Colorless Electrochromic/Electrofluorochromic Dual-Functional Triphenylamine-Based Polyimides: Effect of a Tetraphenylethylene-Based π-Bridge on optoelectronic properties

Electrochromic/electrofluorescence (EC/EFC) bifunctional polymer materials with electrochemically switchable colors and fluorescence intensities have attracted significant research attention due to their promising applications in optoelectronic devices. Tetraphenylethylene (TPE) can improve nature-state transmittance and exhibit solid fluorescence and cyclic stability when used as a π-bridge and fluorescence structure in EC/EFC polymer main chains. Herein, 6 novel polyimides with three different TPE-based π-bridge structures were designed and synthesized to investigate the effect of π-bridge units (short, extended, and interrupted conjugation) on optoelectronic properties. The structure–property relationships of the six polyimides were investigated by electrochemistry, electrospectroscopy, and theoretical calculations. Introduction of the TPE structure effectively reduced the polyimide absorption in the ground state, provided different colored states, and induced aggregation-induced emission (AIE) by changing the conjugation type. The structural influences on the electron redox reaction, energy level, absorption/luminescent spectra, and cycle stabilities were systematically investigated. The designed polyimides were colorless (L* > 97) and showed good EC/EFC dual-switching properties: colorless-to-colorful EC switching performance (including a black-colored state), AIE-enhanced EFC performance (including deep-blue fluorescence), 1.7/2.1 s colored/bleached switch, and EC/EFC cyclic stability over 500/300 cycles.