Role of contact complexes in singlet-triplet energy transfer

(T-T) energy transfer from the acceptor to donor. If the properties of the solvent cage lead to the same probability for these two energy transfer processes between donor and acceptor molecules, there should be no accumulation of triplet acceptor molecules in solution. In this case, acceptor molecules act as a catalyst leading to an increased population of triplet donor states. In addition to the above scheme of energy conversion in a contact complex, one can also expect that during the lifetime of the solvent cage only the S-T stage of electronic energy transfer will be completed. In this case the excited complex decays before the reverse T-T energy transfer from the acceptor to donor takes place and therefore there is a high probability that the unreacted triplet acceptor molecules will migrate beyond the boundary of the solvent cage. Therefore, depending on whether or not the triplet acceptor molecules are found in solution, one can make a conclusion about the number of elementary steps responsible for the electronic excitation energy conversion in a contact complex during its lifetime in the solvent cage. Answering the above-mentioned questions is important for understanding the properties of the solvent cage and mechanisms of energy conversion in excited contact complexes. Therefore, we conducted studies on decay channels for contact complexes, in which one of the partners from a pair was in the singlet excited state. In order to simplify the problem as much as possible and reduce it to considering only two processes: the S-T energy transfer from the donor to acceptor and the reverse T-T energy transfer from the acceptor to donor, the excited complex must fulfill a number of conditions. One of them is the lack of overlap between the fluorescence spectrum of donor and the absorption spectrum of acceptor. The fulfillment of this condition allows us to neglect the sing!et-singlet (from SoD to SoA) energy transfer relative to the efficiency of S-T energy transfer between donor and acceptor. Moreover, some experimental data [i] indicate that the donor triplet level TiD of the studied pair should be considerably lower than the acceptor triplet level TiA (AE T > i000 cm-i). The larger the energy difference between TiD and TiA , the higher the probability of appearance of acceptor molecules in the triplet state in solution at the expense of S-T energy transfer relative to the probability of T-T energy transfer from the donor to acceptor.