In-situ electro-polymerization of fluorescent electrochromic thin films based on charge-transfer complexes

Fluorescent electrochromic (FEC) materials, which reversibly change their both fluorescence and absorption via electrochemical reactions in response to applied potentials, have received extensive attention. Herein we directly obtained poly (2Z,2'Z)-3,3'-(1,4-phenylene)bis(2-(4((6-(9H-carbazol-9-yl)hexyl)oxy)phenyl)acrylonitrile) P (CHDCS) and poly (2Z,2'Z)-3,3'-(2,5-dimethoxy-1,4-phenylene)bis(2-(4((6-(9H-carbazol-9-yl)hexyl)oxy)phenyl) acrylonitrile) P(CHDCSM) as thin films on electrodes via electro-polymerization (EP) from linear donor-acceptor (D-A) monomers with isolated 7c-conjugation. Surprisingly, interchain charge-transfer complexes (CTCs) with mixed D-A stacks were generated between electroactive bis-carbazole (D) and luminescent dicyanodistyrylbenzene (A) units during the EP process. Consequently, significantly redshifted and bright orange fluorescence (similar to 600 nm) originated from intermolecular CT excitons were observed from both polymers, which could be reversibly switched upon applied potentials. This is the first time that the CTCs have been prepared via EP and applied as FEC materials. Moreover, the stronger intermolecular CT effect gives rise to an enhancement of electrical conductivity and FEC response in P(CHDCS), resulting in high optical contrasts (T = 33.6%, I-on/I-off = 28.3) and fast FEC switching (coloring 1.25 s, fading 0.53 s, quenching 2.40 s, brightening 6.17 s) as well as good reversibility. In addition, the CTCs improve the spin delocalization of bis-carbazole radical cations, leading to increased color stability of the FEC films by a decay of only 1.2% in 2 h with no power supply after coloration. This work not only affords an innovative CTC design strategy for FEC materials but also paves the way for constructing highly-performed and energy-efficient FECs via one-step electrochemical methods.

Automated summary

In this study, FEC materials with interchain charge-transfer complexes (CTCs) were prepared via electro-polymerization, showing good reversibility and optical performance.