In Situ Study on the Stimuli-Responsive Chromic Mechanism of Different Intramolecular/Intermolecular Electron Transfer in a Viologen-Based Zn-MOF

Viologen materials have attracted much attention due to the sensitive stimulus response to color change under appropriate external stimuli, yet the chromic mechanism has rarely been explored in depth. Herein, a novel viologen-based Zn-MOF of {[Zn(Vio)(o-PTA)]·2H2O}n has been synthesized by a newly designed viologen derivative (HVio·Br), phthalic acid (o-H2PTA) and Zn2+ ions, exhibiting a sensitive 5-fold responsive color change (photo-/chemo-/hydro-/thermo-/electrochromism). Due to the introduction of an acetophenone substituent on Vio, it can serve as both an electron acceptor and an electron donor. Therefore, Zn-MOF exhibits a photoinduced blue through intramolecular electron transfer (ET), and NH3 stimulated orange through intermolecular ET. o-PTA2– as another donor can form a new donor–acceptor (D–A) system with Vio, which helps to achieve thermally and electrically induced purple through intermolecular ET. All chromic behaviors are visible to the naked eye and have superior reversibility and cycling stability. The different chromic mechanisms have been first studied in situ by crystal structure and X-ray photoelectron spectroscopy (XPS) results before and after color change. Zn-MOF also displays dynamically adjustable fluorescence intensity by UV exposure time, temperature, and acid–base vapors, making it applicable for high-security information applications. This work provides valuable insight for the rational design of next-generation multiresponsive chromic materials.