Mechanism of 2, 3-difurylmaleic anhydride photochromic molecular switch

Abstract

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^–1(3.0945 eV) and 23328 cm^–1(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 S₀), 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. S₁ 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 S₁–S₀ resonance transition wavelength, the C-DFMA transits from S₀ to S₁ state, and then deactivates along the S₁ potential energy curve, until a cross jumping transition occurs at the TS-DFMA structure from S₁ to S₀ and finally the molecule along the S₀ potentioal energy curve returns to the O-DFMA configuration, then the switching action from closed-ring to open-ring is completed. The S₁ 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 S₁–S₀ resonance transition wavelength, O-DFMA transits from S₀ to S₁ state. From the O-DFMA to TS-DFMA structure, there is a relatively “flat” area in the potential energy curve of the S₁ 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 S₁ and approaching to the TS-DFMA configuration, and then DFMA de-excites from the S₁ state potential energy curve along a monotonic decline and a cross jumping transition from S₁ to S₀ 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 S₁ 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.