Synthesis and photophysical properties of borondipyrromethene dyes bearing aryl substituents at the boron center

Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)2 derivatives; where aryl refers to phenyl, anisyl, naphthyl, or pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that the sp3 hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615 and 1.636 A. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)2 compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent dependent and, at least in some cases (aryl = anisyl or pyrenyl), nonradiative decay from the first-excited singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state, in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer from pyrene or naphthalene residues to Bodipy is quantitative.     

Delineation of a fundamental alpha-ketoheterocycle substituent effect for use in the design of enzyme inhibitors

The synthesis and examination of a systematic series of 5-substituted 2-keto oxazoles as inhibitors of fatty acid amide hydrolase (FAAH) defined a fundamental substituent effect that led to the discovery of inhibitors with Ki's as low as 400 pM. The intrinsic basis of the relationship (-log Ki vs sigmap), which relates Ki with the Hammett sigmap constant of the substituent, the magnitude of the effect (rho = 3.01), and its predictive value (R2 = 0.91) suggest a widespread applicability in studies beyond FAAH.     

Trichloromethyl chloroformate ("diphosgene"), ClCOOCCl3: structure and conformational properties in the gaseous and condensed phases

The conformational properties of gaseous trichloromethyl chloroformate (or "diphosgene"), ClC(O)OCCl3, have been studied by vibrational spectroscopy [IR (gas), IR (matrix), and Raman (liquid)] and quantum chemical calculations (MP2 and B3LYP with 6-311G basis sets); in addition, the structure of a single crystal at low temperature has been determined by X-ray diffraction. ClC(O)OCCl3 exhibits only one conformational form having Cs symmetry with a synperiplanar orientation of the C-O single bond relative to the C=O double bond. The calculated energy difference between the syn and anti forms, 5.73 kcal mol(-1) (B3LYP) or 7.06 kcal mol(-1) (MP2), is consistent with the experimental findings for the gas and liquid phases. The crystalline solid at 150 K [monoclinic, P2(1)/n, a = 5.5578(5) angstroms, b = 14.2895(12) angstroms, c = 8.6246(7) angstroms, beta = 102.443(2) degrees, Z = 4] likewise consists only of molecules in the syn form.     

Synthesis of the enantiomers of the dual function 2‐nitroimidazole radiation sensitizer RB 6145

Short, efficient pathways are described for the synthesis of racemic 2‐nitroimidazole radiation sensitizer RB‐6145 ( 2a ) and each of its corresponding (R)‐ and (S)‐enantiomers ( 2b and 2c , respectively). The synthesis of each enantiomer commences with the appropriate chiral epichlorohydrin and utilizes a novel application of 3‐trimethylsilyl‐2‐oxazolidinone ( 3b ) as a mild, safe surrogate for highly toxic aziridine. The synthesis of the (R)‐enantiomer ( 2b ) has been successfully scaled up to provide multi‐kilo quantities of material for early stage preclinical evaluation.     

Bicoherence analysis of model-scale jet noise

Bicoherence analysis has been used to characterize nonlinear effects in the propagation of noise from a model-scale, Mach-2.0, unheated jet. Nonlinear propagation effects are predominantly limited to regions near the peak directivity angle for this jet source and propagation range. The analysis also examines the practice of identifying nonlinear propagation by comparing spectra measured at two different distances and assuming far-field, linear propagation between them. This spectral comparison method can lead to erroneous conclusions regarding the role of nonlinearity when the observations are made in the geometric near field of an extended, directional radiator, such as a jet.     

A Study of Microscale Gas Trapping Using Etched Silicon Micromodels

Immobilization and trapping of carbon dioxide (CO₂) enhances the security of geological storage. Trapping mechanisms have been characterized in four groups: structural, residual, dissolution, and mineralization. While structural trapping acts immediately when injection starts and is well investigated, the contribution of residual and dissolution trapping increases over storage time and these contributions need to be better understood for better predictions. This paper focuses on an experimental pore-scale investigation of residual and capillary trapping. CO₂?Cwater imbibition experiments were conducted in micromodels whose homogenous pore space is geometrically and topologically similar to Berea sandstone. Microvisual data, photographs and video footage, describes the trapping mechanism and, especially, the disconnection and shrinkage of the CO₂ phase. Results show that depending on the flow rate of the imbibing water different trapping mechanisms are observed. Lower flow rates, comparable to the trailing edge of a CO₂ plume, lead to more snap-off events and greater trapped residual saturation, whereas rates comparable to the near wellbore area during enhanced sequestration showed displacement of gas bubbles and greater dissolution that ultimately leads to very low or zero gas saturations. Furthermore, complete dissolution events showed that homogenous as well as heterogeneous dissolution occurs. Whereas the latter is subdivided into microbubble formation and dissolution on crevices or pore roughness, the former occurs without the influence of pore walls. Based on the observations we suggest that the type of rock and its roughness as well as the fines present at the CO₂ brine interface are important factors determining the dissolution mechanism.     

Electrophoresis of sheared polyelectrolytes

ABSTRACT

A shear flow breaks the spherical symmetry of a flexible polymer, which has some interesting consequences for the electrophoresis of polyelectrolytes. In addition to introducing a chain-length dependence of the electrophoretic velocity, there is also the possibility of migration of the molecule perpendicular to the direction of coaxial gradients in pressure and electric potential. This has been shown to produce a rapid and highly localised concentration of DNA within a microfluidic capillary, with a number of potential applications to on-chip preparation and analysis of genomic DNA. In this paper, dedicated to Prof. Daan Frenkel, I will describe a calculation of the electrophoretic motion of a long polyelectrolyte under a coaxial flow and electric field.     

Ultrafast X‐ray imaging of laser–metal additive manufacturing processes

The high‐speed synchrotron X‐ray imaging technique was synchronized with a custom‐built laser‐melting setup to capture the dynamics of laser powder‐bed fusion processes in situ. Various significant phenomena, including vapor‐depression and melt‐pool dynamics and powder‐spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large‐scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts.     

The Critical Fugacity for Surface Adsorption of Self-Avoiding Walks on the Honeycomb Lattice is {1+\sqrt{2}}

In 2010, Duminil-Copin and Smirnov proved a long-standing conjecture of Nienhuis, made in 1982, that the growth constant of self-avoiding walks on the hexagonal (a.k.a. honeycomb) lattice is ${\mu=\sqrt{2+\sqrt{2}}}$ . A key identity used in that proof was later generalised by Smirnov so as to apply to a general O(n) loop model with ${n\in [-2,2]}$ (the case n = 0 corresponding to self-avoiding walks). We modify this model by restricting to a half-plane and introducing a surface fugacity y associated with boundary sites (also called surface sites), and obtain a generalisation of Smirnov’s identity. The critical value of the surface fugacity was conjectured by Batchelor and Yung in 1995 to be ${y_{\rm c}=1+2/\sqrt{2-n}}$ . This value plays a crucial role in our generalized identity, just as the value of the growth constant did in Smirnov’s identity. For the case n = 0, corresponding to self-avoiding walks interacting with a surface, we prove the conjectured value of the critical surface fugacity. A crucial part of the proof involves demonstrating that the generating function of self-avoiding bridges of height T, taken at its critical point 1/μ, tends to 0 as T increases, as predicted from SLE theory.