Unusual size dependence of nonradiative charge recombination rates in acetylene-bridged compounds

Time-resolved fluorescence decays of three asymmetrically substituted phenylethynyl molecules of different size show a sizable solvent dependence. Nonradiative rate constants estimated from quantum yield and lifetime data are consistent with classical electron-transfer theory. The electronic coupling elements, H-RP, derived from fits of theory to the data do not follow the trend of donor-acceptor distance (or molecular size). It is found to be largest for the molecule with a butadiyne linker which is intermediate in size. Our results suggest that the bridge is directly involved in excited-state charge recombination in these conjugated molecules.