Supersonic Jet Studies on the Photophysics of 4-Dimethylaminobenzonitrile (DMABN) and Related Compounds: on the Origin of the Anomalous Fluorescence of DMABN Clusters in a Supersonic Jet

Abstract

Dispersed fluorescence and resonant one-color, two-photon ionization spectra of clusters of six derivatives of 4-dimethylaminobenzonitrile (DMABN) expanded in a supersonic jet are reported. The studied clusters comprise molecules, that show dual fluorescence in solution and others, that do not. The experimental setup allows one to obtain information on the cluster size distribution in the pulsed supersonic expansion by time-of-flight mass spectrometry and to correlate the mass spectra with the species contributing to the dispersed fluorescence emission spectrum. Under jet-cooled conditions, no red-shifted fluorescence is observed for the bare chromophore of all compounds under study except 4-(dimethylamino)-3,5-dimethylbenzonitrile (TMCA). The formation of homogeneous clusters (dimers or small clusters) gives rise to an additional red-shifted emission band, which is attributed to the formation of excimers. The stabilization of the excimer arises from both exciton resonance and charge resonance interaction. The phenomenon of dual fluorescence of aminobenzonitriles in dilute polar solutions is usually attributed to an intramolecular charge transfer induced by a twist of the donor group around the bond between the donor and acceptor moiety [twisted intramolecular charge transfer (TICT)]. Because the red-shifted fluorescence is observed even for the molecules, that exhibit no dual fluorescence in solution, this mechanism cannot account for the observations made in this study. Fluorescence detection with a charge coupled device (CCD) camera in combination with an image intensifier shows the enrichment of the heavier clusters on the jet axis in a supersonic expansion. This angular dispersion of the species with different mass affords special techniques for correlating mass and fluorescence spectra.