The role of the torsion of the phenyl moiety in the mechanism of stimulated ultraviolet light generation in 2-phenylbenzazoles

Abstract

The general difference observed between the photophysical behavior of 2-phenylbenzazoles and parent unsubstituted benzazole is explained through an exchange of the energy order of the two lower excited singlet states of both compounds. The energy variation takes place because S1 -excited 2-phenylbenzazoles suffer a subnanosecond conformational change towards planarity by torsion around the phenyl-heterocycle bond. This torsion is in accordance with theoretical calculations, with ultraviolet (UV)-visible and fluorescence data, and with results obtained on rotationally-constrained 2-phenylindoles. These changes, as well as the usually high fluorescence quantum yields and small Stokes shifts exhibited by unsubstituted benzazoles, make 2-phenylbenzazoles suitable dye lasers for the UV-zone of the spectrum.