Thermally Cross-Linked Copolymer for Highly Transparent to Multicolor-Showing Electrochromic Materials

A triphenylamine derivative STPA was designed and synthesized by introducing thermally cross-linked vinyl group at the end of 4-methoxytriphenylamine. The corresponding polymer pSTPA was prepared by a thermal polymerization reaction without any initiator. CV tests reveal that pSTPA has a pair of distinct redox peaks. Spectroelectrochemical results demonstrates that pSTPA has a maximum absorption peak at 354 nm and a new absorption peak at 730 nm with increasing voltage, accompanied with the polymer from transparent to blue. In order to realize multicolor showing and improve the cycling stability, the corresponding copolymer pSTPA-co-TPA-OCH3 was further prepared via thermally cross-linked vinyl groups by combining STPA with methoxy-modified triphenylamine derivative TPA-OCH3 monomers in a molar ratio of 1:1. As expected, the copolymer film presents a multicolored appearance with four color changes (transparent–dark yellow–sky blue–purple), high optical contrast (45.0% at 480 nm, 56.1% at 700 nm, and 82.4% at 910 nm, respectively), large coloration efficiency of 287 cm2/C (at 910 nm), and good cycling stability (essentially no degradation of optical contrast over 2000 cycles). Therefore, this work puts forward an effective method to achieve a highly transparent to multicolor-showing electrochromic polymer via thermally cross-linked copolymerization, which shows potential applications in smart windows, sunglasses, and electronic tags.