Pressure-induced solvatochromism of the charge-transfer transitions in pyridinium betaines

Pressure is used to induce the solvatochromism of the charge-transfer (CT) absorption bands of several pyridinium N-phenoxide betaine dyes in methanol, ethanol, 2-propanol, poly(methyl methacrylate), and poly(styrene). The major conclusion of this study is that the peak maxima of the CT bands correlate with the solvent polarity function (ε - 1)/(ε + 2). The spectral changes observed are very similar to the solvatochromic behavior found at ambient pressure with solvents of varying polarity. Pressure-induced solvatochromism, though, has several advantages over ambient pressure methods: (1) the dielectric properties of the solvent change in a smooth and continuous fashion and (2) any specific solute-solvent interactions that occur are, to first order, constant with pressure. The experiments reported in this paper are an implementation of a general principle of high-pressure research: by varying the bulk properties of a material, either continuously or discontinuously, the relationship between the bulk and local properties can be determined.