Carbazole-isonicotinoyl\nHydrazone and Its Structurally\nDistinct Zinc(II) Complexes as Reversible Mechanochromic, Mechanofluorochromic,\nand Acidochromic Smart Materials

A new donor–acceptor type hydrazone (<b>HL</b>), namely,\n(E)-<i>N</i>′-((9-ethyl-9H-carbazol-3-yl)methylene)isonico­tinohydrazide\ncomprising carbazole as donor and isonicotinoyl group as acceptor\nunit and its two Zn(II)-complexes <b>1</b> and <b>2</b> have been synthesized and characterized by elemental analysis, FT-IR, <sup>1</sup>H and <sup>13</sup>C NMR, and mass spectral techniques. Complexes <b>1</b> and <b>2</b> have been synthesized using the same\nligand, <b>HL</b>, and zinc metal ion by treating ZnCl<sub>2</sub> and Zn(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O, respectively.\nThe structure of <b>HL</b> was confirmed by single-crystal X-ray\n diffraction analysis. Comprehensive photophysical properties of <b>HL</b>, <b>1</b>, and <b>2</b> revealed remarkable\nreversible mechanochromic luminescent behavior of <b>HL</b> and <b>1</b> on grinding, whereas <b>2</b> remains nonresponsive\ntoward mechanical force stimuli. Notably, under UV light irradiation\n(365 nm), the solid-state emission color of pristine <b>HL</b> changes from blue to yellowish green after grinding, while the luminescence\ncolor of <b>1</b> transforms from yellow to orange. In addition, <b>HL</b> and <b>1</b>-<b>2</b> also exhibit acidochromic\nbehavior on exposure to HCl/NH<sub>3</sub> vapors. Notably, the mechanofluorochromism\nand the acidochromism of <b>HL</b> and <b>1</b> have been\nthoroughly studied by PXRD, TGA, and DSC studies and fluorescence\nlifetime measurements. Further, the structural chemistry involved\nin these behaviors has been thoroughly explored and emphasized that\neven a small structural variation can significantly alter the photophysical\nproperties of organic and inorganic materials.