Theoretical and Experimental Investigation of Functionalized Cyanopyridines Yield an Anolyte with an Extremely Low Reduction Potential for Nonaqueous Redox Flow Batteries

Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of −2.2 V or lower vs. ferrocene^+/0 (Fc^+/0), but the 3-CNPy⋅⁻ radical anion forms a sigma-dimer that is re-oxidized at E≈−1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge-discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu₄][PF₆] shows that 2-CNPy and 4-CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density-functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E_1/2 only about +0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3-CN-6-PhPy and 3-CN-4-PhPy are the most promising as anolytes. They exhibit reversible reductions at E_1/2=−2.19 and −2.22 V vs. ferrocene^+/0, respectively, and retain about half of their capacity after 30 bulk charge-discharge cycles. An improved version of 3-CN-6-PhPy with three methyl groups (3-cyano-4-methyl-6-(3,5-dimethylphenyl)pyridine) has an extremely low reduction potential of −2.50 V vs. Fc^+/0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge-discharge cycling in acetonitrile.     

Classification of stress-intensity factors from isochromatic-fringe patterns

The stresses in the local neighborhood of a crack tip have been used to develop a relation between the isochromatic-fringe orderN, its position parametersr and θ and the stress field expressed in terms of stress intensities,K _I ,K _II , and a far-field stress σ _ox . This relation was programmed and a plotting routine was developed to map isochromatic (σ₁ ? σ₂) fields in the neighborhood of the crack tip. The stress intensitiesK _I andK _II and the far-field stress σ _ox were varied and isochromatic fields were constructed for each combination. As bothK _II and σ _ox influence the size, shape and orientation of the isochromatics loops in a systematic manner, the pictorial representation of the isochromatic fields can be used to classify the state of stress (K _I ,K _II and σ _ox ) at the crack tip. Isochromatics which classify six different states of stress have been illustrated and methods used to determineK _I ,K _II , andσ _ox in five of the six states are given.     

Differential stress-holo-interferometry

In displacement analysis of opaque bodies using holographic interferometry, it is a common practice to record one hologram of the body at some arbitrary load and then to increase the load and record a second hologram on the same photographic plate. The fringes in the reconstructed image correspond to the change in the displacement occurring between the two exposures. A new technique for photoelastic analysis based on this same idea will be presented. This technique, to be referred to as differential stress-holo-interferometry, has several advantages over existing techniques. Using vector-algebra methods, the general intensity equations for a double-exposure hologram of a photoelastic model in which neither of the holograms is of the unstressed model is developed. In general the resulting interferogram is difficult to interpret; however, for selected types and levels of loading, a pattern which is easily interpreted results. It is shown that the isochromatic fringes in these patterns are more precisely defined than those in a conventional double-exposed hologram of a photoelastic model. In addition, the new technique offers the advantages of increased fringe visibility, isochromatic-fringe multiplication, and an aid for the determination of the isochromatic-fringe order. Finally, for certain types of models, a technique for producing an interferogram in which the isochromatics and isopachics are completely independent and the isopachics do not undergo a half-order-fringe shift is demonstrated.     

Computer-simulated stress-optic patterns

A new method for producing theoretical fringe patterns using a small real-time computer interfaced to analog equipment is presented. The method applies equally well to isochromatic, isopachic or interferometric stress patterns of any stress field for which the analytical solution is known.     

On the limitations of interferometric methods in three-dimensional photoelasticity

The photoelastic response of tinree different epoxy materials was measured. Tensile strips, beams, disks and spheres were loaded and the deformations “frozen in.” Slices from these models were used to calibrate and analyze the absolute retardation of the materials. It was found that the absolute retardation of light vibrating in a given principal direction was directly proportional to the principal strain in that direction and to the deviatoric components of stress in that direction. This result was a consequence of a unique value of the ratio of the absolute-retardation constants and is contrary to published suggestions that absolute-retardation methods could be used to separate normal stresses in the interior of stress-frozen models.     

Photonic crystal sensor for organophosphate nerve agents utilizing the organophosphorus hydrolase enzyme

We developed an intelligent polymerized crystalline colloidal array (IPCCA) photonic crystal sensing material which reversibly senses the organophosphate compound methyl paraoxon at micromolar concentrations in aqueous solutions. A periodic array of colloidal particles is embedded in a poly-2-hydroxyethylacrylate hydrogel. The particle lattice spacing is such that the array Bragg-diffracts visible light. We utilize a bimodular sensing approach in which the enzyme organophosphorus hydrolase (OPH) catalyzes the hydrolysis of methyl paraoxon at basic pH, producing p-nitrophenolate, dimethylphosphate, and two protons. The protons lower the pH and create a steady-state pH gradient. Protonation of the phenolates attached to the hydrogel makes the free energy of mixing of the hydrogel less favorable, which causes the hydrogel to shrink. The IPCCA’s lattice constant decreases, which blueshifts the diffracted light. The magnitude of the steady-state diffraction blueshift is proportional to the concentration of methyl paraoxon. The current detection limit is 0.2 μmol methyl paraoxon per liter.     

Realization of an Asymmetric Non-Aqueous Redox Flow Battery through Molecular Design to Minimize Active Species Crossover and Decomposition

This communication presents a mechanism-based approach to identify organic electrolytes for non-aqueous redox flow batteries (RFBs). Symmetrical flow cell cycling of a pyridinium anolyte and a cyclopropenium catholyte resulted in extensive capacity fade due to competing decomposition of the pyridinium species. Characterization of this decomposition pathway enabled the rational design of next-generation anolyte/catholyte pairs with dramatically enhanced cycling performance. Three factors were identified as critical for slowing capacity fade: (1) separating the anolyte–catholyte in an asymmetric flow cell using an anion exchange membrane (AEM); (2) moving from monomeric to oligomeric electrolytes to limit crossover through the AEM; and (3) removing the basic carbonyl moiety from the anolyte to slow the protonation-induced decomposition pathway. Ultimately, these modifications led to a novel anolyte–catholyte pair that can be cycled in an AEM-separated asymmetric RFB for 96 h with >95 % capacity retention at an open circuit voltage of 1.57 V.     

Peptide secondary structure folding reaction coordinate: correlation between uv raman amide III frequency, Psi Ramachandran angle, and hydrogen bonding

We used UV resonance Raman (UVRR) spectroscopy to quantitatively correlate the peptide bond AmIII3 frequency to its Psi Ramachandran angle and to the number and types of amide hydrogen bonds at different temperatures. This information allows us to develop a family of relationships to directly estimate the Psi Ramachandran angle from measured UVRR AmIII3 frequencies for peptide bonds (PBs) with known hydrogen bonding (HB). These relationships ignore the more modest Phi Ramachandran angle dependence and allow determination of the Psi angle with a standard error of +/-8 degrees , if the HB state of a PB is known. This is normally the case if a known secondary structure motif is studied. Further, if the HB state of a PB in water is unknown, the extreme alterations in such a state could additionally bias the Psi angle by +/-6 degrees . The resulting ability to measure Psi spectroscopically will enable new incisive protein conformational studies, especially in the field of protein folding. This is because any attempt to understand reaction mechanisms requires elucidation of the relevant reaction coordinate(s). The Psi angle is precisely the reaction coordinate that determines secondary structure changes. As shown elsewhere (Mikhonin et al. J. Am. Chem. Soc. 2005, 127, 7712), this correlation can be used to determine portions of the energy landscape along the Psi reaction coordinate.     

Investigating excited state dynamics of salinixanthin and xanthorhodopsin in the near-infrared

Excited state dynamics of native Xanthorhodopsin (XR), of an XR sample with a reduced prosthetic group, and of the associated Carotenoid (CAR) salinixanthin (SX) in ethanol were investigated by hyperspectral Near Infrared (NIR) probing. Global kinetic analysis shows that: (1) unlike the transient spectra recorded in the visible, fitting of the NIR data requires only two phases of exponential spectral evolution, assigned to internal conversion from S(2) → S(1) and from S(1) → S(0) of the carotene. (2) The rate of the internal conversion from S(2) → S(1) in the reduced sample is well fit with a decay time of 130 fs, significantly longer than in XR and in SX, both of which are well fit with τ ≈ 100 fs. This increased lifetime is consistent with a ～30% efficiency of ET from SX to retinal in XR. (3) S(1) of salinixanthin is verified to lie ～12,700 cm(-1) above the ground electronic surface, excluding its involvement in the retinal sensitization in XR. (4) The oscillator strength of the S(1) → S(2) transition is determined to be no more than 0.16, despite its symmetry allowedness. (5) No long lived NIR absorbance decay assignable to the carotenoid S* state was detected in any of the samples. Inconsistencies concerning previously determined S(2) lifetimes and kinetic schemes used to model these data are discussed.     

Solvent tuning of a conical intersection: direct experimental verification of a theoretical prediction

We report an ultrafast study of a merocyanine molecule, whose fluorescence lifetime was tuned by changing the solvent's polarity. A recent theoretical prediction that the fluorescence lifetime is considerably shortened upon lowering the polarity of the solvent, due to tuning of the conical intersection properties, is fully confirmed (Xu et al. J. Phys. Chem. A 2009, 113, 9779-9791). This constitutes a direct measurement of a previously predicted tunable property of a conical intersection.