ELECTRONIC-ENERGY TRANSFER IN BICHROMOPHORIC MOLECULAR CLUSTERS

Intramolecular electronic energy transfer (intra-EET) was investigated in supercooled isolated bichromophoric molecular clusters, under the conditions of supersonic beam expansion. Two types of clusters were studied, the first is the benzene-biacetyl complex. The second cluster was composed of naphthalene and anthracene for which previous work has shown that intra-EET at short range is the dominant decay channel. Investigation of the spectroscopic properties of these chromophores separately and loosely bound in a van der Waals complex helps to understand the influence of the initial vibronic level and of cluster's interchromophoric orientation on the EET rate. The fluorescence excitation spectrum of naphthalene in the presence of anthracene shows quenching, simultaneously with the appearance of new spectral features and emission characteristic to anthracene only, which are indicative of an intra-EET process. The quenching of different excitation levels in naphthalene follow a static Stern-Volmer-like kinetics, with the same quenching rate constant. This can be understood as occurring only because of cluster formation. The relative emission from excited levels of the donor (naphthalene) moiety of the bichromophoric complex was measured as function of the amount of added anthracene (acceptor moiety). The emission intensity shows a pressure dependence which varies with the particular vibronic excitation of naphthalene, in agreement with the kinetic model. This is an evidence for the 1:1 cluster composition and suggests that the intra-EET rate differs for different vibronic states of naphthalene. Similar results are obtained for the benzene-biacetyl system. Evidence is given for the formation of a bichromophoric molecular complex between benzene and biacetyl in the jet. Excitation of several vibronic levels of the benzene chromophore shows quenching of benzene emission with simultaneous appearance of biacetyl fluorescence emission. The quenching follows an apparent Stem-Volmer kinetics as a function of added acceptor pressure, indicative of EET in a binary benzene-biacetyl complex. The quenching efficiency depends on the particular vibronic excitation of the benzene moiety which is explained in terms of resonances in the spectral overlap between the two chromophores.