Structural, optical, and photophysical properties of nickel(II) alkylthioporphyrins: insights from experimental and DFT/TDDFT studies

The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio group, NiOMTP (OMTP = 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D(2d) and hybrid (ruffled with a modest degree of saddling) D(2) conformations, both characterized by an up-down (ud) orientation of the vicinal methylthio groups are by far the preferred conformations in the "gas phase". In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and beta-alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthio groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway 1(pi,pi) --> 3(d(z2),d(x2-y2))--> ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands.     

Dynamic thermoelastic effects for half-planes and half-spaces with nearly-planar surfaces

The effects of non-planarity on the dynamic surface temperature changes induced for plane-strain and 3D problems on the nearly-planar surfaces of, respectively, coupled thermoelastic half-planes and half-spaces by surface heat fluxes are treated. The nearly-planar nature of the surfaces allows the problem solutions to be written, following a standard perturbation scheme, as series expansions in a dimensionless surface contour amplitude parameter. The first, or zero-order, terms represent the ideal (planar) surface solutions, while the second, or first-order, terms represent corrections for non-planarity.Because the characteristic thermoelastic time is of O(10^–7)s, large-time asymptotic forms of the exact integral transform solutions can be used. These can be inverted exactly and used in Green's function operations to yield analytic, or integrals of analytic, expressions. Two types of thermal loading for the half-plane and yet a third type of thermal loading for the half-space are considered. Comparison of the zero- and first-order surface temperature changes for each case indicate that non-planarity gives rise for large times to changes in surface regions beyond those predicted by an ideal surface analysis. Moreover, the magnitudes of these changes can be more significant than the ideal surface results.     

Infrared multiphoton dissociation spectroscopy of cationized threonine: effects of alkali-metal cation size on gas-phase conformation

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li+(Thr) and Na+(Thr) are similar and relatively simple, whereas K+(Thr), Rb+(Thr), and Cs+(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1[N,CO,OH], binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K+(Thr), Rb+(Thr), and Cs+(Thr) exhibit evidence of the charge-solvated complex, M3[COOH], in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZW[CO2-], is also present, particularly for the Cs+ complex. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M+(Ser) complexes, although there are subtle distinctions that are discussed.     

Microstrain sensitivity of orbital and electronic phase separation in SrCrO3

An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.     

Excited-state equilibration over 30 A in a platinum(II) quinolinolate-bridge-platinum(II) porphyrin complex

Long-range triplet excited-state equilibration occurs over a nanometric distance between platinum(II) 8-quinolinolate (3Ptq2 = 1.87 eV) and platinum(II) tetraphenylporphyrin (3PtTPP = 1.89 eV). The equilibrium is mediated by a fluorene-thiophene-fluorene bridge (3FTF = 1.92 eV) and is characterized by a double-exponential decay (tau1 = 39 +/- 4 ps; tau2 = 351 +/- 15 ps) that suggests the participation of three separate excited states: 3Ptq2, 3FTF, and 3PtTPP, respectively. Numerical simulation of the dual equilibrium allowed for estimation of the individual rate constants for each of the reversible steps (kET = 3.9 x 10(9)-4.1 x 10(10) s(-1)). As a result of rapid triplet-state equilibration, almost 50% of the excited-state energy is directed from the PtTPP chromophore toward Ptq2, in spite of a small endothermic barrier (0.03 eV).     

Electrochemical recognition of synthetic heparin mimetic at liquid/liquid microinterfaces

Electrochemically controlled molecular recognition of a synthetic heparin mimetic, Arixtra, at nitrobenzene/water microinterfaces was investigated to obtain a greater understanding of interfacial recognition and sensing of heparin and its analogues with biomedical importance. In contrast to unfractionated heparin, this synthetic pentasaccharide that mimics the unique Antithrombin III binding domain of heparin possesses well-defined structure and ionic charge to enable quantitative interpretation of cyclic voltammetric/chronoamperometric responses based on the interfacial recognition at micropipet electrodes. Arixtra is electrochemically extracted from the water phase into the bulk nitrobenzene phase containing highly lipophilic ionophores, methyltridodecylammonium or dimethyldioctadecylammonium. Numerical analysis of the kinetically controlled cyclic voltammograms demonstrates for the first time that formal potentials and standard rate constants of polyion transfer at liquid/liquid interfaces are ionophore dependent. Moreover, octadecylammonium and octadecylguanidinium are introduced as new, simple ionophores to model recognition sites of heparin-binding proteins at liquid/liquid interfaces. In comparison to octadecyltrimethylammonium, the best ionophore for heparin recognition at liquid/liquid interfaces reported so far, these new ionophores dramatically facilitate Arixtra adsorption at the interfaces. With a saline solution at physiological pH, an Arixtra molecule is selectively and cooperatively bound to 5 molecules of the guanidinium ionophore, suggesting hydrogen-bond-directed interactions of each guanidinium with a few of 10 negatively charged sulfo or carboxyl groups of Arixtra at the interfaces.     

The effect of spatial confinement of Nafion in porous membranes on macroscopic properties of the membrane

Polyelectrolytes were incorporated into porous reinforcing materials to study the properties of ionomers in confined spaces and to determine the effect of the porous material on the behaviour of the membranes. Nafion^® was imbibed into porous polypropylene (Celgard^®), ultra-high-molecular weight polyethylene (Daramic^®), and polytetrafluoroethylene (PTFE) films. Through the use of reinforcing materials, it is possible to prepare membranes that are thinner, but stronger than pure ionomer membranes. Thin reinforced membranes have advantages such as lower areal resistance (as low as 0.14 Ω cm² for 57 μm CG3501 + Nafion^® compared to 0.34 Ω cm² for 89 μm cast Nafion^®) and lower dimensional changes due to swelling (as low as a 4% change in length and width for WDM + Nafion^® compared to 13% for cast Nafion^®). Using reinforcing materials results in a reduction in important membrane properties compared to bulk Nafion^®, such as proton conductivity (as low as 0.016 S cm⁻¹ for CG3401 + Nafion^® compared to 0.076 S cm⁻¹ for cast Nafion^®), effective proton mobility (as low as 3.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ CG3401 + Nafion^® compared to 7.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ for cast Nafion^®), and water vapour permeance (as low as 0.036 g h⁻¹ Pa⁻¹ m⁻² for WDM + Nafion^® compared to 0.056 g h⁻¹ Pa⁻¹ m⁻² for cast Nafion^®). By normalizing the membrane properties with respect to ionomer content, it was possible to examine the properties of the Nafion^® inside the pores of the membranes. The proton conductivity (as low as 0.032 S cm⁻¹ for CG3401 + Nafion^®), effective proton mobility (as low as 3.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ for CG3401 + Nafion^®), and water vapour permeability (as low as 2.7 × 10⁻⁶ g h⁻¹ Pa⁻¹ m⁻¹ for PTFE MP 0.1 + Nafion^®) of the ionomer in the membrane are also diminished compared to bulk Nafion^® due to decreased connectivity of the ionomer and a restriction in macromolecular motions caused by the pore walls. A series of porous materials with increasing pore were also examined. As the pore size of the PTFE MP materials increased from 0.1 μm to 10 μm, the proton conductivity (0.022 S cm⁻¹ to 0.041 S cm⁻¹), effective proton mobility ((4.1 to 5.6) × 10⁻⁴ cm² V⁻¹ s⁻¹), and water vapour permeability ((2.4 to 4.3) × 10⁻⁶ g h⁻¹ Pa⁻¹ m⁻¹) of the reinforced membranes improved with increasing pore size and the properties of the ionomer inside the membranes approached the value of bulk Nafion^®.     

IRMPD Action Spectroscopy,ER-CID Experiments,and Theoretical Studies of Sodium Cationized Thymidine and 5-Methyluridine: Kinetic Trapping During the ESI Desolvation Process Preserves the Solution Structure of [Thd+Na]<Superscript>+</Superscript>

Thymidine (dThd) is a fundamental building block of DNA nucleic acids, whereas 5-methyluridine (Thd) is a common modified nucleoside found in tRNA. In order to determine the conformations of the sodium cationized thymine nucleosides [dThd+Na]⁺ and [Thd+Na]⁺ produced by electrospray ionization, their infrared multiple photon dissociation (IRMPD) action spectra are measured. Complementary electronic structure calculations are performed to determine the stable low-energy conformations of these complexes. Geometry optimizations and frequency analyses are performed at the B3LYP/6-311+G(d,p) level of theory, whereas energies are calculated at the B3LYP/6-311+G(2d,2p) level of theory. As protonation preferentially stabilizes minor tautomers of dThd and Thd, tautomerization facilitated by Na⁺ binding is also considered. Comparisons of the measured IRMPD and computed IR spectra find that [dThd+Na]⁺ prefers tridentate (O2,O4',O5') coordination to the canonical 2,4-diketo form of dThd with thymine in a syn orientation. In contrast, [Thd+Na]⁺ prefers bidentate (O2,O2') coordination to the canonical 2,4-diketo tautomer of Thd with thymine in an anti orientation. Although 2,4-dihydroxy tautomers and O2 protonated thymine nucleosides coexist in the gas phase, no evidence for minor tautomers is observed for the sodium cationized species. Consistent with experimental observations, the computational results confirm that the sodium cationized thymine nucleosides exhibit a strong preference for the canonical form of the thymine nucleobase. Survival yield analyses based on energy-resolved collision-induced dissociation (ER-CID) experiments suggest that the relative stabilities of protonated and sodium cationized dThd and Thd follow the order [dThd+H]⁺ < [Thd+H]⁺ < [dThd+Na]⁺ < [Thd+Na]⁺.

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