Theoretical studies on geometrical properties and photochromic mechanism of two photochromic compounds

Two organic photochromic compounds containing pyrazolone-ring as photochromic functional unit (1-phenyl-3-methyl-4-benzal-5-pyrazolone thiosemicarbazone and 1-phenyl-3-methyl-4-benzal-5-pyrazolone 4-methylthioesmicarbazone) are investigated to deepen our understanding of geometrical properties and the mechanism of photoinduced intermolecular proton transfer by using DFT method. Bader's atom-in-molecule (AIM) theory is applied to investigate the nature of various hydrogen bonds and their relative strength. Good correlation between hydrogen bond length and electron density at the bond critical point is obtained. The oxygen atom of the pyrazolone ring and the sulfur atom in the thiosemicarbazone part are sites of the most negative concentration of the electrostatic potential and it is assumed that the oxygen atom should be the preferred site to accept the proton.