Two Zinc-Viologen Interpenetrating Frameworks with Straight and Offset Stacking Modes Respectively Showing Different Photo/Thermal Responsive Characters

Abstract

Two photo/thermal responsive donor–acceptor molecular systems have been designed and synthesized based on interpenetrating metal organic frameworks (MOFs). For compound 1 [Zn­(CPBPY)­(HBTC)]·H2O, the formation of a 4-fold interpenetrating diamond structure displays reversible photochromic behavior upon light/heat stimuli with an offset π stacking figure for the donor carboxylate and acceptor viologen units. By comparison, the 2-fold parallel interpenetrating framework in compound 2 [Zn2(CPBPY)2(BTEC)]·2H2O·2CH3CH2OH exhibits photo coloration and thermal coloration behaviors accompanying long-lived charge separation upon light/heat stimuli with a face-to-face π-stacking figure for the donor carboxylate and acceptor viologen units. A faster response rate and stable charge separation status are observed in the system of 2 due to its large π-conjugation molecular system formed by the sandwich type π–π stacking structure. This example of structure-controlled photoresponsive systems shows a way to drive toward tuning functional system features via the design of a molecular self-assembly approach.

A Multistimuli Responsive Crystalline Cd(II)-Viologen Coordination Polymer with Single-Crystal–Single-Crystal Transformation

“…However, when heated at 130 °C, two new bands centered at about 401 and 550 nm were observed (Figure a), and their intensities increase gradually with the duration of heating, and finally tend to be saturated when heated for 40 min. When heated at 180 °C, two new bands at about 401 and 617 nm appeared (Figure c), which resembles to the reported viologen-based chromic compounds. − To further confirm the generation of the radicals after heating, the electron paramagnetic resonance (EPR) spectra (Figure b,d) were measured before and after heating at 130 and 180 °C. There are no EPR signals before heating, but after heating at 130 and 180 °C, a single line signal with g = 2.0053 and 2.0060 appears, respectively, indicating the formation of viologen free radicals.…”

A Photochromic Zinc–Viologen Framework with a High-Contrast Nonlinear Optical Switchable Behavior

“…In general, free radicals may be generated in the chromic process of viologen-based materials when the electron donor–electron acceptor interactions can satisfy the requirement for electron transfer from the donors to the acceptors. In general, the distance from the donor atom to the acceptor atom should be about 3.7 Å or shorter, which is suitable for photoinduced electron transfer …”

Versatile Photo/Electricity Responsive Properties of a Coordination Polymer Based on Extended Viologen Ligands

“…The coordination polymers (CPs) constructed from metal clusters and organic ligands could provide cooperation between components and the inter−contact structural framework, which can create effective channels for ET, allowing for the transition of viologen units between different redox states under stimuli [ 16 , 17 , 18 , 19 , 20 ]. As a consequence, taking advantage of the great versatility afforded by coordination chemistry, nonphoto−responsive viologen ligands may not only result in photo−responsiveness, but could also integrate photochemical and electrochemical functionalities [ 20 , 21 ]. The viologen derivatives, which consist of two pyridinium rings spaced by conjugating aryl groups, named “extended viologen” (ExV), display similar redox features to the viologen units [ 20 ].…”