Photochemistry in solid matrices: Laser flash photolysis of benzophenone in polymethyl-methacrylate and polystyrene matrices

Benzophenone triplets were produced in matrices of polymethylmethacrylate (PMMA) and polystyrene (PSt) at concentrations up to ca. $\text{6 × 10}{}^{\mathrm{-4}}\text{m}$ upon irradiation with single flashes of 347 nm light. With PMMA matrices a single triplet decay mode following first-order kinetics was observed at T ≤ 150 K and at T ≥ 410 K. Two distinct modes of triplet decay were observed in the temperature range in between: a fast first-order process, the lifetime (ca. 4 μs at 295 K) being independent of the initial triplet concentration [T]₀, and a slow second-order process, the first half-life being proportional to [T]₀⁻¹. Similar results were obtained with PSt matrices. At temperatures between 180 and 350 K two modes of triplet decay were also observed, a fast first-order and a slow second-order process. From these results it is concluded that T-T annihilation is an important deactivation route between the glass temperature range and T_γ, where the rotation of α-methyl groups (PMMA) and of phenyl groups (PSt), respectively, commences. Above T_g, deactivation processes involving matrix molecules become dominant. Hydrogen abstraction, evidenced by the absorption spectrum of ketyl radicals, occurs to some extent here. Moreover energy transfer from vibronically excited triplet levels to chromophores of the matrix is feasible at elevated temperatures.

On the basis that T-T annihilation is a diffusion-controlled process, the application of Smoluchowski's equation yields, for the temperature range between T_γ and T_g, diffusion constants much higher than expected. This indicates a strong reduction of the micro-viscosity of the matrix if benzophenone is admixed.