Shin-ya Takizawa

Benzimidazoline-Dimethoxypyrene. An Effective Promoter System for Photoinduced Electron Transfer Promoted Reductive Transformations of Organic Compounds

2-(p-Methoxyphenyl)-1,3-dimethylbenzimidazoline (ADMBI) and 2-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (HPDMBI) are used as reducing reagents in 1,8-dimethoxypyrene (1,8-DMP) sensitized, photoinduced electron transfer (PET) reactions. This system was effectively used for PET induced, reductive transformations of various organic substrates, including α,β-epoxy ketones, the olefin tethered 2-bromomethyl-1-tetralone, and o-allyloxy-iodobenzene, as well as for the deprotection reactions of dodecyl-2-benzoylbenzoate and N-sulfonylindole. The results of studies show that 1,8-DMP is a more effective sensitizer than the previously used 9-methylcarbazole for deprotection of N-methyl-4-picolinium ester.

Synthesis, characterization and electroluminescence properties of new iridium complexes based on cyclic phenylvinylpyridine derivatives: Tuning of emission colour and efficiency by structural control

A series of phosphorescent cyclometalated iridium(III) complexes 4–6 with new ligands of bicyclic phenylvinylpyridines have been synthesized and systematically examined for their photophysical, electrochemical and electroluminescence (EL) properties. All of the bis- and tris-cyclometalated complexes could be characterized by 1H-NMR, elemental analysis and X-ray crystal structure analysis. The phosphorescence colour of the complexes in solution, the solid state and polymer film is unusually changed depending on the structure of the ligand. EL devices using the complexes exhibited external quantum efficiencies (EQE) of 1.2–8.1% at 100 cd m−2, and EL colour tuning in the range of 571–635 nm was achieved analogous to PL properties of the phosphors. X-Ray analyses of complexes 4a–c demonstrated that the degree of steric hindrance increases with an increase of the cycloalkane ring size, leading to hypsochromic shifts of emission by the distortion of the phenyl groups. X-Ray analyses also suggested that long iridium–carbon bonds are unfavorable for efficient electroluminescence. In addition, we have found a unique and important aspect for the study of EL devices: the EL devices based on complexes 4c, 6a show moderate EL efficiencies (EQE = 4.0 and 6.1%) although these complexes have no emission in solution at room temperature, indicating that the PL properties of the complexes in solution are not directly related to the EL efficiencies.

Photoinduced electron-transfer reaction of α-bromomethyl-substituted benzocyclic β-keto esters with amines: selective reaction pathways depending on the nature of the amine radical cations

Photoinduced electron-transfer reaction of α-bromomethyl-substituted benzocyclic β-keto esters with tertiary amines was investigated. Debrominated β-keto esters and ring-expanded γ-keto esters were obtained as major products. On the basis of mechanistic experiments it was concluded that these products are formed via a reaction sequence of selective carbon–bromine bond cleavage and subsequent competitive hydrogen abstraction and Dowd–Beckwith ring-expansion of the resulting primary alkyl radicals. The characteristic product distribution observed for the type of amine used is rationalized on the basis of selective reaction pathways of generated radical intermediates that depend on the nature of the amine radical cations.

Phosphorescence Color Tunable Iridium Complexes with Ligands of 2-Phenylimidazo[1,2-a]Pyridine Derivatives

New iridium complexes with ligands of 2-phenylimidazo[1,2-a]pyridines were prepared, and their emission maxima were found to be greatly dependent on the substituents. A trifluoromethyl group on the imidazole part made the emission maximum blue-shifted, and combination with the same groups on the phenyl ring resulted in the bluest phosphorescence among the derivatives.

Changeable reactivity of ketyl radicals derived from 2-bromomethyl-2-(3-butenyl)benzocyclic-1-alkanones depending on electron transfer conditions employed

Photoinduced electron transfer reaction of 2-bromomethyl-2-(3-butenyl)benzocyclic-1-alkanones with amines afforded 5-exo radical cyclization products while electron transfer reaction with samarium diiodide produced cyclopropanols.