Increasing the open circuit voltage of bulk-heterojunction solar cells by raising the LUMO level of the acceptor

We report the synthesis, characterization, and electrochemical properties of ten new fullerene derivatives for usage in organic solar cells. The phenyl ring of PCBM was substituted with electron-donating and electron-withdrawing substituents to study their influence on the LUMO level of the parent fullerene. We varied the LUMO level over a range of 86 mV and show a small but significant change of the open circuit voltage upon application in MDMO-PPV:methanofullerene bulk-heterojunction photovoltaic cells. [structure: see text].     

A valence bond study of the oxygen molecule

Ab initio valence bond calculations are performed for the three lowest states of the oxygen molecule (³Σ⁻_g, ¹Δ_g, and ¹Σ⁺_g). One objective of the present study was to make a contribution to previous valence bond discussions about the oxygen “double” bond. Further, we study the origin of a small barrier in the potential energy surface of the ground state. Two compact models are employed to maintain the clear picture that can be offered by the valence bond method. The first model has only the Rumer structures that are essential for bonding and a proper dissociation. The second model, in addition, has structures which represent excited atoms. These prove to be important for the dissociation energies. For both models, the orbitals are fully optimized. The spectroscopic data obtained are significantly better than are the (few) valence bond results on O₂ that have been published and have the quality of multiconfiguration self-consistent field calculations in which the same valence space is used. The “hump” in the potential energy surface of the ground state is shown to arise from a spin recoupling. The free atoms correspond to a spin coupling that is incapable of describing the formation of bonds. Only at short distances, an alternative spin coupling provides bonding and the repulsive curve is converted into an attractive one. Our results on this subject support a valence bond explanation previously given by McWeeny [R. McWeeny, Int. J. Quantum Chem. Symp. 24 , 733 (1990)]. © 1996 John Wiley & Sons, Inc.     

Gradients in valence bond theory

A gradient method for very general valence bond (VB) wavefunctions is presented. This method introduces the electronic energy as a Lagrange multiplier, and evaluates the contributions of the derivatives of the normalisation and of the first- and second-order cofactors present in the VB energy expression. The correctness of the method is illustrated with classic and breathing-orbital VB calculations on the HF molecule.     

SO2-binding properties of cationic η6,η1-NCN-pincer arene ruthenium platinum complexes: spectroscopic and theoretical studies

The SO(2)-binding properties of a series of η(6),η(1)-NCN-pincer ruthenium platinum complexes (NCN = 2,6-bis[(dimethylamino)methyl]phenyl anion) have been studied by both UV-visible spectroscopy and theoretical calculations. When an electron-withdrawing [Ru(C(5)R(5))](+) fragment (R = H or Me) is η(6)-coordinated to the phenyl ring of the NCN-pincer platinum fragment (cf. [2](+) and [3](+), see Scheme 1), the characteristic orange coloration (pointing to η(1)- SO(2) binding to Pt) of a solution of the parent NCN-pincer platinum complex 1 in dichloromethane upon SO(2)-bubbling is not observed. However, when the ruthenium center is η(6)-coordinated to a phenyl substituent linked in para-position to the carbon-to-platinum bond, i.e. complex [4](+), the SO(2)-binding property of the NCN-platinum center seems to be retained, as bubbling SO(2) into a solution of the latter complex produces the characteristic orange color. We performed theoretical calculations at the MP2 level of approximation and TD-DFT studies, which enabled us to interpret the absence of color change in the case of [2](+) as an absence of coordination of SO(2) to platinum. We analyze this absence or weaker SO(2)-coordination in dichloromethane to be a consequence of the relative electron-poorness of the platinum center in the respective η(6)-ruthenium coordinated NCN-pincer platinum complexes, that leads to a lower binding energy and an elongated calculated Pt-S bond distance. We also discuss the effects of electrostatic interactions in these cationic systems, which also seems to play a destabilizing role for complex [2(SO(2))](+).     

A comparison of approaches to estimate the resonance energy

We discuss Ab Initio approaches to calculate the energy lowering (stabilisation) due to aromaticity. We compare the valence bond method and the block-localised wave function approaches to calculate the resonance energy. We conclude that the valence bond approach employs a Pauling–Wheland resonance energy and that the block-localised approach employs a delocalisation criterion. The latter is shown to be more basis set dependent in a series of illustrative calculations.     

Gramicidin S Derivatives Containing cis‐ and trans‐Morpholine Amino Acids (MAAs) as Turn Mimetics

The cyclic decapeptide gramicidin S (GS) was used as a model for the evaluation of four turn mimetics. For this purpose, one of the D ‐Phe‐Pro two‐residue turn motifs in the rigid cyclic β‐hairpin structure of GS was replaced with morpholine amino acids (MAA 2 – 5 ), differing in stereochemistry and length of the side‐chain. The conformational properties of the thus obtained GS analogues ( 6 – 9 ) was assessed by using NMR spectroscopy and X‐ray crystallography, and correlated with their biological properties (antimicrobial and hemolytic activity). We show that compound 8 , containing the dipeptide isostere trans‐MAA 4 , has an apparent high structural resemblance with GS and that its antibacterial activity against a panel of Gram positive and ‐negative bacterial strains is better than the derivatives 6 , 7 and 9 .     

Development of a negligible depletion-solid phase microextraction method to determine the free concentration of chlorpromazine in aqueous samples containing albumin

The addition of proteins to in vitro systems influences the free concentration of the test compound in the medium. The objective of this study was to set up a negligible depletion-solid phase microextraction method, coupled to high-performance liquid chromatography (nd-SPME-HPLC) to measure the free concentration of chlorpromazine (CPZ) in medium containing albumin. The nd-SPME method was optimized for coating thickness (polyacrylate coating) and exposure time, and potential effects from the addition of bovine serum albumin (BSA) were studied. It was shown that the addition of albumin did not cause fouling or influenced the uptake kinetics of CPZ into the fiber. At a realistic in vivo albumin concentration of 40 g/L of albumin, 94% of CPZ was protein bound. This is in line with findings in vivo, reporting a protein binding for CPZ of 92-99%. The nd-SPME-HPLC method described in this study can be used to measure the free concentration of chlorpromazine in medium containing proteins. These findings can be used to correct in vitro data for protein binding of chlorpromazine and this information is essential for the extrapolation to in vivo data.     

Elucidating the sorption mechanism of “mixed-mode” SPME using the basic drug amphetamine as a model compound

We studied the sorption of amphetamine as a model drug to represent small, polar organic cations to a new SPME coating combining C18 and propylsulfonic acid. This combination of hydrophobic and strong cation exchange (SCX) groups was compared to conventional SPME fibers with polyacrylate (PA) or C18 coating. The affinity of amphetamine at physiological pH (PBS) was 20 to 180 times greater for the new C18/SCX coating than for C18 alone and PA of different coating thickness. As amphetamine is a base and >99% protonated at physiological pH, this enhanced affinity is attributed to the ion-exchange phase in the coating. Tests at pH above the pK_a of amphetamine show that, when normalized to the coating volume, neutral amphetamine also has a higher affinity compared to PA. As ion-exchange groups are not unlimitedly present in the coating, amphetamine isotherms level off to a saturation concentration on the C18/SCX fiber at the highest tested aqueous concentrations. Also, other cations (Na⁺, K⁺, Ca²⁺) compete for the SCX sites and decrease the sorption coefficients, e.g. by 1.7 log units when comparing Milli-Q water with PBS. The C18/SCX fiber provides improved sensitivity over some of the classic SPME fibers. However, care should be taken near the cation exchange capacity of the fiber and the fiber should be calibrated in an appropriate matrix so as to eliminate competition effects.     

Methyl α- -fructofuranoside: Synthesis and conversion into carboxylates

Methyl α- -fructofuranoside was synthesized by methylation of -fructose and subsequent isolation of the α-furanoside from the anomeric mixture. This fructofuranoside was used as a starting material for the syntheses of several carboxylates, applying glycolic oxidation, selective oxidation of the primary alcohol function at the C-6 position and carboxymethylation.     

FLUORESCENCE SPECTRA AND ZERO-FIELD MAGNETIC RESONANCE OF CHLOROPHYLL a-WATER COMPLEXES

Abstract. –Fluorescence detected magnetic resonance (FDMR) and fluorimetry have been used to study chlorophyll-water complexes at T = 4.2 K. By combining these methods, zero-field splitting (ZFS)-parameters can be assigned to the triplet state of the various species. It is found that these parameters decrease upon aggregation and/or complexation. These results can be rationalized by using a simple free-electron model for chlorophyll including excitonic interaction. Evidence is presented favouring a structure recently proposed by Shipman and Katz for a chlorophyll-dimer involving hydrogen-bonded water, present in non-polar solutions at low temperature with spectral properties similar to those of chlorophyll in vivo.