Photoexcited States of UV Absorbers, Benzophenone Derivatives

The UV absorption, phosphorescence and phosphorescence-excitation spectra of benzophenone (BP) derivatives used as organic UV absorbers have been observed in rigid solutions at 77 K. The triplet-triplet absorption spectra have been observed in acetonitrile at room temperature. The BP derivatives studied are 2,2',4,4'-tetrahydroxybenzophenone (BP-2), 2-hydroxy-4-methoxybenzophenone (BP-3), 2,2'-dihydroxy-4,4'-dimethoxybenzophenone (BP-6), 5-chloro-2-hydroxybenzophenone (BP-7) and 2-hydroxy-4-n-octyloxybenzophenone (BP-12). The energy levels and lifetimes of the lowest excited triplet (T-1) states of these BP derivatives were determined from the first peak of phosphorescence. The time-resolved near-IR emission spectrum of singlet oxygen generated by photosensitization with BP-7 was observed in acetonitrile at room temperature. BP-2, BP-3, BP-6 and BP-12 show photoinduced phosphorescence enhancement in ethanol at 77 K. The possible mechanism of the observed phosphorescence enhancement is discussed. The T-1 states of 2-hydroxy-5-methylbenzophenone, 4-methoxybenzophenone and 2,4'-dimethoxybenzophenone have been studied for comparison.