A novel series of p - N diblock light-emitting copolymers based on oligothiophenes and 1,4-Bis(oxadiazolyl)-2,5-dialkyloxybenzene

A new series of p - n diblock conjugated copolymers consisting of alternate 1,4-bis-(oxadiazolyl)-2,5-dialkyloxybenzene and oligothiophenes with one to three thiophene rings (P1 - P3) have been synthesized. The polymers have well-defined structures and exhibit good thermal stability with the onset decomposition temperatures in nitrogen at around 300°C. The glass transition temperature (T_g) of the polymers decreases with increasing the length of oligothiophene blocks. Both the absorption spectra and photoluminescence spectra shift to longer wavelength with increase in the length of oligothiophene blocks. The emissive color of the polymers could be tuned from blue to green to orange just by increasing the number of thiophene rings in the oilgothiophene blocks from one to three. No obvious change in reduction potential is found for the polymers with variation in the length of oligothiophene blocks. The reduction potential E^1/2 of the polymers is measured by cyclic voltammetry to be around -1.7 V vs SCE, comparable to those of poly(cyanoterephthalyidene) (CN-PPV) and other good electron-transporting materials. The oxidation potential of the polymers can be reduced remarkably by increasing the length of oligothiophene blocks. The oxidation potential E^1/2 of P3 is measured to be 1.25 V with the onset potential at 1.0 V vs SCE. These values are comparable to those of some hole-injection favorable electroluminescent polymers. The results prove that the p - n diblock structure may be a promising molecular design for synthetically tuning the HOMO and LUMO of conjugated polymers.