Immobilization of Pd Catalysts on Mesoporous Silica for Amine- and Copper-Free Sonogashira Coupling Reactions

Immobilization of catalysts on solid supports is a promising approach to combine the advantages of heterogeneous and homogeneous catalysts. Pd(PPh₃)₂Cl₂, known as an extremely active homogeneous catalyst for the Sonogashira coupling reaction, has been immobilized on high-surface-area MCF (mesocellular foams)–type mesoporous silica powder modified with 3-aminopropyltriethoxysilane and subsequently with diphenylphosphine. The functionalized MCF-type silica and supported catalysts have been characterized by x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR), elemental analysis, nitrogen sorption porosimetry, and scanning electron microscopy (SEM). Such supported Pd catalysts have proven to be useful recyclable reagents for copper- and amine-free Sonogashira coupling reactions of haloaromatic compounds with terminal alkynes.     

One-step synthesis and redox properties of dodecahydro-3a,9a-diazaperylene--the most easily oxidized p-phenylenediamine

Dodecahydro-3a,9a-diazaperylene (DHDAP) was prepared in one step from p-phenylenediamine and 1-bromo-3-chloropropane, and its first redox potential is 292 mV more negative than the first redox potential of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), thus becoming the most easily oxidized p-phenylenediamine homologue.     

Photolithographic Patterning and Doping of Silica Xerogel Films

This study shows that conventional photolithography can be applied for patterning native or organic dye-doped silica films (0.5 m thick) obtained via a base-catalyzed sol-gel process. Photoresist was spin-coated onto high optical quality xerogel films, soft-baked, exposed to UV irradiation through a photomask, and developed with a commercial photoresist developing solution. Etching away of the photoresist-unprotected areas of the silica films was carried out with a dilute HF solution, while the remaining unexposed photoresist was removed with acetone. Interdigitated array patterns with features as small as 0.5 mm show a smooth surface and extremely sharp interfaces. Densification of the films at 550°C for 2 h decreases the film thickness by 11%, increases the refractive index from 1.420 to 1.456, and allows for well-defined patterning down to length scales of 10 m. Since the densification conditions are incompatible with organic dopants, it is demonstrated that sol-gel films can be doped after pattering (post-doping) by adsorption of cationic dyes from solution. Scanning electron microscopy reveals that the microstructure of patterned sol-gel films is similar to that of bulk monoliths, indicating that the photolithographic procedure is not harmful to the film quality. All patterned films demonstrate highly regular light diffraction patterns.     

Synthesis and spectroscopic properties of the elusive 3a,9a-diazaperylenium dication

The 3a,9a-diazaperylenium dication (1) was synthesized for the first time in two steps from p-phenylene diamine. Ab initio calculations show a twisted ground state with a 6.4 degrees tilt between the two quinolizinium building blocks. Dication 1 is photoluminescent in fluid solutions of H(2)O, CH(3)CN, and CH(3)NO(2), but not in rigid matrices of the same solvents. This phenomenon has been attributed to a geometric relaxation of the tilted ground state into an emitting planar lowest singlet excited state. [structure: see text]     

Orientation of pyrylium guests in cucurbituril hosts

According to recent reports, supramolecular complexes of the pyrylium cation with cucurbit[x]urils (CB[x], x = 7, 8) show promising photoluminescence suitable for electroluminescent devices. In turn, photoluminescence seems to be related to the stereochemistry of the complexes; however, that has been controversial. Here, we report that in H(2)O, 2,6-disubsituted-4-phenyl pyryliums (Pylm) form dimers quantitatively (equilibrium constants >10(4) M(-1)), but they enter as such only in the larger CB[8]. In terms of orientation, (1)H NMR shows that Me-Pylm, Ph-Pylm, and t-Bu-Pylm insert their 4-phenyl groups in either the CB[7] or CB[8] cavity. The orientation of iPr-Pylm in the iPr-Pylm@CB[7] complex is similar. Experimental conclusions are supported by DFT calculations using the M062X functional and the 6-31G(d) basis set. In the case of (iPr-Pylm)(2)@CB[8], (1)H NMR of both the guest and the host indicates that both guests might enter CB[8] from the same side with their iPr groups in the cavity, but DFT calculations leave room for ambiguity. In addition to the size and hydrophobicity of the 2,6-substituents of the guests, as well as the size and flexibility of the hosts, theory reveals the importance of explicit solvation (H(2)O) and finite temperature effects (particularly for (1)H NMR shielding calculations) in the determination of the stereochemistry of those complexes.     

Superoxide chemical transformation of diolepoxide polyaromatic hydrocarbon DNA adducts. Determination of benzo[a]pyrene-r-7,t-8,9,c-10-tetrahydrotetrol by gas chromatography.

Benzo[a]pyrene-r-7,t-8,9,c-10-tetrahydrotetrol (100 pg, 342 fmol) was measured using the following sequence of steps: (1) chemical transformation with potassium superoxide to 2,3-pyrenedicarboxylic acid; (2) electrophore derivatization with pentafluorobenzyl bromide; (3) sample clean-up by high-performance liquid chromatography and (4) measurement by gas chromatography with electron-capture detection and by gas chromatography with electron-capture negative-ion mass spectrometry. The overall, absolute yields obtained by the two procedures were 69% and 60%, respectively. This work completes the first stage towards the establishment of a general method for detecting diolepoxide polyaromatic hydrocarbon DNA adducts by gas chromatography.     

Nanoengineered Silica-Polymer Composite Aerogels with No Need for Supercritical Fluid Drying

Owing to their low density, dielectric constant, thermal conductivity, high porosity and chemical inertness, monolithic aerogels could be useful in a variety of electronic, optical and chemical applications [1]. However, practical implementation has been slow, because aerogels are fragile, environmentally sensitive (hydrophilic) and most importantly, the final stage of their preparation involves supercritical fluid (SCF) extraction [1c]. It is reported herewith that for a nominal 3-fold increase in density, typical polymer crosslinked silica aerogels are not only stronger (> 300×) and less hydrophilic (< 10×) than the underlying silica backbone, but they can also withstand the capillary forces exerted upon their nanostructured framework by the residing meniscus of selected solvents, and thus they can be dried under ambient pressure without need for supercritical fluid (SCF) extraction. The best solvent identified for that purpose is pentane, and the resulting aerogels are both microscopically and macroscopically identical to their SCF-CO₂ dried counterparts. Being able to dry monolithic crosslinked aerogels without SCF extraction is expected to facilitate their commercial application.Employed by the Ohio Aerospace Institute.     

Redox-active star molecules incorporating the 4-benzoylpyridinium cation: implications for the charge transfer efficiency along branches vs across the perimeter in dendrimers

We report the redox properties of four star systems incorporating the 4-benzoyl-N-alkylpyridinium cation; the redox potential varies along the branches but remains constant at fixed radii. Bulk electrolysis shows that at a semi-infinite time scale all redox centers are electrochemically accessible. However, voltammetric analysis (cyclic voltammetry and differential pulse voltammetry) shows that only two of the three redox-active centers in the perimeter are electrochemically accessible during potential sweeps as slow as 20 mV s-1 and as fast as 10 V s-1. On the contrary, both redox centers along branches are accessible electrochemically within the same time frame. These results are explained in terms of slow through-space charge transfer and the globular 3-D folding of the molecules and are discussed in terms of their implications on the design of efficient redox functional dendrimers.