Enhancing electrochromic and electrofluorochromic performance of molecular electrochromes by their covalent immobilization to nanoparticles

Abstract

Molecular electrochromes offer the benefit of structural diversification by well-known synthetic means. However, their electrochromic performance lags behind that of their conjugated polymer counterparts. The electrochromic performance of a combined molecular electrochrome/electrofluorochrome can be enhanced by its covalent attachment to ITO nanoparticles via a phosphonic acid linker. A key metric of its covalent attachment to nanoparticles via phosphonic ester and deposition on a transparent electrode is color homogeneity over the entire electrode. This is compounded by the consistent changes in optical transmission in the NIR (890 nm) over 8 hours of switching between the colored and bleached states. Switching between the two colored states is also accelerated upwards of twofold, taking ca. 10 s for >85% transmission percentage difference between each state with a contrast ratio of 6. The electrofluorochromism is also improved when covalently binding the intrinsically fluorescent electrochrome to ITO nanoparticles. A high contrast ratio of 94% is obtained between the intrinsic photoemission and the quenched fluorescence upon switching the applied potential when photoexcited at 450 nm. The fluorescence on/off ratio is maintained upwards of 2 hours of reversibly applying the potential with the electrofluorochromism decaying only 18%, twofold less than in an operating electrofluorochromic device with physisorbed molecular electrochromes.