Molecular Orbital Delocalization/Localization-Induced Crystal-to-Crystal Photochromism of Schiff Bases without ortho-Hydroxyl Groups

Solid-state photochromic compounds can be used in molecular switch and memory, smart window, and so forth. However, a new photochromic mechanism was rarely reported. Photochromic Schiff bases without ortho-hydroxyl groups (thioamide hydrazones) have been prepared, which are drastically different from the traditional photochromic Schiff bases (e.g., salicylal anils and pyrazolone thiosemicarbazones). Systematic experimental and theoretical studies of thioamide hydrazone confirm the crystal-to-crystal photochromism and thermo-enhanced photochromism. The original faint yellow form changes to yellow after UV irradiation and can further be bleached by visible light irradiation. Besides, the decolored form can automatically change to yellow in dark at room temperature or by heating. The structures of the three forms (the original form, the colored form, and the decolored form) show that the intramolecular torsion and vibration lead to the increase/decrease of the donor (nitrogen atom)–acceptor (sulfur atom) distance of the intermolecular hydrogen bond (N–H···S). It further results in the delocalization/localization of the highest occupied molecular orbital, which is verified by the density functional theory calculation. Thus, the absorption intensity increase/decrease is attributed to the reversible switch between the π → π* and n → π* transition. The conclusion was further confirmed by an analogue compound without the thiocarbonyl group. The switch of the transition is accompanied by intermolecular orbital dehybridization/hybridization during the photochromism induced by light because the primitive cell contains more than one molecule.