Spectroscopic and Electrochemical Study of a Novel Blue Electroluminescent p-n Diblock Conjugated Copolymer

A novel p-n diblock copolymer, poly[N-(2′-ethylhexyl)-carbazole-3,6-diyl-1″3″,4″- oxadiazole-2″,5″-diyl-2‴,5‴-dioctyloxy-1‴, 4‴-phenylene-1″″,3″″,4″″-oxadiazole- 2″″,5″″-diyl] (PCOPO) composed of an electron-rich moiety carbazole and an electron-deficient unit aromatic oxadiazole was synthesized aiming at balancing the abilities of conducting holes and electrons. Electrochemical analyses by cyclic voltammetry indicate that PCOPO can be reversibly n-doped and irreversibly p-doped. The cathodic sweep reveals that the reduction involves two-electron process with respect to the successive reduction of oxadiazole rings and carbazole moieties in the polymer chain. The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) energy levels of the polymer are estimated to be 5.60 and 2.66 eV from the onset of oxidation and reduction potentials, respectively. The band gap energy of the polymer estimated by the electrochemical measurement (2.94 eV) is in good agreement with that from the optical method (2.82 eV). The photoluminescence (PL) of film samples shows that the polymer emits greenish-blue light (475 nm). The PL of solutions is concentration-dependent. In dilute solutions, the PL emission is from the singlet exciton transition, whereas in the concentrated solutions, it is mainly originated from excimers. The excimer formation is related to the incorporation of oxadiazole rings into the polymer backbone, which can enhance the interchain interactions. Both photophysical and electronic properties demonstrate that the polymer may be a promising candidate material for the fabrication of an efficient blue light-emitting device.