Phototautomerization on the singlet and triplet surface in o-hydroxyacetophenone derivatives in polar solvents

Nanosecond laser flash photolysis of o-hydroxyacetophenone (1a) and 2,4-dihydroxyacetophenone (1b) in ethanol and acetonitrile results in absorption due to triplet biradicals 2a (λmax 430 nm, τ ≈ 3 μs) and 2b (λmax 400 nm, τ ≈ 1 μs), respectively. Triplet biradical 2a intersystem crosses to form Z-3a (λmax 400 nm, τ ≈ 10 μs), whereas 2b forms both Z-3b and E-3b (λmax 350 nm, τ ≈ 5 and 72 μs). Quenching studies demonstrate that 3a,b are formed on both the singlet and triplet excited surface of 1a and 1b. In ethanol at 77 K, o-hydroxyacetophenone derivatives 1a and 1b show phosphorescence, as is typical for triplet ketones with (n,π*) configuration. The mechanism for the photoreactivity of 1a,b is supported by density functional calculations.