Mechanism of 2, 3-difurylmaleic anhydride photochromic molecular switch
Authors: Zhang Shu-Dong , Wang Chuan-Hang , Tang Wei , Sun Yang , Sun Ning-Ze , Sun Zhao-Yu , Xu Hui ,   School of Physics and Physical Engineering, Qufu Normal University, Qufu  273165, China
Published: June 2021

The photochromic switching mechanism of 2,3-difurylmaleic anhydride (DFMA) is investigated by first-principles calculations. Based on the stable structures of the open-ring (O-DFMA) and closed-ring (C-DFMA) of the DFMA, the minimum energy path (MEP) and the configuration of transition states (TS-DFMA) between the O-DFMA and C-DFMA are found by using the nudged elastic band (NEB) method, the potential barriers of O-DFMA and C-DFMA are 24959 cm<sup>–1</sup>(3.0945 eV) and 23328 cm<sup>–1</sup>(2.8923 eV), respectively, indicating that the DFMA molecule may be a thermally bistable molecule. Along the molecular configuration corresponding to the MEP curve (i.e. ground state S<sub>0</sub>), the potential energy curves of the lowest 8 singlet excited states of DFMA are calculated. Among these energy curves, only the first electronic excited state (i.e. S<sub>1</sub> state) has a minimum value in the transition state (TS-DFMA) configuration. Combined with the molecular orbital transitions and orbital images, the photochromic mechanism of DFMA can be described as follows (1) From C-DFMA to O-DFMA process: under the action of the laser with S<sub>1</sub>–S<sub>0</sub> resonance transition wavelength, the C-DFMA transits from S<sub>0</sub> to S<sub>1</sub> state, and then deactivates along the S<sub>1</sub> potential energy curve, until a cross jumping transition occurs at the TS-DFMA structure from S<sub>1</sub> to S<sub>0</sub> and finally the molecule along the S<sub>0</sub> potentioal energy curve returns to the O-DFMA configuration, then the switching action from closed-ring to open-ring is completed. The S<sub>1</sub> state potential energy curve drops monotonically in this switching process, implying that there will be no fluorescent radiation in this process. (2) From O-DFMA to C-DFMA process: under the action of the laser with S<sub>1</sub>–S<sub>0</sub> resonance transition wavelength, O-DFMA transits from S<sub>0</sub> to S<sub>1</sub> state. From the O-DFMA to TS-DFMA structure, there is a relatively “flat” area in the potential energy curve of the S<sub>1</sub> state, and it decreases significantly only when it is close to the TS-DFMA. This means that O-DFMA needs to be excited with some vibrational modes to pass through the “flat” region of S<sub>1</sub> and approaching to the TS-DFMA configuration, and then DFMA de-excites from the S<sub>1</sub> state potential energy curve along a monotonic decline and a cross jumping transition from S<sub>1</sub> to S<sub>0</sub> occurs in the TS-DFMA configuration, completing the switching action from open-ring to closed-ring. It is also precisely because of the flat region of the potential energy curve of the initial S<sub>1</sub> state that this excitation and switching process is accompanied by fluorescent radiations. The photochromic mechanism of DFMA indicates that it is suitable for making fluorescent molecular switches.