Merocyanine-type compound

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

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