Transfer of Triplet State Energy in Fluid Solutions. III. Reversible Energy Transfer.

Abstract

The reversible energy transfer in the quenching of biacetyl phosphorescence in soln. for 11 systems was studied at 20.0 +- 0.1 Deg. The O-free soln. was irradiated with a short flash of moderate intensity and the rate of decay of the phosphorescence was measured. The solvent was C6H6. The light used for the excitation of biacetyl was limited to the range 3800-5000 A. A mechanism is proposed for biacetyl and quenchers having short lifetimes in the triplet state. Results were obtained for the donor-acceptor systems biacetyl-fluoranthene, biacetyl-1,2-benzopyrene, biacetyl-pyrene, biacetyl-2,2'-dinaphthyl, naphthalene-biacetyl, 1-chloronaphthalene-biacetyl, and 1-bromonaphthalene-biacetyl. The results can be summarized in terms of triplet-state energies of acceptor and donor. The rate const. for transfer of triplet-state energy, ET, in C6H6 is approx. the same for all systems in which ET of the donor is at least 1000 cm.-1 higher than that of the acceptor, indicating a diffusion-controlled process. When the electronic excitation of the donor in the triplet state is less than that of the acceptor, then the rate const. for energy transfer is kr exp(-ET/kT), where kr denotes the rate const. for energy transfer in the opposite direction. Curves based on a diffusion mechanism for energy transfer are in agreement with the exptl. results. Disagreement with the results of Porter and Wilkinson (CA 56, 6800b) was obtained in the case when the triplet energy level of the donor lies only slightly above that of the acceptor: P. and W. found the rate const. for energy transfer to be appreciably reduced. Work reported previously on the quenching of biacetyl phosphorescence by 1-chloronaphthalene, 1-bromonaphthalene, and 1-iodonaphthalene indicated that the value of the rate const. increases with oscillator strength. In that work unsuitably high biacetyl concns. were chosen so that almost complete recovery of energy from the small fraction of impurities in the 1-chloronaphthalene was obtained. These results exclude a dependence between the rate of energy transfer and oscillator strength of the acceptor T -> S transition. Thus, for 1-chloronaphthalene, 1-bromonaphthalene, and 1-iodonaphthalene, approx. the same value for the rate const. k1 was found. [on SciFinder (R)]