Effect of Hyper-Cross-Linking on the Electrochromic Device Properties of Cross-Linkable Carbazole–Diphenylamine Derivatives

Triphenylamine derivatives are promising organic electrochromic (EC) materials due to their easy synthesis, low oxidation potentials, high charge-carrier mobilities, electrochemical stability, coloration efficiency, and tunability of EC properties via substitution. Further, the cross-linking strategy provides an added advantage to the small-molecule-based EC devices in terms of better-quality EC films with enhanced EC properties. Herein, we discuss the EC properties of two carbazole–diphenylamine derivatives, C-Sty2 and C-Sty3, with two and three thermally cross-linkable styryl units, respectively, following a donor−π–donor (D−π–D) design, where both the carbazole moiety and cross-linking styrene groups modulate the photophysical and EC properties. Both styryl derivatives formed rigid, uniform, and transparent films with high solvent resistance and exhibited excellent thermal and electrochemical stability upon cross-linking. C-Sty3, with three cross-linkable styryl units, could form more extended cross-links, leading to hyper-cross-linked films with distinct, regular, and porous morphology compared to C-Sty2. Spectroelectrochemical studies of the films showed a color change from a transparent colorless state to an initial light-yellow color and then a final dark-blue color with a color contrast of 65% at 890 nm for C-Sty2 and 78% at 850 nm for C-Sty3. The hyper-cross-linked films of C-Sty3 demonstrated enhanced coloration efficiency (248 cm2/C), optical contrast, and open-circuit memory compared to C-Sty2. Electrochemical impedance spectroscopy analysis showed lower solution and charge-transfer resistances (Rs and Rct, respectively) for hyper-cross-linked films of C-Sty3, indicating higher conductivity and ion diffusion compared to C-Sty2. Thus, a comparison of the EC properties of two polymers with the same electroactive groups reveals the significance of hyper-cross-linking in the EC properties of these cross-linked polymers. Furthermore, the EC properties of the hyper-cross-linked D−π–D derivative, C-Sty3, were compared to those of previously reported molecules, demonstrating the importance of highly branched conducting polymers for EC applications.