Solvatochromism and Reversible Solvent Exchange Phenomena in Solvatomorphic Organic Chromophore Crystals

Solvatomorphs or solvates are common in pharmaceutical compounds, and solvatochromism is often reported in solution. The present study reports eight solvates of (Z)-3-(3-hydroxy-4-methoxy phenyl)-2-(4-nitrophenyl) acrylonitrile, a pi-conjugated push-pull organic chromophore. The solvates containing (i) chlorobenzene, (ii) ethyl-acetate, (iii) 1,4-dioxane, (iv) acetone, (v) acetonitrile, (vi) N,N-dimethylformamide, (vii) N-methyl-2-pyrrolidine, and (viii) dimethyl sulfoxide as solvents in their crystal lattices display the solvatochromism effect. All the solvates and the parent form are systematically characterized using spectroscopic, crystallization, thermal analysis, single-crystal X-ray diffraction techniques, and structural analysis. The solvatomorphs reported here are either non-isostructural or have a low degree of isostructurality, except for one pair for which a three-dimensional isostructurality is observed. The structural stabilities of the solvates are correlated with their molecular packing, crystal voids, and thermal properties. Moreover, the possible mechanism behind the solvatochromism and the reversible solvent exchange phenomena have been explored. This report is one of the rarest series of solvatomorphs exhibiting reversible solvent exchange properties and solvatochromic effect both in crystal and solution. Such studies are crucial for the dye and pharmaceutical industries dealing with various solvents and the formulation of drugs in solvated forms, respectively.