Experimental and computational studies on the solvatochromism and thermochromism of 4-pyridiniophenolates

Abstract

The observed solvatochromism of the betaine 4-(2,4,6-triphenyl-1-pyridinio)phenolate (2a) in solvents of low and very high relative permittivity has been assessed both experimentally and theoretically using molecular orbital methods. The PM3/COSMO method suggests that there are a number of possible conformations for the betaine involving clockwise or anti-clockwise rotation of the four pendant phenyl groups relative to the central heterocyclic ring. The large thermochromic effect observed for the dye in acetone or tetrahydrofuran on moving from room temperature to −78 °C has been attributed to a combination of conformational changes coupled with an increase in the relative permittivity of the respective solvent. The calculated spectroscopic shifts for the betaine using a multi-electron configuration interaction treatment show similar trends to those found experimentally in aprotic solvents. In solvents with acidic hydrogens, the large hypsochromic shift observed for the betaine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Stable hydrogen bonded structures are predicted to be formed between either water, chloroform, or acetonitrile and the exocyclic oxygen atom of the betaine. The overall shifts observed in these solvents show a good correlation with those calculated for the postulated solvates using a version of the CNDO/S method.