Anionic merocyanine dyes based on thiazol-2-hydrazides: reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts.

Anionic merocyanine colored dyes based on (4-nitro/cyanophenyl)-substituted thiazol-2-hydrazides (THAs), which are donor-π-acceptor type push-pull molecular systems and characterized with a strong intramolecular charge transfer (ICT) in the ground state, have been examined as reverse solvatochromic systems. THAs are shown to exhibit both positive and negative solvatochromism with a reversal of the latter occurring for solvents of E(30) value of ca. 45. The observed behavior is traceable to relative contributions of benzenoid and quinonoid resonance forms of the THAs and their stabilization by solvents to different degrees. Solvatochromic studies in binary mixtures of polar aprotic and protic solvents reveal that the latter are always preferred in the solvation microsphere of THAs. Multiparametric treatment of the E(dye) parameters by Catalán and Kamlet-Taft linear solvation energy relationships (LSERs) compellingly bears out the remarkable influence of hydrogen-bond donating (HBD) acidity, dipolarity and polarizability of the media on the spectral properties of the anionic dyes; hydrogen-bond accepting (HBA) basicity of the solvents is found to influence the least.