Near-infrared and multicolored electrochromism of solution processable triphenylamine-anthraquinone imide hybrid systems

•A series of donor-acceptor molecules based on TPA and AQI-derivatives were prepared.•The energy level and band gaps of these molecules could be effectively tuned by alternating the number of AQI arms.•Multicolored electrochromism was achieved electrochemically by tuning the intramolecular charge transfer interaction.•NIR electrochromism was observed and detailed transitions of these NIR absorptions were calculated.

A series of novel donor-acceptor molecules (TPA-AQI, TPA-AQI2 and TPA-AQI3) with various number of electron-accepting anthraquinone imide (AQI) arms connected to the electron-donating triphenylamine (TPA) core were synthesized and characterized by UV–vis absorption spectroscopy, cyclic voltammetry and spectroelectrochemical measurements. The geometries of these molecules in ground-state as well as the electronic absorption properties on the basis of the optimized geometries were investigated by theoretical calculations. For all the three molecules, there are two main absorptions in the range of 300–450 nm and 450–700 nm, respectively. The former corresponds to ππ* transitions and the latter a nature of intramolecular charge transfer (ICT). Hypsochromic shift of the ICT absorption was observed from the single armed molecule TPA-AQI to the tribranched molecule TPA-AQI3, which may result from the increased steric hindrance between the donor and acceptor segments. Electrochemical studies revealed the ambipolar properties of these molecules and three redox couples, with two quasi-reversible redox couples appearing in the cathodic regime and one redox couple in the anodic regime, on CV curves. CV results also indicated the stabilization of the HOMO levels by increasing AQI arms. Spectroelectrochemical measurements demonstrated the near-infrared (NIR) and multicolor electrochromism of these molecules. When reduced to radical anions or oxidized to radical cations, intense NIR absorptions were observed accompanied with color changes of the solutions. As for TPA-AQI, there existed four different colors at different redox states: Indian-red at the neutral state, bluish green at the radical cationic state, olive green at the radical anionic state, and dark blue at the dianion state. The detailed transitions of the observed NIR absorptions were also discussed.

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