Additivity of substituent effects on the visible absorption spectra of some heteroarylazo compounds: the influence of solvent

Pronounced solvatochromic shifts are observed in the visible spectra of two series of heteroarylazobenzene derivatives based on m-acetylamino- and m-m ethyl-N,N-diethylaniline, measured in various solvents. The heteroaryl azo component of these dyes can be viewed as a thiophene ring substituted by cy ano, nitro, alkoxycarbonyl, or ring aza (to give thiazole and isothiazole d erivatives). Free-Wilson analysis has shown that the influence of each of t he substituents is constant across all molecules within the two series for a given solvent. However, the substituent increments vary between different solvents and correlate with the Kamlet-Taft dipolarity/polarisability para meter pi* of the solvents. There appears to be no intuitive explanation for the relative solvent variation of different substituents, but Onsager theo ry suggests sensitivity to the relative size and shape of the dyes. Visible absorption maxima are predicted for unknown heteroarylazo compounds relate d to those studied. (C) 2000 Elsevier Science Ltd. All rights reserved.