Effect of substituents of naphthalene diimide derivatives on electrochromic behaviours observed in proto-type devices

Several triphenylamine end capped to substituted central naphthalenetetracarboxylic diimide based four donor - acceptor - donor type electro-active monomers (BRTPANDI, CNTPANDI, PHTPANDI, NPYTPANDI) were designed and developed to explore the effect of substituent on the formation of electro-polymers and subsequent the chromic effect of prepared films on the conductive surface. Initially, with the help of density functional theory (DFT) studies, it was observed that HOMO was located over triphenylamine unit, responsible electro-polymerization process upon oxidation, LUMO was residing on the central naphthalene core, and all substituents were not coplanar with naphthalene moiety. Also, the band gap energy was gradually decreased with the effect of strong electron withdrawing substituents on NDI core. In three electrode configuration, reversible multiple colour changes of brown to deep blue by applying voltage 0 to 1.1 V and also brown to deep pink with the voltage change of 0 to −2 V were impressively investigated with relatively good response times, optical contrast, switching stabilities, and coloration efficiencies. Polymer made of CNTPANDI might be switched upto 1200 cycles with the optimum colouration efficiency 560 cm²/C in the anodic process and 300 cycles for the cathodic process in a three electrodes configuration. Proto type devices made of p-CNTPANDI demonstrated the electrochromism operated at a potential range of 0 to 2.2 V. Besides, the device exhibited the EC memory in open-circuit condition with 50% retention of its coloured state until 27 min. The long EC memory as well as high colouration efficiency compared to other TPA based conjugated polymers suggested the potentiality of those polymer films as a power-efficient EC material for modern display applications.