Photophysical processes in the vapour-phase measured by the optic-acoustic effect. Part 3.—Radiationless processes from the lowest singlet and triplet states of [1H6]benzene and [2H6]benzene

Abstract

Optic-acoustic measurements on the triplet state relaxation of [¹H₆] benzene and [²H₆] benzene are described. The respective triplet deactivation rates in s^–1 are 1.7 × 10³+ 6.6 × 10⁷[C₆H₆] and 1.9 × 10³+ 2.1 × 10⁷[C₆D₆] with [C₆H₆][C₆D₆] in mol dm^–3. The concentration dependent term is discussed in terms of triplet excimer formation which may facilitate the passage through an isomeric intermediate to ground state benzene. For excitation at 40400 cm^–1 the triplet quantum yield tends to zero at zero pressure. An explanation of this is given, not involving reversible radiationless processes, but in terms of both pressure-dependent and pressure-independent internal conversion rates out of the vibrationally excited singlet state. With this model internal conversion rates in s^–1 are 1.7 × 10⁷+ 1.5 × 10¹⁰[C₆H₆] and 1.3 × 10⁷+ 1.3 × 10¹⁰[C₆D₆] for excitation at 40400 cm^–1.