Nonradiative processes energy transfer

For energy transfer to occur, the energy level of the excited state of D has to be higher than that for A and the time scale of the energy transfer process must be faster than the lifetime of D. Two possible types of energy transfers are known—namely, radiative and nonradiative (radiationless) energy transfer. 

Nonradiative excitation energy transfer from a donor D to an acceptor A summarized in Figure 13.9 requires interactions between them. These interactions can be of the Coulombic type, or due to an orbital overlap between the two species, or be a combination of both types. Two mechanisms can explain nonradiative energy transfer in donor-acceptor systems. As is the case of electron transfer, energy transfer can also happen in cascade processes. For clarity reasons, however, we will only describe the basic theoretical background for energy dyads comprising one donor D and one acceptor A. 

Instead of the usual decay by a radiative or nonradiative process, an excited molecule may decay by another process, energy transfer. The excited molecule, which is called th donor, may transfer the excitation energy to another molecule, called the acceptor. The transfer is primarily a phenomenon of dipole-dipole interactions between the donor and the acceptor. The rate of energy transfer depends on several factors the overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor (/), the orientation of the donor and the acceptor transition dipoles (k), and the distance between the donor and the acceptor (r). The overlap J is described by the integral... 

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. 

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 ... 

FRET is a nonradiative process that is, the transfer takes place without the emission or absorption of a photon. And yet, the transition dipoles, which are central to the mechanism by which the ground and excited states are coupled, are conspicuously present in the expression for the rate of transfer. For instance, the fluorescence quantum yield and fluorescence spectrum of the donor and the absorption spectrum of the acceptor are part of the overlap integral in the Forster rate expression, Eq. (1.2). These spectroscopic transitions are usually associated with the emission and absorption of a photon. These dipole matrix elements in the quantum mechanical expression for the rate of FRET are the same matrix elements as found for the interaction of a propagating EM field with the chromophores. However, the origin of the EM perturbation driving the energy transfer and the spectroscopic transitions are quite different. The source of this interaction term... 

Photon emission must be a favorable deactivation process of the excited product in relation to other competitive nonradiative processes that may appear in low proportion (Fig. 3). In the case of sensitized CL, both the efficiency of energy transfer from the excited species to the fluorophore and the fluorescence efficiency of the latter must be important. 

Nonradiative energy transfer has a major role in the process of photosynthesis. Light is absorbed by large numbers of chlorophyll molecules in light-harvesting antennae and energy is transferred in a stepwise manner to photosynthetic reaction centres, at which photochemical reactions occur. This fundamental energy-transfer process will be considered in more detail in Chapter 12. 

Nonradiative processes (knr) can occur with a wide range of rate constants. Molecules with high knr values display low quantum yields due to rapid depopulation of the excited state by this route. The measured lifetime in the absence of collisional or energy transfer quenching is usually referred to as To, and is given by to = (kr + knr). ... 

F ure 5.17 Sequential steps for a nonradiative energy transfer process (see the text). 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

Fluorescence (or Forster) resonance energy transfer (FRET) is a process by which energy is transferred nonradiatively from an excited donor to a nearby ground state acceptor. This process arises due to dipole-dipole interactions and is... 

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

The nonradiative energy transfer must be differentiated from radiative transfer which involves the trivial process of emission by the donor and subsequent absorption of the emitted photon by the acceptor ... 

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