Solvent influence on photochemical isomerizations: Photophysics of DODCI

We have studied the photophysics of DODCI dissolved in a series of polar solvents. Through measurements of the temperature dependence of the photoisomer quantum yield, fluorescence lifetime and fluorescence quantum yields we clarify the kinetic mechanism for nonradiative decay. By measuring the isomerization rate as a function of temperature at constant viscosity we are able to separate the innuence of internal barriers and solvent viscous drag. The apparent activation energy observed in solutions is less than the sum of the internal and viscosity activation energies. This is shown to be consistent with the full Kramer's rate expression for diffusive barrier crossing. We also establish the temperature dependence of a second “direct” internal conversion process which does not lead to photoisomer formation and dominates the nonradiative decay of DODCI at low temperatures or in a rigid matrix.