Electrochromic materials based on tetra-substituted viologen analogues with broad absorption and good cycling stability

The design and synthesis of broad absorption electrochromic materials (ECMs) that modulate visible and near infrared light are important for energy-saving smart windows . Most studies on broad absorption ECMs have focused on transition metal oxides , organic polymers and small organic molecules based on triarylamines, while studies on viologen analogues are scarce. In the current study, two tetra-substituted viologen analogues 4,4′,4″,4‴-(benzene-1,2,4,5-tetrayl)- tetrakis(1-butylpyridin-1-ium) (BTTBP) and 1,1,2,2-tetrakis(4-(1-butyl-pyridin-4-yl)- phenyl)ethane (TBPPE) were prepared by introducing benzene and tetraphenylethylene with large conjugation degrees as parent moieties. Compared with the conventional viologen 1,1′-dibutyl-(4,4′-bipyridine)-1,1′-diium without a conjugated parent, BTTBP and TBPPE exhibited high electrochromic performance and broad absorption to the near infrared region . TBPPE in particular exhibited excellent broad absorption electrochromic behavior with high optical contrast (63% at 733 nm and 46% at 997 nm), distinct color change (pale yellow to yellow-green to black-green), good cycling stability (attenuation in optical contrast of 1% after 1000 cycles at 733 nm) and excellent coloration efficiency (251 cm 2 /C) in the near infrared region. These findings provide a strategy for designing high-performance cathode broad absorption ECMs and a promising candidate for energy-saving smart windows.