Carbazole-isonicotinoyl Hydrazone and Its Structurally Distinct Zinc(II) Complexes as Reversible Mechanochromic, Mechanofluorochromic, and Acidochromic Smart Materials

A new donor–acceptor type hydrazone (HL), namely, (E)-N′-((9-ethyl-9H-carbazol-3-yl)methylene)isonicotinohydrazide comprising carbazole as donor and isonicotinoyl group as acceptor unit and its two Zn(II)-complexes 1 and 2 have been synthesized and characterized by elemental analysis, FT-IR, ¹H and ¹³C NMR, and mass spectral techniques. Complexes 1 and 2 have been synthesized using the same ligand, HL, and zinc metal ion by treating ZnCl₂ and Zn(NO₃)₂·6H₂O, respectively. The structure of HL was confirmed by single-crystal X-ray diffraction analysis. Comprehensive photophysical properties of HL, 1, and 2 revealed remarkable reversible mechanochromic luminescent behavior of HL and 1 on grinding, whereas 2 remains nonresponsive toward mechanical force stimuli. Notably, under UV light irradiation (365 nm), the solid-state emission color of pristine HL changes from blue to yellowish green after grinding, while the luminescence color of 1 transforms from yellow to orange. In addition, HL and 1-2 also exhibit acidochromic behavior on exposure to HCl/NH₃ vapors. Notably, the mechanofluorochromism and the acidochromism of HL and 1 have been thoroughly studied by PXRD, TGA, and DSC studies and fluorescence lifetime measurements. Further, the structural chemistry involved in these behaviors has been thoroughly explored and emphasized that even a small structural variation can significantly alter the photophysical properties of organic and inorganic materials.