Solvent Bandwidth Dependence and Band Asymmetry Features of Charge-Transfer Transitions in N-Pyridinium Phenolates

Article abstract of DOI:10.1021/ja00241a007

We have investigated the shape of the solvatochromic absorption band for "Betaine-26" 2,4,6-triphenyl-N-(di-tert-butyl-4-hydroxyphenyl)pyridinium ion in a range of polar, apolar, protic, and aprotic solvents.Multiphonon band theory, including both molecular modes and a vibrationally dispersive solvent, indicates that the solvents fall in three categories: (1) The bandshape for polar, aprotic solvents is well reproduced by that for a structureless continuous dielectric and a single high-frequency molecular mode.Solvent broadening correlates with ε_o^-1 - ε_s^-1, ε_o being the optical and ε_s the static dielectric constant.The molecular frequency, ω_c, and displacement, Δ_c, are not very solvent dependent, emphasizing their molecular character, and the value ω_c ca. 1600 cm^-1 suggests that C-O, C-N, and C-C stretching is involved. (2) Bands for apolar, aprotic solvents correspond to the same model, ω_c and Δ_c are again not very solvent dependent and coincide with the values for polar aprotic solvents.The solvent broadening is solvent independent, and wider than that for a structureless dielectric.This points to multipolar, dispersive, pressure, or pseudopotential forces as coupling mechanisms. (3) The bandshape for normal alcohols can only be reproduced by a model resting on two molecular modes and a vibrational high-frequency solvent "tail".Broadening, asymmetry, molecular frequencies, and deuterium isotope effects trace the protic solvent spectral entanglement to coupling between betaine-26 and a local mode group with features of both O-H stretching and bending and librational solvent motion.