB monolayers incorporating dyads 2 and 3 and reference compound 10: The compounds were assembled by the Langmuir-Blodgett (LB) technique using a Nima Teflon trough with dimensions 720x100 mm 2 housed in a constant temperature (20  1 ºC) clean room. The surface pressure (π) was measured by using a Wilhelmy paper plate pressure sensor, and ultrapure Milli-Q water (resistivity 18.2 Mcm) was used as subphase. Solutions of 2, 3, and 10 (5•10 -6 M) were prepared in chloroform (HPLC grade, 99.9% purchased from Sigma) and spread using a Hamilton micro-syringe held very close to the aqueous surface, allowing the surface pressure to return to a value as close as possible to zero between each addition. The spreading solvent was allowed to completely evaporate over a period of at least 15 min before compression of the Langmuir (L) film at a constant sweeping speed of 0.02 nm 2 •molecule -1 •min -1 . The films were deposited at a constant surface pressure by the vertical dipping method with a dipping speed of 0.6 cm•min -1 . All the films were transferred by the withdrawal of the substrate from the water subphase for which the transfer ratio is close to 1. Deposition of the films by immersion of the substrates into the water surface resulted in a poor transfer ratio (<0.2).

  LB films of dyad 3 and reference compound 10: The electrical properties were recorded with a conductive-AFM (Bruker ICON) under humidity control, ca. 30%, with a N2 flow using the Peak Force Tunneling AFM (PF-TUNA™) mode, and employing a PF-TUNA™ cantilever from Bruker (coated with Pt/Ir 20 nm, ca. 25 nm radius, 0.4 N•m -1 spring constant and 70 kHz resonance frequency). The PF-TUNA™ operation mode for the AFM was chosen in order to avoid lateral forces during the images that would have damaged tip coating and sample surface, while at the same time allowing the use of cantilevers with low spring constant.

Table 1 .

 1 Optical and electrochemical gaps of 2, 3, 9, and 10 determined by UV/Vis spectroscopy and CV in CH2Cl2.

  as a colorless liquid besides two

	side-products		[4,4'-(bicyclo[2.2.2]octane-1,4-diyl)diethyne-2,1-diyl)bis(N,N-
	dioctylaniline	(4S),	colorless	liquid	(17	mg,	7%),	and	1,4-
	bis(trimethylsilylethynyl)bicyclo[2.2.2]octane (5S),

Acknowledgements

This work was supported by the ERC Advanced grant no. 246637 ("OPTELOMAC") and the Swiss National Science Foundation (SNF). ADF acknowledges the NSF-IRFP (U.S.A.) for a postdoctoral fellowship. SM and PC are grateful for financial assistance from Ministerio de Ciencia e Innovación from Spain in the framework of grants CTQ2012-33198 and CTQ2013-50187-EXP.We thank Dr. Nils Trapp for help with the X-ray crystal structures, Dr. Antorrena and Dr. González-Orive for their help with the XPS and AFM experiments, respectively.

Table of Content


Souad Dekar

Kamel Ouari
email: kamel_ouari@univ-setif.dz

Sabrina Bendia

Douniazed Hannachi

Jean Weiss

Mononuclear oxovanadium(IV) Schiff base complex: Synthesis, spectroscopy, electrochemistry, DFT calculation and catalytic activity

Keywords: spectroscopy, electrochemistry, DFT calculation and catalytic activity Oxovanadium complex, Spectroscopy, Crystallography, DFT, Cyclic Voltammetry, Oxidation

published or not. The documents may come

Mononuclear oxovanadium(IV) Schiff base complex:

Synthesis,

Introduction

In recent decades Schiff bases play an important role in inorganic chemistry as they easily form stable complexes with most transition metal ions [1][2][3]. The coordination chemistry of transition metal-Schiff base complexes has been extraordinary developed in different aspects [4][5][6][7] because of their high stability and their important properties in different oxidation states.

Recently much research has been focused on the coordination chemistry of vanadium complexes because of the interesting structural features [8][9][10], biochemical and pharmacological efficiency as insulinomimetic profile [11], antiparasitic [12] and antitumor activities [13]. In medicinal processes, such as haloperoxidation [15], phosphorylation [16], nitrogen fixation [17],

glycogen metabolism [18,19] and insulin mimicking [20,21], spermicidal [22], anti-leukemia [23].

Also, in homogeneous catalysis, vanadium Schiff base complexes formed with chelating ligands involving hetero atoms are found to be used in a variety of systems [14]. Homogeneous catalysts of oxovanadium(IV) complexes induce organic reactions in the oxidation of sulfides and sulfones [24,25], the epoxidation of alkenes [26,27], the hydroxylation of hydrocarbons [28], the oxidation of alcohols to aldehydes and ketones [29,30], phenols [31] and tertiary amines to N-oxides [32].

These studies are indicative that oxovanadium(IV) complexes are potential catalysts to influence the yield and selectivity in chemical transformation [33]. In addition, oxovanadium(IV) complexes, attract considerable attention due to their remarkable catalytic activity and good selectivity for the oxidation of various substrates using H2O2 as a green oxidant [34], thus responding to the increasing demand for more efficient homogeneous and heterogeneous catalytic systems [35][36][37][38]., The epoxidation of alkenes is one of the most widely studied reactions in organic chemistry since epoxides are key starting materials for a wide variety of products and the catalytic epoxidation of olefins by transition metal complexes is an area of intense research activity [START_REF] Jørgensen | Transition-metal-catalyzed epoxidations[END_REF][40] In the last decades, the synthesis, the structural study and catalytic activity of oxovanadium Schiff base complexes in oxidation of different organic substrates have been described in a great number of publications [41,42]. The central metal oxovanadium is surrounded by N2O2 coordination environment. Recent advances have shown vanadium complexes to be efficient catalysts for the activation of peroxides by virtue of vanadium in terms of stereoselectivity, reactivity and specificity [43]. The ability of these complexes to form metalloperoxo species, which in turn effectively transfer an oxygen atom to reductants with a high degree of selectivity, render them synthetically useful for obtaining valuable molecules.

In order to investigate the catalytic activity of oxovanadium Schiff base complexes as catalysts, we have synthesized in this work a new symmetrical tetradentate Schiff base ligand and its corresponding oxovanadium complex have been characterized by spectroscopic methods. Subsequently, the crystal structure of this new Schiff base complex has been determined by X-ray crystallography. We report hereafter the catalytic epoxidation of cyclohexene in dry solvent using hydrogen peroxide and molecular oxygen as oxidants. The electrochemical behavior of the complex has also been investigated.

Experimental

Materials and Measurements

All chemicals were of reagent grade solvents and all commercially available reagents (Aldrich or Merck) and used without further purification. The syntheses were carried out under inert atmosphere (N2) following conventional procedures.

The melting points for the ligand H2L and its complex of VO(IV)L were determined with a Kofler Banc 7779 apparatus and are uncorrected. Infrared spectra were recorded using KBr pellets on a Shimadzu FTIR IR Affinity-1 spectrophotometer. High-resolution mass spectrum (HRMS) for oxovanadium (IV) complex was acquired via electrospray ionization mass spectrometry (ESI-MS).

Electronic spectra were performed on a Shimadzu UV-1800 spectrophotometer using DMF as solvent. Elemental analyses were carried out on an Elementar-Vario EL III CHNSO analyzer. 1 H-NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer using DMSO-d6 as solvent and tetramethylsilane (TMS) as internal standard. The molar conductivities of the compounds were carried out on conductivitymeter MeterLab CDM-210 apparatus. Thermogravimetry (TGA) and differential thermoanalysis (DTA) were carried out by using a PL-1500 apparatus, the measurements were performed in nitrogen atmosphere, the heating rate was held at 10 °C min -1 .

The electrochemical properties of the structures were investigated at room temperature in solutions containing Lithium perchlorate (LiClO4), 0.1 M, as supporting electrolyte on a glassy carbon (GC) working electrode, the counter electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE), the results are given versus SCE. The solvent was DMF with a 10 -3 M concentration for both the ligand and the complex. The scan rate used in all the voltammograms was of 100 mV/s. All solutions were deoxygenated by passing a stream of prepurified nitrogen into the solution for at least 10 min prior to recording the voltammograms. Gas chromatography (GC) analyses were carried out on a GC-2014 Shimadzu instrument; it is equipped with a DB5 capillary column of 30 m length, 0.25 mm diameter and flame-ionization detector.

X-ray crystallography

Single crystals of VO(IV)L complex were grown by slow layer diffusion of DMSO into a MeOH solution at room temperature. The crystals were placed in oil and a brown single crystal of dimensions 0.25 × 0.15 × 0.12 mm 3 was selected, mounted on a glass fiber and placed in a lowtemperature N2 stream. X-ray diffraction data collection was carried out on a Bruker APEX II DUO Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N2 device, using Mo Kα radiation (k = 0.71073 Å). The crystal-detector distance was 38 mm. The cell parameters were determined (APEX2 software) [START_REF]M86-E01078 APEX2 User Manual[END_REF] from reflections taken from tree sets of 12 frames, each at 10 s exposure. The structure was solved by direct methods using the program SHELXS-97 [START_REF] Sheldrick | SHELXS-97 program for crystal structure determination[END_REF]. The refinement and all further calculations were carried out using SHELXL-97 [START_REF] Sheldrick | SHELXL-97[END_REF]. The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted fullmatrix least-squares on F2. A semi-empirical absorption correction was applied using SADABS in APEX2 [START_REF] Dolaz | [END_REF]; transmission factors: Tmin/Tmax = 0.580/0.746.

General procedure of the oxidation reaction

In a typical experiment, 15 mmol of the oxovanadium catalyst were dissolved in 15 mL of DMF, 15 mmol of cyclohexene and 30 mmol of hydrogen peroxide (H2O2 ) or molecular oxygen (O2 ) were added. With O2, the reaction is carried out under bubbling with a needle introduced into the mixture. The reaction mixture was refluxed while being stirred and the reaction progress was monitored at 1 h intervals. The optimized reaction time was obtained by following the reaction progress at 1 h intervals up to 24 h; 6 h was the optimized reaction time.

In the absence of the complex, no oxidation products were observed. In gas chromatography, the retention times for the starting materials and the products were determined by comparison with authentic samples. The conversion percent (%) are calculated by the following equation, in which C initial and C final are initial and final concentration of the substrate, respectively.

Synthesis of the ligand

The tetradentate Schiff base ligand, H2L, was synthesized according to the literature method [48,49] by condensing of 1 mmol (0.108g) of phenylenediamine with 2 mmol (0.402 g) of 5-Bromo salicylaldehyde in 15 ml of methanol .The bright orange solution was stirred and heated to reflux for 2 h under nitrogen atmosphere, scheme 1.

The precipitate was filtered, washed with cold methanol (3x2 ml) and diethyl ether (3x2 ml) and then dried in vacuum over a night. Suitable single crystals for X-ray crystallography were obtained from recrystallization using DMSO/MeOH. Yield: 90%; mp = 234-238 °C; ΛM (Ω -1 cm 2 mol -1 ) =1.47; IR, KBr pellets,  cm

Synthesis of the complex

The title complex, VO(IV)L, was prepared in a 10 ml MeOH by adding dropwise a solution of 0.5 mmol bis (acetylacetonato) oxovanadium in absolute MeOH to 0.5 mmol of the ligand H2L. The mixture was refluxed and stirred for 2 h under nitrogen atmosphere, scheme 1. The green powder was separated from the solution and purified by washing with cold methanol (3x2 ml)

and diethyl ether (3x2 ml). Suitable single crystals for X-ray crystallography were obtained from

Br Br

Scheme.1. Synthesis of the ligand H2L and the corresponding oxovanadium complex VO(IV)L.

Results and discussion

The Schiff base H2L and oxovanadium complex VO(IV)L are stable at room temperature and are soluble in organic solvents such as DMF, DMSO, C5H5N and insoluble in inorganic solvents.

Elemental analysis

Elemental analysis data of the oxovanadium complex show good agreement between found and calculated values and fitted well with a 1:1 metal to ligand.

Molar conductivity measurements

The molar conductivity of ligand and its complex were measured in DMF 10 -3 M as solvent at room temperature, showed a low conductance value, 1.47 and 3.81 ohm -1 cm 2 mol -1 , respectively, suggest a non electrolytic nature and there is no counter ion present outside the coordination sphere [50,51].

Electronic spectral studies

The electronic spectral bands of the Schiff base ligand H2L and its corresponding complex VO(IV)L are presented in Table .1.

In the 200 to 800 nm wavelength range, the Schiff base ligand exhibits two bands at 267 and 344 nm. The first band is attributed to the  _   transitions in the diamine aromatic backbone. The second one is due to n _   transition of the azomethine group [52,53].

In the complex, the three bands localized at 306 and 322 nm are assigned to the intra-ligand π _ π* and n _   and 414 nm due to LMCT transitions [54,55]. The d-d transitions were not observed for the VO(IV)L complex.

Infrared spectra

The IR spectra of the free Schiff base ligand and the complex exhibit several bands in 400-4000 cm -1 region.

The IR spectrum of the ligand, a sharp C=N band was observed at 1621 cm -1 [56]. The ligand displays intense bands at 3457 and 1274 cm -1 which are assigned to (O-H) and (C-O)

stretching vibrations of the phenolic moieties. Bands appearing in the range 1473-1561 cm -1 are related to the aromatic C=C vibrations. Additionally, the band observed at 630 cm -1 is assigned to the C-Br vibration [57].

In the spectrum of the complex, the vibration bands observed in the Schiff bases were decreased in intensity and shifted towards a lower frequency for the C=N band, 1604 cm -1 [58,59],

the frequency of the C-O band is shifted to a higher value, 1293 cm -1 , indicating that the nitrogen atom of the azomethine group and the oxygen of the phenolic moiety are coordinated to the metal ion [60]. The oxovanadium complex shows a band at 968 cm -1 attributed to V=O frequency [61].

NMR spectroscopy

The chemical shifts of the different types of protons in the 1 H NMR spectra of the H2L ligand was reported in Table 2.

The structure of the synthesized ligand is given by the following scheme: The azomethine proton appears at 8.54 ppm. Multiplet signal, due to the aromatic protons, are observed in the range 6.88-8.00 ppm [49 ].

Fig. 1. 1 H NMR spectrum of Schiff base ligand (H2L) in CDCl3.

Mass spectrometry

The The dibrominated complex mass spectrum presents its expected molecular peak cluster, due to the relative abundance of the bromine atome isotopes [62,63].

Fig. 2. The mass spectrum of oxovanadium complex VO(IV)L.

X-ray crystal structure

An ORTEP view of VO(IV)L with the atom-numbering scheme is presented in Fig. 3. Table 3 shows crystal data, data collection and structure refinement parameters. Most relevant bond lengths and angles are given in Table .4. The crystal structure shows the presence of one molecule of mononuclear complex in the asymmetric unit. The VO(IV)L crystallizes in the monoclinic system with space group P21/c and the cell dimensions a = 11.0545 (8) ; b = 18.1170 (12) ; c = 9.6547 (7) Å; á = ã = 90°; â = 108.581

(2). The oxovanadium atom in this complex is coordinated by the two nitrogen and two oxygen donor atoms and one oxygen atom to complete the N2O3 coordination sphere. The geometry can be considered as a distorted square pyramidal coordination with N1, O1, N2 and O2 of the Schiff base complex. The distances between the coordinated nitrogen and oxygen atoms with the metal center do not significantly differ (Table 3).

The C=N bond distances are 1.308( 7) Å (N1=C7) and 1.300( 7) Å (N2=C14), which are consistent with a slight elongation of the C=N double bond in the corresponding ligand [64,49].

The V-O bond lengths of 1.888(4) and 1.929(4) Å and the V-N bond lengths of 2.062(5) and 2.058(5) Å are similar to those given in related Schiff base oxovanadium complexes [64][65][66][67]. The V-O bond distances are shorter than V-N, which indicates stronger coordination of the oxygen atoms.

The β-angle N1-V1-N2 is 78.33 (19) and the α-angle O1-V1-O2 is 89.30 (19). Thus, the phenolate oxygens are displaced out of the "equatorial" plane away from the axial oxo ligand.The oxovanadium V=O bond distance is 1.604(4), which is a typical value for vanadyl compounds [68,69].

The two phenolate ring moieties of the complex are not coplanar with the above coordination plane (N1-N2-O1-O2) and bend to the same side. The packing structure of VO(IV)L consists of a discrete molecule held together by Van Der Waals forces (Fig. 4).

DFT Calculations

The computational study was carried out starting from the X-ray crystallographic molecular structures of VO(IV)L complex and H2L ligand Recent investigations have shown that the Becke's three parameter hybrid functional using the LYP correlation functional [70] (B3LYP) method [71] is reliable for the description of geometrical parameters, electronic structure and optical properties of different molecules and complexes [72][73][74][75][76]. In the present density functional theory (DFT) calculations, the B3LYP functional was adopted together with one set of two polarization functions (TZ2P) as the basis sets and none frozen cores in all calculations. Relativistic corrections were taken into account with the use of the relativistic scalar zero-order-regular approximation (ZORA) method [77,78]. The numerical integration parameter was done with the Beckegrid option fixed to the quality very good [79,80]. The energy convergence criterions were au. All quantum chemistry calculations were performed with Amsterdam Density Functional (ADF) program developed by Baerends and co-workers [81][82][START_REF] Baerends | ADF2014.01[END_REF].

All structures were calculated in their ground state without any symmetry constraints. No imaginary vibrational frequencies were found for VO(IV)L and H2L optimized geometries.

The important optimized geometrical parameters (distance and angle) of the VO(IV)L and H2L calculated using the density functionals B3LYP are presented in Fig. 5 and Table .5. Experimental data are given in green.

* centroid between O-N-N-O

As seen in Fig. 5, the deviations between theoretical result and experimental geometry parameters of H2L ligand are in satisfactory agreement with experimental results. For the VO(IV)L complex, the V-O3 distance is significantly shorter than the V-O1,2, as observed in the solid state.

In addition, the dihedral angle O1-N1-N2-O2 and angles N1-V-N2 and O1-V-O2 are same in the solid state. More generally, the average bond lengths V-O, V-N, Br-C and C-C calculated by B3LYP/TZ2P method are systematically a little longer than the experimental. Our calculated results of distance and angles agree well with the experimental data. This agreement, if it was necessary, clearly supports the ability of B3LYP/TZ2P to accurately reproduce the structure of H2L and VO(IV)L.

In Table .6 The lowest energy transition of H2L (at 373and 344 nm) involves exclusively (99%) the HOMO and HOMO-1,respectevely, localized on the donor part of the and the LUMO, delocalized on the two aromatic rings (see Figure 2.a). Thus, these transition can be classically described as an intramolecular charge transfer (ICT) corresponds to the π →π* with some minor n→π* transition (HOMO→LUMO). The optical transition centered at 325, 289, 271 and 256 nm can be assigned to a π→π* transition (see fig. 2-a). On the other hand the experimental lowestenergy absorption maximum of this ligand are observed at 267 and 343 nm. From these results, we can say that there is a good agreement between the experimental data and excitation calculations.

Our TD-DFT calculations obtained for VO(IV)L complex predict that the transition π→π* and n →π* are located at 410 and 424 nm are dominated by the {SOMO→ SUMO, SOMO→ SUMO+1} and {SOMO-1→ SUMO, SOMO→ SUMO} respectively (see Fig. 2.b).

Furthermore, the electronic transition arise from the metal toward the ligand orbitals (metal to ligand charge transfer MLCT) at 424nm is dominated by the SOMO-2→SUMO and f =0.126

(see table 3). On the other hand, the excitations from the SOMO-2→SUMO+3 and SOMO-2→SUMO+4 are intra-charge metal transfer (ICMT character) bonds at 554 and 589 nm but the oscillator strengths are zero. These results agree well with experimental and confirmed that the ICMT (d→d) transition completely absent in VO(IV)L complex.

Thermal analysis

The thermal properties of the Schiff base and the corresponding complex were investigated by thermo gravimetric analyses TGA and DTA measurements to get information about the thermal stabilities whether the water molecules are inside or outside the inter coordination sphere of the central metal ion and to suggest a general scheme for thermal decomposition of this complex.

At a heating rate of 10 °C/min under a nitrogen atmosphere the temperature was increased from ambient to 950 °C. The absence of weight loss up to 80 °C indicates that there is no water molecule in the crystalline form [90,91]. Also, the TGA showed no weight loss up to 150 °C indicating that the water molecule is not coordinated to complex [92,93].

The complex was decomposed in two steps (Fig. 7). The first step occurs between 343 and 414 °C and was attributed to the release of C6H4N2, as indicated by the mass loss of 19.31% (calculated 18.56%). The second step occurs 414-661°C, a mass loss of 64.15% (calculated 68.26%) due to the elimination of a C14H8O2Br2 group, with the formation of metal oxide [48].

Electrochemical measurements

The electrochemical behavior of the ligand H2L and the complex VO(IV)L was investigated by cyclic voltammetry with a glassy carbon (GC) electrode in DMF solution containing 10 -1 M LiClO4 as the supporting electrolyte, under nitrogen saturated atmosphere in the potential range -2200 to 1600 mV/SCE. The Fc/Fc + , redox couple of ferrocene was used as an internal standard. The A typical cyclic voltammogram of complex in the potential range from 200 to 900 mV/SCE is shown in Fig. 9. Multiple scans which resulted in nearly superposable cyclic voltammograms show that the five coordinates geometry is stable in both oxidation states, at least on the cyclic voltammetry time scale. The result revealed that the redox process of studied vanadium Schiff base complex is a one-electron transfer reaction. The results of cyclic voltammetry (CV) measurements are given in Table 7.

The square root of the scan rate exhibits a linear relation with the current peak Ip (inset of Fig. 9). The Ip/v 1/2 value is almost constant for all scan rates. This establishes the electrode process as diffusion controlled [98]. The variation of the scan rate between 200 and 900mVs -1 only showed a marginal increase in ΔEp. At a scan rate of 100 mV/s, the separation between cathodic and anodic peaks (ΔEp ≈ 72 mV)

is suggestive of a one electron-process. The ipa/ipc ratio was closest to 1 indicating chemical reversibility system.

Rotating disk electrode voltammetric studies

In order to determine the diffusionnal character of the complex, a linear sweep hydrodynamics voltammetry on rotating disk electrode (RDE), using GC working electrode, is carried out in order to determine the diffusion coefficient D. We have studied the evolution of the current cathodic as a function of the potential Ic= f(E) at different rotation rate of the disc, from 250 to 3000 rpm, in the interval 300 to 800 mV/ECS for a scan rate of 20 mV/s. The current potential curves and the Levich plots are shown in Fig. 10.

Under the studied conditions, a linear correlation between the limiting current and the square root of the rotating rate is observed, which leads to the determination of the value of the proportionality constant using Levich equation for a one electron VO V /VO IV reduction [99].

Diffusion coefficient was calculated from the slope of the ilim vs. ω 1/2 plots and its value is 2.10 -6 cm²/s [97]. Rotation rates: from 250 to 3000 rpm. The inset shows the Levich plot.

Catalytic activity

The catalytic activity and selectivity of the VO(IV)L have been studied for the oxidation of cyclohexene with H2O2 and O2 as oxidants in DMF, under similar experimental conditions [ 100 ].

The oxidation reactions give 2-cyclohexenone as the main product, 2-cyclohexenol and cyclohexene epoxide in small yields which are evidently identified using gas chromatography by comparison with standards (Fig. 11). In order to obtain maximum conversion of cyclohexene, the catalytic reactions were carried out using two oxidants.

The results of the catalytic oxidation of cyclohexene by H2O2 and O2 in the presence of VO(IV)L are shown in table 8, which shows the variation of product yield with the oxidant nature. Higher yield was obtained for 2-cyclohexenone than 2-cyclohexenol and cyclohexene epoxide for the H2O2 oxidant. In the case of O2, as oxidant, the yields for cyclohexenol and cyclohexene epoxide were nearly similar. The yield of the three products increased, but level off with time. This level off is most likely due to the degradation of the VO(IV)L catalyst by the oxidant with time.

---

The nature and the relative yields of the products, formed by catalytic oxidation of cyclohexene using oxovanadium Schiff base complex, vary considerably depending with the oxidant nature. This work shows that the VO(IV)L, as catalyst, in the presence of O2 and H2O2 oxidants, favored the formation of 2-cyclohexenone to the rest of the products (fig. 11).

Fig. 11. The oxidant effect on the oxidation of cyclohexene.

The results show that the activity of the oxovanadium complex in the oxidation of cyclohexene in the presence of O2 in DMF is much higher than in the presence of H2O2 in DMF, the trend of the observed oxidant effect was O2 > H2O2. This may be related to the ability of O2 and inability of H2O2 to mix with the organic substrate phase [101].

In the proposed mechanism, it seems that active oxidant is oxovanadium(V) initially formed by oxidation of oxovanadium(IV) in the presence of O2 (Scheme 2) [102]. O2 coordinate to the V V O species to form V V -O-O -intermediate, which can react with olefin to yield oxide product.

Conclusion

New five-coordinate oxovanadium (V) complex derived from Schiff base ligand containing the N2O2 donor group successfully synthesized with high purity and yield, 90% for H2L and 84%

for VO(IV)L , and characterized with the aid of FT-IR, UV-Vis, and mass spectrometric techniques, along with elemental and thermal analyses. The spectroscopic analysis confirmed the composition and the structure of the newly obtained compound.

The non-electrolytic nature of the VO(IV)L complex was confirmed on the basis of their molar conductance value.

The thermo gravimetric analysis TGA and differential thermoanalysis DTA, are consistent with the proposed structures and provided unambiguous evidence for no coordinated water in the ligand and its complex, and showed that the compound is thermally stable up to 340°C for VO(IV)L, 220°C for H2L. X-ray analysis reveals that VO(IV)L has a distorted square-pyramid geometry.

The calculation confirms that calculated geometrical parameters are in agreement with the experimental data.

Electrochemical studies by the cyclic voltammetry analysis revealed a one-electron quasireversible pair of waves assigned to the VO IV /VO V electroactive species, the half-wave potentials near to 606 mV.

The catalyst VO(IV)L favored the formation of 2-cyclohexenone (73.16% and 63.56%) to the rest of the products in the presence of O2 and H2O2 oxidants. The activity of the oxovanadium complex in the oxidation of cyclohexene in the presence of O2 is much higher than in the presence of H2O2 (O2 > H2O2).

Supplementary Material

CCDC-1516191 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif by e-mailing: data_request@ccdc.cam.ac.uk; or by contacting: The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: 44(0)1223-336033.

  -1 : 3457 (O-H), 1610 (C=N), 1480 (C=C), 1274 (C-O); UV-Vis: DMF, λ nm, [ε M -1 cm -1 ]: 267.44 [8738], 343.96 [7095]; 1 H NMR: (CDCl3  ppm): 13.20 (s, 8.54 (s, CH=N), 6.80 -8.00 (m, ArH); 13 C NMR: (CDCl3  ppm): 160.32 (O-H), 162.40 (CH=N), 108-161 (C-Ar).

Table. 2 .The 1 H

 21 Chemical shifts (in ppm) of the proton of the ligand in CDCl3 NMR spectra of Schiff bases shows signals in conformity with the structure. The peak observed at 13.20 ppm in these Schiff bases was assignable to the O-H of the bromophenol form.

  2. below illustrates the mass spectrum of the complex VO(IV)L. The peak appearing at m/z = 538.76 is the molecular peak [M .+ ].

Fig. 3 .

 3 Fig.3. The molecular structure of the oxovanadium complex. Thermal ellipsoids are drawn at 50% probability

Fig. 4 . 4 .

 44 Fig. 4. Schematic drawing of the crystal packing

Fig. 5 .

 5 Fig.5. DFT optimized geometries of H2L and VO(IV)L ,with important bond lengths (Å) of ligand.

  , we display the computed values of the electronic chemical potential (μ), chemical hardness (η), global electrophilicity (ω) indices and IP of H2L and VO(IV)L. The global hardness (η) measures the stability of a system in terms of resistance to electron transfer and the chemical potential (μ) characterises the escaping tendency of electrons from the equilibrium system. Each system's (η) and (μ) are calculated from the energy of HOMO and LUMO [84-87]: ( ) the global electrophilicity index expresses the ability of a molecule to accept electrons from the surroundings [88]:According to the absolute scale of global electrophilicity power (ω) reported by Domingo et al.[89], the H2L ligand can be classified as strong electrophiles (see Table.

Table. 6 .

 6 Global reactivity descriptors: chemical potential (μ, eV), chemical hardness (η, eV) and electrophilicity index (ωfunctional theory (TD-DFT) calculations were performed in order to understand these spectroscopic data. The molecular orbital energy diagrams of H2L and VO(IV)L are shown in Figures.2.a-b, respectively, and the corresponding electronic transitions are listed in

Fig. 2 .

 2 Fig.2.a. Molecular orbital energy diagram for H2L (B3LYP/TZ2P).

Fig. 2 .

 2 Fig.2.b. DFT molecular orbital diagrams of SOMO and SUMO of α and β spin of VO(IV)L complex (the energies are given in eV). B3LYP/TZ2P level of theory.

Fig. 7 .

 7 Fig.7. TGA and DTA spectrum of the oxovanadium complex VO(IV)L.

Fig. 8 .

 8 Fig.8. Cyclic voltammograms of H2L ligand (a), VOL complex (b) and Fc/Fc + (c) in 0.1 M LiClO4/DMF solution at scan rate of 100 mVs -1 .

Fig. 9 .

 9 Fig.9. CV of 10 -3 M of VOL in DMF/10 -1 M LiClO4 at various scan rates from 25 to 300 mV/s. The inset shows the anodic peak current (ipa) vs.  1/2 .

Fig. 10 .

 10 Fig. 10. Voltammograms of VO(IV)L complex at RDE of GC in DMF solution.

Scheme. 2 .

 2 Scheme.2. Proposed catalytic cycle for the oxidation of cyclohexene by VO(IV)L

Table .

 .

			UV-Vis
	Compounds	max nm)	(, M -1 cm -1 )	Assignement
		267	(8738)	π _ π*
	H2L	344	(7095)	n _  
		306	(16054)	π _ π*
	VOL	322	(15800)	n _ π*
		414	(13000)	LMCT

1. Electronic spectra of the H2L and VO(IV)L

Table . 3

 . . Crystallographic and refinement data for VO(IV)L complex.

	Compound	VO(IV)L
	Molecular formula moiety	C20H12Br2N2O3V
	Molecular weight	173(2)
	Temperature (K)	Mo-K (0.71073 Å)
	Radiation	Monoclinic
	Crystal system	P21/c
	Space group	11.0545 (8)
	a/Å	18.1170 (12)
	b/ Å	9.6547 (7)
	c/ Å	90.00
	alpha°	108.581 (2)
	Beta°	90.00
	gamma°	1832.8 (2)
	V/Å 3	4
	Z	1.954
	Dcalc (mg m -3 )	0.25 × 0.15 × 0.12
	Crystal size (mm 3 )	Prism
	Crystal description	Brown
	Crystal colour	4.927
	Absorption coefficient (mm -1 )	1052
	F(0 0 0)	28978/3856 [Rint = 0.044]
	Reflections collected/unique	-15,15; -24,24; -13,8
	Range/indices (h, k, l)	1.944-29.022
	Tetalimit	3856
	No. of observed data, I > 2Sigma(I)	247
	No. of variables	0
	No. of restraints	1.158
	Goodness of fit on F 2	4.02 and -1.77
	Largest diff. Peak and hole (e Å -3 )	0.0710, 0.1724
	R1, wR2 [I ≥ 2Sigma(I)] a	0.0901, 0.1815
	R1, wR2 (all data) a
	a R1 = ∑||F0| -|Fc||/∑|F0|, wR2 = [∑w(F0 2 -Fc 2 ) 2 /∑w(F0 2 ) 2 ] 1/2
	w = 1/[σ 2 (Fo 2 ) + (0.0612P) 2 + 17.1114P] where P = (Fo 2 + 2Fc 2 )/3

Table . 5

 .

	N1-V-N2	78	77
	O1-V-O2	89	88
	O1-N1-N2-O2	0.11	0.
		Exp	B3LYP/TZ2P
	V-N1, 2	1.887	1.945
	V-O1,2	2.063	2.100
	V-O3	1.604	1.579
	C1-C6	1.396	1.407
	Cent * -V	0.582	0.628
	Br-C10,17	1.893	1.915

. Selected Bond Lengths (Å) and Angles (°) of VO(IV)L complex

Table . 7

 . .

	Table.7. Calculated Lowest Excitations TD-DFT for H2L and VO(IV)L. B3LYP/TZ2P level of
	theory
			H2L
	 (nm)	f	Type	composition
	373.5	0.294 π→π*, n→π* HOMO→LUMO
	344.52	0.108 π→π*, n→π* HOMO-1→LUMO
	325.44	0.159 π→π*	HOMO→LUMO+1
	289.21	0.117 π→π*	HOMO-3→LUMO
		0.280 π→π*	HOMO-4→LUMO
	256.101 0.228 π→π*	HOMO-5→LUMO
			VO(IV)L
	410.49	0.055 π→π*	SOMO→SUMO
			π→π*, n→π*	SOMO→SUMO +1
	424.02	0.126 π→π*	SOMO-1→SUMO
			π→π*	SOMO→SUMO
			MLCT	SOMO-2→SUMO
	554.11 0.0001 ICMT	SOMO-2→SUMO+3
	589.79 0.0003 ICMT	SOMO-2→SUMO+4
			MLCT	SOMO-2→SUMO+1

Table . 7

 . . Cyclic voltammetric parameters for the oxovanadium VO(IV)L complex in different scan

	rates
	v							ipa/ipc
	25	639.8	576.1	63.7	607.9	6,4	4.66	1.37
	50	642.6	573.3	69.3	607.9	9	6.70	1.34
	75	645.1	573.1	72.0	609.1	10,5	7.79	1.34
	100	642.6	570.8	71.8	606.7	12,2	9.10	1.34
	150	647.8	571.7	76.1	609.7	14,49	11.00	1.31
	200	648.7	572.6	76.1	610.6	16,5	12.66	1.30
	250	648.7	569.7	79.0	609.2	18	14.14	1.27
	300	650.6	571.7	78.9	611.1	19,82	15.4	1.28

Table .

 .

	8. Catalytic oxidation of cyclohexene by H2O2 and O2 in the presence of VO(IV)L
					%Sélectivity
	Catalyst	Oxidant	%Conversion
				Epoxide Cyclohexene-ol Cyclohexene-one
		H2O2	80	9.71	26.72	63.56
	VO(IV)L	O2	85	3.21	23.62	73.16
		Oxidant Free	-

Acknowledgement

The Authors gratefully acknowledge the financial support from The Algerian Ministry of Higher Education and Scientific Research. They also acknowledge the help to access the NMR Facility and Microanalysis Service at GDS 3648 of the University of Strasbourg -CNRS -France.