Triplet exciton diffusion and phosphorescence quenching in iridium(III)-centered dendrimers

A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay.     

fluorinations with potassium tetrafluorocobaltate(III)—V : The fluorinations of some polycyclic aromatic hydrocarbons over potassium tetrafluorocobaltate(III) and an improved synthesis of perfluoro-phenanthrene and -pyrene

Anthracene, phenanthrene, decafluorophenanthrene, pyrene, biphenyl and decafluorobiphenyl have been fluorinated over potassium tetrafluorocobaltate(III) and some of them over caesium tetrafluorocobaltate(III). All the product mixtures except those from pyrene and decafluorobiphenyl consisted mainly of perfluoro-olefins and -aromatics; the pyrene product was largely a perfluoroper-hydropyrene, and that from decafluorobiphenyl was very largely perfluorobicyclohexyl. Only in the case of biphenyl was any hydrogen-containing product isolated. The mechanistic interpretation of these observations is attempted. Defluorination of the relevant crude fluorination mixtures provided a good route to perfluoro-pyrene and -phenanthrene. In the case of the fluorination product from anthracene the defluorination largely stopped at an intermediate, partially aromatic, stage.     

Nature of the structural heterogeneity in SiH films by small angle neutron scattering

The density and small angle neutron scattering of a variety of silicon hydride films have been measured as function of heat treatment. Anode deposited samples have previously been shown to have a columnar morphology with a dominant column diameter of ～60 Å. This is confirmed and the new studies establish that the matrix separating the columns is of very low density - essentially comprising voids. The mechanism of hydrogen evolution during heat treatment depends on the void morphology. Both plasma deposited and sputtered films may be made which are structurally homogeneous on a scale of ? 10 Å.     

Spin-dependent photoconductivity in n-type and p-type amorphous silicon

It is found that, as in crystalline silicon, the photoconductivity in amorphous silicon prepared by glow discharge is strongly spin-dependent. In this material, the position of the Fermi level can be smoothly varied by phosphorous or boron doping and the magnitude of the spin-dependent recombination has been measured as a function of the doping: it is found to have a large maximum when the material is intrinsic. The similarities with the spin-dependent effects in crystalline and dislocation silicon suggest that the recombination process in amorphous silicon is the same as in the crystalline material and that dislocation-like centres are responsible for the spin-dependent recombination properties in both materials.     

Localized states in amorphous arsenic

Optical absorption and photoconductivity spectra of bulk amorphous arsenic are presented and analyzed using a general density of states model for amorphous semiconductors. It is concluded that localized states do exist in the forbidden gap and at the band edges and that transitions between localized states are significant.     

Small angle x-ray and neutron scattering studies of plasma-deposited amorphous silicon-hydrogen films

Small-angle scattering measurements using both neutrons and X-rays have been made on a series of plasma-deposited a-Si : H films. One class of sample exhibits strong anisotropic scattering indicating rod-like microstructure normal to the film surface with a dominant rod diameter of ~ 60 Å. Other samples show isotropic scattering with a similar dimension or no scattering at all within the resolution of the experiment.     

The fluorination of ethane and ethene over potassium tetrafluorocobaltate(III) and cobalt trifluoride

Fluorination of ethane and ethene with KCoF₄ and CoF₃ over a range of temperatures gave, in all cases, mixtures of polyfluoroethanes, C₂H_6-nF_n n = 1–6, with very little CC bond cleavage. Apart from an initial easy saturation of ethene to CH₂FCH₂F, both substrates had a similar reactivity. Hydrogen appeared to be replaced by fluorine almost randomly in all fluorinations. CH₂FCH₂F is a stable compound, contrary to literature reports.