PHOTOPHYSICS OF THE NAPHTHALENE-ANTHRACENE BICHROMOPHORIC MOLECULAR-SYSTEM 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 loosely bound in a van der Waals comp lex, helps to understand the dependence of the EET rate on the initial ly excited vibronic level and on the cluster's interchromophoric orien tation. 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 sp ectrum at increasing naphthalene pressures one can identify other spec tral features, characterized by different spectral shifts from excitat ions 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 c luster composition, and for a slow intra-EET rate that is associated w ith an unfavoured orientation of the two chromophores in one of the tw o possible conformers of the A-N cluster, as supported by a calculatio n of the cluster geometry and by comparison with a recent study of int ra-EET in the A-(CH2)(n)-N bichromophoric molecules. (C) 1998 Elsevier Science S.A.