Fluorinated benzyl viologens for enhanced electrochromism and remarkable stability in electrochromic devices: An in-situ mass exchange probing through EQCM

Benzyl viologens (BVs) have garnered massive attention due to their remarkable functional ability as a cathodically coloring material and glare reduction agent in electrochromic devices (ECDs). However, BV-based ECDs have some challenges, of which dimer formation due to viologen radicals is foremost, as it extensively damages the device performance. Herein, a new approach is proposed to address this issue in which five novel fluorine-substituted benzyl viologen (FSBV) are synthesized through a multi-step route based on the fluorine-atom count on the viologen moiety. Once synthesized, the FSBVs (0.01 M) were employed as an electrochromic material (ECM) along with tetrabutylammonium tetrafluoroborate/propylene carbonate (0.5 M TBABF4/PC) as a supporting electrolyte and ferrocene (0.05 M Fc) as a counter electrode material in an ECD assembly. We observed that the family of FSBV-based ECDs demonstrated suppressed dimerization tendency, higher optical contrast, fast response time, and enhanced cycling stability compared to the pristine benzyl viologen-based device (BV/Fc ECD). In particular, PFBV/Fc ECD (from the FSBV family), that utilizes 1,1ʹ-bis(2,3,4,5,6-pentafluorobenzyl)-4,4ʹ-bipyridine 1,1ʹ-diium tetrafluoroborate (PFBV) as an ECM, exhibited higher optical contrast (ΔT) of ∼63.6%, high coloration efficiency of ∼304 cm2/C, fast switching time of ∼1.2 s and an excellent ΔT retention of ∼97% after 10,000 cycles, all at 603 nm. The exciting feature of this study lies in the deployment of a highly sensitive and non-invasive technique, electrochemical quartz crystal microbalance (EQCM), to monitor the ultrasmall mass transmutations caused by synthesized chromophores at the electrode/electrolyte interface. The EQCM analysis revealed that the molecular structure of maneuvered viologen derivatives has a decisive role in determining the mass exchange behavior at the quartz crystal surface, which is reflected in a dissimilar degree of electrochromic performance.