Spectroscopy and dynamics of the first excited state of 1- and 2-cyanonaphthalene cooled in a supersonic jet

Abstract

The fluorescence excitation and dispersed fluorescence spectra of 1- and 2-cyanonaphthalene (1-CNN and 2-CNN) cooled in a supersonic expansion have been studied in the region of the first singlet electronic transition. The main vibronic transitions have been tentatively assigned by comparison with unsubstituted naphthalene. In contrast to naphthalene, vibronically induced Herzberg–Teller transitions do not appear very strongly in the spectra, whereas intense Condon-allowed transitions are observed. Mode mixing has been evidenced in both molecules, for E_vib > 700 cm^–1 in 2-CNN and 450 cm^–1 for the 1-derivative, and the onset of intramolecular vibrational redistribution is discussed in 1-CNN.

The vibrational energy dependence of the fluorescence lifetime has been studied in 1- and 2-CNN. While 2-CNN displays a usual slight shortening of the lifetime when the excess energy increases, 1-CNN exhibits a shorter 0 ° level lifetime relative to higher vibronic levels. The sensitized phosphorescence excitation spectrum shows that the intersystem crossing process is favoured for this 0 ° level relative to higher vibronic levels. This effect has been discussed in terms of a coincidence with a close-lying triplet state.