Electron-Deficient Indenofluorene-Based Systems: Multicolor and Visible-To-Near-Infrared (NIR) Electrochromism and OFF-OFF-ON Electrofluorochromism.

Abstract

Multifunctional electrochromic systems with divergent functions are highly sought after but rare and challenging to develop. Here, we report the synthesis and multi-functional electrochromic behaviour (multi-colour modulation, visible-to-near IR absorption switching, and electrofluorochromism) of electron-deficient indenofluorene-based systems. The compounds (IF₁-IF₅) have been obtained via Knoevenagel condensation of indenofluorenone (IF_K) with various nucleophiles (malononitrile, benzylcyanide, dimethyl malonate, methyl cyanoacetate, benzothiazole-2-acetonitrile). IF₁-IF₅ possess strong colours and absorb in the visible region (λ_max=295-388 nm). These compounds are electroactive and exhibit two reversible reduction waves (-0.35--0.88 V and -0.65--1.30 V vs Ag/AgCl), generating radical anion and dianion systems. For the electrochromic studies, upon applying the one-electron reduction potential (-0.35 V vs Ag/AgCl), the absorption maximum of IF₁ redshift from 382 nm to 1240 nm (IF₁ ^-⋅), which subsequently blueshifts to 466 & 745 nm (IF₁ ^2-⋅) upon increasing to two-electron reduction potential (-0.65 V vs Ag/AgCl). The color of the solution was switched from green to dark blue and further to light orange. The electrochromic behaviour is highly reversible and cycling. In addition, unlike the non-fluorescent (IF₁ and IF₁ ^-⋅), the two-electron reduced system (IF₁ ^2-⋅) is red-emissive (λ_em=630 nm) due to the combination of radicals leading to the formation of antiaromatic quinoidal π-conjugated system (IF₁ ^2-).