Photophysical and electrochemical properties of newly synthesized thioxathone–viologen binary derivatives and their photo-/electrochromic displays in ionic liquids and polymer gels

Abstract

Thioxanthones (TXs) and viologens (VIOs) are important photo-sensitizers and electron mediators with interesting opto-electrochemical properties and potential applications for novel opto-electrical devices, displays, switches and sensors. Here, we reported the photophysical and electrochemical properties of newly synthesized TX–VIO binary derivatives, and their photo-/electrochromic displays in the ionic liquids and polymer gels. The precursor of TXBr (without the VIO substituent) gave off a blue light emission centered at about 437 nm with a quantum efficiency (QE) of approximately 0.2. For TX-VIOs, their QE was below 0.01; this significant luminescent quenching was attributed to an intramolecular energy transfer from the excited singlet state of TX to electroactive VIO. Thus, these TX–VIOs displayed good photochromic behaviors under ultraviolet and/or visible light radiation. Cyclic voltammograms of TX–VIOs showed two couples of chemically reversible waves centered at approximately −0.43 and −0.90 V (vs. Hg/Hg₂Cl₂). The former one was attributed to the electron transfer process of TX–VIO²⁺ ↔ TX–VIO⁺˙, and the latter one to that of TX–VIO⁺˙ ↔ TX–VIO⁰ together with that of TX^(red)–VIO²⁺ ↔ TX^(ox)–VIO²⁺. Finally, sandwiched types of transparent and flexible photo- and electrochromic devices were constructed with the use of TX–VIOs in the ionic liquids and polymeric gels as the light-sensitizer and color display layer. These devices displayed a good transmittance and good reversible color (pink, blue and colorless) change behavior under visible light radiation or an applied bias of −2.4 V.