Enhanced fluorescence from o-phthalaldehyde and fluorescamine fluorophores using Triton and β-cyclodextrin

Mexiletine— and lysine hydrochloride—o-phthalaldehyde and mexiletine hydrochloride—, cysteine—, cysteamine—, homocysteine— and lysine hydrochloride—fluorescamine derivatives were subjected to Triton and β-cyclodextrin enhancement treatments. Of several classical fluorescence-enhancing reagents tested (Triton, β-cyclodextrin, sodium dodecyl sulphate, Brij), Triton provided the best results, followed by β-cyclodextrin. Increases in fluorescence emission by a factor of up to about 10 (mexiletine—fluorescamine—Triton X-100) were observed, with a generally negligible influence of the enhancing reagents on the excitation and emission maxima. Fluorescence enhancement by the addition of a suitable reagent solution to the final analyte solution may, in specific instances, enhance the detectability of native or chemically induced fluorophores.     

Thermal decomposition of HO2NO2 (peroxynitric acid, PNA): rate coefficient and determination of the enthalpy of formation

Rate coefficients for the gas-phase thermal decomposition of HO(2)NO(2) (peroxynitric acid, PNA) are reported at temperatures between 331 and 350 K at total pressures of 25 and 50 Torr of N(2). Rate coefficients were determined by measuring the steady-state OH concentration in a mixture of known concentrations of HO(2)NO(2) and NO. The measured thermal decomposition rate coefficients k(-)(1)(T,P) are used in combination with previously published rate coefficient data for the HO(2)NO(2) formation reaction to yield a standard enthalpy for reaction 1 of Delta(r)H degrees (298K) = -24.0 +/- 0.5 kcal mol(-1) (uncertainties are 2sigma values and include estimated systematic errors). A HO(2)NO(2) standard heat of formation, Delta(f)H degrees (298K)(HO(2)NO(2)), of -12.6 +/- 1.0 kcal mol(-1) was calculated from this value. Some of the previously reported data on the thermal decomposition of HO(2)NO(2) have been reanalyzed and shown to be in good agreement with our reported value.     

A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(μ3‐O)(O)2(COO)6] Cluster

The synthesis of titanium–carboxylate metal–organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium‐based MOFs by the use of titanocene dichloride to synthesize COK‐69, the first breathing Ti MOF, which is built up from trans‐1,4‐cyclohexanedicarboxylate linkers and an unprecedented [Ti^IV₃(μ₃‐O)(O)₂(COO)₆] cluster. The photoactive properties of COK‐69 were investigated in depth by proton‐coupled electron‐transfer experiments, which revealed that up to one Ti^IV center per cluster can be photoreduced to Ti^III while preserving the structural integrity of the framework. The electronic structure of COK‐69 was determined by molecular modeling, and a band gap of 3.77 eV was found.     

PPV Polymerization through the Gilch Route: Diradical Character of Monomers

Despite various studies on the polymerization of poly(p‐phenylene vinylene) (PPV) through different precursor routes, detailed mechanistic knowledge on the individual reaction steps and intermediates is still incomplete. The present study aims to gain more insight into the radical polymerization of PPV through the Gilch route. The initial steps of the polymerization involve the formation of a p‐quinodimethane intermediate, which spontaneously self‐initiates through a dimerization process leading to the formation of diradical species; chain propagation ensues on both sides of the diradical or chain termination occurs by the formation of side products, such as [2.2]paracyclophanes. Furthermore, different p‐quinodimethane systems were assessed with respect to the size of their aromatic core as well as the presence of heteroatoms in/on the conjugated system. The nature of the aromatic core and the specific substituents alter the electronic structure of the p‐quinodimethane monomers, affecting the mechanism of polymerization. The diradical character of the monomers has been investigated with several advanced methodologies, such as spin‐projected UHF, CASSCF, CASPT2, and DMRG calculations. It was shown that larger aromatic cores led to a higher diradical character in the monomers, which in turn is proposed to cause rapid initiation.     

Characterization of poly(sodium styrene sulfonate) thin films grafted from functionalized titanium surfaces

Biointegration of titanium implants in the body is controlled by their surface properties. Improving surface properties by coating with a bioactive polymer is a promising approach to improve the biological performance of titanium implants. To optimize the grafting processes, it is important to fully understand the composition and structure of the modified surfaces. The main focus of this study is to provide a detailed, multitechnique characterization of a bioactive poly(sodium styrene sulfonate) (pNaSS) thin film grafted from titanium surfaces via a two-step procedure. Thin titanium films (～50 nm thick with an average surface roughness of 0.9 ± 0.2 nm) prepared by evaporation onto silicon wafers were used as smooth model substrates. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that the titanium film was covered with a TiO(2) layer that was at least 10 nm thick and contained hydroxyl groups present at the outermost surface. These hydroxyl groups were first modified with a 3-methacryloxypropyltrimethoxysilane (MPS) cross-linker. XPS and ToF-SIMS showed that a monolayer of the MPS molecules was successfully attached onto the titanium surfaces. The pNaSS film was grafted from the MPS-modified titanium through atom transfer radical polymerization. Again, XPS and ToF-SIMS were used to verify that the pNaSS molecules were successfully grafted onto the modified surfaces. Atomic force microscopy analysis showed that the film was smooth and uniformly covered the surface. Fourier transform infrared spectroscopy indicated that an ordered array of grafted NaSS molecules were present on the titanium surfaces. Sum frequency generation vibration spectroscopy and near edge X-ray absorption fine structure spectroscopy illustrated that the NaSS molecules were grafted onto the titanium surface with a substantial degree of orientational order in the styrene rings.     

Furan-oxidation-triggered inducible DNA cross-linking: acyclic versus cyclic furan-containing building blocks--on the benefit of restoring the cyclic sugar backbone

Oligodeoxynucleotides incorporating a reactive functionality can cause irreversible cross-linking to the target sequence and have been widely studied for their potential in inhibition of gene expression or development of diagnostic probes for gene analysis. Reactive oligonucleotides further show potential in a supramolecular context for the construction of nanometer-sized DNA-based objects. Inspired by the cytochrome P450 catalyzed transformation of furan into a reactive enal species, we recently introduced a furan-oxidation-based methodology for cross-linking of nucleic acids. Previous experiments using a simple acyclic building block equipped with a furan moiety for incorporation into oligodeoxynucleotides have shown that cross-linking occurs in a very fast and efficient way and that substantial amounts of stable, site-selectively cross-linked species can be isolated. Given the destabilization of duplexes observed upon introduction of the initially designed furan-modified building block into DNA duplexes, we explore here the potential benefits of two new building blocks featuring an extended aromatic system and a restored cyclic backbone. Thorough experimental analysis of cross-linking reactions in a series of contexts, combined with theoretical calculations, permit structural characterization of the formed species and allow assessment of the origin of the enhanced cross-link selectivity. Our experiments clearly show that the modular nature of the furan-modified building blocks used in the current cross-linking strategy allow for fine tuning of both yield and selectivity of the interstrand cross-linking reaction.     

Synthesis of 3-methoxyazetidines via an aziridine to azetidine rearrangement and theoretical rationalization of the reaction mechanism

The synthetic utility of N-alkylidene-(2,3-dibromo-2-methylpropyl)amines and N-(2,3-dibromo-2-methylpropylidene)benzylamines was demonstrated by the unexpected synthesis of 3-methoxy-3-methylazetidines upon treatment with sodium borohydride in methanol under reflux through a rare aziridine to azetidine rearrangement. These findings stand in contrast to the known reactivity of the closely related N-alkylidene-(2,3-dibromopropyl)amines, which are easily converted into 2-(bromomethyl)aziridines under the same reaction conditions. A thorough insight into the reaction mechanism was provided by both experimental study and theoretical rationalization.