INTRAMOLECULAR EXCIPLEX FORMATION IN JET-COOLED BICHROMOPHORIC CHAIN MOLECULES - TORSIONAL CONFORMATIONS AND EXCESS VIBRATIONAL-ENERGY DEPENDENCE

Intramolecular exciplex formation and excess vibrational energy dependence were investigated in the bichromophoric EDA systems of several 1-(4-(dialkylamino)phenyl-3-(1- and 9-anthryl)propanes in a supersonic free jet. Fluorescence excitation spectra suggest that two types of ground-state conformers are involved at low He backing pressure (750 Torr). These conformers were tentatively ascribed to trans (t) and gauche (g) conformers concerning the C1-C2 conformation of trimethylene-substituted anthryl. Fluorescence excitation spectra of the t conformer show well-resolved vibrational structures of the S1 state, while those of g were considerably congested. The dispersed fluorescence spectra of the t conformer exhibit large Stokes shifted exciplex fluorescence depending on the excess vibrational energy above the zero-point level of the S1 state. However, no significant exciplex formation was observed in the g conformers up to an excess vibrational energy of <2000 cm-1. The energy thresholds for the exciplex formation in the t conformers in 1-(4-(dimethyl- and diethylamino)phenyl)-3-(9-anthryl)propanes were determined to be 900 and 700 cm-1, respectively. Those for 1-(4-(dimethyl- and diethylamino)phenyl)-3-(1-anthryl)propanes were 500 and 1400 cm-1. The remarkably different features of exciplex formation between t and g conformers and also the energy threshold for the exciplex formation in the t conformers will be discussed in terms of intramolecular conformations of the ground and exciplex states.