The photophysical properties of some donor-acceptor dye molecules such astrans-4-dimethylamino-4′-cyanostilbene (DCS) and its model bridged single bond derivative (DCSBS) and bridged double bond derivative (DCSBD) with different degrees of flexibility were investigated. This work reports the effect of molecular structure as well as medium viscosity on the fluorescence properties, including fluorescence quantum yields, of this class of molecules. The result could be explained on the basis of a multidimensional excited state which lead to the conclusion thatE/Z isomerization (or double bond twisting) in the excited singlet state is the most important nonradiative deactivation channel to the ground state. Moreover, experimental results obtained by using stationary fluorescence technique reveal the formation of very weak dye-protic solvent exciplexes of different stoichiometries. Furthermore, dynamical relaxation behaviour, as reflected in the fluorescence quantum yield, is correlated with solvent viscosity/free volume. The results point to possible profitable exploitation of this class of molecules as fluorescent probes.