Photoinduced Charge Separation within Metallo-supramolecular

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Photoinduced charge separation within metallo-supramolecular wires built around a [Ru(bpy)3]2+_bisterpyridine linear entity. Rajaa Ghassan Farran, Damien Jouvenot, Béatrice Gennaro, Frederique Loiseau, Jérôme Chauvin, and Alain Deronzier ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05082 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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ACS Applied Materials & Interfaces

Photoinduced Charge Separation within Metallo-Supramolecular Wires Built around a [Ru(bpy)3]2+_Bisterpyridine Linear Entity.

Rajaa Farran, Damien Jouvenot, Béatrice Gennaro, Frédérique Loiseau, Jérôme Chauvin*, Alain Deronzier. Université de Grenoble-Alpes, Département de Chimie Moléculaire, UMR CNRS 5250, CS 40700, 38058 Grenoble cedex 9, France, E-mail : [email protected]

Keywords : Ruthenium ; Photoredox processes ; Surface assemblies ; Photoelectrode ; Coordination polymer ; Inorganic triad.

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Abstract: A [Ru(bpy)3]2+ like complex (L1) bearing two free terpyridine groups at the 5 and 5’ positions of the same bipyridine, linked by the rigid and linear 2,5-dimethyl phenylene bridges has been synthesized to open access to two classes of linear molecular wires with photosensitive properties: a bimetallic coordination polymer and an inorganic triad. In this article we report on the synthesis and characterization of the resulting [{RuII_FeII}n]4n+ alternated bimetallic polymer and the [CoIII_RuII_FeII]7+ triad based on the building block L1. The [{RuII_FeII}n]4n+ polymer is fully characterized in solution. Cyclic voltammetry and emission lifetime measurements show that the bridging ligand allows interaction between the metal centers in the excited state despite the lack of interactions in the ground state. Under visible irradiation, the polymer can be fully oxidized in the presence of a sacrificial electron acceptor in solution. Thin robust films of the polymer are easily obtained on ITO by a simple electrochemical procedure based on an electroreduction adsorption process. The ITO/[{RuII_FeII}n]4n+ modified electrode behaves as a photocathode under irradiation in presence of ArN2+. The magnitude of the photocurrent is dependent on the film thickness, probably limited by the diffusion of charge in thicker film. On the other hand L1 is also used to construct a well-ordered triad in association with Co(III) and Fe(II) metallic centers as electron acceptor and donor respectively. The metallic triad is anchored on ITO or on a SiO2 wafer, starting from a terpyridine phosphonate modified surface. AFM images prove the presence of the triad in a linear upward orientation. Irradiation of the ITO/[CoIII_RuII_FeII]7+ modified surface in the presence of triethanolamine in CH3CN induces the generation of an anodic photocurrent of around 30 µA.cm-2. The photocurrent density generated by the ITO/[CoIII_RuII_FeII]7+ electrode, appears to be more stable than in the case of ITO/[{RuII_FeII}n]4n+ due to the presence of the anchoring group. Moreover, this photocurrent magnitude represents an enhancement of 30% compared to our previous triad (Dalton Trans., 2014, 43, 12156-12159), proving the advantage of a linear and rigid spacer for the construction of such molecular assemblies with photoinduced charge transfer abilities.


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Introduction:

The great diversity of properties depending on the metal center and organic ligands selected offer to molecular wires built around metallo-supramolecular units, a wide range of applications.1 The immobilization of such species on conducting surfaces has attracted great interest in the development of devices for applications dedicated to micro and nano electronics, biotechnologies, catalysis or solar energy conversion etc.2-5 Within this context, ligands containing multiple metal-binding sites allow access to multicomponent arrays by combining different metal centers to give rich redox, optical or magnetic properties. Among the different possible chelating groups, bis (2,2’:6’,2”-terpyridine) derivatives are ideally suited for the construction of such well-defined metallo supramolecular assemblies on surfaces.6-9 These ligands exhibit high binding affinity toward first row transition metal ions resulting in linear structures and prevent the formation of isomers in contrast to bidentate bipyridine or phenanthroline ligands.10-11 Hence, one-dimensional coordination oligomers and polymers can be easily obtained in solution by a mixture of metallic cations and ditopic ligands where the two terpyridine groups are set to coordinate metals in opposite directions. This technique is optimal if only one kind of metallic ion is used, but if the goal is to assemble different cations, statistical distribution is to be expected. Well-alternated heterometallic species can however be obtained with a surface assisted process.6-9, 12 Starting from a surface coated with a terpyridine self-assembled monolayer, the alternated dipping of the surface in solutions of specific metallic salts and ditopic terpyridine ligands allows the systems to grow up at the surface by the successive coordination steps. More recently, the use of ditopic ligands featuring specific properties instead of a conventional organic linker for the synthesis of molecular wire, allowed the preparation of sophisticated nanostructures coupling different properties. For instance, introduction of polymeric side chains between two terpyridine units lead to the synthesis of a metallopolymer with better solubility and mechanical properties compatible with inkjet printing technology.13 Luminescent structures have also been obtained using a perylene diimine bridge between two chelating units and Zn(II)14 or Fe(II)15 as metallic centers, or with a bis(terpyridine) functionalized BODIPY dye.16 Photochromic units such as dithienylethene17 and dimethyldihydropyrene18, 19 were also incorporated between two terpyridines; the nature of the coordinated metal dictates if the photoisomerization can occur. 3 ACS Paragon Plus Environment


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Aiming to reach metallosupramolecular structures containing additional photoredox properties our group has already designed a [Ru(bpy)3]2+ metallo-ligand bearing two free terpyridines on the 4 and 4’ positions on the same bipyridine.20 The [Ru(bpy)3]2+ core offers much better photophysical properties compared to the [Ru(tpy)2]2+ one.21 This building block has been recently used in association with Fe(II) or Zn(II) to obtain alternated bi-metallic coordination polymer.22 These polymers can be deposited by an electroinduced process on a substrate and exhibit luminescence and photoredox properties. Using this approach to construct an efficient charge separated metallic triad system, we designed a ruthenium based metallo-ligand in which one of the bipyridine ligands of the [Ru(bpy)3]2+ photosensitive unit is substituted at the 5 and 5’ positions by two terpyridines via an ether bridge.23 We followed a stepwise technique to elaborate, on an ITO surface, an inorganic triad with a Co(III) center as an electron acceptor and Fe(II) as a donor one. To obtain more linear assemblies, we have now synthesized a novel [Ru(bpy)3]2+-based metallo-ligand in which the two chelating terpyridines are bound on the same bipyridine by a dimethyl phenylene bridge to ensure not only a good rigidity but also a weak electronic coupling as previously demonstrated for other photosensitive systems24-26 (scheme 1). We used this building block in association with other metallic cations to fabricate two distinct kinds of molecular wires. With a single metallic cation like Fe2+ a soluble 1D electrochromic and photoactivable coordination polymer can be easily made, it can be also easily prepared as a thin film on a conductive surface by an electrodeposition precipitation process. This new [Ru(bpy)3]2+-based metallo-ligand has also been used in a stepwise mode to obtain a Co(III)/Ru(II)/Fe(II) triad on ITO and SiO2 wafer. In the present article we report the synthesis and main characterizations of these two molecular wires using this new [Ru(bpy)3]2+-based building block. Both heterometallic structures immobilized on electrode surfaces generated an efficient photocurrent under visible irradiation, with a better stability for the triad and an enhancement of 30% in magnitude compared to our former system.23


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Scheme 1. Structure of the [Ru(bpy)3]2+ - based metallo-ligand (L1)

Experimental part: General procedure: Acetonitrile (CH3CN, Rathburn), dimethyl sulfoxide (DMSO, Acros), dimethyl formaldehyde (DMF, Acros) ethylene glycol (Prolab), ethanol (SDS), diethyl ether (Aldrich) and methanol (SDS) were purchased under their analytical grade and used as received. Tetra-n-butyl ammonium perchlorate ([Bu4N]ClO4, Fluka) was used as received and stored under argon in a dry-glovebox (Jaram). 4-bromophenyl diazonium tetrafluoroborate p-BrC6H4N2(BF4) (ArN2+, Acros), triethanolamine (TEOA, Aldrich), Fe(BF4)2.6H2O (97%, Aldrich) and Co(BF4)2.6H2O (99%, Aldrich) were purchased and used as received. All other chemical reagents used in the synthetic route were obtained from commercial sources as guaranteed-grade reagents and used without further purification. Column chromatography was carried out on silica gel 60 (Merck, 70-230 mesh). Syntheses of air sensitive reactions were performed in oven-dried glassware attached to a vacuum line with Schlenk techniques. 5,5’-dibromo-2,2’-bipyridine27, 4-bromo-2,5-dimethylbenzaldehyde28 and

4’(4-benzylphosphonic

acid)-2,2’:6’,2’’-terpyridine

(ttpy-phosphonate)23

were

synthesized according to previously described procedures. 1

H ̶ NMR and

13

C ̶ NMR spectra were recorded at room temperature on Bruker Avance III

400 MHz spectrometers. 1H chemical shifts were referenced to residual solvent peaks. Coupling constants value (J) are given in hertz and chemical shift (δ) in ppm. The abbreviation used are s = singlet, d = doublet, dd = doublet of doublet, dt = doublet of triplet, t = triplet, m = multiplet, q = quartet). DOSY experiments were performed on a Bruker Avance III 500 MHz spectrometers. The electrospray ionization mass spectrometry (ESI-MS) experiments were performed on a triple quadrupole mass spectrometer Quattro II (Water Micromass). The sampling cone 5 ACS Paragon Plus Environment


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voltage was set to 30 V. Matrix assisted laser desorption/ionization time-of-flight (MALDITOF) mass spectra were recorded on a Bruker-Daltonics Maldi-ToFToF Speed. Synthesis of [{RuII(bpy)2(5,5’-(2,5-dimethylphenyltpy)2-bpy)FeII}n].4n(PF6) denoted as [{RuII–FeII}n]4n+: Metallo-ligand L1 (see supporting information) (50 mg, 0.03 mmol) and [Fe(BF4)2].6H2O (11 mg, 0.03 mmol) were refluxed in 10 mL of dry CH3CN for 2 h under argon. The resulting dark purple solution was cooled down and concentrated under vacuum. Saturated KPF6 aqueous solution was added, leading to the formation of a dark reddish violet precipitate, which was filtered off, washed with water, diethyl ether and dried under air. Yield: 20 mg of [{RuII–FeII}n]4n+ (50%). The polymer has been characterized by 1H MNR and MALDI-TOF (see details in Results and discussion section 1.1).

Stepwise

formation

of

the

triad

ITO/[Co(III)_Ru(II)_Fe(II)]7+

and

SiO2/[Co(III)_Ru(II)_Fe(II)]7+ : Planar ITO substrates were purchased from Solems and cleaned sequentially by sonication in acetone and ethanol, while silicon wafers (SiO2) were treated by ozone for 10 minutes followed by rinsing with ethanol prior to grafting. Then the dried electrodes were first dipped in 10-3 M ethanolic solution of ttpy-phosphonate over a period of 24 hours at room temperature. The terpyridine terminated surface was cleaned by washing with ethanol, dried by a stream of argon and immersed in a 10-3 M ethanolic solution of [Co(BF4)2].6H2O at room temperature for 3 hours. After thorough rinsing with ethanol, the electrode is dipped over night at room temperature, in a 10-3 M ethanol:acetone (8:2) solution containing the photosensitizer moiety L1 that self-assembles on the pending cobalt center. The electrode is then rinsed again and dried under argon followed by dipping in [Fe(BF4)2].6H2O (10-3 M in ethanol for 5 hours) and capping with 4’-ptolyl-2,2’:6’,2”terpyridine (ttpy) (10-3 M in ethanol overnight).

Atomic Force Microscopy: AFM measurements were performed with a Picoplus instrument (Molecular Imaging) equipped with a PicoScan controller and an AC-mode control box. An AFM cantilever with an aluminum coating (BudgetSensors Tap150 Al-G) and a nominal spring constant of 5 N.m-1 was used. For these experiments, the polymers were deposited on clean ITO whereas the triads were deposited on silicon wafers which were pretreated by ozone. The scan sizes were 3 µm2 and 500 nm2 for the tapping and contact mode respectively. Images were treated using a Gwyddion program. The polymer film thickness was determined as follow: A scratch was performed with a needle in a small zone and the difference of 6 ACS Paragon Plus Environment

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altitude between the free electrode and the deposited material was recorded in the tapping mode.

Electrochemistry: Electrochemical measurements were run under an argon atmosphere in a dry-glovebox at room temperature. Cyclic voltammetry (CV) and controlled potential electrolysis experiments were performed using an EG&G PAR model 173 potentiostat/ galvanostat equipped with a PAR model universal programmer 175 and a PAR model 179 digital coulometer. A standard three electrode electrochemical cell was used. Potentials were referenced to an Ag/AgNO3 (10-3M) reference electrode in CH3CN + 0.1 M [Bu4N]ClO4. Potentials referred to that system can be converted to the ferrocene/ferricinium couple by subtracting 87 mV and to SCE or NHE by adding 298 mV or 548 mV, respectively. A Pt wire was used as auxiliary electrode. The working electrode was a 3 mm diameter vitreous carbon electrode polished with 2 mm diamond paste (Mecaprex Presi). The rotating disk electrode (RDE) was a 3 mm diameter carbon disk with a rotation rate ω = 600 rot.min-1. Rectangular ITO plates (8 x 25 mm2) were used for the electrodeposition of thin films (polymer or triads) on a transparent support to allow optical characterization. Electrochemical measurements of the modified surfaces (ITO/[{RuII–FeII}n]4n+ and ITO/[CoIII_RuII_FeII ]7+ were performed using CHI 621 (CH instrument) potentiostat in a three electrode electrochemical cell with a flat glass window. The functionalized ITO surfaces were used as working electrode. The distances between the electrodes were small and ohmic drop were neglected.

Absorption and Emission: Absorption spectra of the metallo-ligand and polymer in solution were obtained using a Cary 300 UV-visible spectrophotometer (Varian) and a 1 cm path length quartz cell. The UV-visible spectra of the initial and electrolyzed solutions were transferred to 1 mm path length quartz cuvette cells and were recorded directly in the glovebox with a Zeiss spectrophotometer (MCS 501 UV-NIR) using optical fiber. Emission spectra were recorded in deoxygenated solvents at room temperature between 500 and 800 nm after irradiation at 460 nm using a Fluoromax 4 (Horiba). Emission quantum yields φem were determined at 25 °C in deoxygenated acetonitrile solutions using [Ru(bpy)3].2Cl in H2O (φem = 0.028) as a standard, and a methodology already presented.29 Emission lifetime measurements were performed after irradiation at 400 nm with a picosecond Nd:YAG laser and using a time correlated single photon counting detection (PicoHarp 300). Experimental



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uncertainties are as followed: absorption maxima, 2nm; molar absorption, 20%; emission maxima, 5nm; emission lifetimes, 10%; emission quantum yields, 20%. Photocurrent Experiments: The photocurrent experiments were carried out in a specific cell with a flat glass window and three compartments using the functionalized ITO as a working electrode connected to a brass wire thanks to a silver conducting paint (Radiospare). Triethanol amine TEOA (15 mM) was employed as an electron donor for the trimetallic triad ITO/[CoIII_RuII_FeII] along with a bias of 0.12 V, and 4- Bromophenyl diazonium tetrafuoroborate p-BrC6H4N2(BF4) (ArN2+ ; BF4) as an electron acceptor for the deposited polymer ITO/ [{RuII–FeII}n]4n+ and a bias of 0.4 V. The samples were illuminated with a Mercury-Xenon lamp (Hamamatsu L9588) operated at maximum power. The electrochemical cell was held 4 cm above the lamp for every measurement. In these conditions, the radiant power landing on the surface of the cell window is 1.79 W.cm-2. During this experiment irradiation was alternatively switched on and off using a shutter.

Results and Discussion: 1. L1 and [{RuII –FeII}n]4n+ polymer. 1.1 Synthesis: The building block (L1) is a ruthenium trisbipyridine-based ditopic metallo-ligand bearing two open terpyridine coordination sites linked on the same symmetrically substituted bipyridine at the 5 and 5’ positions via 2,5-dimethylphenylene bridges. The position of the modification serves to attain a maximal distance between the respective terpyridine moieties, while the 2,5-dimethylphenylene linkers are expected to ensure a weak electronic coupling between the different subunits in the heterometallic structures as well as maintaining a rigid and linear backbone. L1 was prepared in good yields employing a Suzuki cross-coupling reaction on the [Ru(bpy)2(bpy-Br2)].2[PF6] complex (see supporting informations). The soluble one dimensional coordination polymer [{RuII_FeII}n]4n+ was obtained by refluxing equimolar amounts of [Fe(BF4)].6H2O and L1 in CH3CN for 2 h. The reaction is accompanied by a fast change in color from orange to purple indicating the formation of the [Fe(tpy)2]2+ core (Scheme 2). This polymer was isolated as a hexafluorophosphate salt.


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Scheme 2. Synthesis of the polymer [{RuII-FeII}n]4n+, (i) Fe(BF4)2.6H2O, CH3CN, 80 °C, 2 h, KPF6. Alternately, [{RuII_FeII}n]4n+ was also prepared in situ in an electrochemical cell containing CH3CN + 0.1 M [Bu4N]ClO4. Solutions of isolated or in situ prepared polymers were found to exhibit identical UV-visible absorption and electrochemical characteristics. Isolated [{RuII_FeII}n]4n+ polymers were characterized by 1H NMR, 1H DOSY NMR and MALDI-TOF mass spectroscopies. The addition of one equivalent of [Fe(BF4)].6H2O salt in a solution of L1 leads to a significant broadening of the set of the original signals in the aromatic region of the 1H NMR spectrum in accordance with the formation of the coordination polymer. This was further supported by the one order of magnitude shift of the diffusion coefficients between L1 (log D = - 8.5) and the [{RuII_FeII}n]4n+ oligomers (log D = - 9.5) in the 1H DOSY NMR experiment showing the formation of a high molecular weight species (Figure S1). According to the Stockes-Einstein equation (equation 1), (where kB, η, T and R stand respectively for the Boltzmann constant, the dynamic viscosity of the solvent, the temperature and the radius of the molecules), the ratio of the diffusion coefficient suggests a 10 times larger radius for the coordination polymer compared to L1.

(1)

The coordination polymer was also characterized by MALDI-TOF mass spectrometry (Figure 1) showing a series of signals with the m/z range between 3 000 and 18 000 with a consistent pattern corresponding to the masses of oligomers of up to 10 repeating units of the monomeric species represented by M = [RuII–FeII]4+. This indicates that the formed oligomers or polymers contain at least 10 alternating units of both iron and ruthenium fragments. The MALDI-TOF conditions prevent detection of higher molecular weight peaks due to the fragility of the [Fe(tpy)2]2+ unit, as already reported for other [Fe(tpy)2]2+ based polymer compounds.30 In addition, the intensity of the signals decreases as the molecular weight 9 ACS Paragon Plus Environment


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increases, reaching the limit of detection for higher molecular weight. The detected polymer length doubles that of previously reported bimetallic RuII/FeII polymers containing ethyl bridges at the 4,4’ position of the same bipyridine.22 This result shows that 2,5-dimethyl benzene as linkages at the 5 and 5’ positions of the same bipyridine limited the formation of small cyclic oligomers and lead to the formation of more tailored 1D polymers.

1.2. Electrochemical properties in solution: The redox properties of the metallo-ligand L1 and the [{RuII_FeII}n]4n+ polymer are summarized in table 1. L1 shows in the anodic part a reversible system corresponding to the oxidation of the ruthenium center and in the cathodic part three reversible peaks attributed to the reduction of the bipyridines (figure S2). By comparison with the electrochemical behavior of [Ru(bpy)3]2+, the first reduction of the bipyridine ligand (E1/2 = - 1.63 V for [Ru(bpy)3]2+)31 is at a slightly less negative potential for L1 and is then assigned to the reduction of the bipyridine bearing the two terpyridine units. The cyclic voltammogram of [{RuII-FeII}n]4n+ corresponds to the sum of the electroactivity of both parent subunits (i.e.: [Fe(ttpy)2]2+ and L1) in accordance with the absence of any electronic connection between both metallic centers thanks to the 2,5-dimethyl phenylene linkage (Figure 2A). In the positive region, two well separated reversible redox systems are observed at E1/2 = + 0.81 and + 1.03 V assigned to the FeIII/FeII and RuIII/RuII redox couples respectively. The Rotating Disc Electrode (RDE) experiment confirms the 1:1 ratio of the metallic centers in the polymer structure as shown by equal heights under the oxidation peaks (Figure 2B). In the negative region, all the waves are highly distorted and remain difficult to assign due to the deposition of the reduced form of the polymer on the electrode surface. Electrolysis of the solution performed at 0.90 V and 1.05 V lead to the formation of [{RuII_FeIII}n]5n+ and [{RuIII_FeIII}n]6n+ respectively with a yield of 95% as quantified by the RDE experiments (figure 2B). The intermediate [{RuII_FeIII}n]5n+ oxidized form of the polymer is perfectly stable, as a consequence of the large difference of potential between the FeIII/FeII and RuIII/RuII redox processes (∆E1/2 = 220 mV). Both oxidation processes have been followed by UV-visible spectroscopies (Figure 3). The absorption spectrum of [{RuII_FeII}n]4n+ is essentially the superposition of the individual molecular components L1 and [Fe(ttpy)2]2+ in proportion with stoichiometry (table 2). The two bands in the visible region at 456 and 568 nm are ascribed respectively to the RuII-bpy and FeII-tpy MLCT, while the UV part of the spectrum is dominated by π →π* transitions of the ligands. Electrolysis at 10 ACS Paragon Plus Environment

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0.9 V leads to the consumption of one electron per Fe center, and is accompanied by the disappearance of the intense FeII-tpy MLCT at 568 nm, on the behalf of a low intensity band centered at 670 nm typical of the Fe(III) species. After an additional electron was consumed at 1.05 V, the absorption band of the Ru(II) center was replaced by those of the Ru(III) species: a shoulder at around 430 nm and a less intense and broad absorption at 660 nm that merges with the Fe(II) absorption band. Back electrolysis of the [{RuIII_FeIII}n]6n+ solution at 0.60 V regenerates [{RuII_FeII}n]4n+ with a 95 % reaction yield. The reaction was monitored by UV-visible spectroscopy (Figure 3d). 1.3. Elaboration and characterization of modified ITO/[{RuII –FeII}n]4n+ electrode: A rapid film growth of [{RuII –FeII}n]4n+ on Pt, C or ITO electrodes occurs by repeated potential scans in reduction between 0 and -2.0 V (on the beginning of the third reduction process of the ligand) in analogy to previously reported polymer containing metallic subunits.22, 32, 33 The deposition of the electroactive film originates from the low solubility of the reduced species of the polymer in CH3CN (figure 4a). This result is in agreement with the macromolecular structure of the [{RuII_FeII}n]4n+ species since trinuclear complexes (RuII_FeII_RuII) do not adsorb at the surface of the electrode during iterative cycling reduction.29 Cycling only in the potential range including the first two reduction peaks (between 0 and - 1.80 V) shows an inefficient deposition. If the repeated scans are limited on the FeII/FeIII and RuII/RuIII reversible oxidation peaks no deposition is observed. After 15 successive cycles including the three reduction systems, the modified ITO electrode is carefully washed and transfered into a polymer-free CH3CN + 0.1 M [Bu4N]ClO4 solution (figure 4b). As expected, the CV of the electrode shows, in the negative part, the ligand centered reductions and in the positive window, the oxidations of both metallic centers at similar potential than those of [{RuII –FeII}n]4n+ in solution. In addition to these systems, prepeaks are observed at the base of the FeIII/II oxidation and of the first ligand reduction. These prepeaks emerge due to the flow of charge previously delayed during the scan as a consequence of the formation of a resistive film on the electrode surface.34 Three cycles on the oxidation peaks allowed the proper penetration of the electrolyte inside the film, hence eliminating the capacitive peaks, and recording a stable signal similar to the electrochemical behavior observed in solution. Notably the 1:1 ratio between the current intensity under the oxidation peaks of FeII and RuII is maintained proving that no metal release process occurs during the deposition. The apparent surface coverage of the modified ITO/[{RuII –FeII}n]4n+



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after 15 cycles was estimated to Γ = 3 x 10-9 mol.cm-2 by integrating the area of the peak under both electroactive units indicating the deposition of several layers of the polymer. The morphology of the ITO coated by the coordination polymer [{RuII –FeII}n]4n+ was investigated using AFM measurements in the tapping mode (figure S3). Despite the rigidity and linearity of the polymer structure, we were not able to detect a specific lamella pattern on the surfaces as it can appear with long polymer chains based on alternating bis-terpyridine complexes and poly(ethylene)oxide.30 Instead, the modified electrodes appear as nodular and inhomogeneous typical of a thick electrodeposited film without proper order.22,

35

The film

thickness was determined by scratching the modified electrode with a needle and recording the difference of altitudes between the free ITO surface and the deposited film in the tapping mode. The thickness is estimated to 62 nm after a deposition process of 15 iterative potential scans. As expected, a linear relationship was observed between the thickness of the film and the number of cycles performed during the deposition (figure S4). The modified ITO electrodes exhibit a rich electrochromic behavior due to the presence of both Fe(II) and Ru(II) centers. By cycling between 0 and 1.5 V, the film turns from red to brown to pale green. Initially, right after the deposition process, the UV-visible spectrum of the film clearly shows the characteristic MLCT absorption band of the Ru(II) and Fe(II) subunits in the visible at 450 and 573 nm respectively (figure 5), the color of the film is dark reddish. Upon increasing the applied potential to 0.9 V the [Fe(ttpy)2]2+ center is oxidized to [Fe(ttpy)2]3+ which induces a first color change from red to brown. Finally by applying a potential of 1.2 V, a green film is obtained due to the formation of Fe(III) and Ru(III) centers.

1.4. Photophysical properties in solution: Absorption and emission properties of L1 and [{RuII-FeII}n]4n+ were recorded in deoxygenated CH3CN solutions at 25 °C (table 2,). Upon excitation in the MLCT band at 450 nm, L1 exhibits an emission with a maximum at 616 nm, typical of the 3MLCT emitting state of the [Ru(bpy)3]2+ core (figure S5). Both luminescence quantum yield (Φem) and lifetime (τ) were similar to the regular [Ru(bpy)3]2+ complex (τ = 0.89 µs, Φem = 0.059 for [Ru(bpy)3]2+ in CH3CN),36 showing that the chemical modification has only a small influence on the photophysical properties of the ruthenium photosensitizer. However, upon excitation at 450 nm of a deoxygenated CH3CN solution containing the polymeric [{RuII-FeII}n]4n+ species, a very weak luminescence is detected around 616 nm. The low quantum yield (Φem = 0.008) which constitutes only 15% of the emission of L1 (Φem = 0.058) is attributed to a strong 12 ACS Paragon Plus Environment

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deactivation pathway of the [Ru(bpy)3]2+* subunits by the [Fe(ttpy)2]2+ core and proves that the bridge allows interaction between the metal centers at the excited state despite the lack of interaction in the ground state. This strong luminescence inhibition of the [Ru(bpy)3]2+* center was already evidenced in similar covalently connected systems and attributed to an energy transfer (En.T) process, driven by the large overlap between the emission spectrum of [Ru(bpy)3]2+ and the electronic absorption spectrum of [Fe(ttpy)2]2+.22, 29, 37, 38 Quenching by electron transfer can be ruled out since the oxidation of the [Fe(ttpy)2]2+ subunit is an energetically unfavorable process (∆G = E°(FeIII/II) - E°(RuII*/I)= 0.28 V). This value was calculated based on the reduction potential of the excited state of [Ru(bpy)3]2+* (E°(RuII*/I) = 0.83 V vs SCE39 converted vs Ag/AgNO3 by subtracting 298 mV40) and the oxidation potential of the ground state of the Fe(II) subunit E°(FeIII/II) = 0.81 V vs Ag/AgNO3 (table 1). The luminescence lifetime of [{RuII_FeII}n]4n+ is best described by a bi-exponential decay where τ1 and τ2 are respectively the short and long lifetime component of the decay according to equation (2) where A is the fraction of emitted intensity associated with τ1.

exp −

+ 1 − − 2

This bi-exponential decay (τ1 = 3 ns; τ2 = 888 ns) could be the result of the quenched (95% of the overall decay) and unquenched [Ru(bpy)3]2+ units (5%) respectively either by En.T. to the [Fe(ttpy)2]2+ core as evidenced in similar trinuclear and polymeric systems,22, 29, 41, 42 or by energy migration between adjacent photosensitizers as governed by the retaining of the metallic centers in a close environment.42-44 The residual long lifetime (τ2 = 888 ns), could be due to the lability of Fe(II) or its instability under irradiation releasing some free Ru(II) units in solution.41 It could also be the consequence of geometric constraints that do not allow an efficient folding to overlap the molecular orbital of the [Ru(bpy)3]2+* and [Fe(ttpy)2]2+ subunits.22,42 The quenching rate constant (kq) occurring in [{RuII_FeII}n]4n+ is estimated to 3 x 108 s-1 following equation 3.

1 1 − 3

This value falls in the same range of previously reported trinuclear and polymeric complexes holding both [Ru(bpy)3]2+ and iron subunits.22, 29 All our attempts to record the luminescence spectra of the modified ITO electrodes with [{RuII_FeII}n]4n+ failed, probably as a consequence of the very weak emission of the thin layers. As we already showed for other bimetallic polymers containing [Ru(bpy)3]2+ subunits, photoinduced oxidation processes of both metallic centers can occur in presence of a 13 ACS Paragon Plus Environment


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sacrificial electron acceptor such as an aromatic diazonium ArN2+ salt22 (figure 6). The process is highly efficient using p-BrC6H4N2+ for instance due to its fast decomposition after reduction that efficiently competes with the back electron transfer reaction. Under continuous irradiation the photooxidation proceeds in two steps, the first one leads to the formation of the [{RuII-FeIII}n]5n+ oxidized state, in accordance with the decrease of the MLCT band of Fe(II) and the concomitant increase of the band of the Fe(III) centered at 670 nm, followed in a second step, after prolonged irradiation time to the formation of the [{RuIII-FeIII}n]6n+ species, illustrated by the regular decrease of absorbance at 450 nm and the slight increase of the band centered at 660 nm. Equation 4-7, summarized the overall photooxidation process. Under irradiation the [Ru(bpy)3]2+* centers are oxidized by ArN2+ (equations 4,5), the reaction is highly favorable with a ∆G around - 67 mV14 and proceed with a large quantum yield of 0.34.45 Since the photogenerated [Ru(bpy)3]3+ species is a good oxidant toward [Fe(ttpy)2]2+, the photooxidation leads to the formation of the [{RuII_FeIII}n]5n+ species (equation 6). When all the Fe centers have been oxidized, prolongued irradiation leads to the formation of [{RuIII_FeIII}n]6n+ if ArN2+ is added in large excess. In addition, we recently proved that the [Fe(ttpy)2]2+* subunit can be directly oxidized in presence of ArN2+,46 the reaction proceeds with a lower quantum yield estimated to 10-3, but may also contribute to the formation of the [{RuII-FeIII}n]5n+ intermediate. Under visible irradiation of [{RuII-FeII}n]4n+, the [Fe(ttpy)2]2+* subunits can either be populated by direct absorption at 560 nm or by an energy transfer process after excitation of the [Ru(bpy)3]2+* center. *+

Ru## − Fe## %& '(&) ,-----. Ru##∗ − Fe## %& '(&)

Ru##∗ − Fe## %& '(&) + 01) → Ru### − Fe## %& '3&) + 04 + 1

(4) (5)

Ru### − Fe## %& '3&) → Ru## − Fe### %& '3&)

(6)

Ru## − Fe### %& '3&) + 01) ,----. Ru### − Fe### %& '&) 04 + 1

(7)

*+

2. Elaboration and characterization of modified ITO/[CoII_RuII_FeII]6+ electrode: A stepwise assembly of a trimetallic triad based on the metallo-ligand L1 has been carried out on an electrode following our previously described methodology.23 This strategy takes advantage of the strong self-assembling interaction between oxygen-containing functional groups such as phosphonates with ITO and SiO2 surface as well as the efficient chelation abilities of terpyridine ligands with first row transition metals (scheme 3).


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Scheme 3. Stepwise assembling method of the trimetallic triad on surface. For clarity, charges have been omitted (ttpy = 4’-ptolyl-2,2’:6’,2”-terpyridine, and tpy phosphonate = 4’(4-benzylphosphonic acid)-2,2’:6’,2’’-terpyridine).

In a first step, the surface is grafted with a phosphonate-substituted terpyridine. This modified surface is then dipped in an ethanolic solution of Co(BF4)2 at room temperature. After thorough rinsing, the surface is again dipped in a solution containing the photosensitizing moiety (L1) that self-assembles on the pending metallic center leaving a free terpyridine exposed to the outer solution. This free tridentate ligand can then be involved in a coordination process with a metallic center, namely FeII. After rinsing, the system is capped with a final 4’-ptolyl-2,2’:6’,2”-terpyridine. The CV of ITO/[CoII_RuII_FeII]6+ was recorded in CH3CN + 0.1 M [Bu4N]ClO4 (figure 7, table 1). The first reversible peak, around - 0.1 V is characteristic of the CoIII/CoII couple.47 At higher potentials, a reversible monoelectronic peak at +0.75 V indicates the presence of the [Fe(tpy)2]2+ fragment. Finally the oxidation of RuII into RuIII center is observed slightly below +1.0 V. The linear relationship between the intensity of the peaks and the scan rate clearly indicates that the processes observed occur on a grafted surface (figure 7 inset). The intensity of the current under the three oxidation processes is almost similar, proving the formation of a triad with a (1:1:1) ratio between Co, Fe and Ru. A surface coverage value Γ = 0.5 x 10-11 mol.cm-2 is obtained by integration of the different redox waves. This quite low value, less than 10% of the maximum covering, could be the result of the chemical lability of the CoII or FeII terpyridine complexes that leads to their de-coordination after every immersion. Indeed low surface coverage values were also obtained in our previous report23 or with dyads built in 15 ACS Paragon Plus Environment


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a stepwise manner.48 AFM measurements on silicon wafer modified with [CoII_RuII_FeII]6+ assemblies, were performed in tapping mode in order to study the morphologies of the surfaces (figure 8 (a and c)). While the bare SiO2 electrode is flat and homogeneous, the functionalized one shows an increase in roughness with an uneven distribution. The profiles (figure 8d) show three characteristic heights around 1.5 nm; 2.75 nm and 5 nm, which could be attributed to the presence of three different architectures on the surface due to the layer by layer construction: Co-monoterpyridine complex, (Ru-Co) dyad and (Fe-Ru-Co) triad respectively. The results prove that the efficiency of the stepwise construction remains low.

3. Photo-electric conversion: Upon light irradiation in the visible region, all the modified ITO electrodes are able to generate photocurrent with different magnitudes. ITO/[{RuII –FeII}n]4n+ electrodes at a bias of 0.4 V generate a cathodic photocurrent in presence of 15 mM of ArN2+ in CH3CN + 0.1M [Bu4N]ClO4 (figure 9). The photocurrent is the consequence of the irreversible oxidative quenching of the [Ru(bpy)3]2+* centers by ArN2+ (or to a lesser extend of the [Fe(tpy)3]2+* one). As discussed in the photophysics section, the photogenerated RuIII species lead to the formation of a FeIII center that is reduced to FeII on the electrode at 0.4 V. Alternatively the RuIII center can be also directly reduced by the bias. The magnitude of the photocurrent depends on the film thickness. For instance after 10 cycles of potential scan between 0 and - 2.0 V in a solution of [{RuII –FeII}n]4n+ in CH3CN + 0.1 M [Bu4N]ClO4, a film of 47 nm (thickness measured by AFM see experimental part) grows at the surface of the electrode. This modified electrode once transferred in a solution of 15 mM of ArN2+ in CH3CN + 0.1M [Bu4N]ClO4 generates a photocurrent of 6 µA/cm2, whereas the photocurrent density is estimated to 18 µA/cm2 for a modified electrode prepared after 20 cycles of potential sweep resulting in a thickness of 75 nm. This latter thickness was estimated from the linear fit between the number of the potential sweep and the thickness of the resulting thin film (figure S5). When the film is thicker however the photocurrent density does not reach higher value. This phenomenon could be explained by a low rate of charge propagation through a thicker polymer film as by the diffusion of the sacrificial electron acceptor. Similar results were also observed with polypyrrol film of [Ru(bpy)3]2+ where the increase of the photocurrent intensity brought by increasing the film thickness is rapidly cancelled by the resulting electron diffusion limitation.49 In addition, a significant drop of the magnitude of the photocurrent is observed during consecutive light on/off cycles. For 16 ACS Paragon Plus Environment

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example, current density drops from 18 to 10 µA.cm-2 after five irradiation cycles with the 75 nm film thickness. The instability of the signal is accompanied by a drastic loss of the amount of the deposited polymers (up to 70%) as evidenced by the decrease of the surface coverage value calculated from the recorded CV after 8 cycles of irradiation (figure S6). Irradiation of ITO/[CoIII_RuII_FeII]7+ at a bias of 0.12 V lead to the generation of an anodic photocurrent with a magnitude of 25 µA.cm-2 in CH3CN + 0.1 M [Bu4N]ClO4 in the presence of a tertiary amine as sacrificial electron (TEOA, 15mM) in solution (figure 10). The bias has been selected since at this potential the cobalt subunit is under its Co3+ form which is a good electron acceptor towards the [Ru(bpy)3]2+* species.45 Upon irradiation the Ru(II) subunit is promoted to its excited state which is a stronger reductant ( E°(RuIII/II*) = - 0.79 V vs SCE39 converted to - 1.08 V vs Ag/AgNO340 ).The origin of the anodic photocurrent is then the oxidative quenching of the [Ru(bpy)3]2+* center by the Co(III) subunit (scheme 4).

Scheme 4. Mechanism of the electron transfer processes across the triad anchored on ITO electrode. The back electron transfer (BET) within the resulting ITO/[Co(II)–Ru(III)–Fe(II)]7+ assemblies is then short-circuited either by a fast electron injection from the Co(II) center to the ITO electrode at 0.12 V or by an efficient electron transfer process from the Fe(II) subunits to the Ru(III) center (∆G = - 200 mV). Sacrificial electron donor in solution reduces the Fe(III) unit to its original redox state. Without TEOA in solution no photocurrent is observed. The photocurrent generation is also a proof that the photoinduced electron transfer 17 ACS Paragon Plus Environment


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process between Ru(II)* and Co(III) is highly competitive with the energy transfer process between Ru(II)* and Fe(II) species (scheme 4). In this triad, the Co(III) center acts as an electronic relay between the [Ru(bpy)3]2+* unit and the electrode, the Fe(II) center as a redox mediator between the sacrificial electron donor in solution and the photogenerated Ru(III) species. The magnitude of the photocurrent is 30% higher than our previously reported selfassembled triads built with the same metallic centers, where the three subunits are held via tilted ether bridges.23 This enhancement of the photoresponse is attributed to the more linear architecture of the present reported triad, due to the higher rigidity of the 2,5-dimethyl phenylene with respect to ether bridges, hence maintaining a linear upward structure. As a consequence, it most probably reduces the kinetics of the back electron transfer deactivation within the transient [Co(II)–Ru(III)–Fe(II)]7+ species; and by increasing the distance between the Ru(II) center and the surface, decreases also the direct deactivation pathway of the Ru(II)* by the electrode. The photocurrent density for the trimetallic species is more stable than that for the bimetallic polymer upon long term alternative irradiation/dark cycles. This is credited to the covalent linkages between the phosphonates and the ITO surfaces, which limits desorption phenomena upon irradiation. Thus, such engineered assemblies, are capable of producing a significant amount of stable current despite the drastically low surface coverage emphasizing on the importance of keeping control of the orientation and order of molecules in such applications.

Conclusion: We report in this publication the synthesis of a novel rigid [Ru(bpy)3]2+-based metallo-ligand holding two free terpyridine coordination sites. This versatile platform allowed accessing 1D photoactivable bimetallic coordination polymers [{RuII –FeII}n]4n+ soluble in CH3CN which were easily deposited by an electro-induced process onto conducting surfaces leading to rich electrochromic activities (red, brown and green) due to the presence of both Fe(II) and Ru(II) centers. In addition, the Ru(II)-based metallo-ligand opened access to the fabrication, using a stepwise methodology, of linear trimetallic [Fe(II)_Ru(II)_Co(III)]7+ triad assemblies on surfaces with a coverage value estimated to Γ = 0.5×10-11 mol/cm2. Both heterometallic structures immobilized on electrodes were capable of generating photocurrent in the presence of a sacrificial reagent in CH3CN + 0.1 M [Bu4N]ClO4 upon visible irradiation. The ITO/[{RuII –FeII}n]4n+ system behaved as a photocathode in the presence of ArN2+ at an applied potential of 0.4 V. However a long exposure to light lead to the desorption of the film. 18 ACS Paragon Plus Environment

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On the other hand, the ITO/triad system behaved as a photoanode in the presence of TEOA at an applied potential of 0.12 V. The good chemical and photochemical stability of the triad anchored with phosphonate groups on ITO was already controlled over 1.5 hours of alternating irradiation cycles of 20 s.23 As an extension of this work this linear triad could be built between two nano-electrodes in order to form photoactivable molecular junctions taking advantage of the rigid and linear backbone. Ongoing work in our laboratory includes also the formation of 1D coordination polymers with other kind of metallic centers coupling photosensitive and catalytic activities.

Supporting Information Available. Synthesis procedure and electrochemical behaviour of L1, 1H DOSY NMR of [{RuII_FeII}n]4n+, dependence of the film thickness vs. the deposition processes and AFM image of the ITO/ [{RuII_FeII}n]4n+. Acknowledgements. The authors thank the LabEx ARCANE (ANR-11-LABX-0003-01) for financial support, the technical support from the chemistry platform «NanoBio campus» in Grenoble (France), and Hugues Bonnet from DCM UGA-CNRS UMR 5250 for his help in AFM experiments.



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Tables: Table 1. Redox potentials of [Fe(ttpy)2]2+, [Co(ttpy)2]2+, L1, [{RuII –FeII}n]4n+ and of the modified ITO electrodes with the [{RuII –FeII}n]4n+coordination polymer and [CoII_RuII _ FeII]6n+ trimetallic triad. The data are recorded in deoxygenated CH3CN + 0.1 M [Bu4N]ClO4. E1/2/V (∆Ep/mV) vs. Ag/AgNO3 10-2 M. v = 100 mV.s-1 E 1/2Ox , V (∆Ep, mV) CoIII/CoII

FeIII/FeII

[Fe(ttpy)2]2+ 29

__

0.76(60)

__

-1.54(60)

-1.65(60)

̶

[Co(ttpy)2]2+ 45

-0.06(70)

__

__

-1.06 (60)a

-1.90 (60)

-2.26 (50)

Complexes

RuIII/RuII

E 1/2Red , V (∆Ep, mV) Ligand centered reduction processes

L1

__

__

1.03(60)

-1.51(60)

-1.74(80)

-2.03(100)

[{RuII -FeII}n]4n+

__

0.81(60)

1.03(60)

-1.50b

-1.80b

-2.10b

ITO/[{RuII -FeII}n]4n+

__

0.76(10)

0.97(10)

-1.54

-1.73

n. d.

ITO/[CoII-RuII -FeII]6n+

-0.06(10)

0.78(10)

0.98(10)

n. d.

n. d.

n. d.

a

this value corresponds to the Co(II)/Co(I) metal centered reduction process. Epc values, E 1/2 cannot be accurately measured since the waves are strongly distorted by adsorption phenomena of the reduction product. n. d. = non determined

b

Table 2. Absorption and emission properties of [Fe(ttpy)2]2+, L1 and the corresponding [{RuII_FeII}n]4n+ polymer in deoxygenated CH3CN solution at 25 °C. Φem represents the emission quantum yield, and τ1 and τ2 the luminescence lifetimes. Complexes

λabs /nm (ε/M-1.cm-1)

λemi/nm

Φem

τ1 ns (%)

τ2 ns (%)

̶

̶

[Fe(ttpy)2]2+ 29

567(25 900)

̶

̶

L1

453(12 900)

616

0.058

[{RuII -FeII}n]4n+

453(16 500) 568(24 500)

616

0.008

1000 3(95)

888(5)


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Figures:

Figure 1. Partial MALDI-TOF mass spectrum of the polymer [{RuII_FeII}n]4n+, M corresponds to the monomer of the formula [RuII_FeII]4n+.

Figure 2. A: Cyclic voltammogram of [{RuII _FeII}n]4n+ (0.5 mM) in CH3CN + 0.1 M [Bu4N]ClO4 at a vitreous carbon electrode (diam. = 3 mm); v = 100 mV.s-1. B: Voltammograms at a rotating disk electrode (RDE) of the same solution at a vitreous carbon electrode; rotation rate ω = 600 rot.min-1 ; scan rate: 5 mV.s-1, (a) initial solution, (b) after exhaustive electrolysis at + 0.90 V, and (c) after exhaustive electrolysis at +1.05 V. 21 ACS Paragon Plus Environment


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Figure 3. Absorption spectra of a 1 mM solution of [{RuII_FeII}n]4n+ in CH3CN + 0.1 M [Bu4N]ClO4: (a) initial solution; (b) after exhaustive electrolysis at 0.90 V; (c) after exhaustive electrolysis at 1.05 V; (d) after back electrolysis at 0.60 V; l = 1 mm.

Figure 4. (a) Elaboration of a modified ITO/[{RuII_FeII}n]4n+ electrode by 15 successive cycles between 0 and - 2.0 V from a 0.5 mM solution of [{RuII_FeII}n]4n+ in CH3CN + 0.1 M [Bu4N]ClO4 at a scan rate: 50 mV.s-1. (b) Cyclic voltammogram of the resulting modified electrode ITO/[{RuII_FeII}n]4n+ after transfer in CH3CN + 0.1 M [Bu4N]ClO4 v = 50 mV.s-1 22 ACS Paragon Plus Environment

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Figure 5. Absorbance spectra of an ITO/[{RuII_FeII}n]4n+ modified electrode (Γ = 1.3×10-9 mol.cm-2) at different bias: 0 V (full line); 0.9 V (dashed line); 1.2 V (dotted line).

Figure 6. UV-Visible absorption changes under visible irradiation (λ = 450 nm) of a mixture of [{RuII_FeII}n]4n+ (0.025 mM) and ArN2+ (15 mM) in CH3CN, l = 1 cm. One spectrum every 20 s. Firt step photogeneration of [{RuII_FeIII}n]5n+; second step photogeneration of [{RuIII_FeIII}n]6n+ .



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Figure 7. Cyclic voltammogram of the modified ITO/[CoII_RuII_FeII]6+ electrode in CH3CN + 0.1 M [Bu4N]ClO4 v = 20 mV.s-1. Inset: plot of the current intensity under oxidation peak vs. scan rate (blue CoIII/CoII, purple FeIII/FeII, orange RuIII/RuII oxidation peaks).

Figure 8. (a) AFM image in a tapping mode of a 1 µm2 naked silicon oxide wafer; (b) 3 profiles of the naked silicon oxide surface; (c) AFM image in a tapping mode of a 1 µm2 of SiO2/[CoII_RuII_FeII] (Γ = 0.5 x 10-11 mol.cm-2); (d) 3 profiles of the SiO2/[CoII_RuII_FeII] surface.


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Figure 9. Photoresponse of ITO electrode covered with [{RuII_FeII}n]4n+ film of different thickness: 47 nm (red curve), 75 nm (black curve) and 150 nm (blue curve) at a bias of 0.4 V upon irradiation with a xenon lamp in the presence of 15 mM of ArN2+ in CH3CN + 0.1 M [Bu4N]ClO4.

Figure 10. Photoresponse of ITO/[CoIII_RuII_FeII]7+ electrode (Γ = 0.5×10-11 mol.cm-2) at a bias of 0.12 V upon irradiation with a xenon lamp in the presence of 15 mM of TEOA in CH3CN + 0.1 M [Bu4N]ClO4 solution.



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Table of contents graphic :

A [RuII(bpy)3]2+ center bearing two free terpyridines is used as a linear building block to construct alternated bi metallic Ru(II)-Fe(II) coordination polymer and Co(III)-Ru(II)-Fe(II) inorganic triad on surface; under visible irradiation both structures generated photocurrent with higher stability for the inorganic triad. 29 ACS Paragon Plus Environment

Photoinduced Charge Separation within Metallo-supramolecular

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