Temperature-dependent absorption spectrum of betaine-30 in methanol

The absorption spectrum of the strongly solvatochromic dye betaine-30 was examined in methanol as a function of temperature. In contrast to previous studies of betaine-30 in acetonitrile, the temperature-dependent spectrum in methanol displays an isosbestic point, suggestive of two forms of betaine-30 in equilibrium. We propose that the two species are betaine hydrogen-bonded to one and two methanol molecules, and we discuss the implications of the isosbestic point in view of the expected thermo-solvatochromic behavior. The absorption spectrum of betaine-30 in several binary solvent mixtures is examined to investigate the importance of specific interactions between betaine-30 and methanol and ethanol.