Singlet Quenching by Energy Transfer and Exciplex Formation

Singlet Quenching by Energy Transfer and Exciplex Formation.—In a series of papers Shetlar has examined the kinetic consequences of there being more than one quenching process in a system. The non-linear Stern-Volmer type expressions derived will be useful in both luminescence studies and studies of photochemical reactions.108 

Determination of the absolute fluorescence quantum yields for solids is usually much more difficult than for solution systems. A method based on energy transfer has been employed to determine the absolute Of of thiocyanine dye molecules in monomolecular layers.113 

Bromocyclopropane has been used as a heavy-atom quencher of singlet states. The advantage of bromocyclopropane is that the C—Br cleavage reaction, which leads to alkyl radical and bromine atom formation and results in difficulties in the quantitative interpretation of the heavy-atom quenching experiment, is inefficient because cyclopropyl radicals are formed only with difficulty. This technique has been used to show that 1,8-divinylnaphthalene (8) undergoes intramolecular photocycloaddition to give (9) and (10) via the triplet state.114 

The relative rate constants for the fluorescence quenching of benzene and a series of its simple derivatives by some chloro- and fluoro-methanes and CDC13 are found to follow qualitatively but not quantitatively the hypothesis that the rate-determining step in the quenching process is formation of a donor-acceptor exciplex. The study examined the kinetics following excitation to S2 and S3 as well as Si. 115 The lack of quantitative correlation with the theoretical model used may be due to a deficiency in the model for excited-state quenching or due to the 

The critical radius 117 of the resonant energy transfer from the triplet state of benzophenone to the singlet state of perylene at 77 in a mixture of chloroform and ether has been determined to be 68 A, a value similar to those obtained for related pairs of molecules.118 

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