Photodissociation of p-ethyl- and p-(α-hydroxyethyl)toluene in solution

Abstract

Photodissociation of p-ethyl- and p-(α-hydroxyethyl)toluene at 266 nm in n-heptane solution was studied by nanosecond fluorescence and absorption spectroscopy. p-Ethyltoluene dissociates by CH–H bond fission in the CH₂ group. p-(α-Hydroxyethyl)toluene decomposes by CH–OH bond fission in the CH(OH) group. The quantum yields are of the order of ∼10⁻³ for dissociation of p-ethyl- and p-(α-hydroxyethyl)toluene. For p-ethyltoluene, the dissociation rate of 4.0×10⁷ s⁻¹ is equal to the fluorescence decay rate of 4.0×10⁷ s⁻¹. For p-(α-hydroxyethyl)toluene, the dissociation rate of >1.0×10⁹ s⁻¹ is much faster than the fluorescence decay rate of 4.5×10⁷ s⁻¹. One-photon excitation is required for dissociation of p-ethyl- and p-(α-hydroxyethyl)toluene. A model is proposed to interpret the observations. For p-ethyltoluene, since the S₁ [ππ*(benzene)] state does not correlate adiabatically to the σσ*(C–H) state, intersystem crossing proceeds to the T₁ [ππ*(benzene)] state, which crosses to the σσ*(C–H) state, leading to CH–H bond fission. For p-(α-hydroxyethyl)toluene, the S₁ [ππ*(benzene)] state crosses adiabatically to the np(O)σ*(C–O) state, allowing rapid CH–OH bond fission.