ATTENUATION OF INTRAMOLECULAR ELECTRON-TRANSFER RATES IN LIQUID-CRYSTALS

The anisotropic properties of liquid crystals (LC) are the key factor in reducing photoinduced intramolecular electron transfer (IET) rates in donor-spacer-acceptor systems. A rate reduction by 3-4 orders of ma gnitude in LCs, as compared to normal isotropic liquids, enables to ap ply submicrosecond EPR spectroscopy to these reactions. We attribute t his remarkable effect to the existence of a potential barrier, known a s the nematic potential, which characterizes the anisotropic nature of the nematic LC. This nematic potential slows the rotational correlati on time of,the liquid crystalline solvent molecule dipoles. Since IET is coupled to the solvent motion, slowing the rotational correlation t ime reduces the IET rate. Moreover, in all cases examined, the solvent (LC) dynamics control the IET rate over the entire nematic phase, thu s suggesting that the reaction dynamics is brought into the solvent-co ntrolled adiabatic limit.