An alternative method for producing efficient white light-emitting polymer diodes based on a blend of two polymers is reported. The white light emission is composed of a broad blue emission of laddertype (polyparaphenylene) (m-LPPP) and a red-orange emission of a new polymer, poly(perylene-co-diethynylbenzene) (PPDB). The red-orange electroluminescence emission is promoted by an excitation energy and charge transfer from m-LPPP to the PPDB. A concentration of 0.05% PPDB in the polymer blend is required in order to obtain white light emission. By inserting an insulating material in the blend, so that a maximum external quantum efficiency of 1.2% is obtained.

We present experimental and theoretical findings on the geometry of polycrystalline para hexaphenyl via Raman scattering. The planarity of the molecule is affected by hydrostatic pressure and temperature.Our studies indicate that the potential energy curve which governs the torsional motion between neighboring phenyl rings is "W" shaped. We determine the activation energy to promote the molecule from a nonplanar to a planar state to be 0.04 eV, in good agreement with our quantum chemical calculations. From the relative intensities of the 1280 cm 21 to the 1220 cm 21 Raman modes we show that high pressure planarizes the molecules, modifying the "W"-shaped potential energy curve to a "U"-shaped one.[S0031-9007(99)09073-0]

Electric field-induced charge photogeneration in ladder-type poly(para-phenylene) is investigated by field-assisted femtosecond pump-probe experiments carried out on light emitting diodes. The characteristic photoinduced absorption band at 1.9 eV allows one to directly monitor the polaron population. We find that polarons are formed by exciton fission without intermediate states on a time scale of 10 ps. The buildup kinetics of the polaron population suggests a dissociation driven by exciton diffusion during interchain thermalization. [S0031-9007(98)

The steady-state photoinduced absorption ͑PA͒, photoluminescence ͑PL͒, PL-detected magnetic resonance ͑PLDMR͒, and PA-detected magnetic resonance ͑PADMR͒ of poly-and oligo-͑para-phenylenes͒ films is described. In particular, the excitation density ͑laser power͒ N 0 dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons ͑SE's͒ and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-1 2 magnetic resonance conditions reduce the total polaron population. Since the sublinear N 0 dependences of the positive ͑PL-enhancing͒ spin-1 2 PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent -conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.