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Chapter 2:
Mechanisms of Energy Transfer
The idea of a nonradiative transfer of electronic excitation energy was first proposed by J. Perrin in the 1920s and later interpreted in terms of quantum me­chanics by F. Perrin. The theoretical model of F. Perrin showed that excitation could be transferred over distances of hundreds and even thousands of angstroms. In the late 1940s, Th. Forster, based on the quantum-mechanical approach and qualitative experiments on dye solutions, concluded that energy transfer can occur over distances up to 100 A. Sometime later the possibility of an "inductive reso­nance" between the molecules spaced at such distances was advanced by Galanin. For concentrated solutions of dyes with overlapping fluorescence and absorption spectra, Forster and Galanin observed a "nontrivial" fluorescence quenching of do­nor molecules upon increasing their concentration or adding acceptor molecules. The nontriviality consisted in the fact that the decrease in fluorescence inten­sity could not be explained by a trivial reabsorption of the light in the cuvette though reabsorption did take place. Since the quenching occurred at concentra­tions on the order of 1-10 mM, when the mean statistical intermolecular distances were about 100 A or less, these experiments were interpreted as proof of the occur­rence of effective energy transfer over such distances.
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