Influence of Viscous Media on Charge Transfer Reactions

The influence of viscous H2O-Dextran media on two charge transfer reactions has been investigated using laser photolysis coupled with pico and nanosecond time resolved absorption spectroscopy.In the first case, a charge-transfer reaction from a solute (potassium ferrocyanide) to the solvent was studied and the electron solvation dynamics was followed. A solvation time delay, depending on the polymer concentration, is observed which indicates that electrons remain quasi-free for longer periods in these media.In the second case, the quenching process of the triplet state of an excited metallo porphyrin(ZnP4+) by an electron acceptor, methylviologen chloride (MV2+, 2 Cl–) is studied. The measured triplet quenching rate constant values decrease with increasing dextran concentration, but remain higher than those calculated, taking into account a simple viscosity effect. This result is explained in terms of a competition between a decay of the diffusion rate constants of (ZnP4+)* and MV2+ due to a viscosity effect and an enhanced cage effect around the ions (ZnP4+*, MV2+). Once formed, this last effect favours the charge transfer reaction.