Primary photoprocesses in retinol

Pulsed laser photolysis techniques, with nanosecond time-resolution, are applied to solutions of retinol (vitamin A) and retinyl acetate, under varying conditions of solvent polarity and oxygen content. The observed absorbance and conductivity changes are identified as due to: (a) the excited fluorescent state, λ_max = 435 nm; (b) the lowest excited triplet state, λ_max = 405 nm; (c) the retinylic cation (λ_max = 590 nm), formed via: ROH→^hv R⁺ + OH^-. The results constitute the first experimental evidence for ionic photodissociation and for spontaneous, as well as oxygen-induced, intersystem crossing in retinol. Both processes may provide routes for isomerization, being thus relevant to the visual process.