Catalytic 1,4‐Rhodium(III) Migration Enables 1,3‐Enynes to Function as One‐Carbon Oxidative Annulation Partners in C?H Functionalizations

1,3‐Enynes containing allylic hydrogens cis to the alkyne are shown to act as one‐carbon partners, rather than two‐carbon partners, in various rhodium‐catalyzed oxidative annulations. The mechanism of these unexpected transformations is proposed to occur through double C H activation, involving a hitherto rare example of the 1,4‐migration of a Rh^III species. This phenomenon is general across a variety of substrates, and provides a diverse range of heterocyclic products.     

Synthesis and Two‐Photon Absorption Property Characterizations of Small Dendritic Chromophores Containing Functionalized Quinoxaliniod Heterocycles

A series of star‐shaped multi‐polar chromophores (compounds 1 – 3 ) containing functionalized quinoxaline and quinoxalinoid (indenoquinoxaline and pyridopyrazine) units has been synthesized and characterized for their two‐photon absorption (2PA) properties both in the femtosecond and the nanosecond time domain. Under our experimental conditions, these model fluorophores are found to manifest strong and wide‐dispersed two‐photon absorption in the near‐infrared region. It is demonstrated that molecular structures with multi‐branched π frameworks incorporating properly functionalized quinoxalinoid units would possess large molecular nonlinear absorptivities within the studied spectral range. Effective optical‐power attenuation and stabilization behaviors in the nanosecond time domain of a selected representative dye molecule (i.e., compound 2 ) from this model compound set were also investigated and the results indicate that such structural motif could be a useful approach for the molecular design toward strong two‐photon‐absorbing material systems for quick‐responsive and broadband optical‐suppressing‐related applications, particularly to confront long laser pulses.     

Bi-wavelength two dimensional chirped grating couplers for low cost WDM PON transceivers

We propose and demonstrate a bi-wavelength two dimensional (2D) waveguide grating coupler on silicon-on-insulator which has efficient coupling of optical light with two-wavelength bands independently between standard optical single mode fibers and nanophotonic waveguides. The details of design are described and the measurement results as well as system performance are experimentally characterized. The bi-wavelength grating coupler can be used as wavelength-division-multiplexing (WDM) splitter/combiner for monolithically silicon integrated transceivers, potentially meeting the low cost requirements for future WDM passive optical network (PON).     

Silica-clad crystalline germanium core optical fibers

Silica-clad optical fibers comprising a core of crystalline germanium were drawn using a molten core technique. With respect to previous fibers drawn using a borosilicate cladding, the present fibers exhibit negligible oxygen despite being fabricated at more than twice the melting point of the germanium. The counterintuitive result of less oxygen when the fiber is drawn at a higher temperatures is discussed. The measured propagation loss for the fiber was 0.7?dB/cm at 3.39?μm, which is the lowest loss reported to date.     

Structural Basis for the Identification of an i‐Motif Tetraplex Core with a Parallel‐Duplex Junction as a Structural Motif in CCG Triplet Repeats

CCG triplet repeats can fold into tetraplex structures, which are associated with the expansion of (CCG)_n trinucleotide sequences in certain neurological diseases. These structures are stabilized by intertwining i‐motifs. However, the structural basis for tetraplex i‐motif formation in CCG triplet repeats remains largely unknown. We report the first crystal structure of a CCG‐repeat sequence, which shows that two dT(CCG)₃A strands can associate to form a tetraplex structure with an i‐motif core containing four C:C⁺ pairs flanked by two G:G homopurine base pairs as a structural motif. The tetraplex core is attached to a short parallel‐stranded duplex. Each hairpin itself contains a central CCG loop in which the nucleotides are flipped out and stabilized by stacking interactions. The helical twists between adjacent cytosine residues of this structure in the i‐motif core have an average value of 30°, which is greater than those previously reported for i‐motif structures.     

Synthesis of Spirocyclic Enones by Rhodium‐Catalyzed Dearomatizing Oxidative Annulation of 2‐Alkenylphenols with Alkynes and Enynes

The dearomatizing oxidative annulation of 2‐alkenylphenols with alkynes and enynes proceeds with high yields and regioselectivities under Rh^III catalysis. These reactions are successful using Cu(OAc)₂ or air as the stoichiometric oxidant, and provide spirocyclic enones, the basic ring system of which appears in several natural products. Application of this process to the preparation of a highly functionalized tetracycle is also demonstrated.     

Tetrathienoacene copolymers as high mobility, soluble organic semiconductors

Increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that consists of four fused thiophene rings is shown to be a promising route toward high-performance organic semiconductors. We report on a dialkylated tetrathienoacene copolymer that can be deposited from solution to yield ordered films with a short pi-pi distance of 3.76 A and with a field-effect hole mobility that exceeds 0.3 cm2/V.s. This polymer enables simple transistor fabrication at relatively low temperatures, which is particularly important for the realization of large-area, mechanically flexible electronics.     

Reversible UV-light-induced ultrahydrophobic-to-ultrahydrophilic transition in an alpha-Fe2O3 nanoflakes film

We describe a simple and robust approach to fabricating an alpha-Fe2O3 switchable surface. The hydrophobicity of alpha-Fe2O3 nanostructures was observed for the first time. A remarkable surface wettability transition can be easily achieved by ultraviolet (UV) illumination. The distinctive properties of surface defects are disclosed by X-ray photoelectron spectroscopy (XPS) analysis. The nanoscale adsorption and photocatalytic properties of Fe2+ defects account for the highly amphiphilic character of the surfaces. We believe that the experiment will further the molecular-scale understanding and manipulation of the wetting behavior on smart devices.     

Copper-catalyzed silylation of cyclopropenes using (trifluoromethyl)trimethylsilane

A variety of cyclopropenes undergo direct silylation using (trifluoromethyl)trimethylsilane in the presence of a copper-bisphosphine catalyst; under these conditions, cyclopropenes that might otherwise undergo ring-opening are silylated efficiently.