The O–H Stretching Mode of a Prototypical Photoacid as a Local Dielectric Probe

We investigate the OH stretch vibrational frequency shifts of a prototype photoacid, 2-naphthol (2N), when dissolved in solvents of low polarity. We combine femtosecond mid-infrared spectroscopy and a theoretical model based on the Pullin–van der Zwan–Hynes perturbative approach to explore vibrational solvatochromic effects in the ground S₀ and the first electronically excited ¹L_b states. The model is parametrized using density functional theory (DFT), at the B3LYP/TZVP and TD-B3LYP/TZVP levels for the 2N chromophore in the S₀ and ¹L_b states, respectively. From the agreement between experiment and theory we conclude that vibrational solvatochromic effects are dominated by the instantaneous dielectric response of the solvent, while time-dependent nuclear rearrangements are of secondary importance.