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

Dispersed fluorescence and resonant one-color, two-photon ionization s pectra 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 other s, that do not. The experimental setup allows one to obtain informatio n 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 s pectrum. Under jet-cooled conditions, no red-shifted fluorescence is o bserved for he bare chromophore of all compounds under study except 4- (dimethylamino)-3,5-dimethylbenzonitrile (TMCA). The formation of homo geneous clusters (dimers or small clusters) gives rise to an additiona l redshifted emission band, which is attributed to the formation of ex cimers. The stabilization of the excimer arises from both exciton reso nance and charge resonance interaction. The phenomenon of dual fluores cence of aminobenzonitriles in dilute polar solutions is usually attri buted to an intramolecular charge transfer induced by a twist of the d onor group around the bond between the donor and acceptor moiety [twis ted intramolecular charge transfer(TICT)]. Because the red-shifted flu orescence is observed even for the molecules, that exhibit no dual flu orescence in solution, this mechanism cannot account for the observati ons made in this study. Fluorescence detection with a charge coupled d evice (CCD) camera in combination with an image intensifier shows the enrichment of the heavier clusters on the jet axis in a supersonic exp ansion. This angular dispersion of the species with different mass aff ords special techniques for correlating mass and fluorescence spectra.