ultrafast spectroscopy on bimolecular photoinduced electron transfer reactions

in Figure 1. After excitation of one of the reactants, diffusion is required to bring the reaction partners at distances where charge separation (CS) can occur. The nature of the primary quenching product is still debated.3 For weakly exergonic CS, ∆GCS > ~ -0.4 eV, it is generally accepted that the product is a contact ion pair (CIP), that is, two ions at a distance enabling a substantial overlap of their molecular orbitals. Therefore, CIPs often exhibit some charge transfer (CT) fl uorescence. It should be noted, that CIPs could also be generated by direct excitation in the CT band of donoracceptor complexes. As the product of more exergonic CS does in general not exhibit CT emission, it is often believed that it is not a CIP but rather a solvent-separated ion pair (SSIP). In polar solvents, all theses ion pairs can undergo charge recombination (CR) to the neutral ground state or dissociate into solvated free ions. Despite an impressive number of studies, several questions still remain unanswered. Among them, one can mention: • The inverted region predicted by ET theories,4 that is, the decrease of the ET rate constant with increasing driving force in the high exergonicity regime, has been observed for intermolecular5 and intramolecular CR6, for charge shift7 and for intramolecular CS8 processes. There is however no convincing report of the inverted region for photoinduced intermolecular CS reactions. Why? • As ET quenching involves both diffusion and CS, the intrinsic CS rate constant, kCS, is not directly accessible when the process is diffusion controlled. This is the case as soon as ∆GCS < -0.3 eV even in a low viscosity solvent like acetonitrile.1 Therefore, how large can be kCS for an intermolecular process in liquids? • What is the structure of the various intermediates involved in these ET quenching reactions? These questions are almost as old as the fi eld of photoinduced ET reactions. When they were fi rst addressed, the experimental tools available to the photochemists were not as sophisticated as now. This was the beginning of laser fl ash photolysis, and processes occurring in time-scales shorter than several nanoseconds were not experimentally accessible.9 Therefore, only hypotheses that could not really be verifi ed at that time were proposed as answers to the above questions. Thanks to the impressive progress in the fi eld of lasers and optoelectronics, we have now experimental tools that photochemists in the 1960s could not have even dreamed of. It is therefore worth to revisit this fi eld and to see whether answers can now be proposed. In the following, we will describe some of the efforts of our group in this direction.