Synthesis and fast switching electrochromic properties of conjugated microporous polymers based on carbazole-thiophene derivatives

To study the relationship between chemical structure and switching time of electrochromic polymer materials, four star-shape monomers, 3,6-di(thiophen-2-yl)-9-(4-(thiophen-2-yl)phenyl)-9H-carbazole (M1), 2,7-di(thiophen-2-yl)-9-(4-(thiophen-2-yl)phenyl)-9H-carbazole(M2), 3,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-(4-(thiophen-2-yl) phenyl)-9H-carbazole(M3) and 2,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-(4-(thiophen-2-yl)phenyl)-9H-carbazole(M4) were designed and synthesized successfully, their chemical structure were characterized by nuclear magnetic resonance (NMR) and high resolution mass spectrum (HRMS). Their resulting polymer films could be obtained by electropolymerization, all the polymer films showed well-defined electrochemical properties except for PM2, the PM3 and PM4 revealed wider electrochemical response due to the existence of EDOT units. The surface morphology results indicated that all the polymer films consisted of many particles, and PM1 and PM3 showed smaller grain than that of PM2 and PM4 due to the different aggregation structure, which might benefit for the transport of doped ions. Electrochromic kinetic studies confirmed such hypothesis furtherly, PM3 revealed fastest switching time (Tc: 0.72 s and Tb: 0.61 s), the reason could be explained by electrochemical impedance spectroscopy, which indicated that PM3 possessed the smallest charge transfer resistance (Rct) and fastest diffusion coefficient values at the oxidation state. We hope these findings will provide a little effect on designing and synthesizing high performance electrochromic materials and devices.