Photoionization versus photoheterolysis of all-trans-retinol. The effects of solvent and laser radiation intensity

Abstract

The time-resolved formation of the retinyl carbocation from all-trans-retinol and all-trans-retinol acetate was studied by use of picosecond flash photolysis. From both precursors, the retinyl cation is produced by heterolytic C–O bond cleavage in solvents of medium polarity (acetonitrile, tetrahydrofuran, propanol with Reichardt polarity parameter E^N_T ∼ 0.5) and high polarity (EtOH, MeOH, TFE, HFIP, E^N_T > 0.6) during the laser pulse (≤5 ps) where its lifetime is >10 ns. The absorption maximum of the cation at early times (t < 100 ps) is at λ = 590–600 nm; it shifts to shorter wavelengths (Δλ = 5–10 nm) within 1–10 ns. This spectral shift is suggested to be due to contact ion pair → solvent-separated ion pair → free-ion transformation. The quantum yield of cation formation ϕ_cat is independent of excitation wavelength (213, 266 or 355 nm). Photoheterolysis proceeds via a one-quantum process. In chlorinated solvents, i.e. n-BuCl, 1,2-dichloroethane, chloroform or CCl₄, formation of the retinol radical cation (which is characterized by a peak at 610 nm and further absorption maxima at ∼840 and ∼940 nm) by intermolecular electron transfer to the solvent molecules was detected. The radical cation lifetime in all these solvents is 1.5–2 ns, except for CCl₄ where it is 0.25 ns. The formation of the radical cation or cation was not detected in the low polarity solvents: cyclohexane, hexane, dioxane and p-xylene. However, in solvents of medium and high polarity, at high radiation intensities the radical cation may form in addition to the cation (as a result of two-quantum ionization). DFT calculations confirm our experimental results. The rate of retinol S₁ depopulation (k = 0.3–1 × 10⁹ s⁻¹) is almost independent of the solvent polarity in the range from cyclohexane to methanol. In highly polar solvents (E^N_T > 0.9) the rate increases to (0.5–5) × 10¹⁰ s⁻¹.