Satoshi Tokura

Ruthenium and rhenium complexes with silyl-substituted bipyridyl ligands

Abstract The preparation of 5,5′-bis(trimethylsilyl)- ( 1a ) and 5,5′-bis(pentamethyldisilanyl)-2,2′-bipyridines ( 1b ) by dehalogenative coupling of the corresponding 2-bromo-5-silylpyridines is described. Silyl substitution causes broad and red shifted π→π* and σ→π* UV–vis absorption bands; electrochemical reduction is facilitated. With these ligands, a series of ruthenium complexes [Ru(bpy) 2 (L)](PF 6 ) 2 ( 3a , L= 1a ; 3b , L= 1b ) and [RuL 3 ](PF 6 ) 2 ( 4a , L= 1a ; 4b , L= 1b ), as well as rhenium compounds Re (L)(CO) 3 Cl ( 5a , L= 1a ; 5b , L= 1b ) (bpy=2,2′-bipyridine) were synthesized. These complexes give rise to red-shifted metal-to-ligand charge-transfer absorptions in the region of 460–480 nm for the ruthenium complexes and around 400 nm for the rhenium complexes. While the oxidation potentials of ruthenium complexes 3a , 3b , 4a , and 4b are almost the same as that of [Ru(bpy) 3 ](PF 6 ) 2 , reduction of the ruthenium and rhenium complexes occurs at more positive potentials than that of [Ru(bpy) 3 ](PF 6 ) 2 and Re(bpy)(CO) 3 Cl. Band maxima of the metal-to-ligand charge-transfer emission of the ruthenium and the rhenium complexes were observed at 620 and 610 nm, respectively. The results indicate that the LUMO levels of 2,2′-bipyridine and its metal complexes are lowered by electron-accepting effects of trimethylsilyl and pentamethyldisilanyl substituents, while the HOMO level of bpy is elevated by pentamethyldisilanyl substitution due to the effective σ–π conjugation between an SiSi bonding orbital and a bpy π orbital.

Optical limiting and bistability of a σ–π photoconductive copolymer

Abstract Optical nonlinearity of a σ–π photoconductive copolymer, poly(disilanylene-2,2 ′ -bipyridine-5,5 ′ -diyl) ruthenium complex, has been investigated in chloroform solution of various concentrations. The nonlinear optical absorption resulted in optical limiting, optical bistability, and laser pulse compression for nanosecond pulses at the wavelength of 532 nm. Theoretical analyses showed that the optical absorptive nonlinearity was dependent on the photogenerated excitons and exciton–exciton couplings.

Photochemical reactions of germoxanes: Generation of germyl and germoxy radicals

Photochemical reactions of phenyl-substituted digermoxanes, (PhnMe3-nGe)2O (n = 1–3), and a cyclic germoxane, (Me2GeO)4, have been investigated by chemical trapping and laser flash-photolysis. On irradiation, phenylated digermoxanes undergo homolysis of the germanium—oxygen bond to give germyl radicals and germoxy radicals. In the presence of nonhalogenated solvents, these germyl radicals and gerrnoxy radicals couple to yield digermanes and digermyl peroxides, respectively. In polyhalomethanes (CCl4, CHCl3, and BrCCl3), germyl radicals are converted to the corresponding halides by abstraction of a halogen atom. Germoxy radicals do not react with polyhalomethanes; they couple to produce digermyl peroxides which decompose to give either a pair of germoxy radicals or germyl radicals and germylperoxy radicals. Photolysis of a cyclic germoxane also results in cleavage of the germanim—oxygen bond.

Absorption and emission of germylenes (germanediyls); their complexation

Germylenes, R2Ge: (R = Me or Ph), show broad electronic absorption bands with λmax in the region of 416–464 nm; weak fluorescence peaks in the region of 620–651 nm at 77 K, and a large Stokes shift, attributed to a conformational change in the C–Ge–C bond angle between the ground and the lowest excited singlet state; UV–VIS spectra provide evidence for the interaction of dimethylgermylene with the π-system of arylated germanium precursors.

Photochemistry of phenyl-substituted digermanoxane. First direct observation of germoxy radicals probed by laser flash photolysis

Laser flash photolysis of the phenyl-substituted digermanoxanes (PhnMe3-nGe)2O (n = 1—3) results in GeO bond homolysis to give germyl radicals and germoxy radicals.