Base‐Induced Apparent Inverted Solvatochromism in Pyridinium Phenolates

Due to conflicting reports regarding the solvatochromism of π-extended pyridinium-phenolate dyes, the choice of base (N,N,N',N'-tetramethylguanidine or 1 M tetrabutylammonium hydroxide in methanol) used to deprotonate the dyes was investigated. Three known dyes were synthesized and their UV-vis absorbance spectra were measured in a range of solvent using the two bases and the zwitterionic forms of the dyes. It was found that the methanolic tetrabutylammonium hydroxide could cause the appearance of inverted solvatochromism in negatively solvatochromic dyes when used in solvents of sufficiently low polarity. This was attributed to the presence of the polar, hydrogen bond donating methanol which preferentially solvates the dye, increasing the polarity it observes. Informed by these results, recommendations are presented for the methods used to compare solvatochromic dyes.