Enhanced electrochromic write–erase efficiency of a device with a novel viologen: 1,1′-bis(2-(1H-indol-3-yl)ethyl)-4,4′-bipyridinium diperchlorate

A novel cathodically coloring viologen electrochrome: 1,1′-bis(2-(1H-indol-3-yl)ethyl)-4,4′-bipyridinium diperchlorate (IEV), comprising of a 4,4′-bipyridyl core, sandwiched between two indole moieties, was synthesized using 3-(2-bromoethyl)-indole. An electrochromic device (ECD) was assembled using an electrolyte containing an imidazolium imide ionic liquid and characterized by a large electrical conductivity, thermal stability upto 150 °C, and an electrochemical potential stability range of ∼3.6 V. The IEV viologen was dissolved in the electrolyte and Prussian blue was used as the anodic electrochrome. The indole moieties of the IEV2+ salt owing to their electron donating tendency can act like bleaching agents and bleach the viologen faster (IEV+. → IEV2+) and this hypothesis was used for the improved write-erase efficiency of the device. The device switched between colorless and dark violet-blue hues under applied potentials of ±1.5 V. A large transmission modulation (52%, λ = 605 nm), a high electrochromic coloring efficiency of 533 cm2 C-1 at 605 nm and switching times of ∼2 s and good stability during 2000 cycles were reported herein. The electrochemical activity of the ECD improved when it was maintained at an elevated temperature of 70 °C, with no sign of thermal degradation. Furthermore, we also present the ability of this new viologen to function as an excellent redox mediator as we achieved an 86% enhancement in the power conversion efficiency of a solution processed solar cell, by its' addition in the electrolyte. Our studies demonstrate this new viologen to be a highly versatile electroactive material which can be useful for both electrochromics and photovoltaics