Spray Processable Hybrid 3,4-Propylenedioxythiophene: Phenylene Electrochromic Polymers
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The optical and redox properties of a new family of thiophene−phenylene-based molecules are reported. The monomers 1,4-bis[2-(3,3-dimethyl-(3,4-propylenedioxy)thienyl]-2,5-didodecyloxybenzene (BProDOT-Me2-B(OC12H25)2) and 1,4-bis[2-(3,3-dihexyl-(3,4-propylenedioxy)thienyl]-2,5-didodecyloxybenzene (BProDOT-Hex2-B(OC12H25)2) have been synthesized via Negishi coupling of the alkyl-substituted ProDOT and didodecyloxyphenylene units in ca. 40% yields. They were efficiently electropolymerized to form electroactive films which exhibit redox switching at fairly low potentials (+0.1 V vs Fc/Fc+). BProDOT-Hex2-B(OC12H25)2) was polymerized via ferric chloride-mediated chemical oxidation and the polymer isolated after dedoping with hydrazine. A number-average molecular weight of 14 600 g/mol was estimated using size exclusion chromatography, and the polymer structure was confirmed by MALDI mass spectrometry. The polymer is highly soluble in chloroform, THF, and toluene, and homogeneous films were prepared by spin-coating or spray-casting from a chloroform solution. Electronic band gaps of 2.1 eV have been characterized spectroelectrochemically for both the methyl- and hexyl-substituted polymers from the onset of the π to π* transition. PBProDOT-Hex2-B(OC12H25)2 electrochemically switches between orange, blue, and highly transmissive-gray colors, making it potentially useful in large area electrochromic displays. It also exhibits solvatochromic properties, being yellow in good solvents and red in poorly solvating solvents. Polymer light-emitting diodes have been prepared with PBProDOT-Hex2-B(OC12H25)2 and have shown a strong yellow emission with λmax = 570 nm. The current−voltage characteristics of PBProDOT-Hex2-B(OC12H25)2-based hole-only devices have also been measured and fitted to the space charge limited current model, allowing a direct measurement of the hole mobility which has been determined to be ca. 10-7−10-6 cm2 V-1 s-1.