Reversible vis-NIR electrochromic/electrofluorochromic switching in dual-functional devices modulated by different benzothiadiazole-arylamine anodic components

Abstract

Redox active materials, whose optical emission and absorption spectra are both electrically switchable, are said to be dual functional electrochromic and electrofluorochromic materials. They are intriguing for a wide range of applications, e.g., displays, smart-windows, sensing, information storage, and encryption/anticounterfeiting devices. Herein, we investigated the performance of benzothiadiazole-arylamine compounds serving as either anodic components or electroactive fluorophores, in dual functional electrochromic/electrofluorochromic solid state devices, fabricated as all-in-one ITO/gel/ITO sandwiches. We systematically investigated the electrochromic and electrofluorochromic responses of the devices, by varying the anode among a set of structurally different benzothiadiazole-arylamines and using the ethyl viologen as a cathodic component. All the devices show an interesting vis-NIR electrochromism with a pink/orange to deep dark color switching, arising from the superposition of the electrochromic bands of the viologen and arylamine, with contrasts up to 36%/75% in the NIR/vis ranges and switching times from fractions of a second up to several seconds. Moreover, they show a panchromatic fluorescence from about 450 nm up to 850 nm, due to the intramolecular charge transfer character of the emission typical of these arylamine–benzothiadiazole-arylamine compounds with donor–acceptor–donor architecture. Notably, the relatively strong fluorescence of the devices (fluorescence quantum yields up to 38%) due to the aggregation induced emission (AIE) of the fluorophores in the gel (fluorescence enhancement of up to 63 times with respect to the solution phase with comparable polarity) undergoes a voltage-dependent quenching, with electrofluorochromic contrast ratios of up to 9, and a shift of the emission from NIR/red to yellow. Such an electrofluorochromic response is due to an uneven fluorescence quenching across the whole emission band, the twisted charge transfer states being majorly quenched at lower energy (red-NIR range). Interestingly, the voltage threshold for achieving the above electrochromic and electrofluorochromic responses increases with the first oxidation potential of the anodic component. Mechanistic insights provided by electrochemical impedance spectroscopy clearly show that the above threshold corresponds to the onset for electron injection/ejection at the cathode and anode, respectively, and depends on the difference between the viologen reduction potential and the oxidation potential of the arylamine. Finally, the devices show high stabilities with more than 2000 life cycles.