Quantum yields of triplet formation

The measurement of triplet quantum yields PT = lrftSC is difficult because triplet states are transient intermediates. Phosphorescence quantum yields f ph represent a lower limit for PT, because d ph =1i isc3, 7ph but the measurement of Pph requires low temperatures. Photothermal methods (Section 3.11) can be applied to the determination of triplet 

Two self-calibrated methods are available that do not rely on the knowledge of of a reference compound. Horrocks et al. described an accurate method based on the measurement of triplet triplet absorption by flash photolysis, in combination with Stem Volmer analysis of fluorescence quenching (Section 3.9.8).238 Bromobenzene was used as a heavy-atom quencher of the fluorescence of 9-phenylanthracene. More recently, time-resolved measurements of delayed fluorescence (Section 2.2.4) were analysed to give accurate triplet quantum yields.239 

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If et is not known, it is possible to obtain more accurate values of d>lsc than above by using a lower flash intensity such that all the molecules are not excited during the flash (70 20-100 J). For this method the intensity of the light absorbed Ia must be accurately determined from the absorption spectrum and the incident light intensity 70 determined by actinometry. The concentration of triplet molecules [A ] can be determined from A[A ] as above. Since Ia and [A ] are smaller than in the previous case, errors due to the underlying T0 -> Tx absorption are reduced. The quantum yield of triplet formation is now... 

The limiting < ao, for large [A] is obtained as the intercept in Fig. 11.4. The value 0.6 adds up neatly with R — 0.4 to give a total of unity suggesting no loss of A by internal conversion. Thus this limiting value may be identified with the quantum yield of triplet formation ... 

The formation of triplets via excimers is still an area which is little explored. Medinger and Wilkinson (1966) have shown that pyrene excimer formation leads to a decrease in the quantum yield of triplet formation. A similar result has been obtained for benz(a)anthracene (Heinzelmann and Labhart, 1969). On the other hand, benzene excimer formation leads to an increased triplet yield (Cundall et al., 1972 Hentz and Thibault, 1973). There has been a recent theoretical study of intersystem crossing in molecular pairs (Bowman and Norris, 1978). 

The quantum yield of fluorescence is about 0.18 so that about 80% of the relaxation from the thermal vibrational levels is by nonradiative channels. Evidence in this section indicates that at least most of that nonradiative relaxation is intersystem crossing to the triplet state. The quantum yield of triplet formation is about 0.7, and within the uncertainties of the data, this combined with the small but finite collision-induced decay may account for all nonradiative decay. Triplet formation also occurs in CgDe, and the data again suggest that it may account for essentially all of the nonradiative decay. 

The Absolute Quantum Yields of Triplet Formation in Benzene Vapor at Total Pressures above Ten Torr as Derived from Butene-2 Sensitization Experiments (The yields of Noyes and Harter and of Cundall and Davies have been recalculated from their original data with a branching fraction b( — 0.50)... 

Terazima, M., Hirota, N. Measurement of the quantum yield of triplet formation and short triplet lifetimes by the transient grating technique. J. Chem. Phys. 95, 6490-6495 (1991)... 

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