Photoionization of aniline in aqueous solution and its photolysis in cyclohexane

Abstract

Photophysical and photochemical processes of aniline (AN) in aqueous solution together with those in cyclohexane (CH) were investigated by means of laser flash photolysis, time-resolved thermal lensing techniques and fluorimetry. In CH at 298 K, the relaxation processes in the S₁ state of AN consisted of intersystem crossing (Φ_isc= 0.68), fluorescence (Φ_f= 0.17), internal conversion (Φ_ic= 0.08) and anilino radical formation (Φ_AN˙= 0.07) by β-bond fission. In aqueous solution at 298 K, a significant decrease in the values of Φ_f(=0.03), Φ_isc(=0.24) and the fluorescence lifetime (τ^°_f= 1.0 ns) was observed in contrast to those in CH. The formation of hydrated electron, e^–_aq[quantum yield, Φ_e^–aq= 0.18], and internal conversion (Φ_ic= 0.54, k_ic= 6.4 × 10⁸ s^–1) became dominant in aqueous solution. The laser power dependence of e^–_aq concentration showed that the photoelectron ejection from the S₁ state of AN in aqueous solution resulted from the one-photon absorption process under relatively low laser power (I_L⩽ 30 mJ pulse^–1 cm^–2). The value of Φ_e^–aq was weakly dependent on the excitation laser wavelengths (0.18 and 0.16 at 266 and 308 nm, respectively) within the manifold of the S₁ state of AN and also little effect of Cs⁺ on Φ_e^–aq was observed. Furthermore, according to picosecond laser experiments for aqueous AN, e^–_aq was formed within the duration (<20 ps) of the laser pulse. It can be shown from these observations that the photoionization takes place from the non-relaxed S₁ state (¹AN*). The values of Φ_f and Φ_isc decreased significantly with increasing temperature. The remarkable temperature effect could be attributed to an increase in Φ_e^–aq and Φ_ic.