Solvent and substituent effects on ultraviolet absorption properties of Schiff bases—models of molecular-core of mesogenic compounds

Solvent and substituent effects on the ultraviolet absorption properties of Schiff bases were targeted both experimentally and computationally. A wide set of X-PhCH = NPhCH = CHPh-Y were prepared for this study, and their maximum absorption wavelength ( λ max ) and the corresponding energy (υ max ) in thirteen solvents were measured. The results show that the λ max of the compounds ranges from 347 to 388 nm. With increase of electron withdrawing or electron donating ability of X or Y, a bathochromic shift is observed. Furthermore, the λ max values are sensitive to the properties of solvents, while the polarity of solvents alone fails to account for the trends of solvatochromism . Computational analyses based on density functional theory were conducted to elucidate the phenomena. The polarization effect of solvent on solute can be used to measure the effect of solvent, and polarizability parameter Δη is proved to be more effective to scale the solvent effects on UV absorption of the compounds. Finally, a quantitative model has been developed to express the effects of substituent and solvent on υ max . The derived model enables practical expression of the relationship between substituents, solvents and UV absorption.