Two‐Photon Absorption and Time‐Resolved Stimulated Emission Depletion Spectroscopy of a New Fluorenyl Derivative

The synthesis, comprehensive linear photophysical characterization, two‐photon absorption (2PA), steady‐state and time‐resolved stimulated emission depletion properties of a new fluorene derivative, (E)‐1‐(2‐(di‐p‐tolylamino)‐9,9‐diethyl‐9H‐fluoren‐7‐yl)‐3‐(thiophen‐2‐yl)prop‐2‐en‐1‐one ( 1 ), are reported. The primary linear spectral properties, including excitation anisotropy, fluorescence lifetimes, and photostability, were investigated in a number of aprotic solvents at room temperature. The degenerate 2PA spectra of 1 were obtained with open‐aperture Z‐scan and two‐photon induced fluorescence methods, using a 1 kHz femtosecond laser system, and maximum 2PA cross‐sections of ～400–600 GM were obtained. The nature of the electronic absorption processes in 1 was investigated by DFT‐based quantum chemical methods implemented in the Gaussian 09 program. The one‐ and two‐photon stimulated emission spectra of 1 were measured over a broad spectral range using a femtosecond pump–probe‐based fluorescence quenching technique, while a new methodology for time‐resolved fluorescence emission spectroscopy is proposed. An effective application of 1 in fluorescence bioimaging was demonstrated by means of one‐ and two‐photon fluorescence microscopy images of HCT 116 cells containing dye encapsulated micelles.     

Thermally stable polymer composites with improved transparency by using colloidal mesoporous silica nanoparticles as inorganic fillers

The colloidal mesoporous silica nanoparticles with small particle sizes (namely, CMS) are used as inorganic fillers of polymers (i.e. epoxy and silicone). From simple calculation, almost all polymers are estimated to be confined in the mesopores. To clarify the superiority of CMS over nonporous silica particles and mesoporous silica particles with much larger size (TMPS-4) as inorganic fillers, a systematic study on mechanical strength and transparency of polymer-silica nanocomposites was conducted. Compared with nonporous silica particles, similar to TMPS-4, CMS shows a greater effect on lowering the CTE. In addition, obtained polymer-CMS nanocomposites show improved transparency than polymer-TMPS-4 nanocomposites.     

The performance of MP2.5 and MP2.X methods for nonequilibrium geometries of molecular complexes

Here we test the performance of the newly developed MP2.5 and MP2.X methods in terms of their abilities to generate accurate binding energies for noncovalently bound complexes at points away from their minimum energy structures and in terms of the accuracy of their potential energy minima. The MP2.X method is a scaled version of MP2.5 that allows for the use of smaller basis sets for the most computationally demanding (MP3) term, significantly reducing its computational cost. MP2.5 and MP2.X binding energy errors are compared to those of the reference CCSD(T)/CBS method on the dissociation curves associated with the S66 dataset of noncovalent complexes (S66x8). It is found that both the MP2.5 and MP2.X methods produce binding energy errors, as well as potential energy minima, that are significantly more accurate than those of MP2 methods. Thus, these methods are appropriate choices when very high quality geometries of noncovalent complexes are required.     

Does the influence of substituents impact upon the surface composition of pyrrolidinium-based ionic liquids? An angle resolved XPS study

The surface chemistry of a series of four pyrrolidinium based ionic liquids, [C(n)C(1)Pyrr][Tf(2)N] where n = 4-10, is investigated by angle resolved X-ray photoelectron spectroscopy (ARXPS). The importance of sample purity is demonstrated and the stability of the ionic liquids under X-ray irradiation investigated. It is apparent that the surface chemistry and orientation is broadly analogous to that of similar imidazolium-based systems.     

Application of surface enhanced Raman spectroscopy to the study of SOFC electrode surfaces

SERS provided by sputtered silver was employed to detect trace amounts of chemical species on SOFC electrodes. Considerable enhancement of Raman signal and lowered detection threshold were shown for coked nickel surfaces, CeO(2) coatings, and cathode materials (LSM and LSCF), suggesting a viable approach to probing electrode degradation and surface catalytic mechanism.     

Divalent metal phosphonate coordination polymers constructed from a dipiperidine-based bisphosphonate ligand

The ligand 4,4'-dipiperidine-N,N'-bis(methylenephosphonic acid), H(4)L, has been reacted with divalent metal salts under solvothermal conditions to yield seven new metal phosphonate coordination polymers. The compounds have been characterized by elemental analyses and their structures determined by single-crystal X-ray diffraction. Zn(2)(L)(H(2)O)(2) and Co(2)(L)(H(2)O)(2) have (different) layered structures, while Mn(2)(L)(H(2)O)(3) has a chain motif. In these compounds, the N atoms of the ligand bind to the metal ions. α-Co(2)Cl(2)(H(2)L), formed from CoCl(2)·6H(2)O and H(4)L in ethanol, is also layered but the N atoms of the ligand are protonated. The Co atoms are tetrahedral, coordinated by three phosphonate oxygen atoms and a chloride ion. A polymorph of this compound, β-Co(2)Cl(2)(H(2)L), was obtained from a mixed ionic liquid under microwave irradiation. The primary difference between the polymorphs is the orientation of the phosphonate group relative to the dipiperidine. When reacted hydrothermally with Sn(II)C(2)O(4), H(4)L partially decomposes, producing phosphate ions which are incorporated into the structure of Sn(6)O(2)(H(2)L)(PO(4))(2)·4H(2)O. In this compound, the N atoms of the ligand are protonated, and two oxide anions are incorporated for charge balance. A second phase is obtained from the same reaction, which was determined to be Sn(7)O(L)(3). This compound has a layered structure which contains relatively large voids within the inorganic portion of the layer. These structures are discussed, as well as factors influencing the state of protonation in the final compounds. The choice of solvent and temperature were found to have a significant influence on the type of structure obtained.     

A new dimension in cyclic coinage metal pyrazolates: decoration with a second ring of coinage metals supported by inter-ring metallophilic interactions

When pyrazolate ligands with thioether chelate arms are used in cyclic coinage metal pyrazolates [Au(μ-pz)](n), the inner gold ring can be framed with an outer silver ring to give novel heterometallic double-crowned complexes [AuAg(μ-L(x))(BF(4))](4). They feature short intramolecular in-plane Ag-Au interactions, are stable as octanuclear species in solution, and show promising luminescence properties.     

Two-photon fluorescence microscopy imaging of cellular oxidative stress using profluorescent nitroxides

A range of varying chromophore nitroxide free radicals and their nonradical methoxyamine analogues were synthesized and their linear photophysical properties examined. The presence of the proximate free radical masks the chromophore's usual fluorescence emission, and these species are described as profluorescent. Two nitroxides incorporating anthracene and fluorescein chromophores (compounds 7 and 19, respectively) exhibited two-photon absorption (2PA) cross sections of approximately 400 G.M. when excited at wavelengths greater than 800 nm. Both of these profluorescent nitroxides demonstrated low cytotoxicity toward Chinese hamster ovary (CHO) cells. Imaging colocalization experiments with the commercially available CellROX Deep Red oxidative stress monitor demonstrated good cellular uptake of the nitroxide probes. Sensitivity of the nitroxide probes to H(2)O(2)-induced damage was also demonstrated by both one- and two-photon fluorescence microscopy. These profluorescent nitroxide probes are potentially powerful tools for imaging oxidative stress in biological systems, and they essentially "light up" in the presence of certain species generated from oxidative stress. The high ratio of the fluorescence quantum yield between the profluorescent nitroxide species and their nonradical adducts provides the sensitivity required for measuring a range of cellular redox environments. Furthermore, their reasonable 2PA cross sections provide for the option of using two-photon fluorescence microscopy, which circumvents commonly encountered disadvantages associated with one-photon imaging such as photobleaching and poor tissue penetration.     

A unified route to the welwitindolinone alkaloids: total syntheses of (-)-N-methylwelwitindolinone C isothiocyanate, (-)-N-methylwelwitindolinone C isonitrile, and (-)-3-hydroxy-N-methylwelwitindolinone C isothiocyanate

As part of a comprehensive strategy to the welwitindolinone alkaloids possessing a bicyclo[4.3.1]decane core, we report herein concise asymmetric total syntheses of (-)-N-methylwelwitindolinone C isothiocyanate (2a), (-)-N-methylwelwitindolinone C isonitrile (2b), and (-)-3-hydroxy-N-methylwelwitindolinone C isothiocyanate (3a) from a common tetracyclic intermediate. The crucial vinyl chloride moiety was installed through electrophilic chlorination of a hydrazone, but only after adjustment of reactivity to circumvent a facile skeletal rearrangement. Selective desulfurization and oxidation of 2a provided access to 2b and 3a, respectively. Notably, this work provides corrected (1)H and (13)C NMR spectral data for 3a.     

Entropic and electrostatic effects on the folding free energy of a surface-attached biomolecule: an experimental and theoretical study

Surface-tethered biomolecules play key roles in many biological processes and biotechnologies. However, while the physical consequences of such surface attachment have seen significant theoretical study, to date this issue has seen relatively little experimental investigation. In response we present here a quantitative experimental and theoretical study of the extent to which attachment to a charged-but otherwise apparently inert-surface alters the folding free energy of a simple biomolecule. Specifically, we have measured the folding free energy of a DNA stem loop both in solution and when site-specifically attached to a negatively charged, hydroxylalkane-coated gold surface. We find that whereas surface attachment is destabilizing at low ionic strength, it becomes stabilizing at ionic strengths above ~130 mM. This behavior presumably reflects two competing mechanisms: excluded volume effects, which stabilize the folded conformation by reducing the entropy of the unfolded state, and electrostatics, which, at lower ionic strengths, destabilizes the more compact folded state via repulsion from the negatively charged surface. To test this hypothesis, we have employed existing theories of the electrostatics of surface-bound polyelectrolytes and the entropy of surface-bound polymers to model both effects. Despite lacking any fitted parameters, these theoretical models quantitatively fit our experimental results, suggesting that, for this system, current knowledge of both surface electrostatics and excluded volume effects is reasonably complete and accurate.