Charge separation versus recombination in dye-sensitized nanocrystalline solar cells: the minimization of kinetic redundancy

In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices.     

Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations

Poor solubility of single-walled and multiwalled carbon nanotubes (NTs) in water and organic solvents presents a considerable challenge for their purification and applications. Macromolecules can be convenient solubilizing agents for NTs and a structural element of composite materials for them. Several block copolymers with different chemical functionalities of the side groups were tested for the preparation of aqueous NT dispersions. Poly(N-cetyl-4-vinylpyridinium bromide-co-N-ethyl-4-vinylpyridinium bromide-co-4-vinylpyridine) was found to form exceptionally stable NT dispersions. It is suggested that the efficiency of macromolecular dispersion agents for NT solubilization correlates with the topological and electronic similarity of polymer-NT and NT-NT interactions in the nanotube bundles. Raman spectroscopy and atomic force and transmission electron microcopies data indicate that the polycations are wrapped around NTs forming a uniform coating 1.0-1.5 nm thick. The ability to wind around the NT originates in the hydrophobic attraction of the polymer backbone to the graphene surface and topological matching. Tetraalkylammonium functional groups in the side chains of the macromolecule create a cloud of positive charge around NTs, which makes them hydrophilic. The prepared dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for electronic materials and sensing. Positive charge on their surface is particularly important for biological and biomedical applications because it strengthens interactions with negatively charged cell membranes. A high degree of spontaneous bundle separation afforded by the polymer coating can also be beneficial for NT sorting.     

A practical, fast, and high-yielding aziridination procedure using simple Cu(II) complexes containing N-donor pyridine-based ligands

Four-coordinate dichlorocopper(II) complexes derived from di(2-pyridyl)methanes or pyridine itself exhibit high catalytic activity in aziridination of regular olefins with PhINTs in weakly coordinating chloroform in the presence of 1-2 equiv of NaBArF4 (BArF4- = tetra[3,5-di(trifluoromethyl)phenyl]borate). High yields of aziridines exceeding 90% can be obtained with a 1:1 olefin/PhINTs ratio and 1-5 mol % catalyst loading for such reactive olefins as styrene, tri- and tetramethylethylene. For cis-cyclooctene, indene, methyl acrylate, methyl methacrylate, vinyl methyl ketone, tert-butylethylene, and neopentylethylene, as well as for 1-hexene and cyclopentene, yields of corresponding aziridines vary from 44% to 83%. The catalytic activity and efficiency of the reported copper complexes decrease moderately in the absence of NaBArF4.     

The first enantioselective total synthesis of the anti-Helicobacter pylori agent (+)-spirolaxine methyl ether

The first enantioselective synthesis of the anti-Helicobacter pylori agent (+)-spirolaxine methyl ether has been carried out in a convergent fashion by heterocycle-activated Julia olefination of a spiroacetal-containing sulfone fragment with a phthalide-containing aldehyde fragment. The total synthesis of (+)-spirolaxine methyl ether establishes the absolute stereochemistry of the natural product to be (3R,2'R,5'R,7'R).     

On the parallel mechanism of the dissociation of energy-selected P(CH3)3+ ions

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.     

Comparative study of guest charge-charge interactions within silica sol-gel

We investigated the effect of charge-charge interactions on the mobilities of rhodamine 6G (R6G), Nile Red, sulforhodamine B, and Oregon Green 514 (ORG) guest molecules within a silica sol-gel host as the guest charge progressed from positive to neutral to negative. Through classification of the mobility as fixed, tumbling, or intermediate behavior, we were able to distinguish differences in surface attraction as the guest charge was varied. On the basis of our results, an attractive charge (as tested by cationic R6G) does not contribute significantly to mobility within dry films. However, an increase in the cationic influence is observed in water-equilibrated environments. A comparison of ORG in dry and water- and phosphate-buffer-equilibrated films indicates that charge repulsion does significantly increase dye rotational mobility (to a maximum of 24 +/- 3% tumbling molecules). However, in view of the percentage of tumbling molecules found, charge-charge interactions do not appear to be the dominant force controlling guest mobility.     

Evaluation of Hofmeister effects on the kinetic stability of proteins

Dissolved salts are known to affect properties of proteins in solution including solubility and melting temperature, and the effects of dissolved salts can be ranked qualitatively by the Hofmeister series. We seek a quantitative model to predict the effects of salts in the Hofmeister series on the deactivation kinetics of enzymes. Such a model would allow for a better prediction of useful biocatalyst lifetimes or an improved estimation of protein-based pharmaceutical shelf life. Here we consider a number of salt properties that are proposed indicators of Hofmeister effects in the literature as a means for predicting salt effects on the deactivation of horse liver alcohol dehydrogenase (HL-ADH), alpha-chymotrypsin, and monomeric red fluorescent protein (mRFP). We find that surface tension increments are not accurate predictors of salt effects but find a common trend between observed deactivation constants and B-viscosity coefficients of the Jones-Dole equation, which are indicative of ion hydration. This trend suggests that deactivation constants (log k(d,obs)) vary linearly with chaotropic B-viscosity coefficients but are relatively unchanged in kosmotropic solutions. The invariance with kosmotropic B-viscosity coefficients suggests the existence of a minimum deactivation constant for proteins. Differential scanning calorimetry is used to measure protein melting temperatures and thermodynamic parameters, which are used to calculate the intrinsic irreversible deactivation constant. We find that either the protein unfolding rate or the rate of intrinsic irreversible deactivation can control the observed deactivation rates.     

Ab initio and direct dynamics studies of the reaction of singlet methylene with acetylene and the lifetime of the cyclopropene complex

The energetics of the (1)CH(2) + C(2)H(2) --> H + C(3)H(3) reaction are accurately calculated using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. The reaction enthalpy (0 K) is predicted to be -20.33 kcal/mol. This reaction has no classical barrier in either the entrance or exit channel. However, there are several stable intermediates-cyclopropene (c-C(3)H(4)), allene (CH(2)CCH(2)), and propyne (CH(3)CCH)-along the minimum energy path. These intermediates with zero-point energy corrections lie below the reactants by 87.11 (c-C(3)H(4)), 109.69 (CH(2)CCH(2)), and 110.78 kcal/mol (CH(3)CCH). The vibrationally adiabatic ground-state (VAG) barrier height for c-C(3)H(4) isomerization to allene is obtained as 45.2 kcal/mol, and to propyne as 37.2 kcal/mol. In addition, the (1)CH(2) + C(2)H(2) reaction is investigated utilizing the dual-level "scaling all correlation" (SAC) ab initio method of Truhlar et al., i.e., the UCCSD(SAC)/cc-pVDZ theory. Results show that the reaction occurs via long-lived complexes. The lifetime of the cyclopropene intermediate is obtained as 3.2 +/- 0.4 ps. It is found that the intermediate propyne can be formed directly from reactants through the insertion of (1)CH(2) into a C-H bond of C(2)H(2). However, compared to the major mechanism in which the propyne is produced through a ring-opening of the cyclopropene complex, this reaction pathway is much less favorable. Finally, the theoretical thermal rate constant exhibits a negative temperature dependence, which is in excellent agreement with the previous results. The temperature dependence is consistent with the earlier RRKM results but weaker than the experimental observations at high temperatures.     

Competing non-covalent interactions in alkali metal ion-acetonitrile-water clusters

Competitive ion-dipole, ion-water, and water-water interactions were investigated at the molecular level in M+ (CH3CN)n(H2O)m cluster ions for M = Na and K. Different [n,m] combinations for two different n + m cluster sizes were characterized with infrared predissociation spectroscopy in the O-H stretch region and MP2 calculations. In all cases, no differences were observed between the two alkali metal ions. The results showed that at the n + m = 4 cluster size, the solvent molecules interact only with the ion, and that the interaction between the ion and the large dipole moment of CH3CN decreases the ion-water electrostatic interactions. At the n + m = 5 cluster size, at least two different hydrogen-bonded structures were identified. In these structures, the ion-dipole interaction weakens the ability of the ion to polarize the hydrogen bonds and thus decreases the strength of the water-water interactions in the immediate vicinity of the alkali metal ion.     

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.