Sensitization hydrocarbon crystals

Very early studies of dye sensitization have been on metal electrodes and aromatic hydrocarbon crystals [6,7]. On metal electrodes, the excited states of dye molecules are rapidly deactivated (by energy transfer to the broad continuum states of the metal) and there are no evidences of electron transfer quenching. Organic crystals are insulators with a wide bandgap (> 3eV) and narrow conduction and valence bands. Hole injection in the fully occupied valence bands of organic crystals such as anthracene, perylene or phenanthrene through excited dye molecules (e.g, Rhodamine B) was discovered in 1963 and the process has been examined extensively. The efficiency of sensitized hole generation in these cases is directly related to the... 

Because of the existence of numerous isomers, hydrocarbon mixtures having a large number of carbon atoms can not be easily analyzed in detail. It is common practice either to group the constituents around key components that have large concentrations and whose properties are representative, or to use the concept of petroleum fractions. It is obvious that the grouping around a component or in a fraction can only be done if their chemical natures are similar. It should be kept in mind that the accuracy will be diminished when estimating certain properties particularly sensitive to molecular structure such as octane number or crystallization point. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Tetrakis(trimethylsilyl)lead 94a can be isolated as pale yellow crystals moderately sensitive to light, soluble in hydrocarbons and ethers, decomposing above 80 °C. 

The discussion thus far has emphasized sensitivity of the frequency of C02 s v3 mode to local stress, sensitivity of its absorption intensity to IR polarization, and sensitivity of both properties to resonant coupling of dimers. For the type of crystals under consideration, which consist mostly of simple hydrocarbon groups, these factors probably dominate in determining the IR spectral characteristics. Other factors can be involved, however, and although they can make simple interpretation of the spectra more problematic, they can also provide additional information about the environment of the C02 probe molecule. The following examples illustrate how one can track the motion of C02 over distances of 1-15 A by observing its proximity to free radical centers or to halogen or deuterium substituents in the crystal lattice. This information complements the previously discussed structural studies, which related to structure within the dimer rather than to the location of the C02 in the crystal matrix. 

Figure I. Difference in the phase region of the lamellar liquid crystal (black) when an aromatic hydrocarbon (left) is replaced by an aliphatic one (right) demonstrates the sensitivity of the lyotropic liquid crystalline structure to weak intermodular forces. The emulsifier is a polyoxyethylene (9) nonyl phenol ether.

Reactions Exhibiting Strong Structure Sensitivity. Reactions for which there is at least an order of magnitude difference in activity as a function either of particle size or of exposed crystal planes include the ammonia synthesis reaction and the hydrogenolysis of hydrocarbons. 

The purple crystals are freely soluble in all common organic solvents except aliphatic hydrocarbons, in which they are only sparingly soluble. Solutions of the compound are highly sensitive to oxidation, and rigorous exclusion of air is necessary. The appearance of yellow material, insoluble in hydrocarbons, indicates the onset of oxidation. The purple solid is pyrophoric in air but is stable indefinitely when stored under an inert atmosphere at room temperature. 

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