Abstract
The dependence of excited-state lifetime, fluorescence quantum yield and isomerization rate of organic dye molecules on solvent viscosity has been a subject of numerous experimental and theoretical investigations. To explain the viscosity dependence of excited-state lifetime in this paper the temporal behavior of the excited state population is calculated for several models of the molecular relaxation process by a computer simulation incorporating both the stochastical motion of large molecular parts in the excited state and relaxation transitions. The described method is applicable to calculating the probability of changing the electronic state as a function of time and internal rotation coordinate.