Quantum efficiencies of triplet formation in aromatic molecules

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Abstract
A knowledge of the quantum yields of fluorescence, φf, and phosphorescence φp, alone does not make it possible to separate the two non-radiative processes, φF1, and φP1, which quench respectively the first excited singlet state, S1, and the lowest triplet state, T1. To do this, the quantum yeield of triplet formation, φT, must be known. We have measured the extinction coefficients, φT, of triplet-triplet absorption for a series of aromatic hydrocarbons in EPA at 77°K. From the φT data plus chemical actinometry, we have obtained values of φT. In addition, we have made accurate absolute measurements of fluroescence yield, φF, in solution at 25°C. From our φF data and room-temperature values of φT reported by Lamola and Hammond, we can calculate values of φF1. Benzene, naphthalene, phenanthrene, chrysene and 1, 2-benzanthracene have significant values of φF. Perdeuteration causes no significant change in the value of φF1. We conclude, therefore, that internal conversion from S1 to S0 is determined largely by processes other than coupling of the electronic excitation to C-H vibrations. At 77°K, φF data to combine with our φT values are available only for anthracene. Depending on which of two φF values is used, the value obtained for φF, is either 0.20 or 0.12 with an uncertainty of ±0.10. Clearly there is a great need for accurate low-temperature measurements of fluorescence yield in order to determine whether significant internal conversion from S1 to S0 occurs at 77°K.