Water-Assisted Photoinduced Tautomerization in a Potential Bioactive Semicarbazone Schiff Base

This article epitomizes the design and synthesis of a potential bioactive Semicarbazone Schiff base (E)-2-(4-(diethylamino)-2-hydroxybenzylidene)hydrazine-1-carboxamide (DHHC) and its water assisted photoinduced tautomerization. DHHC is feebly emissive in non-polar solvents but the fluorescence intensity intensifies (at 406 nm) in alcohols possibly due to intermolecular H-bonding. However, in protic polar water, the scenario is markedly different, the fluorescence intensity decreases a bit as compared to alcohols and the emission maxima shifted from 406 nm to 430 nm probably due to solvent (water) assisted excited state proton transfer reaction (ESPT). In contrast to the reported excited state intramolecular proton transfer reaction (ESIPT) in various compounds, DHHC on photoexcitation loses a proton from its phenolic -OH group to the water molecule, which insists a subsequent proton transfer from the hydronium ion to the imine nitrogen of DHHC resulting 24 nm red-shifted fluorescence emission from the tautomeric species of DHHC, in aqueous media. The mechanism of ESPT reaction is explained theoretically through the reaction kinetics of the DHHC-water complex (in ratio 1 : 3) in an explicit CPCM solvent model. The illustration of the findings may open up enormous possibilities for Semicarbazone Schiff bases to be used as a probe for water sensing inside biomacromolecules like proteins and enzymes. The graphical abstract explains the water-assisted excited state proton transfer (ESPT) reaction in Semicarbazone Schiff base (DHHC). The sun provides the required excitation wavelength to excite the DHHC submerged in the pool of water, which then undergoes photoinduced tautomerization in the presence of water.image