Dithieno[3,2‐b:2′,3′‐d]pyrrole–alkylthiophene–benzo[c][1,2,5]thiadiazole‐based highly stable and low band gap polymers for polymer light‐emitting diodes

A series of new low band gap π‐conjugated polymers containing N‐alkyldithieno3,2‐b:2′,3′‐d]pyrrole, benzoc]1,2,5]thiadiazole, and alkylthiophenes are reported. The polymerization condition was standardized and the use of CuO to obtain high‐molecular‐weight polymer was also realized. The molecular weight of the polymers was found to be in the range of 45,000–53,000. All the polymers were found to be soluble in most of the common organic solvents, such as chloroform, dichloromethane, THF, and chlorobenzene with excellent film forming properties. The λ_max of the polymers was found to be in the range of 687–663 nm with band gap in the range of 1.35–1.43 eV. The oxidation potential of the polymers from cyclic voltammetry was determined to be 0.5–0.75 V. The HOMO levels of the above synthesized polymers were found to be between 5.24 and 5.54 eV. All the polymers exhibited a PL emission in between 755 and 773 nm. The polymers were found to be thermally stable above 277 °C with only a 5% weight loss. From the thermal stability values, it is expected that the current set of polymers are stable enough for the application in electronic devices. To realize the potential use of the polymers, EL devices were fabricated and found to show red emission with comparatively low threshold voltage. A brightness of 54 cd m⁻² for the device with polymer PC could be reached. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6514–6525, 2009