Effect of trifluoromethyl substituents in benzyl-based viologen on the electrochromic performance: Optical contrast and stability

Abstract

Viologen (V) based molecules are classic and prestigious electrochromic (EC) materials which have dramatic optical contrast and tunable coloring states in solution-type electrochromic devices (ECDs). However, the stability of viologen based ECDs was significantly reduced by the dimer formation, and the resultant aggregation of viologens, which may occur once viologen dication (V2+) was reduced to viologen radical cation (V+•). To diminish this phenomenon, two novel symmetric benzyl-based viologens including 1,1′-bis(3,5-bis(trifluoromethyl)-benzyl)-4,4′-bipyridine-1,1′-diium (DTFMBzV) and 1,1′-bis(4-(trifluoromethyl)benzyl)-4,4′-bipyridine-1,1′-diium (TFMBzV) with various numbers of trifluoromethyl (CF3) substituents were synthesized and studied in this work. Substituent of CF3 in benzyl-based viologens not only enhance its optical property by increasing the π‒conjugation of bipyridine, but also avoid the aggregation by repulsing the viologen with fluorine atoms. The electrochromic performance of ECDs consisted of benzyl viologen (BzV/Fc ECD) and benzyl-based viologens with various numbers of CF3 substituents (DTFMBzV/Fc ECD and TFMBzV/Fc ECD) incorporated with ferrocene (Fc) were studied. According to the result of UV–vis absorbance spectra, the absorbances of benzyl viologen-based ECDs were strengthened at 399 and 605 nm by increasing the number of CF3 substituents. The DTFMBzV/Fc ECD gives a high transmittance change (ΔT) of 63.5% at 605 nm initially with a short response time (<3 s) by biasing the cell potential from 0 to 1.1 V. As for the stability, the DTFMBzV/Fc ECD exhibits an extremely high retention (97% of its initial ΔT) after switching for 10,000 cycles, which implied the dimerization of viologen was diminished by introducing the CF3 substituents with stronger electrostatic repulsion. As the result, DTFMBzV/Fc ECD exhibited the best electrochromic performance in terms of optical contrast and long-term stability. This study provides a simple yet effective strategy to tailing the electrochromic properties of viologen molecules for further application in electrochromism.