Ionization-induced π → H site switching dynamics in phenol–Ar3

Abstract

Electronic excitation spectra of the S₁ ← S₀ transition obtained by resonance-enhanced two-photon ionization (REMPI) are analysed for phenol–Ar_n (PhOH–Ar_n) clusters with n ≤ 4. An additivity rule has been established for the S₁ origin shifts upon sequential complexation at various π binding sites, which has allowed for the identification of two less stable isomers not recognized previously, namely the (2/0) isomer for n = 2 and the (2/1) isomer for n = 3. Infrared (IR) spectra of neutral PhOH–Ar_n and cationic PhOH⁺–Ar_nclusters are recorded in the vicinity of the OH and CH stretch fundamentals (ν_OH, ν_CH) in their S₀ and D₀ ground electronic states using IR ion dip spectroscopy. The small monotonic spectral redshifts Δν_OH of about −1 cm⁻¹ per Ar atom observed for neutral PhOH–Ar_n are consistent with π-bonded ligands. In contrast, the IR spectra of the PhOH⁺–Ar_n cations generated by resonant photoionization of the neutral precursor display the signature of H-bonded isomers, suggesting that ionization triggers an isomerization reaction, in which one of the π-bonded Ar ligands moves to the more attractive OH site. The dynamics of this isomerization reaction is probed for PhOH⁺–Ar₃ by picosecond time-resolved IR spectroscopy. Ionization of the (3/0) isomer of PhOH⁺–Ar₃(3π) with three π-bonded Ar ligands on the same side of the aromatic ring induces a π → H switching reaction toward the PhOH⁺–Ar₃(H/2π) isomer with a time constant faster than 3 ps. Fast intracluster vibrational energy redistribution prevents any H → π back reaction.