Non radiative energy transfer

Therefore, it is possible to compare the experimental lifetimes with estimated radiative lifetimes obtained from the luminescence spectra, and thus derive the importance ofnon-radiative relaxations, at least in the absence of energy transfer. Non-radiative transitions from the excited Do state to the groimd manifold are quite efficient in the presence of OH groups energy can be relaxed to just 3-5 OH stretching quanta. 

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)...

In addition, when the electron and hole recombine, all of their energy, Eg + 3kT, is assumed to be transferred non-radiatively to a third carrier via Auger recombination. But this also becomes excess energy, i.e., more than is needed to maintain the temperature T, and is, therefore, transferred to the lattice atoms. The Auger recombination rate is yne3, where y is the Auger coefficient. 

Energy transfer studies, which are similar to quenching studies, have also led workers to the conclusion that singlet energy migration takes place in PS Energy transfer refers to the process by which singlet excitation is transferred non-radiatively by Forster transfer to an impurity or dopant that can fluoresce The dopant or impurity may be either part of the polymer chain or dispersed in the system ... 

Molecular rotors are fluorophores characteristic for having a fluorescent quantum yield that strongly depends on the viscosity of the solvent [50], This property relies on the ability to resume a twisted conformation in the excited state (twisted intramolecular charge transfer or TICT state) that has a lower energy than the planar conformation. The de-excitation from the twisted conformation happens via a non-radiative pathway. Since the formation of the TICT state is favored in viscous solvents or at low temperature, the probability of fluorescence emission is reduced under those conditions [51]. Molecular rotors have been used as viscosity and flow sensors for biological applications [52], Modifications on their structure have introduced new reactivity that might increase the diversity of their use in the future [53] (see Fig. 6.7). 

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).

The fluorescence quenching occurs when dye molecules are close to the metal. The energy from the first excited fluorophores can be consumed through a non-radiative path to the metal. A spacing layer is usually required to avoid this energy transfer process. In addition, the concentration of the dispersed dye molecules should be suitable to avoid self quenching [34, 81]. 

Internal conversion is a non-radiative transition between two electronic states of the same spin multiplicity. In solution, this process is followed by a vibrational relaxation towards the lowest vibrational level of the final electronic state. The excess vibrational energy can be indeed transferred to the solvent during collisions of the excited molecule with the surrounding solvent molecules. 

Finally, there is a specific red-edge effect related to non-radiative energy transfer between a donor fluorophore whose emission spectrum overlaps the absorption spectrum of an acceptor fluorophore in rigid polar solutions, there is a lack of energy transfer upon excitation at the red-edge. This effect, called Weber s effect, will be described in Section 9.4.3. 

Perrin s model has been used in particular for the interpretation of non-radiative energy transfer in rigid media (see Chapter 9). 

The use of Forster non-radiative energy transfer for measuring distances at a supramolecular level (spectroscopic ruler) will be discussed in detail in Chapter 9. 

Further details of non-radiative energy transfer will be presented in Chapter 9, together with various applications. 

We have considered so far non-radiative energy transfer from a donor to an acceptor that is a different molecule (heterotransfer). Energy transfer between like molecules is also possible (homotransfer) if there is some overlap between the absorption and fluorescence spectra. 

Section 3.5.1 described the various effects observed upon excitation at the red-edge of the absorption spectrum. In particular, a lack of energy transfer was first observed by G. Weber (1960) (and is called Weber s effect for this reason). This effect can be explained in terms of inhomogeneous broadening of spectra. In a rigid polar solution of fluorophores that are close enough to undergo non-radiative en-... 

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