Behaviour of triplet benzil and a related compound in polyhydric matrices. Formation of cisoid geometry and phosphorescence from the second triplet state

Abstract

Benzil shows dual phosphorescence in polyhydric solvent matrices at 77 K. The short-lived phosphorescence (τ= 5.8 ms) has a maximum at 504 nm and the long-lived one (τ= 20 ms) has maxima at 402, 427 and 460 nm. Unlike the former, the latter phosphorescence appears only in specific matrices which have at least two hydroxylic hydrogen atoms per molecule, such as ethylene glycol. The 504 nm phosphorescence arises from the lowest triplet state of benzil having skewed geometry. From a comparison of the emission band in the short-wavelength region with that of 10,11-dihydro-5H-dibenzo[a,d]cycloheptene-10,11-dione (‘bridged benzil’), which has a rigid near-planar cisoid conformation, the shorter-wavelength phosphorescence observed for the first time in this study can be assigned to a medium imposed near-planar cisoid conformer of benzil. Furthermore, bridged benzil has been found to show phosphorescence from the second triplet state (T₂) as well as T₁ emission (530 nm). The lifetimes of the T₁ and T₂ emissions are 9.0 and 21 ms, respectively. Phosphorescence of cisoid benzil corresponds to the T₂ emission of bridged benzil, thus indicating that the shorter-wavelength emission of benzil can also be assigned to T₂ phosphorescence.