Reichardt's dye: the NMR story of the solvatochromic betaine dye

Reichardt's dye, 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio) phenolate (1), has a very large negative solvatochromism in the long wavelength absorption in the UV-vis spectrum when going from nonpolar to polar solvents. This shift provides the basis of the important and widely used E-T(30) scale of solvent polarity. While many papers have investigated the properties of this dye, only a few describe the H-1 and C-13 NMR assignments in any detail. We report herein, our detailed analysis of the proton and carbon chemical shift assignments for this molecule based on 1D and 2D NMR measurements, as well as those of the protonated and methoxy derivatives 2 and 3, respectively. Much to our surprise, some of the critical chemical shift values we observed were significantly different from those previously reported. In addition, we discovered a good correlation not only between the solvent polarity and the chemical shifts of carbons C1 and C4 of Reichardt's dye (1), but also between the concentration of the dye and these chemical shifts.