The C À B bond of organoboronic acids is totally covalent and therefore inert to ionic reactions; however, the nucleophilicity of organic groups attached to a boron atom is enhanced significantly by quaternarization with an anionic ligand. [1] Thus, tetracoordinated ate complexes have been used successfully for most nucleophilic addition and coupling reactions of organoboron compounds, including metal-catalyzed reactions of organoboronic acids. [1, 2] Although the in situ preparation of such complexes from an organoborane and a base is most common, the method has limitations. Therefore, attempts have been made to prepare stable ate complexes. Air-and water-stable trifluoroborates M[RBF 3 ] (M = K, NR 4 ) [3] are typical compounds that exist as pure and waterstable crystalline materials and have advantages over boronic acids in terms of their preparation and handling. However, their metal-catalyzed bond-forming reactions are very slow in the absence of a base because of the low nucleophilicity of the organic group owing to the high electronegativity of the fluorine atoms.[3] Sodium trihydroxyborates [RB(OH) 3 ]Na were synthesized recently as isolated discrete species for cross-coupling in anhydrous solvents without the aid of an additional base.[4] Other isolable ate complexes are 1-alkynyl borates Li[RCCB(OiPr) 3 ][5] and tetraaryl borates M-[Ar 4 B].[6] We describe herein novel cyclic triolborates [7] that are exceptionally stable in air and water and more soluble in organic solvents than potassium trifluoroborates. We also demonstrate the high transmetalation efficiency of lithium and potassium triolborates in palladium-and copper-catalyzed CÀC [2] and CÀN [8] bond-forming reactions. We developed methods for the synthesis of the cyclic triolborates 4 and 6 (Scheme 1). The azeotropic removal of water upon the treatment of organoboronic acids 1 with the triol 2 a gave boronic esters 3, which were converted readily into triolborates 4 by treatment with KOH. Compounds 4 were also obtained in high yields when the esterification was followed directly by quarternization with KOH in the same flask without the isolation of 3. The triolborates 4 are insoluble in toluene (the solvent used) and precipitated as white solids in high yields. The corresponding lithium salts 6 were synthesized by the alkylation of B(OMe) 3 (5 a) or B(OiPr) 3 (5 b) with RLi, followed by the removal of MeOH or iPrOH through ester exchange with 2 a. By using this protocol, the 2-pyridyl borate 6 c, which is sensitive to BÀC bond cleavage in the presence of water, was obtained in high yield. The treatment of PhB(OH) 2 with the triol 2 b followed by nBu 4 NOH afforded a single crystal of the ammonium salt monohydrate [9] (NBu 4 )[C 6 H 5 B(OCH 2 ) 3 CCH 2 CH 3 ]·H 2 O (7) for X-ray crystal analysis.An ORTEP plot of 7 showed that the molecular structure contains a bicyclo[2.2.2]octane ring that includes a tetrahedral boron atom (Figure 1). The C À B bond (1.60 ) is slightly longer than that of neutral PhB(OR) 2 (R = H, alkyl; 1.56 ) as a result of ...

Nanostructure-based photovoltaic devices have exhibited several advantages, such as reduced reflection, extraordinary light trapping, and so forth. In particular, semiconductor nanostructures provide optical modes that have strong dependence on the size and geometry. Metallic nanostructures also attract a lot of attention because of the appealing plasmonic effect on the near-field enhancement. In this study, we propose a novel design, the metal-core/semiconductor-shell nanocones with the core radius varying in a linearly gradient style. With a thin layer of semiconductor absorber coated on a metallic cone, such a design can lead to significant and broadband absorption enhancement across the entire visible and near-infrared solar spectrum. As an example of demonstration, a layer of 16 nm thick crystalline silicon (c-Si) coated on a silver nanocone can absorb 27% of standard solar radiation across a broad spectral range of 300-1100 nm, which is equivalent to a 700 nm thick flat c-Si film. Therefore, the absorption enhancement factor approaching the Yablonovitch limit is achieved with this design. The significant absorption enhancement can be ascribed to three types of optical modes, that is, Fabry-Perot modes, plasmonic modes, and hybrid modes that combine the features of the previous two. In addition, the unique nanocone geometry enables the linearly gradient radius of the semiconductor shell, which can support multiple optical resonances, critical for the broadband absorption. Our design may find general usage as elements for the low cost, high efficiency solar conversion and water-splitting devices.

One promising technique for working toward practical photonic quantum technologies is to implement multiple operations on a monolithic chip, thereby improving stability, scalability and miniaturization. The on-chip spatial control of entangled photons will certainly benefi t numerous applications, including quantum imaging, quantum lithography, quantum metrology and quantum computation. However, external optical elements are usually required to spatially control the entangled photons. Here we present the fi rst experimental demonstration of on-chip spatial control of entangled photons, based on a domain-engineered nonlinear photonic crystal. We manipulate the entangled photons using the inherent properties of the crystal during the parametric downconversion, demonstrating two-photon focusing and beam-splitting from a periodically poled lithium tantalate crystal with a parabolic phase profi le. These experimental results indicate that versatile and precise spatial control of entangled photons is achievable. Because they may be operated independent of any bulk optical elements, domain-engineered nonlinear photonic crystals may prove to be a valuable ingredient in on-chip integrated quantum optics.