A series of mixed-ligand 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) rhenium(I) dicarbonyl complexes that are emissive in fluid solution has been prepared, which includes a new class of the type cis-[Re(CO)2(P-P)(N-N)]+ (where P-P is a chelating diphosphine and N-N is a chelating polypyridine ligand). The four synthetic routes that have been developed rely on either reactive triflate displacement or abstraction of labile chloro ligands, followed by the use of the strong trans-labilizing effect of P donors or direct use of the trans effect of P donors. The spectroscopic, photophysical, and electrochemical properties of these new complexes systematically vary with the net donor ability of the ligands in the coordination sphere, as shown by correlations with Lever's E(L) parameters. Lifetimes and quantum yields of the bipyridine complexes encompass a broad range, 25-1147 ns and ca. 0.002-0.11, respectively.

A new class of long-lived luminescent complexes of the type cis-ReI(CO)2(N-N)(P-P)+ (where N-N is a chelate polypyridine ligand and P-P is a chelate phosphine) has been prepared. The new Re excited states are of extraordinary stability and exhibit red-shifted spectral responses relative to the well-studied fac-Re(bpy)(CO)3L(+/0) series (L is a variety of neutral and anionic ligands). Of primary interest are their extremely long lifetimes in CH2Cl2 solution, which could be a consequence of an energy gap law different from that of the tricarbonyl complexes.

This study explored the effect of radical size, chain length, and mass on the cage recombination efficiency of photochemically generated radical cage pairs. Radical cage pairs containing long-chain radicals of the type [(CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(CO)(3)Mo*, *Mo(CO)(3)(CpCH(2)CH(2)(CH(3))NC(O)(CH(2))(n)CH(3))] were generated in hexanes/squalane solution by photolysis (lambda = 546 nm) of the Mo-Mo bonds in (CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(2)Mo(2)(CO)(6) (n = 3, 8, 18). The cage recombination efficiencies (denoted as F(cP), where F(cP) = k(cP)/(k(cP) + k(dP)), k(dP) is the diffusion rate constant, and k(cP) is the radical recombination rate constant) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl(4) (a metal-radical trap) as a function of solvent system viscosity. The results show that F(cP) increases as the length of the chain on a radical center increases. This finding likely provides at least one of the reasons why the quantum yields for photolytic polymer degradation (and long-chain molecules, in general) decrease as the polymer chains get longer. In quantitative terms, plots of k(dP)/k(cP) were linearly proportional to mass(1/2)/radius(2), in agreement with the prediction of Noyes' cage effect theory. The "radius" of a long-chain radical, such as those studied herein, is rather vague, and for that reason a less ambiguous structural parameter was sought to replace the r(2) term in the Noyes expression. Plots of k(dP)/k(cP) vs mass(1/2)/surface area suggest that surface area can be used in place of the radius(2) term in the Noyes expression. The significance of being able to use a particle's surface area in the Noyes expression is that the expression becomes useful for nonspherical particles. The new expression allows the approximate prediction of F(cP) values for radicals of different sizes and masses.

The photochemistry of the [(CpR)Mo(CO)(3)](2) molecules, where CpR = eta(5)-C(5)H(4)(CH(2))(2)C(O)NCH(3)(CH(2))(n)CH(3) (n = 3, 8, 13, and 18), was examined using femtosecond pump-probe transient absorption spectroscopy. The goal of this study was to investigate the importance of radical size and mass on the dynamics and efficiency of geminate radical-radical recombination. The femtosecond results demonstrated the lack of any size/mass dependence of the recombination efficiency. This finding contrasts with results from a prior study that did find a size/mass dependence using a steady-state photochemical technique. To explain these conflicting results, it is proposed that the femtosecond pump-probe results are a measurement of the efficiency of primary geminate recombination whereas the steady-state method results are a measurement of the sum of primary and secondary geminate recombination efficiencies. The size/mass dependence is evident in the latter because secondary geminate recombination is a slower diffusive recombination process and therefore depends on the steric properties of the radicals. Although the existence of primary and secondary recombination channels is often taken for granted, experimental differentiation of primary and secondary caging has proven to be difficult because it is not possible for a single experimental technique to span the entire timescale for recombination of a radical cage pair and adequately resolve these recombination pathways.

This research was focused on the design and execution of new synthetic routes to low-temperature-curable poly(silarylene-siloxane)polyimides. The synthesis of individual oligoimide and silarylene-siloxane blocks was followed by hydrosilylation polymerization to produce crosslinked copolymers. The silarylene-siloxane and polyimide blocks were structurally characterized by IR and 1 H NMR spectroscopy and size exclusion chromatography. The high-temperature resistance of the copolymers was evaluated through the measurement of heat distortion temperatures (T HD 's) via thermomechanical analysis and by the determination of the weight loss at elevated temperatures via thermogravimetric analysis. Glass-transition temperatures (T g 's) of the silarylene-siloxane segments were measured by differential scanning calorimetry. Hydrosilylation curing was conducted at 60 8C in the presence of chloroplatinic acid (H 2 PtCl 6 ). The copolymers displayed both high-temperature resistance and low-temperature flexibility. We observed T g of the silarylene-siloxane segment as low as À77 8C and T HD of the polyimide segment as high as 323 8C. The influence of various oligoimide molecular weights on the properties of copolymers containing the same silarylene-siloxane was examined. The effect of various silarylene-siloxane molecular weights on the properties of copolymers containing the same oligoimide was also examined. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: [4922][4923][4924][4925][4926][4927][4928][4929][4930][4931][4932] 2005