Nonradiative interactions

Aslan, K. and Perez-Luna, V. H. (2006). Nonradiative Interactions between Biotin Functionalized Gold Nanoparticles and Fluorophore-Labeled Antibiotin/ /asmonto 1 111-119. 

Nonradiative energy transfer is induced by an interaction between the state of the system, in which the sensitizer is in the excited state and the activator in the ground state, and the state in which the activator is in the excited and the sensitizer in the ground state. In the presence of radiative decay, nonradiative decay, and energy transfer the emission of radiation from a single sensitizer ion decays exponentially with time, /. 

Noncontact Interactions (Nonradiative and Radiative Energy Transfer).197... 

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 ... 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

Nonradiative transfer of excitation energy requires some interaction between donor and acceptor molecules and occurs if the emission spectrum of the donor overlaps the absorption spectrum of the acceptor, so that several vibronic transitions in the donor must have practically the same energy as the corresponding transitions in the acceptor. Such transitions are coupled, i.e., they are in resonance, and that is why the term resonance energy transfer (RET) or electronic energy transfer (EET) are often used. 

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

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... 

Figure 4. Schematic energy level diagram for a catenane based on charge-transfer (CT) interactions and for its separated components. The wavy lines indicate nonradiative decay paths of the electronic excited states.

Fig. 6 Modified Jablonski diagram for illustrating metal-fluorophore interactions, (a) the transition of dye excited by the incident light, (b) the enhanced excitation according to enlarged electromagnetic field, (c) the fluorescent emission of dye molecule, (d) the nonradiative relaxation, (e) the enhanced emission of the fluorophores and metal coupling in far field. Reproduced with permission from Ref. [77]...

In contrast, the nonradiative decay rate k r may be viewed to be determined by the localized environment of the luminescent molecule. The localized environment perturbs the natural electronic configuration of the sensor molecule increasing the probability of its decay. The functional form of knr is determined by the nature of the interaction between the excited sensor and its surrounding perturbation. For example, the knr may be proportional to the concentration, partial pressure, or value of a [Parameter] of interest ... 

There are several conformational arrangements with different interactions of the fluorophores with surrounding groups of atoms. Such interactions may affect differently nonradiative deexcitation processes in the excited state, and the decay times for these conformational states will differ. If each of these states is characterized by singleexponential decay kinetics, then the number of constants f, will correspond to the number of aromatic groups in the protein that are in conformationally different states. 

Interaction between an Excited Electronic State and a Microsphere Radiative and Nonradiative Decay Rates... 

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