Hong-Xing Zhang

Aurophilic Attraction and Luminescence of Binuclear Gold(I) Complexes with Bridging Phosphine Ligands: ab initio Study

Electronic structure and spectroscopic properties of [Au2(dpm)2]2+ (dpm = bis(diphosphino)methane) were studied by ab initio calculations. The absorption and emission spectra of this binuclear gold(I) complex in acetonitrile and in the solid state were calculated by single excitation configuration interaction (CIS) method. In the calculations, the solvent effect was taken into account by the weakly solvated [Au2(dpm)2]2+ x (MeCN)2 complex. The ground state structures of [Au2(dpm)2]2+ and [Au2(dpm)2]2+ x (MeCN)2 were optimized by the second-order Møller-Plesset perturbation (MP2) method, while the emissive triplet excited state structures were optimized by the CIS calculations. The results reveal that coordination of acetonitrile to the gold atom in the 3[d(sigma*)s(sigma)] excited state causes a significant red shift in emission energy. The weak aurophilic attraction exists in the ground states of [Au2(dpm)2]2+ and [Au2(dpm)2]2+ x (MeCN)2, and is greatly enhanced in their 3[d(sigma*)s(sigma)] excited states. In acetonitrile, the 3Au(s(sigma)) --> 1Ag(d(sigma*)) transition (phosphorescence) of [Au2(dpm)2]2+ was calculated at 557 nm, in consistent with the observed emission of [Au2(dppm)2](ClO4)2 (dppm = bis(diphenylphosphino)methane) at 575 nm. A high energy emission at 331 nm is predicted for [Au2(dpm)2]2+ in the absence of the interaction between the gold atom and solvent molecule and/or neighboring anion in the excited state. The CIS calculations on the excited states also reveal that the two absorption bands at 278 and 218 nm recorded for [Au2(dcpm)2](ClO4)2 in acetonitrile can be attributed to the 1Ag(d(sigma*)) --> 1Au(p(sigma)) and 1Ag(d(sigma*)) --> 1Au((sp)sigma) transitions, respectively.

A novel soft template strategy to fabricate mesoporous carbon/graphene composites as high-performance supercapacitor electrodes

A novel soft template method is developed to synthesize a mesoporous carbon/graphene (MCG) composite. The resulting MCG composite exhibits a outstanding capacitance as high as 242 F g−1 in 6 M KOH electrolyte at the current density of 0.5 A g−1, which is much higher than mesoporous carbon, graphene and a sample made by mechanical mixing of mesoporous carbon with graphene. A series of experimental results show that the thickness, BET surface area and carbonized temperatures seriously affect the structure and energy storage performance of the as-prepared MCG composite. Remarkably, the synthesized MCG composite displays good cyclic stability, and the final capacitance was up to 105% compared to the initial capacitance after 2000 cycles of the composite. The mesoporous carbon in the MCG composite is beneficial to the accessibility and rapid diffusion of the electrolyte, and the graphene in MCG can facilitate the transport of electrons during the processes of charging and discharging owing to its high conductivity, which leads to an excellent energy storage performance.

DFT/TD‐DFT investigation on Ir(III) complexes with N‐heterocyclic carbene ligands: Geometries, electronic structures, absorption, and phosphorescence properties

Iridium(III) complexes with N‐heterocyclic (NHC) ligands including fac‐Ir(pmb)3 (1), mer‐Ir(pmb)3 (2), (pmb)2Ir(acac) (3), mer‐Ir(pypi)3 (4), and fac‐Ir(pypi)3 (5) [pmb = 1‐phenyl‐3H‐benzimidazolin‐2‐ylidene, acac = acetoylacetonate, pypi = 1‐phenyl‐5H‐benzimidazolin‐2‐ylidene; fac = facial, mer = meridional] were investigated theoretically. The geometry structures of 1–5 in the ground and excited state were optimized with restricted and unrestricted DFT (density functional theory) methods, respectively (LANL2DZ for Ir atom and 6‐31G for other atoms). The HOMOs (highest occupied molecular orbitals) of 1–3 are composed of d(Ir) and π(phenyl), while those of 4 and 5 are contributed by d(Ir) and π(carbene). The LUMOs (lowest unoccupied molecular orbitals) of 1, 2, 4, and 5 are localized on carbene, but that of 3 is localized on acac. The calculated lowest‐lying absorptions with TD‐DFT method based on Perdew‐Burke‐Erzenrhof (PBE) functional of 1 (310 nm), 2 (332 nm), and 3 (347 nm) have MLcarbeneCT/ILphenyl→carbeneCT (MLCT = metal‐to‐ligand charge transfer; ILCT = intraligand charge transfer) transition characters, whereas those of 4 (385 nm) and 5 (389 nm) are assigned to MLcarbeneCT/ILcarbene→carbeneCT transitions. The phosphorescences calculated by TD‐DFT method with PBE0 functional of 1 (386 nm) and 2 (388 nm) originate from 3MLcarbeneCT/3ILphenyl→carbeneCT excited states, but those of 4 (575 nm) and 5 (578 nm) come from 3MLcarbeneCT/3ILcarbene→carbeneCT excited states. The calculated results showed that the carbene and phenyl groups act as two independent chromophores in transition processes. Compared with 1 and 2, the absorptions of 4 and 5 are red‐shifted by increasing the effective π‐conjugation groups near the Ccarbene atom. We predicated that (pmb)2Ir(acac) is nonemissive, because the LUMO of 3 is contributed by the nonemissive acac ligand.

Stability and reactivities based on moment analysis

The truncated expansion of a function ¦x¦ is used to obtain the total Hückel π-electron energy partitioned into various sums, in terms of moments as well as molecular fragments. The additivity is in general satisfactory for acyclic and cyclic systems, which exhibit a regularity called the generalized Hückel rule which indicates whether a fragment plays the role of stabilization or of destabilization. A unified treatment based on the energy partitioning is proposed for rationalizing aromaticity, reactivities and bond lengths of conjugated hydrocarbons. The relationships between molecular properties and topology can be deduced from inspecting, enumerating or summing the relative contributions of various fragments.

Efficient blue-emitting Ir(III) complexes with phosphine carbanion-based ancillary ligand: a DFT study.

We report a theoretical study on a series of heteroleptic cyclometalated Ir(III) complexes for OLED application. The geometries, electronic structures, and the lowest-lying singlet absorptions and triplet emissions of [(fppy)(2)Ir(III)(PPh(2)Np)] (1), and theoretically designed models [(fppy)(2)Ir(III)(PH(2)Np)] (2) and [(fppy)(2)Ir(III)Np](-)(3) were investigated with density functional theory (DFT)-based approaches, where, fppyH = 4-fluorophenyl-pyridine and NpH = naphthalene. The ground and excited states were, respectively, optimized at the M062X/LanL2DZ;6-31G* and CIS/LanL2DZ:6-31G* level of theory within CH(2)Cl(2) solution provided by PCM. The lowest absorptions and emissions were evaluated at M062X/Stuttgart;cc-pVTZ;cc-pVDZ level of theory. Though the lowest absorptions and emissions were all attributed as the ligand-based charge-transfer transition with slight metal-to-ligand charge-transfer transition character, the subtle differences in geometries and electronic structures result in the different quantum yields and versatile emission color. The newly designed molecular 3 is expected to be highly emissive in deep blue region.

Structural and Dynamic Basis of Human Cytochrome P450 7B1: A Survey of Substrate Selectivity and Major Active Site Access Channels

Cytochrome P450 (CYP) 7B1 is a steroid cytochrome P450 7α-hydroxylase that has been linked directly with bile salt synthesis and hereditary spastic paraplegia type 5 (SPG5). The enzyme provides the primary metabolic route for neurosteroids dehydroepiandrosterone (DHEA), cholesterol derivatives 25-hydroxycholesterol (25-HOChol), and other steroids such as 5α-androstane-3β,17β-diol (anediol), and 5α-androstene-3β,17β-diol (enediol). A series of investigations including homology modeling, molecular dynamics (MD), and automatic docking, combined with the results of previous experimental site-directed mutagenesis studies and access channels analysis, have identified the structural features relevant to the substrate selectivity of CYP7B1. The results clearly identify the dominant access channels and critical residues responsible for ligand binding. Both binding free energy analysis and total interaction energy analysis are consistent with the experimental conclusion that 25-HOChol is the best substrate. According to 20 ns MD simulations, the Phe cluster residues that lie above the active site, particularly Phe489, are proposed to merge the active site with the adjacent channel to the surface and accommodate substrate binding in a reasonable orientation. The investigation of CYP7B1-substrate binding modes provides detailed insights into the poorly understood structural features of human CYP7B1 at the atomic level, and will be valuable information for drug development and protein engineering.

Theoretical investigation of triphenylamine-based sensitizers with different π-spacers for DSSC

The molecular geometries, electronic structures, and absorption spectra of two organic dyes, 3-(5-(4-(IDB)phenyl)thiophene-2-yl)-2-cyanoacrylic acid (IDB-1), and 3-(5-(4-(IDB)styryl) thiophene-2-yl)-2-cyanoacrylic acid (IDB-2), before and after binding to TiO2 cluster were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods to understand the effect of enhanced π-conjugation of organic dye on the energy-to-electricity conversion efficiency (η) of dye-sensitized solar cell (DSSC), where, IDB=10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl. The introduction of vinyl unit into the π-spacer enhances the coplanarity and subsequently red-shifts, intensifies, and broadens the absorption spectrum of IDB-2, resulting in the stronger electronic coupling between dye and TiO2 conduction band, thus the more efficient electron transfer. From the theoretical evaluation of electron injection driving force (D), light-harvesting efficiency (LHE), and shift of TiO2 conduction band (ΔEcb), we successfully interpreted the experimentally observed efficiency difference between IDB-1 and IDB-2. Under this theoretical procedure, several novel D-π-A dyes namely IDB-3, IDB-4, and IDB-5, were designed. Our calculated results reveal that IDB-5 has the improved Jsc and Voc compared with IDB-2 because it performs nicely on the three key parameters (D, LHE, and ΔEcb). This work highlight the importance of using dimethyl-substituted cyclopentadithiophene group as π-spacer in achieving more efficient dyes for DSSC. We hope these discussions can provide fundamental guidelines for the optimization of existing cell efficiency as well as the design of novel high-efficiency organic dyes.

What Makes Hydroxamate a Promising Anchoring Group in Dye- Sensitized Solar Cells? Insights from Theoretical Investigation

We report, from a theoretical point of view, the first comparative study between the highly water-stable hydroxamate and the widely used carboxylate, in addition to the robust phosphate anchors. Theoretical calculations reveal that hydroxamate would be better for photoabsorption. A quantum dynamics description of the interfacial electron transfer (IET), including the underlying nuclear motion effect, is presented. We find that both hydroxamate and carboxylate would have efficient IET character; for phosphate the injection time is significantly longer (several hundred femtoseconds). We also verified that the symmetry of the geometry of the anchoring group plays important roles in the electronic charge delocalization. We conclude that hydroxamate can be a promising anchoring group, as compared to carboxylate and phosphate, due to its better photoabsorption and comparable IET time scale as well as the experimental advantage of water stability. We expect the implications of these findings to be relevant for the design of more efficient anchoring groups for dye-sensitized solar cell (DSSC) application.

Very high penetrance and occurrence of Leber’s hereditary optic neuropathy in a large Han Chinese pedigree carrying the ND4 G11778A mutation

We report here the clinical, genetics and molecular characterization of a five-generation Han Chinese family with Lebers hereditary optic neuropathy (LHON). Strikingly, this family exhibits very high penetrance and occurrence of optic neuropathy. In particular, 25 (10 males/15 females) of 30 matrilineal relatives exhibited the variable severity, ranging from profound to mild of visual impairment. This penetrance of optic neuropathy in this Chinese family is much higher than those in many families with LHON worldwide. The age-at-onset for visual impairment in matrilineal relatives in this Chinese family varied from 7 to 24years old, with the average of 15 years old. Furthermore, the ratio between affected male and female matrilineal relatives is 1:1.5 in the Chinese family. This observation is in contrast with the typical features in LHON pedigrees that there was predominance of affected males in LHON in many families from different ethnic origins. Molecular analysis of mitochondrial genome identified the known ND4 G11778A mutation and 51 variants, belonging to Asian haplogroup C4a1. The absence of other known secondary LHON-associated and functionally significant mtDNA mutations in this Chinese family suggested that mitochondrial variants may not play an important role in the phenotypic manifestation of the G11778A mutation in this Chinese family. Therefore, nuclear modifier gene(s) may be responsible for very high penetrance and occurrence of optic neuropathy in this Chinese pedigree.

Molecular Dynamics Simulations Suggest Ligand’s Binding to Nicotinamidase/Pyrazinamidase

The research on the binding process of ligand to pyrazinamidase (PncA) is crucial for elucidating the inherent relationship between resistance of Mycobacterium tuberculosis and PncA’s activity. In the present study, molecular dynamics (MD) simulation methods were performed to investigate the unbinding process of nicotinamide (NAM) from two PncA enzymes, which is the reverse of the corresponding binding process. The calculated potential of mean force (PMF) based on the steered molecular dynamics (SMD) simulations sheds light on an optimal binding/unbinding pathway of the ligand. The comparative analyses between two PncAs clearly exhibit the consistency of the binding/unbinding pathway in the two enzymes, implying the universality of the pathway in all kinds of PncAs. Several important residues dominating the pathway were also determined by the calculation of interaction energies. The structural change of the proteins induced by NAM’s unbinding or binding shows the great extent interior motion in some homologous region adjacent to the active sites of the two PncAs. The structure comparison substantiates that this region should be very important for the ligand’s binding in all PncAs. Additionally, MD simulations also show that the coordination position of the ligand is displaced by one water molecule in the unliganded enzymes. These results could provide the more penetrating understanding of drug resistance of M. tuberculosis and be helpful for the development of new antituberculosis drugs.