Knowledge Bank

Large configuration change in the excited state of a molecular system can be an important factor in causing large discrepancies between observed radiative lifetimes and lifetimes calculated from the corresponding integrated absorption intensity. We have measured fluorescence lifetimes and quantum yields in different solvents for a number of molecules. Our results indicate that $ro$, the radiative fluorescence lifetime, increases with an increase of the polarity of the solvent. At low temperatures, when the solvent cage reorientation cannot occur during the lifetime of the excited state, the emission arises from a Franck-Condon state which has an equilibrium configuration similar to that of the ground state. Under these conditions $\tau o$ decreases and the emission becomes blue shifted.