Long-Range Nonradiative Transfer of Electronic Excitation Energy

Abstract

A molecule in an electronically excited state can transfer its excitation energy to another molecule in a variety of ways. Under appropriate conditions the energy donor and acceptor do not have to come in direct contact with one another for the energy transfer to occur, and the migration of energy may take place over distances even in excess of 50A1. The energy transfer is readily demonstrated when the donor and acceptor molecules are not of the same kind and have different absorption and emission spectra; one can then selectively excite the donor and observe the enhanced emission of the acceptor due to light absorption by the donor. Energy can, however, migrate also among molecules of the same kind, which is manifested by a depolarization of the emitted light when the transition moments of the participating molecules are not aligned parallel to one another. Long-range nonradiative energy transfer has also been demonstrated in some photochemical reactions, in which the species which has undergone the reaction was not the one which absorbed the light. A classical example is the finding that light absorbed by the aromatic amino acids of a heme protein complexed to carbon monoxide was 100% effective in splitting off the ligand from the heme2–4. Since it was obvious that it is not possible for all the aromatic amino acids to be in contact with the heme group, long-range transfer was indicated.