Experimental and computational studies on the solvatochromism and thermochromism of 4-pyridiniophenolates
作者:John O. Morley、James Padfield
DOI:10.1039/b204654g
日期:2002.10.1
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.
Two new salts of 4-(2,4,6-Triphenyl-1-pyridinio)phenolate with p-toluenesulfonic acid in molar ratio 2:1 (“red salt”) and 1:1 (“colourless salt”) were synthesised. The solvatochromic effect, 1H NMR spectra and crystal structure of the salts were investigated. In the crystalline “red salt” two molecules of the betaine dye are joined by a strong, asymmetric hydrogen bond contrary to previous suggestions
合成了摩尔比为2:1(“红色盐”)和1:1(“无色盐”)的4-(2,4,6-三苯基-1-吡啶基)酚盐与对甲苯磺酸的两种新盐。研究了盐的溶剂溶变色效应,1 H NMR光谱和晶体结构。与以前的建议相反,在结晶的“红色盐”中,甜菜碱染料的两个分子通过牢固的不对称氢键连接。在极性溶剂中观察到类似的行为,但氢键是对称的。在低极性溶剂中,其他分子间电荷转移相互作用是可能的。
Inverting the Solvatochromism of Pyridinium-<i>N</i>-phenolate Dyes by the Addition of a Second Pyridinium Unit
solvatochromic core results in an unexpected change in the spectral behavior from negative solvatochromism (displacement of the absorption band to shorter wavelengths) to inverted solvatochromism, characterized by the transition from negative to positivesolvatochromism (displacement of the absorption band to longer wavelengths) at moderate solvent polarities.