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.