A three-state effective Hamiltonian for symmetric cationic diarylmethanes

We analyze the low-energy electronic structure of a series of symmetric cationic diarylmethanes, which are bridge-substituted derivatives of Michler's Hydrol Blue. We use a four-electron, three-orbital complete active space self-consistent field and multi-state multi-reference perturbation theory model to calculate a three-state diabatic effective Hamiltonian for each dye in the series. We exploit an isolobal analogy between the active spaces of the self-consistent field solutions for each dye to represent the electronic structure in a set of analogous diabatic states. The diabatic states can be identified with the bonding structures in classical resonance-theoretic models of cyanine dyes. We identify diabatic states with opposing charge and bond-order localization, analogous to the classical resonance structures, and a third state with charge on the bridge. While the left- and right-charged structures are similar for all dyes, the structure of the bridge-charged diabatic state, and the Hamiltonian matrix elements connected to it, change significantly across the series. The change is correlated with an inversion of the sign of the charge carrier on the bridge, which changes from an electron pair to a hole as the series is traversed.     

A highly diastereoselective series of Nicholas cyclisation reactions of N-Boc-protected propargylamines

The reaction of N-Boc-protected propargylamine with salicylaldehyde derivatives and their subsequent Nicholas cyclisation reaction to provide a range of novel benzopyrans is reported. The cyclisation reactions proceeded with excellent levels of diastereoselectivity to afford compounds with cis-relative stereochemistry. As far as we are able to ascertain these are the first reported examples of Nicholas cyclisation reactions of propargyl alcohols that bear a terminal alkynyl N-protected amino motif.     

An experimental and in situ IR spectroscopic study of the lithiation-substitution of N-Boc-2-phenylpyrrolidine and -piperidine: controlling the formation of quaternary stereocenters

A general and enantioselective synthesis of 2-substituted 2-phenylpyrrolidines and -piperidines, an important class of pharmaceutically relevant compounds that contain a quaternary stereocenter, has been developed. The approach involves lithiation-substitution of enantioenriched N-Boc-2-phenylpyrrolidine or -piperidine (prepared by asymmetric Negishi arylation or catalytic asymmetric reduction, respectively). The combined use of synthetic experiments and in situ IR spectroscopic monitoring allowed optimum lithiation conditions to be identified: n-BuLi in THF at -50 °C for 5-30 min. Monitoring of the lithiation using in situ IR spectroscopy indicated that the rotation of the tert-butoxycarbonyl (Boc) group is slower in a 2-lithiated pyrrolidine than a 2-lithiated piperidine; low yields for the lithiation-substitution of N-Boc-2-phenylpyrrolidine at -78 °C can be ascribed to this slow rotation. For N-Boc-2-phenylpyrrolidine and -piperidine, the barriers to rotation of the Boc group were determined using density functional theory calculations and variable-temperature (1)H NMR spectroscopy. For the pyrrolidine, the half-life (t(1/2)) for rotation of the Boc group was found to be ～10 h at -78 °C and ～3.5 min at -50 °C. In contrast, for the piperidine, t(1/2) was determined to be ～4 s at -78 °C.     

Stacey Crossed Products Associated to Exel Systems

There are many different crossed products by an endomorphism of a C*-algebra, and constructions by Exel and Stacey have proved particularly useful. Here we consider Exel crossed products associated to transfer operators which extend to be unital on the multiplier algebra. We show that every Exel crossed product is isomorphic to a Stacey crossed product, though by a different endomorphism of a different C*-algebra. We apply this result to a variety of Exel systems, including those associated to shifts on the path spaces of directed graphs.     

Anomalous excitation spectra of frustrated quantum antiferromagnets

We use series expansions to study the excitation spectra of spin-1/2 antiferromagnets on anisotropic triangular lattices. For the isotropic triangular lattice model (TLM), the high-energy spectra show several anomalous features that differ strongly from linear spin-wave theory (LSWT). Even in the Néel phase, the deviations from LSWT increase sharply with frustration, leading to rotonlike minima at special wave vectors. We argue that these results can be interpreted naturally in a spinon language and provide an explanation for the previously observed anomalous finite-temperature properties of the TLM. In the coupled-chains limit, quantum renormalizations strongly enhance the one-dimensionality of the spectra, in agreement with experiments on Cs2CuCl4.     

Harmonic entanglement with second-order nonlinearity

We investigate the second-order nonlinear interaction as a means to generate entanglement between fields of differing wavelengths and show that perfect entanglement can, in principle, be produced between the fundamental and second-harmonic fields in these processes. Neither pure second-harmonic generation nor parametric oscillation optimally produce entanglement; such optimal entanglement is rather produced by an intermediate process.     

Generation and contrasting gas-phase reactivity of 2-(2-alkenylpyrrol-1-yl)phenoxyl and thiophenoxyl radicals

The pyrrolylacrylates 9 and 10 were synthesised and subjected to flash vacuum pyrolysis (FVP) at 650-700 degrees C to generate the radicals 11 and 18, respectively. The phenoxyl 11 underwent hydrogen capture to give a mixture of the phenol 12 and the pyrrolobenzoxazine 13 in low yields, which were also obtained by a Wittig reaction of the 2-formylpyrrole 14. The thiophenoxyl 18 gave a single major product in 41% yield which was identified as the pyrrolo[1,2-a]quinoline 17 by a sequence of NMR experiments. A mechanism for the formation of 17 by a rearrangement-sulfur extrusion sequence is proposed.     

Asymmetric synthesis of 3-hydroxy-pyrrolidines via tin-lithium exchange and cyclization

We report a method for the synthesis of chiral pyrrolidines using tin-lithium exchange and cyclization reactions. The precursors are formed readily from simple starting materials and undergo tin-lithium exchange by treatment with n-butyllithium. Subsequent intramolecular carbolithiation is stereoselective to give highly enantiomerically enriched pyrrolidines in excellent yields.     

Radical ion pair mediated triplet formation in polymer-fullerene blend films

Efficient triplet formation is observed for films of high ionisation potential polythiophenes blended with a fullerene derivative, and assigned to formation via geminate charge recombination of bound radical ion pair states.     

Single-molecule electrical studies on a 7 nm long molecular wire

A self-assembled arylene-ethynylene molecular wire with a rigid 7 nm long backbone exhibits symmetrical current-voltage (I-V) characteristics and a single-molecule current of 0.35 +/- 0.05 nA at 0.3 V; these data are supported by theoretical calculations.