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 (Φisc= 0.68), fluorescence (Φf= 0.17), internal conversion (Φic= 0.08) and anilino radical formation (ΦAN˙= 0.07) by β-bond fission. In aqueous solution at 298 K, a significant decrease in the values of Φf(=0.03), Φisc(=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, Φe–aq= 0.18], and internal conversion (Φic= 0.54, kic= 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 (IL⩽ 30 mJ pulse–1 cm–2). The value of Φe–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 Φe–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 Φf and Φisc decreased significantly with increasing temperature. The remarkable temperature effect could be attributed to an increase in Φe–aq and Φic.