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 st udied, the first is the benzene-biacetyl complex. The second cluster w as composed of naphthalene and anthracene for which previous work has shown that intra-EET at short range is the dominant decay channel. Inv estigation of the spectroscopic properties of these chromophores separ ately and loosely bound in a van der Waals complex helps to understand the influence of the initial vibronic level and of cluster's interchr omophoric orientation on the EET rate. The fluorescence excitation spe ctrum of naphthalene in the presence of anthracene shows quenching, si multaneously with the appearance of new spectral features and emission characteristic to anthracene only, which are indicative of an intra-E ET process. The quenching of different excitation levels in naphthalen e follow a static Stern-Volmer-like kinetics, with the same quenching rate constant. This can be understood as occurring only because of clu ster formation. The relative emission from excited levels of the donor (naphthalene) moiety of the bichromophoric complex was measured as fu nction of the amount of added anthracene (acceptor moiety). The emissi on intensity shows a pressure dependence which varies with the particu lar vibronic excitation of naphthalene, in agreement with the kinetic model. This is an evidence for the 1:1 cluster composition and suggest s that the intra-EET rate differs for different vibronic states of nap hthalene. Similar results are obtained for the benzene-biacetyl system . Evidence is given for the formation of a bichromophoric molecular co mplex between benzene and biacetyl in the jet. Excitation of several v ibronic levels of the benzene chromophore shows quenching of benzene e mission 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-bia cetyl complex. The quenching efficiency depends on the particular vibr onic excitation of the benzene moiety which is explained in terms of r esonances in the spectral overlap between the two chromophores.