Fluorene-Bridged Viologen Derivatives Exhibiting Strong Fluorescence and Electrochromic Properties

Abstract

In this study, fluorene was integrated as a bridging moiety within the pyridine ring to synthesize a bifunctional viologen derivative exhibiting both electrochromic and photoluminescent properties. The impact of substituents on the performance of the viologen derivative was also investigated. Upon application of an external voltage, electrochromic devices based on the viologen derivative demonstrated reversible switching between colorless and green states, achieving a remarkable optical contrast of ΔT = 70.2%. The coloring duration is remarkably brief, measured at τc = 6.5 seconds, while the optical contrast (ΔT) consistently maintains a value of approximately 70.2% even after an extended period of 3000 seconds, indicating exceptional stability. Additionally, the incorporation of the fluorene bridging group endows the viologen with enhanced fluorescence properties. It is noteworthy that both solvent polarity and standing time exert significant influences on the fluorescence intensity of the fluorene-viologen. As the solvent polarity increases, the fluorescence intensity of the fluorene-viologen derivative initially enhances and subsequently diminishes. After a period of 5 days, the fluorescence intensity of this derivative reaches its peak, approximately five times greater than its initial value. This fluorene-viologen derivative exhibits promising potential for applications in areas such as smart windows, flexible displays, energy storage, and beyond.

Keywords: Viologen Derivative, Fluorene, Bridging, Electrochromism, Photoluminescence