INTRAMOLECULAR ELECTRONIC-ENERGY TRANSFER IN THE NAPHTHALENE-ANTHRACENE BICHROMOPHORIC MOLECULAR-COMPLEX 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 a t different anthracene pressures shows bands that can be assigned to d imers of anthracene. From measurement of the anthracene excitation spe ctrum at increasing naphthalene pressures one can identify other spect ral features, characterized by different spectral shifts from excitati ons of the bare molecule. Some transitions are probably due to a 13.5- cm(-1) progression associated with an interchromophore cluster bond. P ressure dependence of fluorescence intensity gives evidence for 1:1 cl uster composition, and for a slow intra-EET rate that is associated wi th an unfavored orientation of the two chromophores in one of the two possible conformers of the A-N cluster, as supported by a calculation of the cluster geometry and by comparison with a recent study of intra -EET in the A-(CH2)(n)-N bichromophoric molecules.