No abstract

We have prepared a new series of mixed thiophene-pyrrole oligomers to investigate the electronic benefits arising from the combination of these two heterocycles. The oligomers are functionalized with several hexyl and aryl groups to improve both processability and chemical robustness. An analysis of their spectroscopic (absorption and emission), photophysical, electrochemical, solid state, and vibrational properties is performed in combination with quantum-chemical calculations. This analysis provides relevant information regarding the use of these materials as organic semiconductors. The balance between the high aromatic character of pyrrole and the moderate aromaticity of thiophene allows us to address the impact of the coupling of these heterocycles in conjugated systems. The data are interpreted on the basis of the aromaticity, molecular conformations, ground and excited electronic state structures, frontier orbital topologies and energies, oxidative states, and quinoidal versus aromatic competition.

Oligothiophene sulfones of up to six rings can be conveniently prepared by the direct oxidation of butyl-substituted thiophene oligomers with m-CPBA in dichloromethane. Reverse selectivity of oxidized rings is observed relative to previously reported systems without beta-substitution. The selectivity in the trimer and hexamer is confirmed with single-crystal X-ray structure data. The sulfones possess red-shifted absorptions and increased electron affinities relative to the parent oligomers.

No abstract

Thiophene derivatives R 0090Reverse Selectivity in m-CPBA Oxidation of Oligothiophenes to Sulfones. -Direct oxidation of β-alkyl-substituted oligothiophenes affords symmetrical products with oxidation on the central rings. The method is applied to tri-and hexamers. This result is in the contrast to the previously observed terminal ring oxidation of β-unsubstituted substrates. The central ring sulfones show red-shift absorption and increased electron affinities compared to the parent compounds. -(PAPPENFUS*, T. M.; MELBY, J. H.; HANSEN, B. B.; SUMPTION, D. M.; HUBERS, S. A.; JANZEN, D. E.; EWBANK, P. C.; MCGEE, K. A.; BURAND, M. W.; MANN, K. R.; Org. Lett. 9 (2007) 19, 3721-3724; Div. Sci. Math., Univ. Minn., Morris, MN 56267, USA; Eng.) -R. Steudel 05-103

Over 20% of electricity in US is used by lighting. Solid state lighting (SSL) efficiency can theoretically surpass that of incandescent and fluorescent lighting techniques. Nonetheless SSL efficiency is greatly reduced at high temperatures that result from inadequate heat dissipation. SSL requires blue and green light emitting diodes (LEDs) made from Gallium Nitride (GaN) and Indium Gallium Nitride (InGaN) to eventually generate white light. Using an infrared thermal imaging camera, temperatures of working blue and green LEDs with different efficiencies were measured. The results show that higher efficiency LEDs have lower active region temperatures when driven with the same power. Further, they motivate our study of thermal properties of the individual thin films that compose the LEDs, since earlier studies show that conduction is the primary dissipative mechanism for heat in LEDs. Bulk thermal properties are poor estimates of thin film properties due to increased boundary and defect scattering of phonons in the films. By examining real LED structures with the 3-omega technique, thin film thermal conductivities can be measured. For this technique, a thin metal line was fabricated onto a smooth dielectric sample surface. This thin metal line works as both a heater and a thermometer. Benchmark studies on Pyrex 7740 were used to validate our 3-omega setup. Data from real GaN/InGaN LED structures show that the effective thermal conductivities of the AlN buffer layer and multi-quantum-well active region are substantially suppressed relative to their anticipated values based on bulk properties.

The electrical response of hydrogen sensors based on Al0.30Ga0.70N/GaN high-electron-mobility transistors (HEMTs) with Pt catalytic gate electrodes was measured in a flowing gaseous stream consisting of hydrogen in a pure nitrogen diluent at ambient and elevated temperatures. The sensor response was found to monotonically increase for a wide range of hydrogen concentrations (500 ppb to 5 vol%). Various models based upon Langmuir adsorption were investigated to describe the sensor response in this regime. A simple two-state model involving two distinct hydrogen binding states that have previously been observed in surface studies was found to adequately describe sensor response from 500 ppb to 5 vol% hydrogen in nitrogen. The relevance of other modified Langmuir models to adequately describe the sensor response as a function of hydrogen concentration is also discussed.