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

Electronic excitation spectra of the S1 ← S0 transition obtained by resonance-enhanced two-photon ionization (REMPI) are analysed for phenol–Arn (PhOH–Arn) 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–Arn and cationic PhOH+–Arnclusters are recorded in the vicinity of the OH and CH stretch fundamentals (νOH, νCH) in their S0 and D0 ground electronic states using IR ion dip spectroscopy. The small monotonic spectral redshifts ΔνOH of about −1 cm−1 per Ar atom observed for neutral PhOH–Arn are consistent with π-bonded ligands. In contrast, the IR spectra of the PhOH+–Arn 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.