A new bipyridine-type ligand which allows the selective perturbation of ligand field (LF) excited-state energies is introduced. Any metal-to-ligand charge-transfer (MLCT) excited states occur at nearly the same energy as in the bipyridine complex. The syntheses of the corresponding ruthenium(I1) complexes and their photophysical properties are reported. The results clearly support a current model for the excited-state behavior of polypyridine complexes in which the lowest MLCT decays by thermal population of a higher lying LF state. This method of selective LF-state perturbation nicely complements the perturbation of MLCT states achieved by modification of the ligand's reduction potential.
Inorganimrganic hybrid polymers represent a very active area of current research activities, which has considerable potential for providing new advanced materials. In our continuing interest in high-temperature polymers and polymeric precursors for ceramics, we are especially interested in polymers containing boron, silicon, and acetylene moieties. Numerous polymers containing carborane cage structures have been reported.'" Polymers containing the carborane unit within the backbone exhibited appreciable enhancement in their thermal stability. The development of carborane-siloxane polymers in the early 1960s was a major breakthrough in the search for high-temperature elastomers. These elastomeric materials show superior thermal and oxidative stability at elevated temperatures compared to siloxane-based polymers.6r7 The carborane-containing systems retained their elastomeric properties after aging in air at 315 "C for 300 h.8 Because of the superior thermal and oxidative stability of the poly-(carborane-siloxane) elastomers, we have studied the effects of incorporating acetylenic units into the structure for cross-linking purposes. The presence of acetylenic units in the backbone of a polymeric material provides many attractive advantages. The acetylene remains dormant during processing under ambient conditions. During reaction by thermal or photochemical means, a conjugated cross-link is formed by an addition polymerization reaction without the formation of volatile byproducts.We report the development of a successful route to poly-(carborane-siloxane-acetyleneb, a new class of inorganicorganic hybrid polymers, which exhibit outstanding thermal and oxidative properties and can be easily processed into shaped configurations (see Scheme 1). A recent method for preparing silylene-diacetylene polymers via dilithioacetylenes wasadapted for the synthesis of these hybrid p o l y m e r~.~J~ The synthesis of 1 is a one-pot, twostage reaction. Dilithiobutadiyne (3) is generated by reacting hexachlorobutadiene (2) in THF with 4 equiv of n-butyllithium at -78 "C. The reaction is allowed to warm to ambient temperature and then recooled to-78 "C. Equal molar amounts of dilithiobutadiyne and 1,7-bis(chlorotetramethyldisi1oxy)-rn-carborane (4), purchased from Dexil Corp., were reacted to generate the linear polymer 1 in high yield. Molar calculations were based on 3 being formed quantitatively. After the addition of 4, the reaction mixture was allowed to slowly warm to room temperature. The reaction mixture was then poured into dilute hydrochloric acid and extracted with ether. The ethereal extract was washed with water until neutral and dried over anhydrous sodium sulfate. The ether was removed at reduced pressure to afford a dark brown, viscous liquid 1 in quantitative yield. The polymer 1 is soluble in most common organic solvents. GPC analysis indicated the presence of low molecular weight species ( 4 0 0 ) as well as higher average molecular weight polymers (M, = 4900, M,, -24001, averaging to about 10 repeat units. Heating ...
The excited-state properties of several polypyridine ruthenium complexes have been examined. The complexes were selected on the basis of the energy of their low-lying metal-to-ligand charge-transfer (MLCT) state. The dependence of radiative and radiationless rates on the energy of this excited state is discussed. Also, the amount of metal to solvent charge transfer in this lowest state is shown to be independent of the state's energy. Finally, the increase in the activation energy for population of a higher excited ligand field state was directly correlated with the decrease in the MLCT state's energy.
A processable new thermosetting polymer containing carborane and siloxane groups with acetylenic linkages was examined for antioxidative properties during high‐temperature oxygen exposure. Samples of the polymer were annealed in argon or air at 320–400°C or argon at 900°C and oxidized at 400 and 500°C. Their structure and composition were analyzed by scanning electron microscopy, x‐ray photoelectron spectroscopy, scanning Auger microprobe scattering and Raman microprobe scattering. Samples heated to 400°C in argon exhibited little or no inorganic segregation, but developed surface layers of boron and silicon oxide when heated to 400°C in air. Samples annealed in argon to 900°C and then oxidized at 500°C for up to 100 h developed a continuous silicon oxide surface layer with almost no underlying boron oxide. The layer retarded oxidation of the sample bulk at 500°C.
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