Donor-acceptor type dithienylethenes derivatives based on anthraquinone and anthracene moieties: Synthesis, photochromic properties and theoretical calculations

Developing novel photoelectric functional materials based on dithienylethenes (DTEs) building block has drawn increasing attention due to their excellent photochromic properties. In this contribution, two D-A type DTE derivatives 3 and 4 bearing triphenylamine (TPA) and anthraquinone (or anthracene) groups have been synthesized, and their structures have been characterized by means of NMR and MS. The UV–vis spectroscopy results show that compounds 3 and 4 exhibited reversible photochromism in toluene and THF solutions under UV/vis light irradiation, featuring good fatigue resistance and responsiveness. However, almost no photochromic properties for DTEs 3 and 4 were observed in larger polar DMSO. Interestingly, DTE 3 with obvious push-pull characteristics also displayed visible-light driven photochromic behavior in toluene. Meanwhile, fluorescence spectroscopy experiments have shown that DTE 4 with the anthracene fluorescent group showed fluorescent switching properties upon UV/vis light irradiation in toluene and THF solutions. The electrochemical results revealed a certain decrease in the half-wave potential of TPA-based 3 and 4 after UV irradiation, suggesting that the increasing conjugation degree of the ring-closed isomers results in the increase of HOMO orbital energy and the decrease of HOMO-LUMO gap and is beneficial for the occurrence of oxidation process, which are further proved by DFT calculations. TDDFT predicts that the transition absorption of the ring-closed isomers 3c and 4c respectively at 615 nm and 651 nm mainly come from π-π* mixed with TPA→anthraquinone CT or TPA→anthracene CT transition, which is consistent with the experimental results.