Haoran Sun

The strength of weak interactions: Aromatic fluorine in drug design

Selective aromatic fluorine substitution can increase the affinity of a molecule for a macromolecular recognition site through non-covalent interactions. These effects are evaluated most accurately by direct comparison of binding affinities of selectively fluorinated compounds with their corresponding hydrocarbons. In cases where structural data confirm similar binding geometries for the fluorocarbon and hydrocarbon analogues, reliable estimates for the impact of fluorination upon arene-pi...X and C-F...X interaction energies are possible. Existing studies show that fluorinations impact on any individual molecular interaction is quite modest. Upon binding to a protein receptor, cumulative fluorinated aromatic quadrupolar and C-F...X dipolar interaction energies rarely differ from those the corresponding hydrocarbons by more than 1.3 kcal/mol, and most individual interactions appear to be in the 0.1-0.4 kcal/mol range. Similarly, non-ideal selective fluorination is rarely associated with a dramatic decrease in affinity, because the impact of weak repulsive interactions in the bound state is counterbalanced by increased lipophilicity.

Arene Trifluoromethylation: An Effective Strategy to Obtain Air-Stable n-Type Organic Semiconductors with Tunable Optoelectronic and Electron Transfer Properties

Modulation of organic semiconductor band gap, electron affinities (EA), ionization potentials (IP), and reorganization energies (λ) associated with charge transfer is critical for its applications. We report here that trifluoromethylation not only increases both IP and EA significantly as expected but also narrows the HOMO-LUMO band gaps and increases considerably the air-stability of arene-based n-type organic semiconductors. The increased air-stability results from relatively high EA energies and a change in oxidation mechanism. Calculated EAs and IPs show that trifluoromethylated arenes are excellent candidates for n-type semiconductor materials; though a moderate increase of inner-sphere reorganization energy (λi) associated with charge transfer is the penalty for the improved performance of the trifluoromethylated compounds. However, since λi decreases as the π conjugation increases, a rational design to produce air-stable n-type semiconductor materials with reasonably small λi is simply to prepare trifluoromethylated arenes with extended π conjugation. Furthermore, we found that structural isomerization can fine-tune the optoelectronic and electronic transfer properties of the corresponding aromatics.

A Method for Detecting Water in Organic Solvents

The potent dehydrating ability of difluoro(aryl)-lambda (3)-iodanes is exploited to develop a convenient (19)F-NMR-based aquametry method that is more sensitive than coulometric Karl Fischer titration. The key difluoro(aryl)-lambda (3)-iodane reagents are synthesized readily from commercially available and inexpensive precursors.

Electrochemical and in-situ UV-visible-near-IR and FTIR spectroelectrochemical characterisation of the mixed-valence heteropolyanion PMo12O40n− (n=4, 5, 6, 7) in aprotic media

The mixed-valence heteropolyanion PMo12O40n− (where n=4, 5, 6, 7) in aprotic media was investigated by cyclic voltammetry and in-situ FTIR and UV-visible-near-IR spectroelectrochemical methods. A new optically transparent thin layer cell with adjustable optical path length was designed and characterised by thin layer cyclic voltammetry and double step chronoamperometry using ferrocene as a test redox system. The results indicate that this cell has small ohmic drop and good spectral response. The cell was used for both in-situ FTIR and UV-visible-near-IR spectroelectrochemical measurements. The cyclic voltammetric results indicate that the heteropolyanion PMo12O403− undergoes two reversible one-electron transfer reductions (first and second redox waves) and two quasi-reversible one-electron transfer reductions (third and fourth redox waves). The mixed-valence heteropolyanion PMo12O40n− (where n=4, 5, 6, 7) was formed after electroreduction. In-situ FTIR and UV-visible-near-IR spectroelectrochemical preliminary results indicate that the electronic structure of electrogenerated mixed-valence species PMo12O404− corresponds to class II system in Robin and Day’s classification of mixed-valence compounds. In-situ FTIR spectroelectrochemical studies also suggest that the bond energy of the MoOd double bond and Mo–Ob–Mo and MoOc–Mo bridge bonds was reduced after reduction of the original compound.

Effects of aromatic trifluoromethylation, fluorination, and methylation on intermolecular π-π interactions.

Marcus theory states that the rate of charge transfer is directly proportional to the amount of intermolecular orbital overlap. Theoretically optimizing the electronic coupling through the orientation and distance which both can increase the frontier orbital overlap between molecules is an attractive route to potentially provide theoretical insight for discovering new high performance semiconductor materials. To investigate how these parameters qualitatively affect charge transfer of model systems, unconstrained dimer optimizations with MP2 and dispersion-corrected DFT methods were used to probe the π-π interactions of methylated, fluorinated, and trifluoromethylated benzene, pyridine, and bipyridine dimers. These systems can serve as simplified models representing weak noncovalent interactions in organic semiconductor materials. Enhanced intermolecular interaction energies, reduced π-π distances, and more favorable cofacial orientations were found with the trifluoromethylated dimers compared to fluorinated and methylated dimers studied. Similar effects were found with donor-acceptor pairs that represent organic p-n heterojunction systems. These enhanced π-π interactions are likely caused by increased molecular quadrupole moment and dispersion interaction associated with trifluoromethylation. This computational study illustrates the strong potential of trifluoromethylation and, possibly perfluoroalkylation of acenes and heteroacenes, leading qualitatively to enhanced electron transfer through better π-π stacked structures, making them viable candidates for use as n-type organic semiconductor materials. The findings also provide insight for fundamental interactions between drug molecules that include fluorinated and trimethylfluorinated aromatics binding to protein receptors.

Differential substituent effects of β-halogens in water-soluble porphyrins

The first water-soluble β-octafluorinated porphyrins, 5,10,15,20-tetrakis(4-sulfonatophenyl)-2,3,7,8,12,13,17,18-octafluoroporphyrin, 1, and 5,10,15,20-tetrakis(2,6-difluoro-3-sulfonatophenyl)-2,3,7,8,12,13,17,18-octafluoroporphyrin, 2, have been prepared and their aqueous aggregation, acid–base, and optical properties have been characterized. The porphyrins are tetraanionic at neutral pH (at pH = 3–11 for 1 and pH = 0–9 for 2). Semiempirical (AM1) calculations provide evidence that somewhat unusual acidity characteristics of the fluorinated compounds (with respect to similar brominated porphyrins) can be rationalized solely on the basis of chemical hardness and electronegativity arguments. These results indicate that the large conformational differences seen in the structures of brominated and fluorinated water-soluble porphyrins have little impact upon N–H acidity. Metalation of 1 and 2 with ZnCl2 yielded the zinc complexes, which were characterized by optical spectroscopy and electrochemistry.

Photosensitization effects of porphyrin on n-Si(111) and n-GaAs(100)

Abstract Photosensitization effects of 5,10,15,20-tetrakis-(4-trimethylaminephenyl) porphyrin tetraiodide(TAPPI) on n-Si(111) and n-GaAs(100) were studied by surface photovoltage spectroscopy, n-Si can be sensitized only in the Soret band absorption region of the TAPPI molecule, n-GaAs can be sensitized in the whole irradiation region ( hv > E g ). The diagrams of energetic level correlation between the TAPPI molecule and the two semiconductor substrates were determined by cyclic voltammetric measurement and, on the basis of them, the different photosensitization effects of TAPPI on n-Si and n-GaAs are explained reasonably.

A new dinuclear molybdenum(V)-sulfur complex containing citrate ligand: Synthesis and characterization of K2.5Na2NH4[Mo2O2S2(cit)2].5H2O

The complex, K2.5Na2NH4[Mo2O2S2(cit)(2)]. 5H(2)O (1), was obtained by crystallization from a solution of (NH4)(2)MoS4, potassium citrate (K(3)cit) and hydroxyl sodium in methanol and water under an atmosphere of pure nitrogen at ambient temperature. The crystals are triclinic, space group , a = 7.376 (3)Angstrom, b = 14.620 (2) Angstrom, c = 14.661 (1) Angstrom, alpha = 71.10 (1)degrees, beta = 81.77 (1)degrees, gamma = 78.27(2)degrees, R = 0.0584 for 2545 observed (I > 2 sigma (I)) reflections. Single crystal structure analysis reveals that citrate ligand coordinated to molybdenum atom through two carboxylato oxygens and one deprotonated hydroxyl oxygen together with two bridging sulfur atoms and a terminal oxygen atom completes distorted coordination octahedron around each molybdenum atom. Principal dimensions are: Mo = O-t, 1.707 Angstrom (av); Mo-S-b, 2.341 Angstrom (av); Mo-O-(hydroxyl), 2.021 Angstrom (av); Mo-O(alpha-carboxyl), 2.1290 Angstrom (av) and Mo-O(beta-carboxyl), 2.268(av) Angstrom. IR spectrum is in agreement with the structure.

K6[As6V15O42(H2O)].6H2O

The crystal structure of hexapotassium dotetracontaoxo(hexaarsenio)pentadecavanadate(IV) heptahydrate, K6[As6V15O42(H2O)].6H2O, is composed of an [As6V15O42(H2O)]6− anion, which has D3 symmetry, and six seven-coordinate potassium ions. There is an H2O molecule at the centre of the heteropolyanion. This anion consists of VO5 pyramids, linked by As2O5 units through shared O atoms. The V—O distances range from 1.604 (4) to 2.009 (4) A, As—O from 1.772 (4) to 1.793 (3) A, and K—O from 2.676 (6) to 3.311 (4) A.

Fluorinated Porphyrins and Corroles: Synthesis, Electrochemistry, and Applications

Fluorination alters the electronic structure of aromatic systems in a predictable manner, and these effects can be exploited in the design of aromatic, macrocyclic ligand systems for a variety of applications. Equally important is that ring fluorination has an exceptionally modest steric effect. Thus, fluorination permits electronic effects to be severed from structural changes that confound interpretations of other substituent effect studies. Here the preparation, physical properties, and applications of fluorinated and perfluoroalkylated porphyrins and corroles are briefly reviewed. Special attention is given to the preparation of the most useful fluorinated heterocyclic building blocks underlie most of the synthetic approaches to these macrocycles. Among the predictable changes fluorination engenders in the properties of these aromatic, tetrapyrrole ligands, the most commonly exploited in applications are: (1) an increase in oxidative stability, and (2) a decrease in ring nitrogen basicity that is reflected in more Lewis-acidic, electron-deficient chelated metal ions.