Solid-state bonding technique for template-stripped ultraflat gold substrates

A simple procedure using gold diffusion bonding for the preparation of template-stripped gold (TSG) surfaces is described. TSG surfaces are useful for surface studies because a very consistent flat gold surface with few defects can be easily prepared. We have developed a method of producing TSG surfaces that relies only on gold diffusion bonding rather than epoxies. The resulting substrates are free from concerns of solvent compatibility, heat stability, and impurities. Bonding of centimeter-sized substrates is performed at 300 degrees C for 2 h using a vise and aluminum foil.     

Synthetic chlorins bearing auxochromes at the 3- and 13-positions

Synthetic chlorins bearing diverse auxochromes at the 3- and 13-positions of the macrocycle are valuable targets given their resemblance to chlorophylls a and b, which bear 3-vinyl and 13-keto groups. A de novo route has been exploited to construct nine zinc chlorins bearing substituents at the 3- and 13-positions and two benchmark zinc chlorins lacking such substituents. The chlorins are sterically uncongested and bear (1) a geminal dimethyl group in the reduced pyrroline ring, (2) a H, an acetyl, a triisopropylsilylethynyl (TIPS-ethynyl), or a vinyl at the 3-position, (3) a H, an acetyl, or TIPS-ethynyl at the 13-position, and (4) a H or a mesityl at the 10-position. The synthesis of the 13-substituted chlorins relied on p-TsOH x H2O-catalyzed condensation of an 8,9-dibromo-1-formyldipyrromethane (eastern half) and 2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin (western half), followed by metal-mediated oxidative cyclization, affording the 13-bromochlorin. Similar use of a bromo- or TIPS-ethynyl-substituted western half provided access to 3-substituted chlorins. A 3-bromo, 13-bromo, or 3,13-dibromochlorin was further transformed by Pd-coupling to introduce the vinyl group (via tributylvinyltin), TIPS-ethynyl group (via TIPS-acetylene), or acetyl group (via tributyl(1-ethoxyvinyl)tin, followed by acidic hydrolysis). In the 10-mesityl-substituted zinc chlorins, the series of substituents, 3-vinyl, 13-TIPS-ethynyl, 3-TIPS-ethynyl, 13-acetyl, 3,13-bis(TIPS-ethynyl), 3-TIPS-ethynyl-13-acetyl, or 3,13-diacetyl, progressively causes (1) a redshift in the absorption maximum of the B band (405-436 nm) and the Q(y) band (606-662 nm), (2) a relative increase in the intensity of the Q(y) band (I(B)/I(Q) = 4.2-1.5), and (3) an increase in the fluorescence quantum yield phi(f) (0.059-0.29). The zinc chlorins bearing a 3-TIPS-ethynyl-13-acetyl or a 3,13-diacetyl group exhibit a number of spectral properties resembling those of chlorophyll a or its zinc analogue. Taken together, this study provides access to finely tuned chlorins for spectroscopic studies and diverse applications.     

Microwave-assisted synthesis of 3,6-di(pyridin-2-yl)pyridazines: unexpected ketone and aldehyde cycloadditions

3,6-Di(pyridin-2-yl)pyridazines are an interesting class of compounds because of their metal-coordinating ability resulting in the self-assembly into [2x2] gridlike metal complexes with copper(I) or silver(I) ions. These and other substituted pyridazines can be prepared by the inverse-electron-demand Diels-Alder reactions between acetylenes and 1,2,4,5-tetrazines. In this contribution, the effect of (superheated) microwave conditions on these generally slow cycloadditions is described. The cycloaddition of acetylenes to 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine could be accelerated from several days reflux in toluene or N,N-dimethylformamide to several hours in dichloromethane at 150 degrees C. In addition, the unexpected cycloaddition of the enol tautomers of various ketones and aldehydes to 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine is described in detail providing an alternative route for the synthesis of (substituted) pyridazines.     

Swelling and deswelling of adsorbed microgel monolayers triggered by changes in temperature, pH, and electrolyte concentration

The formation and characterization of close-packed monolayers of negative, poly(N-isopropylacrylamide)-based microgel particles onto positively charged silicon wafers is described. The silicon wafers were rendered positive by first oxidizing their surface to silica and then adsorbing a layer of poly(ethyleneimine). The thickness of the deposited microgel monolayers (under aqueous conditions) has been determined by spectroscopic ellipsometry as a function of temperature (20-60 degrees C), pH (3-8), and added NaCl concentration (0-1 M). No actual desorption of the microgel particles was evident on changing the conditions, but a swelling/deswelling transition was observed around 32 degrees C. The thickness of the monolayer has been compared with the hydrodynamic diameter of the free microgel particles, dispersed in aqueous solution. For the poly(N-isopropylacrylamide) microgel particles, without any bulk ionisable comonomer groups present, the temperature dependence of the ellipsometric thickness of the monolayer mirrors closely that of the hydrodynamic diameter of the free particles. When ionizable (-COOH) groups are introduced into the microgel particles, however, this correspondence is largely lost because the microgel particles forming the deposited monolayer now contract strongly onto the oppositely charged substrate surface.     

Evidence for beta-chlorocarbenium and beta-bromocarbenium ions

Isotopic perturbation of degenerate equilibrium is used to determine whether tetramethylethylenechloronium and tetramethylethylenebromonium ions are closed 1,2-bridged structures or rapid equilibria of open beta-halocarbenium ions. The observed 13C NMR isotope shifts are consistent with a combination of large equilibrium shifts and small upfield intrinsic shifts. The presence of equilibrium shifts in both halonium ions indicates that these ions are not closed 1,2-bridged structures. Rather, they are best represented by equilibria of beta-halocarbenium ions.     

Mesoporous silica nanoparticles for intracellular delivery of membrane-impermeable proteins

An MCM-41-type mesoporous silica nanoparticle (MSN) material with a large average pore diameter (5.4 nm) is synthesized and characterized. The in vitro uptake and release profiles of cytochrome c by the MSN were investigated. The enzymatic activity of the released protein was quantitatively analyzed and compared with that of the native cytochrome c in physiological buffer solutions. We found that the enzymes released from the MSNs are still functional and highly active in catalyzing the oxidation of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) by hydrogen peroxide. In contrast to the fact that cytochrome c is a cell-membrane-impermeable protein, we discovered that the cytochrome c-encapsulated MSNs could be internalized by live human cervical cancer cells (HeLa) and the protein could be released into the cytoplasm. We envision that these MSNs with large pores could serve as a transmembrane delivery vehicle for controlled release of membrane-impermeable proteins in live cells, which may lead to many important biotechnological applications including therapeutics and metabolic manipulation of cells.     

Theoretical investigation of lone pair inversions, ring openings, and hydride shifts in O-methylated epoxides

Mechanisms associated with the isomerization of the O-methylethylene oxonium ion and its tetramethyl-substituted analogue have been explored using correlated electronic structure calculations. The minima and transition states associated with inversion at the oxygen atom, as well as those associated with opening of the epoxide ring, have been characterized. The calculated barrier to inversion at the oxygen atom for the O-methylethylene oxonium ion, 15.7 kcal/mol, agrees well with the experimentally determined value, 10+/-2 kcal/mol. Our calculations indicate that a significantly higher barrier exists for the ring-opening mechanism that leads to more thermodynamically stable structures. This work includes the first known calculations on the O-methyl-2,3-dimethyl-2-butene oxonium ion along with transition states and intermediates associated with ring opening and inversion at the oxygen atom. Results show that there is a significantly lower barrier to ring opening as compared to the O-methylethylene oxonium ion species, leading to a lower probability of isolating this species. The effects of basis sets and correlation techniques on these ions were also analyzed in this work. Our results indicate that the B3LYP/6-31G* level is reliable for obtaining molecular geometries for both minima and transition states on the C3H7O+ and C7H15O+ potential energy surfaces.     

Modulating the light switch by (3)MLCT-(3)ππ* state interconversion

The spectroscopic, electronic, and DNA-binding characteristics of two novel ruthenium complexes based on the dialkynyl ligands 2,3-bis(phenylethynyl)-1,4,8,9-tetraaza-triphenylene (bptt, 1) and 2,3-bis(4-tert-butyl-phenylethynyl)-1,4,8,9-tetraaza-triphenylene (tbptt, 2) have been investigated. Electronic structure calculations of bptt reveal that the frontier molecular orbitals are localized on the pyrazine-dialkynyl portion of the free ligand, a property that is reflected in a red shift of the lowest energy electronic transition (1: λ(max) = 393 nm) upon substitution at the terminal phenyl groups (2: λ(max) = 398 nm). Upon coordination to ruthenium, the low-energy ligand-centered transitions of 1 and 2 are retained, and metal-to-ligand charge transfer transitions (MLCT) centered at λ(max) = 450 nm are observed for [Ru(phen)(2)bptt](2+)(3) and [Ru(phen)(2)tbptt](2+)(4). The photophysical characteristics of 3 and 4 in ethanol closely parallel those observed for [Ru(bpy)(3)](2+) and [Ru(phen)(3)](2+), indicating that the MLCT excited state is primarily localized within the [Ru(phen)(3)](2+) manifold of 3 and 4, and is only sparingly affected by the extended conjugation of the bptt framework. In an aqueous environment, 3 and 4 possess notably small luminescence quantum yields (3: ?(H(2)O) = 0.005, 4: ?(H(2)O) = 0.011) and biexponential decay kinetics (3: τ(1) = 40 ns, τ(2) = 230 ns; 4: τ(1) ～ 26 ns, τ(2) = 150 ns). Addition of CT-DNA to an aqueous solution of 3 causes a significant increase in the luminescence quantum yield (?(DNA) = 0.045), while the quantum yield of 4 is relatively unaffected (?(DNA) = 0.013). The differential behavior demonstrates that tert-butyl substitution on the terminal phenyl groups inhibits the ability of 4 to intercalate with DNA. Such changes in intrinsic luminescence demonstrate that 3 binds to DNA via intercalation (K(b) = 3.3 × 10(4) M(-1)). The origin of this light switch behavior involves two competing (3)MLCT states similar to that of the extensively studied light switch molecule [Ru(phen)(2)dppz](2+). The solvent- and temperature-dependence of the luminescence of 3 reveal that the extended ligand aromaticity lowers the energy of the (3)ππ* excited state into competition with the emitting (3)MLCT state. Interconversion between these two states plays a significant role in the observed photophysics and is responsible for the dual emission in aqueous environments.     

Dynamics of ice nucleation on water repellent surfaces

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.