Correlating photoacidity to hydrogen-bond structure by using the local O-H stretching probe in hydrogen-bonded complexes of aromatic alcohols

To assess the potential use of O-H stretching modes of aromatic alcohols as ultrafast local probes of transient structures and photoacidity, we analyze the response of the O-H stretching mode in the 2-naphthol-acetonitrile (2N-CH₃CN) 1:1 complex after UV photoexcitation. We combine femtosecond UV-infrared pump-probe spectroscopy and a theoretical treatment of vibrational solvatochromic effects based on the Pullin perturbative approach, parametrized at the density functional theory (DFT) level. We analyze the effect of hydrogen bonding on the vibrational properties of the photoacid-base complex in the S₀ state, as compared to O-H stretching vibrations in a wide range of substituted phenols and naphthols covering the 3000-3650 cm^-1 frequency range. Ground state vibrational properties of these phenols and naphthols with various substituent functional groups are analyzed in solvents of different polarity and compared to the vibrational frequency shift of 2N induced by UV photoexcitation to the ¹L_b electronic excited state. We find that the O-H stretching frequency shifts follow a linear relationship with the solvent polarity function F₀ = (2ε₀ - 2)/(2ε₀ + 1), where ε₀ is the static dielectric constant of the solvent. These changes are directly correlated with photoacidity trends determined by reported pK_a values and with structural changes in the O···N and O-H hydrogen-bond distances induced by solvation or photoexcitation of the hydrogen-bonded complexes.