Polarized luminescence and absorption of highly oriented, fully conjugated, heterocyclic aromatic rigid-rod polymer poly-p-phenylenebenzobisoxazole

Poly-p-phenylenebenzobisoxazole (PBO) is a fully conjugated, heterocyclic aromatic rigid-rod polymer having a collinear and coplanar backbone leading to excellent thermo-oxidative stabilities, mechanical tenacity, and optoelectronic properties. PBO is inherently a liquid-crystalline polymer and can be obtained as uniaxially oriented fiber or film by extrusion of acidic solution. Schemes for making uniaxial freestanding films and monolayered luminescence devices for polarized optical absorption and emission were processed from a uniaxial PBO fiber. Wide-angle x-ray scattering revealed that the starting fiber and the resulting films were highly oriented. UV-Vis transmission spectra over 200 to 800 nm indicated that the PBO films also had polarized absorption with an attenuation coefficient ratio ${\alpha }_{\perp }{\mathrm{/\alpha }}_{\parallel }\text{=1.7.}$ The photoluminescence (PL) of the PBO films was excited at 363 nm, showing an emission intensity ratio $I_{\parallel }{\mathrm{/I}}_{\perp }$ as high as 5. The anisotropic PBO film was sandwiched between an indium tin oxide substrate and an Al electrode for monolayer light-emitting diodes (LEDs), showing an electroluminescence (EL) intensity ratio $I_{\parallel }{\mathrm{/I}}_{\perp }\text{=1.6}$ together with a markedly decrease in threshold voltage. PL and EL results demonstrated that a highly polarized luminescence emission was achieved by using the uniaxially oriented PBO films. The polarization effects in absorption and luminescence were absent for isotropic PBO films or LEDs.