Twisted intramolecular charge transfer, nonlinear optical, antibacterial activity, and DFT analysis of ultrasound processed (E)-N′-(4-isopropylbenzylidene)nicotinohydrazide

[Display omitted] •Cuminaldehyde-derived hydrazone INH was synthesized efficiently using ultrasonication.•In all solvents, INH hydrazone exhibited dual emissions.•In polar protic solvents, TICT emission at a longer wavelength for INH was observed.•INH showed higher NLO and antibacterial properties.•Experimental results have been explained successfully based on DFT results. Hydrazones have attracted considerable interest for their antimicrobial and nonlinear optical (NLO) applications. In the present work, a new hydrazone (E)-N′-(4-isopropylbenzylidene)nicotinohydrazide (INH) was synthesized by a condensation reaction of cuminaldehyde with nicotinic hydrazide via ultrasonication. The hydrazone was characterized by FT-IR spectroscopy, mass spectrometry, NMR (1H, 13C, HOMO-COSY, HSQC, and DEPT-135), elemental analysis, and FESEM analyses. Phase-pure and non-existent isomorphic phase transition in INH was demonstrated by X-ray diffraction and thermogravimetric analysis. The absorption maximum of INH in an aqueous solution was red-shifted compared to that of the reactant molecules, indicating that the CN bond formation efficiently enhances the resonance interaction towards the pyridine ring. Fluorescence emission of INH at a shorter wavelength is caused by a locally excited state. A longer wavelength band is caused by intramolecular proton transfer in nonpolar/aprotic solvents, while it is caused by TICT emission in protic solvents. The direct and indirect bandgaps of the INH crystals were assessed using Tauc plots and the Kubelka–Munk algorithm. The efficacy of INH for the inactivation of E. coli and B. subtilis were examined through a plate colony counting assay, and the inactivation was 62.7 ± 0.2 % and 98.3 ± 0.4 %, respectively. The geometrical structure, Mulliken, MEP surface, and electronic properties of INH are theoretically optimized using density functional theory (DFT) at the B3LYP/6-31G(d,p) level and showed good concurrence with the experimentally defined structure. The INH displays higher NLO properties compared to urea as a reference, as determined by molecular hyperpolarizability analysis. The frontier molecular orbitals and global reactivity descriptors showed that INH is more reactive and reasonably stable.