Synthesis of digitoxigenin by remote functionalization

Treatment of a 21-hydroxy-20-keto steroid with the mixed anhydride of trifluoroacetic acid—diethylphosphonoacetic acid leads directly to cardenolides by an intramolecular Horner-Emmons reaction. Photolysis of 3β-acetoxy-5β,14α-card-20(22)-enolide and iodobenzenedichloride in benzene solution then yields 3β-acetoxy-5β-carda-14,20(22)-dienolide (β-anhydrodigitoxigenin acetate).     

The use of discharge lamps as directly modulated atom reservoirs for selective line modulation in atomic emission spectrometry

A simple method is described for selectively modulating the atomic or ionic resonance lines emitted by an excitation source. A modified discharge lamp with a cylindrical hollow cathode is used to generate a modulated atom cloud. The emission from the excitation source is then imaged through the modulated atom cloud within the cylindrical cathode lamp, to yield a selectively modulated signal. The ability of the system to reduce spectral interferences in inductively-coupled plasma emission spectrometry is assessed and discussed.     

Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S(1) <-- S(0) origin transitions of oES and pES occur at 32 369 and 33 407 cm(-1), respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm(-1)) were confirmed by UVHB spectroscopy to be S(1) <-- S(0) origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm(-1)) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis --> trans isomerization in m-ethynylstyrene of 990-1070 cm(-1). Arguments are given for the lack of a kinetic shift in the measurement. The analogous trans --> cis barrier is in the same range (989-1065 cm(-1)), indicating that the relative energies of the zero-point levels of the two isomers are (E(ZPL)(cis) - E(ZPL)(trans))= -75 to +81 cm(-1). Both the barrier heights and relative energies of the minima are close to those determined by DFT (Becke3LYP/6-31+G) calculations.     

Simple methods for the direct assembly, functionalization, and patterning of acid-terminated monolayers on Si111

This article describes mild methods to directly assemble, functionalize, and pattern monolayers of undecylenic acid on hydrogen-terminated Si(111). These monolayers were assembled under very mild conditions from a neat solution of undecylenic acid containing 0.1 mol % 4-(decanoate)-2,2,6,6-tetramethylpiperidinooxy at room temperature without the need for UV light. Because of these mild conditions, monolayers exposing carboxylic acids could be assembled in one step without the need to protect the acid prior to its assembly. The monolayers were extensively characterized by horizontal attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle goniometry. The monolayers bonded to the silicon surface preferentially through the olefin with no detectable bonds between the carboxylic acids and silicon. The crystallinity of the monolayer was studied by infrared spectroscopy through the antisymmetric--v(a)(CH(2))--and symmetric--v(s)(CH(2))--stretches for methylene. Because it is important for future applications to assemble functional surfaces, methods to react the acid-terminated monolayers with trifluoroacetic anhydride and triethylamine to yield a symmetric anhydride on the monolayer were studied. These anhydrides were reacted with a variety of milligram-quantity amines to yield amide-terminated surfaces. This method was general, and a variety of amines could be bonded to the monolayer. The stabilities of these monolayers upon exposure to ambient conditions and under a variety of solvents were described. Because patterned monolayers have found wide applications, we have developed methods to pattern 1-octadecylamine and poly(ethylenimine) on the micrometer scale using soft lithography. In addition, polymer brushes of polynorbornene with thicknesses from 32 to 150 nm were grown from monolayers patterned with the Grubbs' catalyst. The patterned surfaces were imaged by scanning electron microscopy, scanning probe microscopy, and ellipsometry to determine the thicknesses of the patterns and the fidelity of the method.     

Bromination of 2,1,3-benzothiadiazoles

The bromination of 2,1,3-benzothiadiazoles in 47% hydrobromic acid at elevated temperature has led to a general preparative method for the synthesis in high yield of otherwise difficulty accessible brominated 2,1,3-benzothiadiazoles. The typical addition reaction is apparently eliminated under these reaction conditions and substitution takes place exclusively. Bromination of 2,1,3-benzothiadiazole occurs successively at positions 4 and 7. 4-Substituted 2,1,3-benzothia-diazoles are selectively brominated at position 7. 5-Bromo- and 5-methyl-2,1,3-benzothiadiazole are brominated consecutively at positions 4 and 7.     

Polymerizable benzophenone derivatives for labeling vinyl acetate‐butylacrylate latex particles

We describe the synthesis and characterization of three new polymerizable benzophenone derivatives [2‐acryloxy‐5‐methyl benzophenone ( 8 ), 4′‐dimethylamino‐2‐acryloxy‐5‐methyl benzophenone ( 9 ), and 4′‐dimethylamino‐2‐(β‐acryloxyethyl)oxy‐5‐methyl benzophenone ( 10 )]. We show that these monomers can successfully be incorporated into vinyl acetate (VAc) copolymer latex particles. These particles were prepared by semicontinuous emulsion polymerization and mini‐emulsion polymerization of VAc with butylacrylate (BA) for VAc/BA = 4/1 by weight. The two monomers 9 and 10 bearing the 4′‐dimethylamino group satisfy the important spectroscopic criteria required of a dye to serve as an acceptor chromophore for nonradiative energy transfer from phenanthrene (Phe) as the donor. Their UV absorption spectra suggest significant overlap with the emission spectrum of Phe, which can be incorporated into P(VAc‐co‐BA) latex through copolymerization with 9‐acryloxymethyl Phe ( 2 ). In addition, these chromophores provide a window in their absorption spectra for excitation of the Phe chromophore at 300 nm. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3001–3011, 2002     

Synthesis and photochromic properties of molecules containing [e]-annelated dihydropyrenes. Two and three way pi-switches based on the dimethyldihydropyrene-metacyclophanediene valence isomerization

The syntheses of several new simple negative, a simple positive, and multiple negative photochromes containing the dihydropyrene-cyclophanediene photochromic system are described. The photo-openings of the negative photochromes, the [e]-annelated benzo (7), naphtho (9), anthro (11), furano (19), and triphenyleno (15) derivatives of the parent 2,7-di-tert-butyl-trans-10b,10c-dimethyl-dihydropyrene (5), as well as its 4,5-dibromo derivative (13), are described to give the corresponding cyclophanedienes, as well as their photoclosures and thermal closures back to the dihydropyrenes. These are compared to the results obtained for the positive photochrome dibenzo[e,l]dihydropyrene (21) and to the bis(dihydropyreno)chrysene (44) and the (dihydropyrenobenzo)(benzo)metacyclophanediene (47) photochromes, which have more than one photochromic switch present and thus have more than a simple "on-off" state. Thermodynamic data are obtained for the thermal closing reactions. The anthrodihydropyrene (12) has the fastest thermal closing (tau(1/2) = 20 min), while the furanodihydropyrene (19') has the slowest (tau(1/2) = 63 h) at 46 degrees C. An electrochemical readout of the state of the switch is demonstrated for the benzodihydropyrene (7).     

Universal quenching of common fluorescent probes by water and alcohols

Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H₂O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation.

Overtones and combinations of O–H vibrations in the solvent efficiently quench red-emitting fluorophores by resonant energy transfer.     

Evaluation of molecule-microbe interactions with capillary electrophoresis: procedures, utility and restrictions

Understanding the interactions between molecules and living organisms is of paramount importance for the evaluation of pharmaceutical activity, chemical toxicity and all manner of microbiological studies. The capability of capillary electrophoresis (CE) in the evaluation of molecule-microbe interactions is examined in the present paper. The fundamental chemical concept of the binding or association constant for molecular systems measured in free solution is discussed for biological systems where microorganisms uptake or associate with molecules from their environment. The heterogeneity of the living organisms must be understood and accounted for including differences related to semantics such as concentration units and the nature of the associations between two entities and large differences in the size and number of microorganisms as compared to molecules. Finally, the added complexity and even inhomogeneity of a cell compared to most molecular systems must be considered and possibly controlled. The binding of specific molecules to viruses is discussed. CE can be utilized to quickly determine if a molecule binds very strongly or not at all to a cell (i.e., a binary yes/no answer). This could be useful for initial high-throughput screening purposes when using capillary arrays, for example. CE can be useful for determining unusual (large) molecule/microbe stoichiometries. Finally, CE can sometimes be used to determine the size of binding constants (K(RL)) within certain limits provided experimental conditions can be formulated that minimize problems of biological heterogeneity.