Electrochromic near-infrared charge transfer and Lewis acid–base pairing for color tuning

The virtue of molecular electrochromes is the effect of molecular structure on their absorption with applied potential can accurately be established. This has allowed general design rules of electrochromes to be developed, and they include a redox active moiety conjugated with a conjugated chromophore. Indeed, conjugated benzothiadiazoles with triarylamines have emerged as ideal electrochromes owing to their spectral changes in both the visible and the NIR regions with applied potential. Toward further modulating the color of both the bleached (off) and electrochemically colored (on) states, unsymmetric 3- and 4-pyridines conjugated with benzothiadiazoles were investigated as electrochromes. The color could be modulated with absorptions at 445, 805, and 1525 nm. The absorption could further be adjusted by 65 nm by coordinating pyridine with tris(pentafluorophenyl)borane (BCF). The near-IR charge transfer at ca. 1525 nm could be enhanced with applied potentials of 0.6 V for both pyridine regioisomers upon depleting the radical cation, whereas the charge transfer in the absorption spectrum was quenched with the addition of methylviologen for both regioisomers and with BCF for only the 3-pyridine electrochrome.