Vapor-phase fluorescence emissions of pyridine and its methyl derivatives: Excitation-energy dependence of nonradiative electronic relaxation

Abstract

Vapor-phase fluorescence from the S1 (n, π*) states of pyridine-h5 and -d5, α- and β-picolines, and 2,6-lutidine has been studied. The fluorescence quantum yields of these compounds are, respectively, 5.9×10−5, 6.0×10−5, 3.5×10−5, 5.4×10−5, and 2.5×10−5 for excitation to the zero-point level of S1. The quantum yields of the first four compounds decrease sharply as the excitation energy is raised to the value corresponding to the S2 (π, π*) state, whereas the yield of 2,6-lutidine is fairly constant throughout the S1←S0 and S2←S0 absorption regions. The fluorescence quantum yields and their excitation-energy dependence undergo no significant change upon pressure increase from 0.5 to 20 Torr or addition of a foreign gas up to 760 Torr. The rate constants for the intersystem crossing (kISC) and the other nonradiative processes (kFQ) are evaluated as functions of the excitation energy for pyridine-h5 and α-picoline with the aid of the intersystem-crossing yield values which are available from the literature. The results show that kFQ increases rapidly with increasing excitation energy, while kISC is nearly constant. The fast nonradiative decay responsible for the sharp decrease of the fluorescence quantum yield is attributed to the S1→S0 internal conversion.