Edmond Amouyal

Photochemical production of hydrogen and oxygen from water: A review and state of the art

Abstract Photochemical hydrogen production is potentially one of the most fascinating ways for solar energy conversion and storage. Since 1977, several homogeneous, quasi-homogeneous or microheterogeneous model systems of hydrogen or oxygen generation from water, by visible-light irradiation, have been proposed and are briefly reviewed. These half photosystems are based on different approaches: (i) multimolecular systems, (ii) systems involving a supramolecular structure of polyad type, and (iii) systems incorporated in organized and constrained or confined media. A survey of the different attempts for complete water splitting into hydrogen and oxygen is also made.

Excited-state electron-transfer quenching by a series of water photoreduction mediators

Rate constants kq for the quenching of the excited state of Ru(bipy)32+ by a series of viologen salts having different redox potential E12 have been determined in deaerated aqueous solutions at pH = 5 by laser flash photolysis. The kq values are found to decrease with increasing —E12 and to correlate with the reaction free-energy change ΔG. Such a correlation is shown to be consistent with the Rehm—Weller model for electron-transfer reactions.

Towards molecular switching: Photophysical properties of N,N′-bis(4-cyanophenyl)piperazine, a bridging TICT molecule

Abstract The properties of N,N′-bis(4-cyanophenyl)piperazine are described. This molecule has been devised to exhibit bridging properties, together with the possibility of giving twisted internal charge transfer (TICT) states following irradiation. The crystal structure shows that the molecule is almost planar in the ground state. Photochemical excitation on the charge transfer band at 300 nm gives rise to a luminescence which is very sensitive to the solvent polarity. In butanol, the dual luminescence characteristic of TICT is observed, which is attributed to the twisting motion of the acceptor(s) part of the molecule with respect to the donor. Transient absorption measurements have been also performed, which characterize the triplet state. A comparison with dimethylaminobenzonitrile is made.

The triplet state of azobenzene

Abstract The quenching rate constants ( k q ) for triplet energy transfer from aromatic hydrocarbons to trans and cis azobenzene have been measured by flash kinetic spectrophotometry. For both isomers the plot log k q versus E T of the donors shows a “non-classical” behaviour. A model of the energy-transfer process, which accounts for the experimental k q values, allows the energy of the lowest triplet state of both isomers to be determined.

Spectroscopic and theoretical studies of the excited states of fenofibric acid and ketoprofen in relation with their photosensitizing properties

A study of the different excited states of fenofibric acid (FB) and ketoprofen (KP), two aromatic acids derived from benzophenone, has been achieved by absorbance and low temperature emission spectroscopy to take into account their photochemical reactivity and their in vitro and in vivo photosensitizing properties. These experiments have shown that FB mainly exhibits a singlet-singlet transition of ππ* character, the nπ* transition being undetectable, in contrast with KP. In phosphate buffer as well as in ethanol or in isopentane, the lowest triplet excited state of fenofibric acid appears to be, as for KP, an nπ* triplet state with characteristics similar to those of benzophenone. These assignments were fairly well supported by theoretical calculations performed at the TD-DFT level. The transient formation of a ketyl radical detected in flash photolysis experiments performed in ethanol confirms that the photochemical reactivity of FB towards hydrogen abstraction in ethanol is typically that of an nπ* triplet state. The good agreement between the experimental and theoretical results is discussed with respect to those previously obtained by other theoretical methods.

Ruthenium dioxide: a redox catalyst for the generation of hydrogen from water

Ruthenium dioxide, already known to be a catalyst for the oxidation of water to oxygen, has been shown to mediate effectively the generation of hydrogen from water in Ru(bipy)2+3, methyl viologen and EDTA model systems, with efficiencies even higher (pH > 5) than those of previously investigated platinum catalysts. The main factor limiting the evolution of hydrogen is found to be the destruction of the organic electron-transfer relay, and this side-reaction is attributed to H˙(ads) species formed on the particles of catalyst.

Twisted internal charge transfer (TICT) effect in bis(4-cyanophenyl)piperazine, a pseudo-dimer of dimethylaminobenzonitrile: a comparative study

Abstract Dimethylaminobenzonitrile (DMABN) and N,N′-bis(4-cyanophenyl)piperazine (BCPPZ) have been comparatively studied by X-ray diffraction, fluorescence and time-resolved emission and absorption. BCPPZ can be considered as a pseudo-dimer made up of two weakly interacting DMABN moieties. The crystal structure of DMABN is reported for the first time: it shows an almost perfect planarity. By contrast, BCPPZ is appreciably twisted around the C-N bonds connecting the piperazine part and the cyanophenyl group. The twist is assigned to intramolecular H·H repulsions and thus should also be present in solution. The photophysical properties of BCPPZ and DMABN are very similar and, in particular, the dual luminescence typical of the formation of a twisted internal charge transfer (TICT) state is also observed in BCPPZ. The only difference lies in the values of the kinetic and thermodynamic parameters describing the interconversion between the planar excited state and the TICT state: this latter state is thermodynamically more stable for BCPPZ than for DMABN. The general analogy in the photophysical behaviour implies that, while the ground state of BCPPZ is centrosymmetric, the excited states exhibit a broken symmetry which can be qualitatively attributed to the weak orbital interaction between the constituent parts.

Picosecond fluorescence study of a semirigid Zn-porphyrin—viologen dyad. Evidence for two dyad conformers

Abstract Determined from picosecond time-resolved measurements the fluorescence decay of a semirigid Zn-porphyrin-viologen dyad in polar solvents is biexponential with a short-lived and a long-lived component. The long-lived component with lifetime close to that of the reference (Zn-meso-4-benzoxyphenyl-tris (4-tolyl) porphyrin) is responsible for the decay of the porphyrin singlet excited state in the opened conformer of the dyad with a long distance between the porphyrin and viologen moieties. The lifetime of the short-lived component is shortened by the intramolecular electron transfer quenching of the porphyrin singlet excited state by viologen in closed conformer of the dyad with a short distance between the porphyrin and viologen moieties.

Effect of vesicular interfacial potential on electron transfer between N,N,N′,N′-tetramethylbenzidine and quinones

Abstract Photoinduced electron transfer reactions between N,N,N′,N′-tetramethylbenzidine (TMB) and various quinones (duroquinone (DQ), benzoquinone (BQ)) and vitamin K1(VK1) embedded in the lipidic phase of dihexadecylphosphate vesicles (DHP), and the corresponding back electron transfer reactions, are studied by laser flash spectroscopy. The results show that TMB+ cation and Q− anion formation are principally a result of the direct electron transfer between the TMB singlet excited state and Q. The back reaction between TMB+ cation and Q− anion takes place by two decays. The fast decay, corresponding to a reaction site located in the bilayer, is independent of the interfacial electrical potential ΔΨ, and depends on the redox potential of the quinone. The best electron acceptor (VK1) yields the slowest rate. A slow back-reaction rate is observed when the two reactants are separated by the vesicular interface after migration of Q− anions towards the aqueous phase, and the transfer rate constant turns out to be an exponential function of ΔΨ.

Photophysical properties of osmium( II ) complexes with the novel 4′- p -phenylterpyridine-triarylpyridinium ligand

A new family of electron acceptor ligands complexed with osmium(II) and associated with appropriate electron donor subunits within heteroleptic compounds, results in multicomponent assemblies of potential interest for both synthetic chemistry and supramolecular photochemistry.