Effects of long-wavelength fluorescence excitation in liquid solutions of N-pyrrolobenzonitrile

Abstract

The absorption, dual fluorescence, and fluorescence excitation spectra of N-pyrrolobenzonitrile (P5C) in a set of solvents with different polarities are studied upon irradiation of solutions by light with different photon energies. The dual fluorescence from the locally excited Frank-Condon and charge-transfer states are recorded in all cases. The change in the excitation photon energy does not affect the fluorescence band positions, but noticeably changes the intensity ratio between the bands in favor of the long-wavelength band belonging to the charge-transfer state. The effects observed are explained using the data of quantum-mechanical calculations, which demonstrate that the solutions of these systems can very likely contain rotational isomers with different pyrrole orientations with respect to the benzene ring. In the excited state, these isomers have different charge-transfer reaction rates, which leads to different intensity ratios of the recorded fluorescence bands.