A refreshing perspective on electrochromic materials: Phenoxazine as an opportune moiety towards stable and efficient electrochromic polyimides

In our attempt to develop appealing EC materials, here we address the electrochromic materials field by employing the aryl-substituted phenoxazine (POZ) as a new member of triarylamine family with excellent potential for electrochromic applications. Thus, a simple synthetic strategy was followed to build a novel POZ-based diamine that was further used in polycondensation reaction with commercially available aromatic dianhydrides to develop three novel polyimides (PIs). Complementarily, POZ-based model compounds were synthesized to get insights into the electrochemical and electrochromic behavior of the synthesized PIs. With the support of the computational quantum mechanical modeling methods (Density-Functional Theory – DFT and Time-Dependent (TD)–DFT), the mechanism responsible for the absence of the dimerization process upon POZ oxidation was established. Without resorting to any structural modification to block the para position of POZ, the present PIs demonstrated exquisite electrochemical and electrochromic performance, including high redox stability during 100 repetitive cyclic voltammetry scans, unique deep-purple coloration during oxidation, high optical contrast, up to 62%, response time below 6.5 s for coloring and 5.1 s for bleaching, excellent electrochromic efficiency, up to 193 cm2/C and a maximum 21% decay of the electrochromic efficiency after 500 cycles. The most efficient EC polyimide was used as an active layer in electrochromic prototype devices to survey the full applicative potential of these PIs. The devices rendered an optical contrast of 51%, 7.9 s for coloring, and 7.2 s for bleaching, electrochromic efficiency of 178 cm2/C, and decay of the electrochromic efficiency of 6.1% after 100 cycles, placing for the first time the POZ-based PIs among the perspective electrochromic materials.