INTRAMOLECULAR ELECTRONIC-ENERGY TRANSFER IN A SUPERSONIC JET EXPANSION

Intramolecular electronic energy transfer (intra-EET) was investigated in an isolated bichromophoric naphthalene (N) and anthracene (A) 1:1 molecular cluster. Investigation of the spectroscopic properties of th ese chromophores, separately and loose:ly bound in a van der Waals com plex, helps to understand the dependence of the EET rate on the initia lly excited vibronic level and on die cluster's interchromophoric orie ntation. Measurement of fluorescence excitation spectra of anthracene, at different anthracene pressures shows bands that can be assigned to dimers of anthracene. From measurement of the anthracene excitation s pectrum at increasing naphthalene pressures one can identify other spe ctral features, characterized by different spectral shifts from excita tions of the bare molecule. Some transitions are probably due to a 13. 5 cm(-1) progression associated with an interchromophore cluster bond. Pressure dependence of fluorescence intensity gives evidence for 1:1 cluster composition, and for a slow intra-EET rate that is associated with an unfavoured orientation of the two chromophores in one of the t wo possible conformers of the A-N cluster, as supported by a calculati on of the cluster geometry and by comparison with a recent study of in tra-EET in the A-(CH2)(n)-N bichromophoric molecules.