Opto-electronic emission of heterocyclic aromatic coil-like and rigid-rod polymer blends

Poly-2,2′-m-phenylene-5,5′-bibenzimidazole (Pbi) with a partially conjugated coil-like backbone and poly-p-phenylenebenzobisthiazole (PBT) with a fully conjugated rigid-rod backbone are two heterocyclic aromatic polymers which were codissolved and made into freestanding films or spun onto an indium tin oxide (ITO) substrate. Ultraviolet-visible light absorption λmax for Pbi and PBT were 363 nm and 437 nm, respectively, and for the Pbi/PBT blends were a superposition of individual absorption indicating no intermolecular energy transfer. However, the photoluminescence of the blends excited at 363 nm demonstrated a red shift with increasing PBT content. This was attributed to a template effect of PBT rigid-rod backbone confining coil-like Pbi into an extended chain configuration leading to improved charge delocalization. Aluminum electrode was vacuum evaporated onto the polymer/ITO system for light emitting diodes. The diode threshold voltage varied from 4 V to 14 V with decreasing PBT content. Electroluminescence emission spectra of the blends seemed to be a combination of those of Pbi and PBT with emission λmax tunable for 60 nm.