A carbazole-naphthoyl hydrazone conjugate and its Zn(<sc>ii</sc>)-complex as stimuli-responsive smart materials: detection of VOCs and Cu2+, AIE insights and bioimaging applications

A readily accessible hydrazone ((E)-N '-((9-ethyl-9H-carbazol-3-yl)methylene)-3-hydroxy-2-naphthohydrazide ligand (HL) and its luminescent Zn(ii)-complex (1) have been synthesized and thoroughly characterized using several spectro-analytical techniques. The photophysical properties of HL and 1 have been explored in both solid and solution states. Interestingly, HL displayed reversible mechanochromism under ambient light and reversible mechanofluorochromism under 365 nm UV irradiation. Complex 1 comprising HL does not show any sharp visual color changes under ambient light; however, it exhibits noteworthy reversible mechanofluorochromism under 365 nm UV light (bright yellow to golden yellow). Besides solid-state UV-vis and photoluminescence spectroscopy, the underlying mechanism of phase transformation has further been explored by SEM, PXRD, and TGA/DSC analyses. As anticipated, 1 demonstrated remarkable J-type AIE characteristics in a THF/H2O mixture and H-type AIE in DMSO/H2O, which correlated well with the UV-vis, luminescence, and nanoparticle tracking analysis (NTA) results. Furthermore, when comparing HL and 1, only Zn(ii)-complex 1 effectively and selectively detects Cu2+ in aqueous DMSO medium, and the limit of detection was determined to be 9.1 x 10-7 M. Moreover, both HL and 1 were investigated for their biological applications and the MTT assay revealed that 1 is less cytotoxic than HL toward the MCF-7 breast cancer cell line. Furthermore, 1 showed strong fluorescence in the cytosol of the cells compared to HL, and thus it can be used as a cell staining agent. Overall, the present research findings offer a straightforward mutual comparison between HL and 1 and their multifunctional behavior for a wide range of applications in environmental sensing, data storage devices, and bioimaging studies.