Anomalous fluorescence lifetimes of organic molecules

Abstract

Anomalously short radiative fluorescence lifetimes have been observed for naphthylamines which undergo intramolecular configurational changes upon excitation. The excited-state equilibrium configuration has more charge-transfer character than the Franck-Condon state. This makes the transition from the upper state more allowed than calculated from absorption intensities. τom is about half τoe for β-naphthylamine.The radiative fluorescence lifetimes increase with the polarity of the solvent. Such an increase is interpreted in terms of the relaxation of the solvent cage during the lifetime of the excited state; relaxation occurs due to dipole moment changes of the solute upon excitation.At low temperatures when the solvent cage reorientation cannot occur before emission, the emission arises from a Franck-Condon state which has an equilibrium configuration similar to that of the ground state. Under these conditions, the natural fluorescence lifetime shortens and the fluorescence maximum shifts to the blue.