Nonradiative electronic decay process

Emission from the upper electronic excited states of polyatomic molecules, in violation of Kasha s rule which allows emission only from the lowest excited states Q), have been observed in a reasonable variety of molecules (2 11). With notable exceptions of azulene and thioketone, however, such emission is usually very weak, because the rates of nonradiative decay processes greatly exceed the rates of radiative processes when excited states other than the lowest excited states are involved. 

In the treatment of radiationless transitions presented above, we have mainly considered the case of a closed channel decaying into a single open channel, which latter consists of the dense vibronic manifold of some one electronic state (statistical limit). That description is obviously incomplete, since both radiative and nonradiative decay processes occur simultaneously. Clearly, a complete theoretical description of the radiationless transition... 

Figure 11.10 Energy level diagrams for photoinduced electron transfer processed based on 11.8, showing excitation (--), luminescence (---) and nonradiative decay (—).

Electron transfer was interpreted in Ref. [54] in terms of the nonradiative decay process [20-24,44]. For an up-to-date review of theoretical works on electron transfer see the relevant chapter in this volume (R. A. Marcus — Recent developments in fundamental concepts of PET in biological systems). 

In the electron-transfer process generalized in Eq. 1, one of the components of the reactant state may be fluorescent. This spin-allowed radiative process will thus be in competition with the nonradiative electron-transfer reaction and the two processes will contribute to the overall decay of the reactant state. The intrinsic lifetimes of fluorescent molecular states range typically from 10 to longer than 10 s. The occurrence of electron transfer involving the fluorescent state will shorten its lifetime and measurement of this quantity will therefore allow computation of the rate constant for electron transfer. 

Electronically excited dye molecules can undergo a number of decay processes including radiative deactivation by fluorescence or phosphorescence and nonradiative deactivation by internal conversion and intersystem crossing. Because the marking event is so fast (<50 ns), triplet-state processes can be ignored and only the singlet-state manifold need be considered. 

The main conceptual advance made in the last few years is the acceptance that electron-transfer process at dye-sensitised systems under barrierless conditions can be purely electronic. A measurement of the nonradiative decay channel due to electron transfer under these conditions gives a direct determination of the electronic coupling. Subsequent to the initial work pointing this out, there have been a number of determinations of extremely fast electron-transfer times at dye-sensitised surfaces. For dye-derivatised TiOi electron-transfer times from 10 fs to 100 fs have been reported by a number of groups (Rehm et al, 1996 Tachibana et al, 1996 Hannappel et al,... 

A class of futuristic solar cells, often called hot carrier solar cells, seeks to harvest the full energy of solar photons. Such cells would utilize the additional energy content of a blue photon relative to ared one.126 In present-day solar cells, equilibrated carriers are collected and hence all absorbed photons with energy greater than the bandgap contribute equally to the measured efficiency. The realization of such hot carrier solar cells therefore requires electron transfer processes that are competitive with nonradiative decay of molecules or phonon relaxation in solids.126 Literature data indicate that such relaxation occurs on a femtosecond timescale. The ultrafast... 

Electron-transfer processes, as well as energy-transfer processes, can be viewed as special cases of the nonradiative decay of an electronic state. In the framework of perturbation theory [1,2], the probability for a transition from a discrete initial state j/ (corresponding to the reactants) to a discrete final state if (corresponding to the products of the reaction) writes under application of a perturbation V to first order ... 

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