Electron-proton transfer mechanism of excited-state hydrogen transfer in phenol−(NH3) n (n = 5) studied by delayed ionization detected femtosecond time-resolved NIR spectroscopy

Highlights: • Transient near infrared absorption after the S₁ excitation was traced in real-time. • Absorptions from Franck-Condon, intermediate, and final states were identified. • Time constants of the excited state hydrogen transfer reaction were determined. • Electron-proton decoupling results from a long-lived charge transfer intermediate. • Both of the πσ^∗ and charge transfer states are involved in the hydrogen transfer reaction. The reaction mechanism of a hydrogen transfer reaction has a fundamental importance in wide ranges of chemistry, such as redox reactions and enzymatic reactions. The excited-state hydrogen transfer (ESHT) of phenol–(NH₃)_n clusters is a benchmark system to study solvation effects on the ESHT reaction mechanism. Recently, we reported that the mechanism of the ESHT reaction changes to electron–proton decoupled transfer for phenol–(NH₃)₅, from a concerted hydrogen atom transfer for clusters with n < 5, based on observations of picosecond time-resolved NIR/IR spectroscopy (Miyazaki et al., 2018). However, the dynamics of the initial electron-transfer process has not been addressed because the rate is faster than the time-resolution of the picosecond time-resolved measurement. In this study, femtosecond time-resolved NIR spectroscopy was applied to the phenol–(NH₃)₅ to elucidate the initial electron-transfer process. Time evolutions probed in the range of 6000–9000 cm⁻¹ detected two rise components that can be ascribed to electronic absorptions of the Franck-Condon region of the excitation and the transient charge-transfer complex, respectively. A kinetic analysis determined the time-scale of the initial charge-transfer process to be τ_CT = 370 ± 55 fs. The fast reaction time supports (almost) a barrier-less charge transfer process predicted by a theoretical calculation that shows solvation-induced strong mixing of ππ^*-πσ^* states.