Fluorescence excitation and fluorescence spectra of jet-cooled acridine molecules: acridine dimer formation and structure

The fluorescence excitation and fluorescence spectra from a supersonic heli um jet seeded with acridine molecules have been obtained. The presence of l ow-frequency vibrational transitions in the fluorescence excitation spectru m, which can be assigned as intermolecular vibrational modes, clearly indic ates that acridine dimers are formed under jet-cooling conditions. The acri dine dimer ground-state geometry determined by AMI calculations is a head-t o-tail-like structure with an almost collinear arrangement of the short mol ecular axes of both molecular components. The results of calculations of sp ectral characteristics predict a large enhancement of the oscillator streng th for the transition from the ground to first excited singlet state of the dimer (as compared to acridine molecule). Such enhancement is entirely con nected with structure of the dimer (which presumably is also stabilized by the formation of a hydrogen bond). (C) 2000 Elsevier Science B.V. All right s reserved.