Acridine: an investigation of its molecular and crystalline photophysical behaviour

Abstract

The absorption spectrum of acridine in various solvents together with the fluorescence spectra and lifetimes in a rigid matrix confirms the presence of a weak, intrinsic π–π^* emission in this material with its origin at 26 300 cm^–1 akin to a similar emission in its aromatic analogue anthracene. The measured “molecular” acridine lifetime is ∼ 8 ns at 298 K. The temperature dependence of the emission spectra and lifetimes of several crystallographic modifications of acridine are also reported. The properties of acridine V are characteristic of a “monomeric” structure whereas acridine III, known to possess discrete, antiparallel molecular pairs, exhibits “excimeric” emission. Two different acridine molecular pairs comprising the acridine II structure also display excimeric emission with their respective maxima appreciably red-shifted from the molecular emission level, and exhibiting different polarisation behaviour. Triplet states are believed to offer facile non-radiative pathways following thermal activation from both “monomeric” and “excimeric” crystalline states.