Luminescence of monolayer thin films of the fully conjugated rigid-rod polymer poly(p-phenylenebenzobisthiazole)

Poly(p-phenylenebenzobisthiazole) (PBT) is a heterocyclic, aromatic rigidrod polymer with a fully conjugated backbone and excellent dimensional, thermooxidative, and solvent stabilities. A PBT polymer with an intrinsic viscosity of 18.0 dL/g was dissolved in methanesulfonic acid or Lewis acid. The PBT solution was spin-coated and doctor-bladed for freestanding films or onto an indium tin oxide (ITO) substrate. The acid was removed via coagulation. Scanning electron microscopy determined that the resultant film thicknesses were about 340 and 60 nm for PBT freestanding films and films on the ITO substrate, respectively. X-ray scattering demonstrated that the freestanding films were in-plane isotropic without long-range order. The freestanding films were excited with a He-Cd laser at 325 nm for photoluminescence (PL) response. PL spectra showed a distinct intensity maximum at 580 nm, regardless of the film-forming conditions. After the films cooled to 67 K, the PL maximum shifted to 566 nm with enhanced intensity. Aluminum was evaporated onto the monolayer PBT thin film on the ITO substrate as an electron injector for electroluminescence (EL) response. Diodic electric behavior was observed for all monolayer PBT EL devices for the first time. A threshold voltage as low as 4 V was achieved for the monolayer EL devices. In addition, PBT EL spectra were tunable, with a maximum intensity at 570 nm at a bias voltage of 4.5 V changing to 496 nm at 7.5 V (i.e., a blueshift) with greatly increased intensity. (C) 2002 Wiley Periodicals, Inc.