A solvatochromic dye for probing significantly the dipolarity/polarizability of HBD (hydrogen bond donating) environments

The solvatochromism of the novel dye 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-Δ3-thiazoline (1) has been investigated in 26 solvents of different polarity. 1 exhibits a positive solvatochromism, its solvent-induced bathochromic UV/Vis absorption band shift ranges from n-hexane (λmax = 566 nm) to dimethyl sulfoxide (λmax = 640 nm). The molar absorption energy of the solvatochromic band shift of 1 can be well correlated with Kamlet–Taft's and Catalan's solvent polarity scales. 1 is mainly sensitive to the solvent's dipolarity/polarizability (π* of SPPN term) rather than of its HBD (hydrogen-bond donating) or HBA (hydrogen-bond accepting) property. It is emphasized that the UV/Vis absorption band maximum of 1 in the strong HBD solvents acetic acid, 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol fit well in the LSE (linear solvation energy) relationship with Kamlet and Taft's π* and Catalan's SPPN scale, respectively, which makes this dye important as a dipolarity/polarizability indicator for various solid acids.