Efficient electron transfer through a triazole link in ruthenium (II) polypyridine type complexes

Spectroscopic, electrochemical and theoretical characterisations of photoactive systems readily assembled via click-chemistry show an efficient bi-directional charge shift through the triazole link

Monoanionic dipyrrin-pyridine ligands: Synthesis, structure and photophysical properties

A novel monoanionic tetradentate N4 ligand (F5DPPy) based on a dipyrromethene skeleton as a molecular platform and decorated with pyridine rings at the 1- and 9-positions of the dipyrrin motif has been prepared and characterized. Interestingly, although this ligand is weakly fluorescent, it presents a chelation-enhanced fluorescence effect of around 150 times upon coordination to ZnII. Time-dependent (TD) DFT calculations reproduce nicely the spectroscopic features of both the ligand and the complex, and analysis of the electron density redistribution in the excited state suggests that a better orbital overlap of the HOMO and LUMO in F5DPPyZnCl compared with F5DPPy is responsible for the more intense transitions observed with the former system. As such, this ligand opens interesting perspectives in the design of ratiometric sensors

Reversible double oxidation and protonation of the non-innocent bridge in a nickel(II) salophen complex

Substitution on the aromatic bridge of a nickel(II) salophen complex with electron-donating dimethylamino substituents creates a ligand with three stable, easily and reversibly accessible oxidation states. The one-electron-oxidized product is characterized as a nickel(II) radical complex with the radical bore by the central substituted aromatic ring, in contrast to other nickel(II) salen or salophen complexes that oxidize on the phenolate moieties. The doubly oxidized product, a singlet species, is best described as having an iminobenzoquinone bridge with a vinylogous distribution of bond lengths between the dimethylamino substituents. Protonation of the dimethylamino substituents inhibits these redox processes on the time scale of cyclovoltammetry, but electrolysis and chemical oxidation are consistent with deprotonation occurring concomitantly with electron transfer to yield the mono- and dioxidized species described above

Redox noninnocence of the bridge in copper(II) salophen and bis(oxamato) complexes

Two square-planar copper(II) complexes of 1,2-bis(2-hydroxy-3,5-di-tert-butylbenzimino)-4,5-bis(dimethylamino)benzene (1) and N-[4,5-bis(dimethylamino)-2-(oxalylamino)benzene]oxamate (22-) were prepared. The crystal structures of the proligands H2L1 and Et2H2L2, as well as the corresponding complexes, are reported. The proligands each display a one-electron-oxidation wave, which is assigned to oxidation of the bis(dimethylamino)benzene moiety into a π radical. Complexes 1 and 22- exhibit reversible one-electron-oxidation waves in their cyclic voltammograms (E1/21 = 0.14 and E1/22 = 0.31 V for 1 and E1/21 = -0.47 V vs Fc+/Fc for 22-). The first process corresponds to oxidation of the bis(dimethylamino)benzene central ring into a π radical, while the second process for 1 is ascribed to oxidation of the π radical into an α-diiminoquinone. The one-electron-oxidized species 1+ and 2- exhibit intense visible-near-IR absorptions, which are diagnostic of π radicals. They display a triplet signal in their electron paramagnetic resonance spectra, which stem from magnetic coupling between the ligand-radical spin and the copper(II) spin. The zero-field-splitting parameters are larger for 2- than 1+ because of greater delocalization of the spin density onto the coordinated amidato N atoms. Density functional theory calculations support a π-radical nature of the one-electron-oxidized complexes, as well as S = 1 ground spin states. The electrogenerated 12+ comprises a closed-shell diiminoquinone ligand coordinated to a copper(II) metal center. Both 1 and 2 catalyze the aerobic oxidation of benzyl alcohol, albeit with different yields

Photoinduced electron transfer in a molecular dyad by nanosecond pump-pump-probe spectroscopy

International audienceThe design of robust and inexpensive molecular photocatalysts for the conversion of abundant stable molecules like H2O and CO2 into an energetic carrier is one of the major fundamental questions for scientists nowadays. The outstanding challenge is to couple single photoinduced charge separation events with the sequential accumulation of redox equivalents at the catalytic unit for performing multielectronic catalytic reactions. Herein, double excitation by nanosecond pump-pump-probe experiments was used to interrogate the photoinduced charge transfer and charge accumulation on a molecular dyad composed of a porphyrin chromophore and a ruthenium-based catalyst in the presence of a reversible electron acceptor. An accumulative charge transfer state is unattainable because of rapid reverse electron transfer to the photosensitizer upon the second excitation and the low driving force of the forward photodriven electron transfer reaction. Such a method allows the fundamental understanding of the relaxation mechanism after two sequential photon absorptions, deciphering the undesired electron transfer reactions that limit the charge accumulation efficiency. This study is a step toward the improvement of synthetic strategies of molecular photocatalysts for light-induced charge accumulation and more generally, for solar energy conversion