jp8b07408_si_002.cif (690.75 kB)
Molecular Orbital Delocalization/Localization-Induced Crystal-to-Crystal Photochromism of Schiff Bases without ortho-Hydroxyl Groups
dataset
posted on 2018-09-09, 00:00 authored by Sheng Ding, He Lin, Yuming Yu, Lang Liu, Caiming Deng, Jianzhang Zhao, Dianzeng JiaSolid-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.