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The use of optically thick samples for the quantification of photoluminescence quantum yields of light-scattering materials is preferred whenever possible because simple approximate solutions of the problem are obtained as it will be demonstrated in what follows. The problem was first addressed by Oelkrug and Kort��m [46]. Working with their equations one finds for the probability that an emitted photon escapes from the sample: (7) It may be demonstrated that both equations are equivalent. The function ��(��, ��0) is a correction factor http://www.selleckchem.com/products/obeticholic-acid.html for the emission spectrum: (9) Extension of the theory for a system composed of different compounds absorbing at the excitation wavelength, one of them qualified as the emitter yields the following http://www.selleckchem.com/products/Adriamycin.html expression for the observed fluorescence quantum yield [49]: (11) Eqs.?(7)-(13) rest on the following assumptions: (1) the surface of the optically thick sample is Lambertian; (2) the Kubelka�CMunk theory [25] holds; (3) the reabsorption probability is the same irrespective of the reemission cycle; and (4) the scattering coefficient of the sample is independent of wavelength. The last assumption is fulfilled for limited wavelength intervals by samples composed of particles with dimensions larger than wavelength. In a rigid environment collisional processes are precluded, but other ways are active for concentration quenching. Molecular aggregates lead generally to static quenching as aggregates may act as traps of the excitation energy. They may be also acceptors in energy transfer processes, enhancing the quenching effect. Currently, aggregation leads to a dependence of the absorption or remission function spectrum with concentration. In certain cases, however, concentration quenching is observed without evidence of spectroscopic changes. It is attributed in these cases to weakly interacting molecular pairs http://www.selleck.cn/products/sch772984.html constituting excitation energy traps��statistical traps��resulting from the quasi random distribution of dye molecules [50-53]. Energy transfer mechanisms can be classified as radiative or nonradiative, depending on whether the transfer is mediated by a real photon or not. Radiative energy transfer has been considered in the previous section. This type of energy transfer takes place at relatively large absorbances, no matter how long intermolecular distances are. On the other hand, nonradiative energy transfer requires close interaction among donor and acceptor and, for a random distribution of molecules, large local acceptor concentrations. In both cases, strong spectral overlap between donor fluorescence and acceptor absorption is needed. Both energy transfer mechanisms can be dealt with together [54], but the current approach consist in treating them as separate phenomena. Short-range processes (