Influence of Viscous Media on Charge Transfer Reactions

Abstract

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.