Intermolecular nonradiative processes

Quenching The deactivation of an excited molecular entity intermolecularly by an external environmental influence (such as a quencher) or intramolecularly by a substituent through a nonradiative process. When the external environmental influence (quencher) interferes with the behavior of the excited state after its formation, the process is referred to as dynamic quenching. Common mechanisms include energy transfer, charge transfer, etc. When the environmental influence inhibits the excited state formation the process is referred to as static quenching. 

There are many photophysical processes that are responsible for the de-excitation of molecules. The few examples of intermolecular photophysical processes that induce fluorescence quenching are electron transfer, proton transfer and energy transfer. The following section will focus on energy transfer and most specifically on nonradiative energy 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. 

Fig. 1 Jablonski diagram of energy level for describing processes absorption, fluorescence and phosphorescence in complex molecules where kf and /c arc the radiative and nonradiative rates of fluorescence, respectively, kj and kTnr are the radiative and nonradiative rates of phosphorescence, respectively, k-lsc is the interconversion rate, and kmt is the rate of intermolecular processes Av denotes the Stokes shift of fluorescence...

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). 

As seen from (1) and (2), intermolecular processes may reduce essentially the lifetime and the fluorescence quantum yield. Hence, controlling the changes of these characteristics, we can monitor their occurrence and determine some characteristics of intermolecular reactions. Such processes can involve other particles, when they interact directly with the fluorophore (bimolecular reactions) or participate (as energy acceptors) in deactivation of S) state, owing to nonradiative or radiative energy transfer. Table 1 gives the main known intermolecular reactions and interactions, which can be divided into four groups ... 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

The intermolecular transfer of electronic excitation energy is a common phenomenon in photochemistry. It is called photosensitization and may occur by a number of mechanisms, both radiative and nonradiative. In the radiative process, also called the trivial mechanism, the acceptor, A, absorbs a quantum emitted by a donor, D (Equations 13.11 and 13.12). 

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... 

The excitation (absorption of a photon) and the red-shifted emission are two distinct events that are separated by a time window ranging from units to hundreds of nanoseconds depending on the fluorophore and the host system. This enables monitoring fast kinetics, because a number of molecular processes proceed on this timescale in small volumes delimited by distances comparable with the range of intermolecular interactions and affect the time-dependent emission characteristics. They include translational and rotational diffusion of the fluorophore, reorientation of molecules in the solvation shell, segmental dynamics of flexible macromolecules, and nonradiative excitation energy transfer, etc. 

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... 

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