Electronic Excitation Transfer and Relaxation

One purpose of this paper is to point out that so-called intermolecular resonance transfer, intermolecular nonresonance transfer, and intramolecular electronic relaxation in solid media are simply all special cases of radiationless transitions between nonstationary states of the entire system of molecules plus environment. Intermolecular resonance transfer is also a special case of the pure crystal exciton problem. The theoretical results from a previous paper are applied to radiationless transitions in π-electron systems, and the importance of the Franck—Condon factors is emphasized. One role which the vibrational factors play in these processes is illustrated by the large isotope effects which can arise when the radiationless transition converts a large amount of electronic energy into vibrational energy of the system. Quantitative calculations are made of the radiationless transition probability for 1B2u→3E1u(S1→T2) and 1B2u→3B1u(S1→T1) ``intersystem crossing'' in C6H6. It is further shown that the 3B1u→1A1g(T1→S0) and 1B2u→1A1g(S1→S0) processes and other similar processes are slow because of the small vibrational factors which accompany the large energy gap between initial and final electronic states. Calculations cannot rule out the relative importance of the S1→S0 radiationless transition compared with fluorescence and singlet—triplet nonradiative processes. An empirical method by which the Franck—Condon factors may be ascertained from the electronic-energy gaps in π-electron molecules is presented, and the results are used to estimate radiationless transition probabilities for a number of systems. The enhancement of multiplicity forbidden radiationless transitions by a heavy-atom environment is treated, and, in the case where the environment is a solid rare gas, a third-order mechanism accounts for the observed effect. Temperature effects and other kinds of environmental effects upon nonradiative transitions are also dealt with. It is shown that the use of high-energy excitation may increase, but never decrease, QP/QF, the relative quantum yields of phosphorescence to fluorescence. Radiationless transitions for the interesting case of azulene and the enhancement by a heavy-atom environment of the triplet—triplet emission spectrum of this molecule are discussed. Many needed experiments are suggested throughout the paper.