Low temperature X-ray diffraction analysis, electronic density distribution and photophysical properties of bidentate N,O-donor salicylaldehyde Schiff bases and zinc complexes in solid state

Zinc complexes (ZnL2) with bidentate N,O-donor Schiff bases (HL = HSH1, HSH2, HSH3) derivatives of salicylaldehyde and para-substituted anilines were synthesized and fully characterized. The crystallographic parameters of HSH2 and ZnL2 were refined. Low temperature X-ray diffraction analysis including topological analysis of electron density function for crystals of ZnL2 reveals that the geometrical parameters of all investigated compounds as well as the charges in chelate rings of complexes remain almost unchanged upon variation of the substituents (R) in para-position of the aniline fragment, going from -CH3 (HSH1) to -N(CH3)2 (HSH2) and -OCH3 (HSH3). Zinc complexes are thermally stable up to ∼190 °C as shown by thermogravimetric analysis under nitrogen atmosphere. The photoluminescent (PL) properties of HL and ZnL2 were investigated in the solid state while its absorption spectra were measured in methanol. The theoretical calculation of the investigated compounds confirmed firstly the existence of enol-imine and keto-enamine tautomeric forms; and secondly the influence of the introduction of R on the PL properties of the compounds. The study of excited state dynamics of ZnL2 in the solid state shows bi-exponential fluorescence decays and relatively high PL quantum yield, making them potentially adequate for being used as manifold optical materials. The interpretation of photophysical properties are substantiated by DFT and TD-DFT theoretical calculations of the Schiff bases and zinc complex species.