Motional narrowing of broadband absorption of solvatochromic indicator Betaine 30

Temperature dependence of the absorption spectra of Betaine 30 (2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate, widely known as Reichardtʹs dye) is compared with that of the broadband spectra of other dyes. The double value of the half-width (2 hwhm) of Betaine 30 absorption band between 10 and 293 K in a polymer decreases by 130±20 cm−1, from 4220 to 4090 cm−1. This unprecedented narrowing with increasing temperature is ascribed to hydrogen bond symmetrization due to thermally activated proton tunneling in a pigment–solvent complex. The averaging of charge distribution along the proton coordinate causes motional narrowing of inhomogeneous width.