STRUCTURAL MEASUREMENTS OF ISOLATED CLUSTERS INVOLVING AROMATIC MOLECULES: OBSERVATION OF DIFFERENT ISOMERS AND VIBRONICALLY INDUCED PHOTOISOMERIZATION

1. Work supported by the NSF (CHE-91-20767) and by the ACS-PRF (24816-AC6)Author Institution: Department of Chemistry, University of PennsylvaniaVan der Waals complexes of perylene with 1-chlorobutane and 1chloropentane, prepared under supersonic jet conditions, both exist in three isomeric forms. Following vibronic excitation at $355 cm^{-1}$, the spectra evolve in the picosecond time domain due to vibrational coupling and conformational change. The time evolution of the emission spectra has been followed via monochromator-resolved spectroscopy and by picosecond-resolution stimulated emission spectroscopy. Following vibrational relaxation, two of the isomers give similarly shaped and broadened fluorescence bands, suggesting rapid isomerization. A third isomer of each complex undergoes similarly rapid vibrational relaxation but does not relax structurally, even at a vibrational energy of $700 cm^{-1}$. Picosecond-resolution rotational coherence experiments have measured the structures of all three isomers for both complexes. This study provides for the first time, insights into the isomerization channels that involve large-amplitude motion