Visible to near-infrared broadband electrochromic switching of π-extended viologen exhibiting highly efficient photothermal conversion

This study describes a new π-extended viologen molecule (TFBV) characterized by an electron-deficient structure inserted between two pyridinium groups. The single-layered electrochromic devices (ECDs) using TFBV all-in-one ion gel exhibit remarkable transparency under neutral conditions and display strong absorption bands in the visible and near-infrared (NIR) regions. The TFBV ECDs undergo a black coloration at a low operating voltage (−1.4 V), and at higher voltages (>−1.5 V), they display notable absorption in the NIR-II region. The two-electron reduction to neutral TFBV was confirmed through the chemical reduction of TFBV using Na2S, monitored through UV–vis spectra and 1H NMR titrations. The TFBV ECDs demonstrate an impressive color contrast of up to 96 % and a coloration efficiency of 150.18 cm2/C. The ECDs show outstanding kinetic stability through the 2000th cycle and moderate switching stability up to the 10000th cycle, accompanied by fast coloration times. Flexible TFBV ECDs, built on ITO PET, sustain superior performance even during flat and semicircle bending maneuvers, ensuring stability across repeated bending cycles. A uniform color transition is achieved in large-area ECDs of 10 cm × 10 cm. The strong NIR absorption of TFBV ECDs enables the photothermal heater to harvest the heat from the light energy, inducing a temperature rise of 45 °C. Furthermore, these devices perform a photothermal conversion efficiency of 52.5 % during multiple on–off cycles. TFBV ECDs, offering high transparency and intense darkness while blocking UV–Vis-NIR spectra, prove promising candidates for broadband ECDs with integrated photothermal conversion capabilities.