Substituent Effects on Solvent Dependence of the Bandshape of Charge-Transfer Transitions in N-Pyridinium Phenolates

Article abstract of DOI:10.1021/ja00220a025

The solvatochromic absorption bands for "betaine-1", "betaine-22", and "betaine-29" (2,4,6-triphenyl-N-(4-hydroxyphenyl)-, 2,4,6-triphenyl-N-(2,6-dimethyl-4-hydroxyphenyl)- and 2,4,6-triphenyl-N-(3,5-dodecamethylene-4-hydroxyphenyl)pyridinium ions, respectively) in a range of polar, apolar, protic, aprotic solvents have been investigated.The bands can be accurately fitted by a single harmonic high-frequency mode Franck-Condon envelope of Gaussian solvent-broadened sub-bands.Multiphonon band analysis including both molecular modes and the solvent dynamics indicates that the solvent broadening for betaine-29 in polar solvents correlates well with ε_o^-1 - ε_s^-1, ε_o being the optical and ε_s the static dielectric constant, not only for aprotic solvents but also for normal alcohols.This is different from the behavior of 2,4,6-triphenyl-N-(3,5-di-tert-butyl-4-hydroxyphenyl)pyridinium ion ("betaine-26") for which the high-frequency solvent vibrational spectral part was previously found to be important.Bands for apolar solvents are independent of the solvent and are much wider than what corresponds to a structureless dielectric medium, pointing to other than purely electrostatic coupling mechanisms.Both the molecular frequencies and coordinate displacements are largely independent of the solvent, emphasizing their molecular character, and the frequency value of about 1600 cm^-1 suggests that C-O or C-N stretching is involved.Spectral data for "betaine-1" and "betaine-22" could also be obtained for alcohol solvents and chloroform.The C-O/C-N mode at 1600 cm^-1 can also be identified for these compounds.In addition, the band features suggest that coupling both to O-H stretching modes and to less isotope sensitive solvent modes is important.