New Development in Nonradiative Transition by a Thermal Lensing Method at High Pressure

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The mechanism for the nonradiative transition from the first excited singlet state was investigated by determining the quantum yield of triplet state formation with a time-resolved thermal lensing. The first group of molecules studied was anthracene carbonyl compounds. The triplet yield for 9-acetylanthracene was approximately unity in solvents besides alcoholic solvents, indicating that the nonradiative deactivation proceeds only through intersystem crossing and the internal conversion process is absent. In contrast, the triplet yield for 9-benzoylanthracene was 0.2 in methylcyclohexane, indicating that the internal conversion is a main nonradiative process, and the triplet yield was strongly dependent on the solvent, showing a tendency that the contribution of internal conversion is smaller in polar solvents. The occurrence of internal conversion was also found out for anthracene carbonyl compounds such as 9-bromo-10-benzoylanthracene, 9-cyano-10-benzoylanthracene and 9-naphthoylanthracene.The instrument for thermal lensing at high pressure was constructed, and thermal lens signals were measured for 9-acetylanthracene and 9-benzoylanthracene in methylcyclohexane at pressures up to 4000 atm. The triplet yield for 9-acetylanthracene increased with pressure, and this result cannot be theoretically explained because the triplet yield is already unity at 1 atm. It must be examined whether the problem inherent to the measurement at high pressures is present. The triplet yield for 9-benzoylanthracene increased remarkably with pressure, indicating that the contribution of internal conversion is decreased by pressure.In measurements at a normal pressure for other carbonyl aromatic molecules such as acetylnaphthalene, benzoylnaphthalene, acetylphenanthrane and acetylpyrene, the contribution of internal conversion was confirmed. Further, the occurrence of internal conversion for 9,10-dimethoxyanthracene in polar solvents was found out.