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2. Alternative energies

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The team is involved in the elaboration of photoactive molecular architectures displaying long-lived photo-induced charge separation states. These systems include hetero-poly-nuclear complexes (V. Marvaud) and photosensitized POMs (G. Izzet, A. Proust). Owing to the electron reservoir properties of dendrimers and POMs, we are particularly focused on charge photo-accumulation, which is a key step in the elaboration of functional molecular artificial photosynthetic devices. Specific attention is also paid to electron and energy transfer processes (antenna effect). Finally the integration of the photoactive systems into photovoltaic or photoelectrocatalytic devices is foreseen.

The realization of organic photovoltaic cells (OPV), mainly relying on oligothiophenes derivatives is also developed in the group (L. Tortech). To this end, the strategy is the following : 1/ synthesis of original (in-)organic molecules and their insertion into the P/N junction of a photovoltaic device and 2/ building up of interfacial layers (especially at the electrodes). In both approaches, the objectives consist in a deep understanding of the optoelectronic processes as well as an improvement of the performance of the photovoltaic cells.

Keywords : artificial photosynthesis, dendrimer-based antenna, photosensitized POMs, organic photovoltaics, thin films.

- Tuning Photoinduced Electron Transfer in POM-bodipy Hybrids by Controlling the Environment ; Experiment and Theory
Toupalas, G. ; Karlsson, J. ; Black, F. A. ; Masip-Sánchez, A. ; López, X. ; Ben M’Barek, Y. ; Blanchard, S. ; Proust, A. ; Alves, S. ; Chabera, P. ; Clark, I. P. ; Pullerits, T. ; Poblet, J. M. Gibson, E. A. ; Izzet, G. Angew. Chem. Int. Ed. 2021, 60, 6518-6525.
The optical and electrochemical properties of a series of polyoxometalate (POM) oxoclusters decorated with two bodipy (boron-dipyrromethene) light-harvesting units were examined. Evaluated here in this polyanionic donor-acceptor system is the effect of the solvent and associated counterions on the intramolecular photoinduced electron transfer. The results show that both solvents and counterions have a major impact upon the energy of the charge-transfer state by modifying the solvation shell around the POMs. This modification leads to a significantly shorter charge separation time in the case of smaller counterion and slower charge recombination in a less polar solvent. These results were rationalized in terms of Marcus theory and show that solvent and counterion both affect the driving force for photoinduced electron transfer and the reorganization energy. This was corroborated with theoretical investigations combining DFT and molecular dynamics simulations.

- Photocurrent generation from visible light irradiation of covalent polyoxometalate–porphyrin copolymers
Huo, Z. ; Liang, Y. ; Yang, S. ; Zang, D. ; Farha, R. ; Goldmann, M. ; Xu, H. ; Bonnefont, A. ; Matricardi, E. ; Izzet, G.* ; Proust, A. ; Ruhlmann, L.* - Electrochim. Acta. 2021, 368, 137635.

Four hybrid polyoxometalate–porphyrin copolymer films were obtained by the electrooxidation of zinc- β-octaethylporphyrin (ZnOEP) or 5,15-ditolylporphyrin (H2T2P) in the presence of organosilyl functionalized Keggin-type POMs and Dawson-type POMs bearing two remote pyridyl groups. The electropolymerization process of the four copolymers was monitored by EQCM. The obtained copolymers were characterized by UV/Vis spectroscopy, X-ray photoelectron spectroscopy, electrochemistry, and AFM. Their impedance properties (EIS) were studied and their photovoltaic performances were also investigated by photocurrent transient measurements under visible light irradiation. These studies showed a correla- tion between impedance and photovoltaic performances, the films based on Dawson type POMs and Zn porphyrins giving the best results. This last system displayed one of the best photocurrent efficiency for a reported POM photosensitized hybrid.

- Acid-triggering of light-induced charge-separation in hybrid organic/inorganic molecular photoactive dyads for harnessing solar energy
Benazzi, E. ; Karlsson, J. ; Ben M’Barek, Y. ; Chabera, P. ; Blanchard, S. ; Alves, S. ; Proust, A. ; Pullerits, T. ; Izzet, G. ;* Gibson, E. A.* Inorg. Chem. Front. 2021, 8, 1610.

H+ modulated charge-transfer in photoexcited covalent polyoxometalate-bodipy conjugates is described. The hybrid organic/inorganic molecular photoactive dyads are based on Keggin-type polyoxometalates (POMs) covalently grafted via an organotin linker to a bodipy (BOD) photosensitizer. The relative potentials of the photosensitizer and POM are aligned such that light-induced electron transfer from BOD to POM is permitted for the polyoxomolybdate but not effective for the polyoxotungstate analogue. In both cases, the addition of acid shifts the redox potential of the POM only, to increase the driving force for electron transfer. This leads to charge-separation being switched on for the photosynsitized polyoxotungstate in the presence of acid. The addition of acid to photosynsitized polyoxomolybdate accelerates charge-separation by an order of magnitude (from 2 ns to 200 ps) and is accompanied by a deceleration of charge recombination, leading to a charge-separated state lifetime of up to 1.3 μs. This behaviour is consistent with proton coupled electron transfer, which has previously been observed electrochemically for POMs, but this study shows, for the first time, the impact of protonation on photoinduced electron transfer.

- Dye-sensitized photocathodes : boosting photoelectrochemical performances with polyoxometalate electron transfer mediators
Ben M’Barek, Y. ; Rosser, T. ; Sum, J. ; Blanchard, S. ; Volatron, F. ; Izzet, G. ; Salles, R. ; Fize, J. ; Koepf, M. ; Chavarot-Kerlidou, M.* ; Artero, V. ; Proust, A.* - ACS Appl. Energy Mater. 2020, 3, 163-169.

The amplification effect of polyoxometalates (POMs) on the efficiency of dye-sensitized nano-ITO cathodes is disclosed. The use of hybrid polyoxometalates of the type [PW11O39(SnC6H4≡C6H4F)]4–, F standing for a carboxylic group (POM-COOH) or a diazonium unit, allows control of the loading of the POMs on the electrode and investigation of key parameters. Even at very low loading, POM-COOH has a substantial effect on the photocurrent response with up to 25-fold increase. Besides ensuring the stability toward leaching, the anchoring function of the POM hybrids was also found to play an intricate role in the competition between the multiple events involved.

- Thermodynamics, Electrode Kinetics, and Mechanistic Nuances Associated with the Voltammetric Reduction of Dissolved [n-Bu4N]4[PW11O39(Sn(C6H4)C≡C(C6H4)(N3C4H10))] and a Surface-Confined Diazonium Derivative
Rahman, Md Anisur ; Guo, Si-Xuan. ; Laurans, M., Izzet, G. ; Proust, A. Bond A. M, Zhang, J. - ACS Appl. Energy Mater. 2020, 3, 4. 3991-4006.

The power of Fourier-transformed large amplitude alternating current voltammetry (FTACV) has been applied to parameterize the reduction of the phosphotungstate [PW11O39(Sn(C6H4)C≡C(C6H4)(N3C4H10))]4– polyoxometalate (POM) (KWSn[N3C4H10]4–/5–/6– processes) at glassy carbon (GC), gold (Au), and platinum (Pt) electrodes as well as its GC surface-confined KWSn[−]4–-grafted diazonium derivative in acetonitrile (0.10 M [n-Bu4N][PF6]). The thermodynamics (E0) and heterogeneous electron-transfer kinetics (k0 and α) were estimated using the Butler–Volmer relationship. FTACV provides access to significantly more detailed mechanistic information related to nonconformance to the theory than widely used DC voltammetric methods, especially with the more intricate surface-confined electrochemistry. Parameterization, the level of agreement, and systematic variations between experimental and simulated data were established by both an experimenter-controlled heuristic method and by a computationally efficient data optimization approach that employed parameter space searches restricted in scope by knowledge of the heuristically based estimations. The first electron transfer process for both acetonitrile-soluble KWSn[N3C4H10]4– and surface-confined KWSn[−]4– is always significantly faster than the second. The electrode dependence order is kGC0 > kAu0 > kPt0 for the KWSn[N3C4H10]4–/5– process. The relatively slower electrode kinetics found for reduction of KWSn[N3C4H10]4– as compared to some other monomeric Keggin POMs may be due to the long organic chain hindering the approach of the POM to the electrode surface, although differences in ion-pairing and other factors also may play a role. Subtle, but systematic, differences identified in comparisons of experimental and simulated voltammetry give rise to apparently data analysis method dependent parameterization and are discussed in terms of nuances not accommodated in the modeling. In the solution-phase voltammetry, data obtained by an electrochemical quartz crystal microbalance and other techniques are consistent with solid adhering to and modifying the electrode surface following reduction of KWSn[N3C4H10]4– to KWSn[N3C4H10]5–. Kinetic and thermodynamic dispersions present in the heterogeneous KWSn[−]4–-grafted electrode are probable causes of nonideality detected in the surface-confined voltammetry of this material. Thus, FTACV gives valuable insights into what is needed to provide a more realistic description of the polyoxometalate/electrode interface in polyoxometalate electrochemistry by revealing subtle nuances that are often overlooked.

- Conductivity via Thermally Induced Gap-States in a Polyoxometalate Thin Layer
Zhu, Qi. ; Paci, B. ; Generosi, A. ; Renaudineau, S. ; Gouzerh, P. ; Liang, X. Mathieu, C. ; Rountree, C. ; Izzet, G. ; Proust, A. ; Barrett, N. ;* Tortech, L.* - J. Phys. Chem. C, 2019, 123, 1922-1930.

We report a study of α-[P2W18O62]6–, Wells–Dawson polyoxometalate (POM) layers deposited on indium tin oxide (ITO)-coated glass substrates. A variety of techniques have been used including atomic force microscopy for surface topography characterization, current mapping, and current–voltage characteristics, X-ray photoemission spectroscopy for chemical analysis, UV–visible photoemission spectroscopy for determination of band line-ups, and energy dispersive X-ray reflectivity for determination of layer thicknesses and scattering length densities. The conditions of film deposition and subsequent thermal annealing strongly affect the film characteristics. In particular, we show that nanostriped films a few tens of nm thick can be obtained in a reproducible manner and that such structuring is accompanied by the appearance of gap states and by a switch from an insulating to a conductive state. Current–voltage characteristics demonstrate that highly ordered films of K6[P2W18O62] allow electron flow only from ITO to [P2W18O62]6–, thus showing a rectifying effect. Finally, we integrate the POM layer into an organic photovoltaic device and show the conduction through it thanks to favorable band alignment between ITO, the gap states, and the active photovoltaic layers.

- Rapid photoinduced charge injection into covalent polyoxometalate–bodipy conjugates
F. A. Black, A. Jacquart, G. Toupalas, S. Alves, A. Proust, I. P. Clark, E. A. Gibson*, G. Izzet,* Chem. Sci., 2018, 9, 5578–5584.

Controlled design of photoactive hybrids would provide highly active materials for solar energy conversion and photo(electro) catalysis. We describe the kinetics of photoinduced electron transfer in a series of photoactive hybrids based on Keggin-type polyoxometalates (POMs) covalently grafted to bodipy photosensitizers. We show how the electronic properties and corresponding dynamics of these hybrids can be readily tuned by varying the POM metal ion, the anchoring functionalization and the spacer length. Ultrafast visible and IR transient absorption spectroscopy, supported by spectroelectrochemical measurements, reveals that photoinduced electron transfer from the bodipy chromophore to the organosilyl POM derivative occurs as rapidly as τ = 54 ps to generate a long-lived (τ = 4.8 ns) charge- separated (CS) state, making this system appropriate for applications in photoelectrochemical devices.

- Red-light-driven photocatalytic hydrogen evolution using a ruthenium quaterpyridine complex
E. Rousset, D. Chartrand, I. Ciofini, V. Marvaud, G. S. Hanan, Chem. Commun., 2015, 51, 9261-9264.
A high-temperature, microwave synthesis of [Ru(qpy)3]2+ (qpy = 4,4′:2′,2′′:4′′,4′′′-quaterpyridine) affords the photosensitiser in quantitative yield. The complex produces H2 photocatalytically in a range extending from the UV region of the spectrum to the red with greater efficiency when compared to [Ru(bpy)3]2+.

- Charge transfer interactions in self-assembled single walled carbon nanotubes/Dawson–Wells polyoxometalate hybrids
C. B.-Navarro,B. Matt, G. Izzet, C. Romero-Nieto, K. Dirian, A. Raya, S. I. Molina, A. Proust, D. M. Guldi, C. Marti-Gastaldo, E. Coronado, Chem. Sci., 2014, 5, 4346-4354
We demonstrate the success in self-assembling pyrene-modified Dawson–Wells-type polyoxometalates (POMs) with single walled carbon nanotubes (SWCNTs) by means of π–π interactions. In this context, the immobilization of POMs onto SWCNTs is corroborated by aberration-corrected high-resolution electron microscopy, thermogravimetric analysis, and Raman spectroscopy. From steady-state and time-resolved photophysical techniques we derived evidence for mutual interactions between SWCNTs and POMs in the excited states. The latter are the inception to a charge transfer from the SWCNTs to the POMs. Our results corroborate the suitability of POM–SWCNTs assemblies for photoactive molecular devices.

- Enhancement of Photovoltaic Efficiency by Insertion of a Polyoxometalate Layer at the Anode of an Organic Solar Cell
M. Alaaeddine, Q. Zhu, D. Fichou, G. Izzet, J. E. Rault, N. Barrett, A. Proust, L. Tortech, Inorg. Chem. Front. 2014, 1, 682–688.
Wells–Dawson polyoxometalate K6[P2W18O62] is grown as an interfacial layer between indium tin oxide and bulk heterojunction of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The structure of the POM layers depends on the thickness and shows a highly anisotropic surface organization. The films have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy (XPS) to gain insight into their macroscopic organization and better understand their electronic properties. Then, they were put at the anodic interface of a P3HT : PCBM organic solar cell and characterized on an optical bench. The photovoltaic efficiency is discussed in terms of the benefit of the polyoxometalate at the anodic interface of an organic photovoltaic cell.

- Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(III)-photosensitized polyoxotungstate
B. Matt, J. Fize, J. Moussa, H. Amouri, A. Pereira, V. Artero, G. Izzet, A. Proust, Energy Environ. Sci., 2013, 6, 1504-1508.
Steady-state irradiation under visible light of a covalent Ir(III)-photosensitized polyoxotungstate is reported. In the presence of a sacrificial electron donor, photolysis leads to a very efficient photo- reduction of the polyoxometalate. Successive formation of the one- electron and two-electron reduced species, which are unambiguously identified by comparison with spectroelectrochemical measure- ments, is observed with a significantly faster rate reaction for the formation of the one-electron reduced species. The kinetics of the photoreduction, which are correlated with the reduction potentials of the polyoxometalate (POM), can be finely tuned by the presence of an acid. Indeed light-driven formation of the two-electron reduced POM is considerably facilitated in the presence of acetic acid. The system is also able to perform photocatalytic hydrogen production under visible light without significant loss of performance over more than 1 week of continuous photolysis and displays higher photocatalytic efficiency than the related multi-component system, outlining the decisive effect of the covalent bonding between the POM and the photosensitizer. This functional and modular system constitutes a promising step for the development of charge photo- accumulation devices and subsequent photoelectrocatalysts for artificial photosynthesis.

- Long lived charge separation in iridium(III)-photosensitized polyoxometalates : synthesis, photophysical and computational studies of organometallic-redox tunable oxide assemblies
B. Matt, X. Xiang, A. L. Kaledin, N. Han, J. Moussa, H. Amouri, S. Alves, C. L. Hill, T. Lian, D. G. Musaev, G. Izzet, A. Proust, Chem. Sci., 2013, 4, 1737-1745.
Keggin and Dawson-type polyoxometalates (POMs) covalently grafted to heteroleptic cyclometalated iridium(III) complexes (POM-[Ir] dyads) have been prepd. by postfunctionalization of organosilyl and organotin POM derivs. Electronic properties of these 4 photosensitized POM-[Ir] dyads were evaluated by electrochemical measurements and theoreticals calculations. These studies reveal that the electron acceptor character of the POMs vary with structural class (Keggin vs. Dawson) and chem. anchorage (organosilyl vs. organotin) ; they reveal the poor electronic interaction between the POMs and the chromophores. Combined transient absorption and spectroelectrochem. measurements provide evidence for the formation of photoinduced electron transfer from the chromophore to the POM. The lifetimes of the charge-separarated states (ranging from ns to hundreds of ns) are the longest values reported for covalently bonded photosensitized POMs. The functionalization of the heteroleptic cyclometalated iridium(III) on the picolinate ligand provides directionality to the photoinduced electron transfer by enhancing charge sepn. and delaying charge recombination The kinetics of the photoinduced electron transfers are rationalized by Marcus theory. We conclude that the charge separation and charge recombination respectively occur in the Marcus normal and inverted regions.

- Elegant Approach to the Synthesis of A Unique Heteroleptic Cyclometalated Ir(III)-Polyoxometalate Conjugate.
B. Matt, J. Moussa, L.-M. Chamoreau, C. Afonso, A. Proust, H. Amouri, G. Izzet., Organometallics, 2012, 31, 35-38.
A novel heteroleptic cyclometalated iridium(III) complex with one picolinic acid derivative bearing a pendant terminal alkynyl tether has been prepared following a new synthetic route. This pendant alkynyl tether can be further engaged in palladium C-C coupling reactions, allowing its grafting to a Keggin-type polyoxometalate and thus providing a unique iridio-POM conjugate.

- Elaboration of Covalently Linked Polyoxometalates with Ruthenium and Pyrene Chromophores and Characteriation of Their Photophysical Properties
B. Matt, C. Coudret, C. Viala, D. Jouvenot, F. Loiseau, G. Izzet, A. Proust, Inorg. Chem., 2011, 50, 7761–7768
Keggin and Dawson-type polyoxometalates (POMs) decorated by organometallic [cyclometalated ruthenium(II) polypyridine complex] or organic (pyrene) chromophores were prepared by postfunctionalization of hybrid disilylated POM platforms. The connection is made in a very efficient and modular way via Sonogashira coupling reactions, which provide a rigid linkage between the POM and the photoactive centers. Electronic properties have been inferred from electrochemical and photophysical studies and reflect poor electronic interactions between both partners. The presence of the POM leads to luminescence quenching of the chromophores, which was attributed to an intramolecular electron transfer from the chromophore to the POM. The rate of this process is much faster in the POM-pyrene than in the POM-Ru system. It depends on the driving force dictated by the redox potentials of both partners but also in the case of the POM-Ru system on the presence of the metallacycle, which acts as a molecular insulator and delays the intramolecular electron transfer. In the POM-Ru system, a comparative study of the luminescence quenching showed that the electron transfer is still more important in the covalently bonded hybrids than in systems where the POM and the ruthenium complexes are assembled via electrostatic interactions.

- Hybrid Polyoxometalates : Keggin and Dawson Silyl Derivatives as Versatile Platforms
B. Matt, S. Renaudineau, L.-M. Chamoreau, C. Afonso, G. Izzet, A. Proust, J. Org. Chem. 2011, 76, 3107-3112.
A new series of polyoxometalate-based hybrids has been synthesized. These covalently linked organic-inorganic materials represent valuable elementary building blocks ready for postfunctionalization, using classical organic reactions and couplings. This approach is exemplified by the grafting of an organic chromophore via a Sonogashira coupling.

- Electroactive Benzothiazole Hydrazones and Their [Mo6O19]2- Derivatives : Promising Building Blocks for Conducting Molecular Materials
S. Gatard, S. Blanchard, B. Schollhorn, P. Gouzerh, A. Proust, K. Boubekeur., Chem. Eur. J., 2010, 16, 8390-8399.
Synthesis of electroactive benzothiazole hydrazones has been achieved and they have been successfully coupled to [Mo6O19]2- to give rare examples of diazoalkane-hexamolybdates. The molecular packing arrangements of the hydrazone precursors and the electronic features of the covalent organic-inorganic hybrids showing communication between the organic pi-system and the metal centers make these compounds very promising building blocks for the synthesis of conducting molecular materials.

- Straightforward Synthesis of New Polyoxometalates-based Hybrids Examplified by the Covalent Bonding of a Polypyridyl Ligand
V. Duffort, R. Thouvenot, C. Afonso, G. Izzet, A. Proust, Chem. Commun., 2009, 6062–6064.
A new polyoxometalate-based organic/inorganic plateform has been designed for further facile derivatization and covalent attachment of organic linkers. This is examplified by the grafting of a polypyridyl ligand.

- Functionalization of polyoxometalates : towards advanced applications in catalysis and materials science
A. Proust, R. Thouvenot, P. Gouzerh, Chem. Commun., 2008, 1837-1852.
Functionalization via covalent grafting of organic functions allows to tune the redox and acid–base properties, and the solubility of polyoxometalates, to enhance their stability and biological activity and to reduce their toxicity, to facilitate their implementation in extended structures and functional devices. We discuss herein the electronic and binding connections, and the various synthesis methodologies. We emphasize on organonitrogen, organosilyl and organophosphonyl derivatives with special attention to synthesis, characterization and potential applications in catalysis and materials science. We also consider the giant molybdenum oxide-based clusters especially the porous capsule-type clusters (Keplerates) which have high relevance to this context.