A novel series of efficient thiophene-based light-emitting conjugated polymers and application in polymer light-emitting diodes

A novel series of soluble conjugated polymers, poly[1,4-bis(4-alkyl-2-thienyl)-2,5-disubstituted phenylene]s, with different substituents on the phenylene ring (hydrogen, methyl group, or decyloxy group) and on the thiophene ring (n-hexyl chain or cyclohexyl group) were synthesized through FeCls-oxidative polymerization. The structures of the polymers were fully verified by FT-IR, 'H and 13C NMR, and element analysis. The high regioregularity of head-to-head linkage between two adjacent thiophene rings in the polymers was demonstrated. The electronic spectra of the polymers can be tuned by changing the substitution on both the phenylene and thiophene rings, emitting blue to green light. The electronic structures of the polymers can also be tuned by different substitutions, as demonstrated by electrochemical measurements (cyclic voltammetry). The absolute photoluminescence (PL) efficiencies of the polymers in neat films, which can be remarkably effected by the substitution on both the phenylene ring and the thiophene ring, can be up to 29%, much higher than the PL efficiencies in other polythiophene-based light-emitting materials. It was demonstrated that inserting substituted phenylene rings into the backbone of polythiophene is a useful approach to improve the PL efficiency of thiophene-based conjugated polymers. Blue to green electroluminescence (EL) was realized from some of the new polymers in single-layered light-emitting diodes with the configuration of ITO/polymer film/Ca or Al. One green light-emitting diode fabricated from the green-emissive polymer with the highest PL efficiency gave rise to the best EL performance, turning on at ∼8V and reaching an external EL quantum efficiency of 0.1%.