Département de chimie moléculaire

A series of dyads of general formula Ru(bpy)2(bpy-phn-DQ)4+ (n = 1-5), based on a Ru(II) polypyridine unit as photoexcitable donor, a set of oligo-p-phenylene bridges with 1-5 modular units, and a cyclo-diquaternarized 2,2'-bipyridine (DQ2+) as electron acceptor unit, have been synthesized. Their spectroscopic and photophys. properties have been investigated in CH3CN and CH2Cl2 by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. The exptl. study has also been complemented with a computational investigation carried out on the whole series of dyads. The absorption spectra of the dyads show new spectroscopic transitions, in addn. to those characteristic of the donor, bridge, and acceptor fragments. DFT calcns. suggest the assignment of such bands as bridge-to-acceptor ($π$ phn) → ($π$* DQ) charge-transfer transitions. This assignment is consistent with the solvatochromic and spectroelectrochem. behavior of the new bands. For all the dyads at room temp. in fluid soln., the typical 3MLCT luminescence of the Ru(II) polypyridine unit is strongly ({\textgreater}90{%}) quenched, supporting the occurrence of an efficient intramol. photoinduced electron transfer. The study has revealed, however, that the photophys. mechanism is actually more complex than presumed on the basis of a simple photoinduced electron-transfer scheme. For n = 1, very fast (few picoseconds) photoinduced electron transfer from the MLCT state localized on the substituted bpy ligand to the DQ unit has been obsd., followed by slower interligand hopping and charge recombination. For n = 2-5, MLCT excited-state quenching takes place without transient detection of charge-sepd. product, indicating that charge recombination is faster than charge sepn. This behavior can be rationalized in terms of the superexchange couplings expected through this type of bridges for the two processes. The kinetics of MLCT quenching in the dyads with n = 1-5 does not follow the usual exponential falloff with bridge length: after a regular decrease for n = 1-3, the rate consts. become almost insensitive to bridge length for n = 3-5. The rationale of this uncommon behavior, as suggested by DFT calcns., lies in a switch in the MLCT quenching mechanism with increasing bridge length, from oxidative quenching by the DQ acceptor to reductive quenching by the bridge. [on SciFinder(R)]