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.

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We introduce a version of the cw photomodulation technique, measured far from the steady state, for obtaining the quantum efficiency, , of long-lived photoexcitations in -conjugated polymers. We apply this technique to films of a ladder-type poly͑para-phenylene͒ ͓mLPPP͔ for studying the photogeneration action spectra, (E), and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the (E) spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap (Ӎ2.6 eV), both triplet and polaron (E) spectra show, in addition, a monotonous rise at higher E. The rise for triplets is explained by singlet exciton fission into triplet pairs, and from a model fit we get the triplet exciton energy (Ӎ1.6 eV). For polarons this rise is modeled by an electron intersegment tunneling process. The electroabsorption spectrum is also measured and analyzed in terms of Stark shift of the lowest lying exciton, 1B u , and enhanced oscillator strength of the important mA g exciton. A consistent picture for the lowest excited state energy levels and optical transitions in the neutral ͑singlet and triplet͒ and charged manifolds is presented. From both the exciton binding energy of Ӎ0.6 eV and the singlet-triplet energy splitting of Ӎ1 eV, we conclude that the e-e interaction in mLPPP is relatively strong. Our results are in good agreement with recent ab initio band structure calculations for several -conjugated polymers. ͓S0163-1829͑99͒13531-8͔

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Time-resolved femtosecond pump and probe experiments were conducted on films of poly( paraphenylene)-type ladder homopolymers featuring a high degree of intrachain order. We find stimulated emission in the region of the interband luminescence which is of the same magnitude as the photoinduced absorption. The latter is located energetically below the interband luminescence and therefore does not compete with the stimulated emission. We conjecture this to be an intrinsic property of conjugated polymers of high intrachain order and a narrow distribution of long conjugation lengths. From the decay dynamics we conclude that stimulated emission and photoinduced absorption originate from different species. PACS numbers: 78.47.+p, 42.65.Re, 78.55.Kz The yield and fate of the various photoexcitations in conjugated polymers and their implications for electroluminescent, photoconductive, and nonlinear optical devices attracted a lot of interest recently [1]. The increase of conjugation and the reduction of defects was the main motivation to incorporate a poly( para-phenylene) (PPP) backbone into a ladder polymer structure (LPPP) [2,3] (Fig. 1). The high intrachain order [4], the small Stokes shift [5] coupled with a high quantum yield [6], and the excellent solubility due to the large sidegroups made these materials promising candidates for application in light-emitting devices, which were in fact built [7,8]. Both for the fundamental understanding of the states involved and for potential applications the mutual interdependence of absorbing and emitting species is vital. Nevertheless, the electronic properties of the excited states of these compounds have not yet been investigated. Previous studies on poly(paraphenylene vinylene) (PPV) have shown that upon photoexcitation a competition between stimulated emission and photoinduced absorption (PA) can be observed [9] which is attributed to singlet excitons and spatially indirect excitons, respectively. Substantial differences in the photophysics of LPPP's with regard to conjugated polymers with a broad distribution of conjugation lengths [10] have to be expected. The narrow distribution of long conjugation lengths in LPPP's suppresses the exciton migration process, which is considered dominant at small time scales in PPV [11]. The high intrachain order (low defect concentration and planarity) leads to an effective one-dimensional semiconducting behavior, which can support ultrafast intrachain charge separation.The objective of the present study is to shed light on the question of initially formed photoexcitations in LPPP's. Since the electronic properties within the group of LPPP's are known to vary slightly as a function of the substituents and chain length [8,12,13] as well as ring position [5], we chose the material with the highest intramolecular order presently available, which we denote as m-LPPP due to the methyl substituent at the methin bridge (symbol Y in Fig. 1). The temporal evolution of the photogenerated states is monitored by their contribution to transient ...