A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.

The use of light-emitting diodes (LEDs) continues to increase dramatically, for example in color computer screens based on flat LED boards which are intended to replace the large and expensive cathode-ray tubes. In the past, it has proved difficult to economically extend the emission spectra of the LEDs into the blue spectral region.The story of the LED goes back to 1907,L'I and in the last 35 years much effort has been invested in the study of inorganic semiconductors e.g. GaN, ZnS, ZnSe and S i c in order to produce an effective blue LED. However, large scale applications have not been practicable due to the problems in fabrication and the rather low luminescence efficiency. Therefore, the use of alternative materials was proposed[21 in order to extend the spectral region into the blue.Organic materials have always been considered for use in electroluminescent devices,[31 but up to now they have not been widely used except for applications in scintillation detectors. Only recently has there been a noticable effort in electroluminescent device fabrication with organic laye r~. [~. ' 1 The observed transfer function between the current input and luminescence output of organic materials is linear, which allows optocouplers without any distortion to be devised. This feature cannot be provided by existing optocoupling devices. A blue light-emitting electroluminescent device, based on low-molecular-weight organic materials with a substantial efficiency was presented recently.I6] A serious disadvantage of devices consisting of organic materials of rather low molecular weight is the tendency of the materials to recrystallize (initiated by the heat produced in the device), which leads to a drastic decrease in quantum efficiency.16] Intensive research activities on the physical properties and synthesis of conjugated polymers[71 have shown these materials to be promising candidates for applications in stable optoelectronic devices. The first encouraging results on LEDs based on semiconducting polymers were reported by Burroughes et al.[*l and confirmed by Braun et al.'91 These devices emitted in the yellowish ( x 600 nm) sprectral range.In the search for new materials for blue LEDs and linear optocouplers, we have investigated the polymer polybphenylene) (PPP, see Fig.

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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 first-principles local-density band structure calculations for one-dimensional and three-dimensional crystalline poly(para-phenylene) (PPP) using the full-potential linearized augmented-plane-wave and the pseudopotential methods. Optimized structural parameters for PPP chains and for orthorhombic crystalline phases with space groups Pbam and Pnnm are determined.A torsion angle of 27 is predicted in PPP chains and 17' in the crystals. The dielectric tensor and the absorption coefFicient are calculated. We find very good agreement with experimental data, indicating that the excitations are extended band states. The interchain coupling leads to energy band splittings of the order of 0.5 eV. It is shown that the energy gap can be varied over a wide energy range by relatively small structural modifications.

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The performance of organic thin-film transistors (OTFT) for flexible, low cost and disposable "plastic" electronic products advances rapidly: various organic semiconductors display hole or electron carrier mobilities [1] that compare favorably with those of hydrogenated amorphous silicon, [2] the inorganic counterpart for such applications as flexible displays, [3,4] smart cards and radio frequency identification tags, [5,6] nonvolatile memories [7] and sensors. [8,9] The possibility for tailoring functional organic materials, bears potential towards novel electronic products such as smart skins, [10] smart textiles [11] and "invisible electronics", [12] where multiple functionalities, portability and ubiquitous integration is requested. In this context diverse properties of organic thin-film devices are inevitable such as lightweight, low power consumption, low operationvoltage and compatibility with diverse substrates.[12]Reducing the threshold voltage and the subthreshold swing is essential for operating OTFTs at low-voltage levels. When combined with very low gate leakage currents, OTFTs may also become a key element in high-end sensor applications, such as flexible touch pads and screens or thermal imaging tools for night vision, surveillance or for the detection of undesired heat loss paths in buildings.The aforementioned transistor parameters not only critically depend on the thickness and the dielectric properties of the gate insulator, [12][13][14] but also on the trapped charge densities at the interface between these materials. The selection of semiconductors and gate insulators with excellent interface properties is currently the challenge in the quest for improving the performance of OTFTs.Here we show that bottom-gate OTFTs based on the organic semiconductor pentacene and high-k nanocomposite gate dielectrics, exhibit transistor performances with very low gate leakage currents, subthreshold swings close to the theoretical limit, and low-voltage battery operation. The subthreshold swings of OTFTs with different organic and hybrid gate dielectrics follow an inverse dependence on the gate capacitance as is expected by standard MOS theory. The trapped charge carrier density at the interface between the semiconductor and the dielectric surpasses that of the SiO 2 -pentacene interface, being close to the average trap densities in the SiO 2 -Si interface in metal oxide semiconductor transistors. [15] We also report the first application of these OTFTs in an optothermal light sensor. We describe the transistor, the temperature sensitive fluorinated polymer, their combination in an integrated circuit, and the application of this circuit as a thermal infrared sensor and as a switch that can be operated by a laser pointer. Figure 1 shows the structure of low-voltage organic transistors with high dielectric constant (high-k) oxide-polymer nanocomposites. Al 2 O 3 or ZrO 2 were chosen as high-k dielectric materials, combined with poly(a-methyl styrene) (PaMS) or poly(vinyl cinnamate) (PVCi) to form a smooth and ...

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]