Amphiphilic phenylene-ethynylene polymers and oligomers for polarized electroluminescence

Abstract

We investigate the electroluminescence of polymer and oligomer derivatives of p(phenyl-ethynylene) bearing amphiphilic side chains. The polymer and the heptamer show liquid crystalline properties, which are characterized by X-ray diffraction. A periodic structure is observed, which corresponds to a lamellar system with a layer spacing of about 3.8 nm. Two other diffuse bands correspond to a liquid-like order of the side chains and a weak interaction of the π–π interactions between phenyl rings. We have investigated the absorption and the photoluminescence (PL) properties of the polymer and the heptamer films prepared by the Langmuir–Blodgett (LB) and by spin-coating techniques. The polymer film shows an absorption peak at 408 nm with a shoulder at 436 nm; while the heptamer shows a small blue shift of the main peak at 382 nm and the shoulder at 415 nm. Based on the liquid crystalline properties, the two materials can be oriented by rubbing, giving rise to a good dichroic absorption spectrum. The ordering parameter at the peak absorption is better for the polymer than for the heptamer, OP=0.58 and 0.45, respectively. The PL of the two materials are observed at 539 and 516 nm, respectively. The PL yields, measured by the integration sphere method, gives a result of 0.23 and 0.20 emitted photon per absorbed photon respectively. A good dichroic photoemisson is also observed on the oriented rubbed films prepared by spin-coating. We demonstrate that the oligomer can be used in a blue-green light emission device by using the LB multilayer or by spin-coating techniques. We investigate the polarized electroluminescence in devices with the configuration ITO/film/LiF–Al. The oligomer (or the polymer) films are oriented by rubbing. A polarized emission parallel or perpendicular to the sliding direction is observed. The measured dichroism reaches a value of 8 at 531 nm for the heptamer.