Triplet-triplet transfer of electronic excitation energy in vapors of organic molecules

The rate constants of the triplet-triplet (T-T) transfer of energy of the electronic excitation obtained in the gas phase for two sets of donor-acceptor pairs with various spectral characteristics are compared with those predicted by the theories of nonradiative transitions. The rate constants K-TT and efficiencies beta(DA) of the T-T transfer are estimated from quenching of the phosphorescence of biacetyl by aromatic hydrocarbons (anthracene, 9-methylanthracene, 9,10-dimethylanthracene, 9-phenylanthracene, pyrene. and chrysene) and from quenching of the delayed fluorescence of benzophenone, 4-phenylbenzophenone, anthraquinone, carbazole, and naphthalene by biacetyl, as well as from the sensitization of phosphorescence of biacetyl by these compounds. The causes of the broad variations of beta(DA) in the gas phase from 0.35 (biacetyl-anthracene) to 10(-4) (biacetyl-chrysene) are discussed for the studied donor-acceptor pairs, which differ in the exothermicity of the process and in the configuration of the lowest triplet levels of the molecules participating in the transfer. The dependences of the transfer rate constants K-TT on the free energy change DeltaG (the range from 0.4 to -1.7 eV) are analyzed. It is shown that, in gas-phase systems, K-TT increase with the exothermicity of the transfer. attain a highest value, and then decrease in the range of negative -DeltaG. The observed differences in the experimental values of beta(DA) are discussed in terms of changes in the free energy DeltaG of the process, in the energy of rearrangement of the nuclear configuration of the molecules during the transfer, and in the matrix elements of the interaction. (C) 2002 MAIK "Nauka/Interperiodica".