Nonradiative transitions, intermolecular

The evaluation made in the preceding section shows the possibility of a natural explanation of the regulation of low-temperature solid-state reactions in a model accounting for barrier parameter oscillations resulting from intermolecular vibrations. A consistent analysis of such a model is required. A mathematical body used for this purpose is, conceptually, close to the common theory of nonradiative transitions, but unlike the latter it enables us to exceed the limits of the one-dimensional Franck-Condon approximation which is inapplicable in treatment of heavy-particle transfer. 

The intermolecular nonradiative transition can be caused by the direct collisional quenching (or transfer) and/or by the so-called collision-induced radiationless transition. The collision-induced process is particularly important in the spin-forbidden transition and is not well understood. 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

The intermolecular reaction of BPHhDj) with the solvent molecules and the unimolecular cleavage of the O—H ketyl bond of BOH-GT) yielding BP and a hydrogen atom have been observed in the microsecond time scale [112-114]. Thus, the decay of BPH Di) can be attributed to the combination of a chemical reaction and nonradiative and radiative transition processes... 

To analyze these processes in more detail, one should take into account the photoexcitation of the molecule that results in electron, oscillatory, and rotational transitions, followed by different radiative and nonradiative, intramolecular and inter-molecular processes, like luminescence, internal conversion, and intermolecular energy or charge transfer. The oscillatory relaxation times are in the range 10 -10 s, lifetimes of the excited singlet states are lower than 10 s, and the intetmole-cular and intramolecular transitions occur in the time scale of nanoseconds and picoseconds therefore, to investigate these phenomena one needs tools, which allow the experiments to be performed in the time scale of the same order. This became... 

---