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 t hree thiophene rings (P1-P3) have been synthesized. The polymers have well- defined structures and exhibit good thermal stability with the onset decomp osition temperatures in nitrogen at around 300 degrees C. The glass transit ion 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 oligothi ophene 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 pot ential is found for the polymers with variation in the length of oligothiop hene blocks. The reduction potential E-1/2 of the polymers is measured by c yclic voltammetry to be around -1.7 V vs SCE, comparable to those of poly(c yanoterephthalyidene) (CN-PPV) and other good electron-transporting materia ls. The oxidation potential of the polymers can be reduced remarkably by in creasing 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. T hese values are comparable to those of some hole-injection favorable electr oluminescent 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.