Electrochromic Properties of Tetracyanohexatriene Push-Pull Chromophores

Tetracyanoethene (TCNE)-integrated π-systems exhibit strong donor-acceptor (D-A) interactions and display intriguing optical and electronic properties, making them ideal candidates for electrochromism. However, such studies have never been carried out. Here, we report the synthesis and electrochromic properties of novel 1,1,6,6-tetracyanohexatriene (TCHT)-based chromophores (TS1-3 & TA1-2). These compounds have been developed via double [2+2] cycloaddition-retroelectrocyclization (CA-RE) of various electron-rich alkynes (triphenylaminyl, ferrocenyl, and anisyl) with TCNE. The chromophores exhibit intense intramolecular charge transfer (ICT) resulting in strong colors, low-energy absorption extending into the near-infrared (NIR) region (∼900 nm), narrow band gaps (∼1.90 eV) and low LUMO levels (∼-3.5 eV). Upon applying the reduction potential of ∼-1.0 V, TS1-3 and TA1-2 exhibit reversible electrochromism in solution and the solid state with an associated color switching from dark blue↔purple, yellow↔pink, blue↔yellow and green↔blue, purple↔blue, respectively. The process is rapid with a switching time of 1-2 s and stability over 48 redox cycles. The study also reveals a unique electrochromic color tuning behavior of TS1 upon varying the solvent medium (CH₂Cl₂, THF, and CH₃CN) from blue to purple, and then to yellow. These findings establish TCHT-based chromophores as promising candidates for advanced electrochromic devices.