Electropolymerization and spectroelectrochemical properties of poly(4,7-dithien-2-yl-2,1,3-benzothiadiazole) films in three 1-butyl-3-methylimidazolium ionic liquids

Spectroelectrochemical properties of electrodeposited poly(4,7-dithien-2-yl-2,1,3-benzothiadiazole) (PDTBT) films in 1-butyl-3-methylimidazolium tetrafluoroborate (IL1), hexaflurophosphate (IL2), and bis(trifluromethylsulfonyl)imide (IL3) are reported. The net charges of polymerization, cyclic voltammograms, UV/Vis spectra, and scanning electron micrographs indicated the respective films PDTBT1, PDTB2 and PDTBT3 were nano-textured and the yield increased in the order PDTBT2 < PDTB1 < PDTBT3. The formal potentials of the p-doping and n-doping redox couples, and the band gaps estimated from doping onset-potentials (Epd0′;End0′;Eg−ec) were: PDTBT1/IL1 (1.03 V; −1.23 V; 1.67 eV), PDTBT2/IL2 (0.95 V; −1.49 V; 1.49 eV), and PDTBT3/IL3 (1.00 V; −1.38 V; 1.83 eV). The corresponding UV/Vis absorption maxima and the band gaps estimated from absorption onset wavelengths (λmax; Eg-opt) were (540 nm; 1.58 eV), (579 nm; 1.42 eV), and (578 nm; 1.39 eV). Thus, more conjugated PDTBT molecules were formed in the hydrophobic IL2 and 3 than in the hydrophilic IL1. In a double potential-pulse (−0.6 V/−1.0 V) redox cycling monitored at 1000 nm, the optical contrast and coloration efficiency (Δ(%T); CER→Ox) were PDTBT1 (11%; 158 C cm−2), PDTBT (17%; 91 C cm−2) and PTBT3 (34%; 187 C cm−2) with similar color/bleach (1-1.3 s) or reverse (3-4 s) switching times. IL3, the least viscous and most conductive of the three ILs, enabled the most efficient PDTBT electrodeposition and a PDTBT/IL system with superior electrochromic properties.