Electronic energy transfer radiative

As well as returning to the ground state by radiative or radiationless processes, excited states can be deactivated by electronic energy transfer. The principal mechanisms for this involve dipole-dipole interactions (Forster mechanism) or exchange interactions (Dexter mechanism). The former can take place over large distances (5 nm in favourable cases) and is expected for cases where there is good overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor and where there is no change in the spin... 

Several different mechanisms of electronic energy transfer are believed to operate under different circumstances. The first of these is the so-called trivial mechanism of radiative transfer, which can be represented by the processes... 

Arrowsmith et al used the crossed beam reaction F+Na— NaF+Na (3 P) to study radiative transfer and electronic energy transfer (E — E, V) in the Na (3 P)-1-NajCX S ) system. Previous studies of the Na2 system have utilized high-pressure cells or heat pipes in which radiation trapping is strong and Na + Na2 collisional energy transfer dominates. Time-resolved emission, following pulsed dye-laser excitation, has been used by Husain and his coworkers in a systematic survey of the excited-state behaviour of Mg(3 Pj), Ca(4 P,), and Sr(5 Pj). Dye-laser excitation of Mg vapour at 457.1 nm resulted in the observation of slow spontaneous emission from Mg(3 P,) which... 

Figure 4 Orbital scheme illustrating the quenching of a photo-excited fluorophore FI by a nearby metal centre M via an electronic energy transfer (ET) mechanism. A simultaneous exchange of two electrons takes place, one from FI to M, one from M to FI. Following this circular electron motion, FI is deactivated. The excited M centre which is obtained can emit and relax to its ground state, but in most cases undergoes a non-radiative decay.

Radiative transfer is an unfortunate complication in many electronic energy transfer experiments and it is difficult either to eliminate or make satisfactory allowance for this effect. Martinho and d Olveira have studied in detail the influence of radiative transport on observations of electronic excitation energy transfer. In particular they have analyzed the effects of radiative transport on measured fluorescence decay curves for concentrated solutions. An experimental study of the influence of radiative transport on energy transfer from excited fluorene to pyrene it occurs n-hexane relates closely with this work . Kawski et have... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

For the fate of the excited states in condensed media, we must add to this list energy transfer processes. These are broadly classified as radiative (or trivial ), coulombic (mainly dipole-dipole interaction), or electron-exchange processes. 

Figure 2. Principles of reversible luminescence sensing using photochemical quenching processes (electron, energy or proton transfer). Dye = luminescent indicator Q = quencher species dotted arrow non-radiative deactivation processes. The luminescence intensity (and excited state lifetime) of the indicator dye decreases in the presence of the quencher. The indicator dye is typically supported onto a polymer material in contact with the sample. The quencher may he the analyte itself or a third partner species that interacts with the analyte (see 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-... 

Fig. 2.6. Simplified sketch of electron band structme of a semiconductor mineral, showing the processes of excitation (energy absorption), non-radiative energy transfer and generation of luminescence (after Nasdala et al. 2004)...

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