Transparent-to-black electrochromic smart windows based on N, N, N′, N′-Tetraphenylbenzidine derivatives and tungsten trioxide with high adjustment ability for visible and near-infrared light

N,N,N?,N?-tetraphenylbenzidine (TPB) and N,N,N?,N?-tetra(p-tolyl)benzidine (TPB-4Me), were synthesized. Their optoelectrochemical and electrochromic properties were discussed. Both TPB and TPB-4Me exhibited a reversible color change between colorless and dark-green under applied bias voltages. Furthermore, TPB or TPB-4Me and ethyl viologen (EV) or tungsten trioxide (WO3) were used as anode and cathode electrochromic materials, respectively; a series of color complementary ambipolar gel electrochromic devices (ECDs), ECD-3, ECD-4, ECD5, and ECD-6, were fabricated and investigated. All of ambipolar ECDs displayed reversible color changes between transparent and black under different voltages. In addition, they exhibited high optical contrast in both visible and near-infrared regions; their corresponding optical contrasts were 74.6%, 87.9%, 78.2%, and 78.5% at 750 nm, respectively. In comparison, ECDs based on WO3 exhibited higher optical contrast (in near-infrared region) and shorter bleaching time than ECDs based on EV. The optical contrasts of ECD-5 and ECD-6 based on WO3 were greater than 57.6% and 54.0% in the wavelength range of 420?1000 nm, respectively. Their corresponding coloring/bleaching times were found to be 6.6 s/21.7 s and 6.0 s/24.6 s, respectively. The transparent-to-black attributes of these ambipolar ECDs are promising for electrochromic smart windows.