A Rotation-Inversion Dual-Motion Molecular Switch: Race for NMR Signaling.

The interplay between the thermal helical inversion (THI) of the stiff-stilbene moiety and the rotation of the dimethylamino (DMA) group in 1 results in a dependence of the DMA NMR signals on the THI kinetics in (E)-1 but the rotation kinetics in (Z)-1, because the faster motion mode is responsible. Consequently, the photochemical switching from (E)-1 to (Z)-1 illustrates the phenomenon of "switchable motion detection" by the same set of NMR signals in a dual-motion molecular system.

A computational study of the Fenton reaction in different pH ranges.

The reaction of iron(ii) and hydrogen peroxide, namely the Fenton reaction, is well-known for its strong oxidizing capability. While the Fenton reactions are ubiquitous and have wide applications in many areas, the detailed mechanism, especially the nature of the reactive intermediates responsible for oxidation, is not completely clear. In this work, the performances of various density functional theory (DFT) methods on the relative energies of key Fenton intermediates are evaluated. The DFT method selected from the benchmark study is then exploited to investigate the aqueous Fenton reactions in different pH conditions. The results show that at pH > 2.2, the major Fenton oxidants are high-valent oxoiron(iv) aquo complexes. However, depending on the pH conditions, these complexes can exist in three protonation states that display quite different oxidation reactivities. The oxidizing power of FeIV[double bond, length as m-dash]O is found to be principally determined by the total charge of the ligands and is less influenced by the axial ligand effect. Moreover, the calculations reveal that the presence of the hydronium ion can stabilize the intermediate of the hydroxyl radical and further inhibit oxoiron(iv) formation via proton transfer. The contribution of hydroxyl radicals could compete with the oxoiron(iv) species at pH below 2.2. In addition, high-level ab initio calculations question the existence of the iron(iv)-dihydroxo intermediate suggested in the literature. The implications of the computational results for the Fenton oxidation process, cytochrome P450, and catalyst design are discussed.

Redox-Gated Tristable Molecular Brakes of Geared Rotation.

p-Bis(arylcarbonyl)pentiptycenes 2 (aryl = 4-(trifluoromethyl)phenyl) and 3 (aryl = mesityl) have been prepared and investigated as redox-gated molecular rotors. For 2, rotations about the pentiptycene-carbonyl bond (the α rotation) and about the aryl-carbonyl bond (the ß rotation) are independent, and the rotation barriers are 11.3 and 9.5 kcal mol-1, respectively, at 298 K. In contrast, the α and ß rotations in 3 are correlated (geared) in a 2-fold cogwheel pathway between the aryl and the pentiptycene groups with a much lower rotation barrier of 6.5 kcal mol-1 at 298 K in spite of the bulkier aryl groups. Electrochemical reduction of the neutral forms led first to radical anions (2•- and 3•-) and then to a bis(radical anion) for 22- but a dianion for 32-. The redox operations switch the independent α and ß rotations in 2 into a geared rotation in both 2•- and 22- and result in a slow-fast-stop rotation mode for 2-2•--22-. The two redox states 3•- and 32- retain the geared α and ß rotations and follow a fast-slow-stop mode for 3-3•--32-. Both molecular systems mimic tristable molecular brakes and display 8-9 orders of magnitude difference in rotation rate through the redox switching.

TADF-based NIR-II semiconducting polymer dots for in vivo 3D bone imaging.

Intraoperative fluorescence imaging in the second near-infrared (NIR-II) region heralds a new era in image-guided surgery since the success in the first-in-human liver-tumor surgery guided by NIR-II fluorescence. Limited by the conventional small organic NIR dyes such as FDA-approved indocyanine green with suboptimal NIR-II fluorescence and non-targeting ability, the resulting shallow penetration depth and high false positive diagnostic values have been challenging. Described here is the design of NIR-II emissive semiconducting polymer dots (Pdots) incorporated with thermally activated delayed fluorescence (TADF) moieties to exhibit emission maxima of 1064-1100 nm and fluorescence quantum yields of 0.40-1.58% in aqueous solutions. To further understand how the TADF units affect the molecular packing and the resulting optical properties of Pdots, in-depth and thorough density-functional theory calculations were carried out to better understand the underlying mechanisms. We then applied these Pdots for in vivo 3D bone imaging in mice. This work provides a direction for future designs of NIR-II Pdots and holds promising applications for bone-related diseases.

A pentiptycene-derived molecular brake: photochemical E→Z and electrochemical Z→E switching of an enone module.

The synthesis and brakelike performance of a new molecular system (1) consisting of a pentiptycene rotor and a 2-methyleneindanone brake are reported. The rotation kinetics of the rotor was probed by both variable-temperature (1)H and (13)C NMR spectroscopy and DFT calculations, and the switching between the brake-on and brake-off states was conducted by a combination of photochemical and electrochemical isomerization. Because of the greater steric hindrance between the rotor and the brake units in the Z form ((Z)-1) than in the E form ((E)-1), rotation of the rotor is slowed down 500-fold at room temperature (298 K) on going from (E)-1 to (Z)-1, corresponding to the brake-off and brake-on states, respectively. The (E)-1→(Z)-1 photoisomerization in acetonitrile is efficient and reaches an (E)-1/(Z)-1 ratio of 11:89 in the photostationary state upon excitation at 290 nm, attributable to a much larger isomerization quantum efficiency for (E)-1 versus (Z)-1. An efficient (Z)-1→(E)-1 isomerization (96%) was also achieved by electrochemical treatment through the radical anionic intermediates. Consequently, the reversibility of the E-Z switching of 1 is as high as 85%. The repeated E-Z switching of 1 with alternating photochemical and electrochemical treatments is also demonstrated.

Toward a four-toothed molecular bevel gear with C2-symmetrical rotors.

The design, synthesis, conformational analysis, and variable-temperature NMR studies of pentiptycene-based molecular gears Pp(2)X, where Pp is the unlabeled (in 1H) or methoxy groups-labeled (in 1OM) pentiptycene rotor and X is the phenylene stator containing ortho-bridged ethynylene axles, are reported. The approach of using shape-persistent rotors of four teeth but C(2) symmetry for constructing four-toothed molecular gears is unprecedented. In addition, the first example of enantioresolution of chiral pentiptycene scaffolds is demonstrated. Density functional theory (DFT) and AM1 calculations on these Pp(2)X systems suggest two possible correlated torsional motions, geared rocking and four-toothed geared rotations, which compete with the uncorrelated gear slippage. The DFT-derived torsional barriers in 1H for rocking, four-toothed rotation, and gear slippage are approximately 2.9, 5.5, and 4.7 kcal mol(-1), respectively. The low energy barriers for these torsional motions result from the low energy cost of bending the ethynylene axles. Comparison of the NMR spectra of 1OM in a mixture of stereoisomers (1OM-mix) and in an enantiopure form (1OM-op) confirms a fast gear slippage in these Pp(2)X systems. The effect of the methoxy labels on rotational potential energy surface and inter-rotor dynamics is also discussed.

Pentiptycene-derived light-driven molecular brakes: substituent effects of the brake component.

Five pentiptycene-derived stilbene systems (1 R; R = H, OM, NO, Pr, and Bu) have been prepared and investigated as light-driven molecular brakes that have different-sized brake components (1 H<1 OM<1 NO<1 Pr<1 Bu). At room temperature (298 K), rotation of the pentiptycene rotor is fast (k(rot)=10(8)-10(9)  s(-1)) with little interaction with the brake component in the trans form ((E)-1 R), which corresponds to the brake-off state. When the brake is turned on by photoisomerization to the cis form ((Z)-1 R), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)-1 NO, (Z)-1 Pr, and (Z)-1 Bu, the rotation is nearly blocked (k(rot)=2-6 s(-1)) at 298 K. It is also demonstrated that the rotation is slower in [D(6)]DMSO than in CD(2)Cl(2). A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)-1 H, (Z)-1 OM, and (Z)-1 NO, and it levels off on going from (Z)-1 NO to (Z)-1 Pr and (Z)-1 Bu. DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E-Z photoswitching is up to 46%, and both the E and Z isomers are stable under the irradiation conditions.

A Molecular Rotor That Probes the Helical Inversion of Stiff-Stilbene.

Probing the inversion kinetics of a molecular helix is inherently a challenging task. We demonstrate herein that a fast-rotating pentiptycene component could function as an external NMR probe to afford the kinetic information on the inversion of a neighboring helical stiff-stilbene unit.

Site specificity of the (alpha)C--H bond dissociation energy for a naturally occurring beta-hairpin peptide-An ab initio study.

A naturally occurring beta-hairpin peptide (PDB ID 1UAO) was used as a model to study the backbone oxidation of a protein with ab initio calculation at the B3LYB/6-31G(d) without any constraints. The (alpha)C--H bond dissociation energy of three different glycyl radicals located at different sites on the beta-hairpin peptide was calculated to evaluate the site specificity of backbone oxidation. The molecular and electronic structures of these glycyl radicals were analyzed to rationalize this site specificity. The overall molecular structure of the alpha-H abstracted beta-hairpin peptide remained almost unchanged with the exception of the local conformation of the attacked residue. However, the (alpha)C--H bond strength varied dramatically among these different sites.

Dicyclopenta[ghi,pqr]perylene as a Structural Motif for Bowl-Shaped Hydrocarbons: Synthetic and Conformational Studies.

Dicyclopenta[ghi,pqr]perylene (DCPP) is a substructural fragment on the surface of C70 yet not on C60. Unlike its intensely investigated buckybowl cousins, corannulene and sumanene, DCPP is largely ignored due to the lack of synthetic accessibility. This communication describes the first preparation of a DCPP derivative from bay substituted perylene bis(4-(trifluoromethyl)phenyl)methanol as the key cyclization precursor. Further incorporation of indeno substitutions at peri positions was accomplished through Suzuki-Heck benzannulation. DCPP derivatives 4 adopts a planar structure in crystal. On the contrary, indeno DCPP 5 and bis-indeno DCPP 6 adopt the bowl-shaped conformation in both the solid state and solution. Density functional theory (DFT) calculation reveals that the lowest-energy conformations of 4, 5, and 6 are all bowl-shaped. Nevertheless, small bowl-to-bowl inversion barrier for 4 (3.4 kcal/mol) is overcome by the crystal packing force, which leads to its observed planar structure. However, the bowl-shaped structures of 5 and 6 are affirmed by DFT calculation with intermediate and high bowl-to-bowl inversion barriers (10.4 and 18.5 kcal/mol, respectively).

Synthesis and Physical Study of Perylene and Anthracene Polynitrile as Electron Acceptors.

Attaching electron-withdrawing nitrile groups to π-conjugated systems is an effective approach toward electron acceptors. By combining various cyanation methodologies, perylene and anthracene polynitriles with up to eight nitrile substituents on one aromatic scaffold were synthesized. This strategy produces stable electron acceptors with lowest unoccupied molecular orbital (LUMO) levels comparable to benchmark acceptors. A very stable octanitrile anion was also produced serendipitously. Reactivity patterns of these acceptors were rationalized by density functional theory (DFT) calculation.

Carrier Transport and Recombination Mechanism in Blue Phosphorescent Organic Light-Emitting Diode with Hosts Consisting of Cabazole- and Triazole-Moiety.

In this study, we demonstrated a blue phosphorescent organic light-emitting diode (BPOLED) based on a host with two carbazole and one trizole (2CbzTAZ) moiety, 9,9'-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-1,3-phenylene)bis(9H-carbazole), that exhibits bipolar transport characteristics. Compared with the devices with a carbazole host (N,N'-dicarbazolyl-3,5-benzene, (mCP)), triazole host (3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole, (TAZ)), or a physical mixture of mCP:TAZ, which exhibit hole, electron, and bipolar transport characteristics, respectively, the BPOLED with the bipolar 2CbzTAZ host exhibited the lowest driving voltage (6.55 V at 10 mA/cm2), the highest efficiencies (maximum current efficiency of 52.25 cd/A and external quantum efficiency of 23.89%), and the lowest efficiency roll-off, when doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as blue phosphor. From analyses of light leakage of the emission spectra of electroluminescence, transient electroluminescence, and partially doped OLEDs, it was found that the recombination zone was well confined inside the emitting layer and the recombination rate was most efficient in a 2CbzTAZ-based OLED. For the other cases using mCP, TAZ, and mCP:TAZ as hosts, electrons and holes transported with different routes that resulted in carrier accumulation on different organic molecules and lowered the recombination rate.

Global and local structural changes of cytochrome c and lysozyme characterized by a multigroup unfolding process.

Equilibrium unfolding behaviors of cytochrome c and lysozyme induced by the presence of urea (0-10 M) as well as changes in temperature (295-363 K) or pH (1.8-7) are examined via small-angle x-ray scattering and spectroscopic techniques, including circular dichroism and optical absorption. Denaturant and temperature effects are incorporated into the free energy expression for a general multigroup unfolding process. Results indicate that there are at least four unfolding groups in the temperature-, urea-, or pH-induced unfolding of cytochrome c: two of these are related to the prosthetic heme group, and the other two correspond, respectively, to the unfolding of alpha-helices and global changes in protein morphology that are largely unaccounted for by the first two groups. In contrast, the unfolding of lysozyme approximately follows a simple one-group process. A modified mean-field Ising model is adopted for a coherent description of the unfolding behaviors observed. Thermodynamic parameters extracted from simple denaturing processes, on the basis of the Ising model, can closely predict unfolding behaviors of the proteins in compounded denaturing environments.

A pentiptycene-derived light-driven molecular brake.

A room-temperature light-driven molecular brake (1), consisting of a pentiptycene rotator, a 3,5-dinitrophenyl brake, and a photoisomerizable ethenyl spacer, is reported. The rotation rates of the rotator differ by about 9 orders of magnitude between the brake-on (cis-1) and brake-off (trans-1) states.

A rotary molecular motor gated by electrical energy.

DFT calculations predict that the chiral pentiptycene derivative E-1 possesses distinct rotational potential energy surfaces in the neutral vs the radical anionic (E-1(•-)) form such that continued electrochemical switching between E-1 and E-1(•-) could lead to a directional rotation of the pentiptycene rotor about the exocyclic C-C bond. The time scale of random Brownian rotation is ∼10(6) s for E-1 and ∼170 s for E-1(•-) at 150 K, and thus a switching time scale of 0.2 s could readily bias the rotation direction to >99% at 150 K. The synthetic feasibility, line-shape analysis on the VT (1)H NMR spectra, and electrochemical redox switching of E-1 are demonstrated.

Oxygen radical-mediated oxidation reactions of an alanine peptide motif - density functional theory and transition state theory study.

BACKGROUND: Oxygen-base (O-base) oxidation in protein backbone is important in the protein backbone fragmentation due to the attack from reactive oxygen species (ROS). In this study, an alanine peptide was used model system to investigate this O-base oxidation by employing density functional theory (DFT) calculations combining with continuum solvent model. Detailed reaction steps were analyzed along with their reaction rate constants. RESULTS: Most of the O-base oxidation reactions for this alanine peptide are exothermic except for the bond-breakage of the Cα-N bond to form hydroperoxy alanine radical. Among the reactions investigated in this study, the activated energy of OH α-H abstraction is the lowest one, while the generation of alkylperoxy peptide radical must overcome the highest energy barrier. The aqueous situation facilitates the oxidation reactions to generate hydroxyl alanine peptide derivatives except for the fragmentations of alkoxyl alanine peptide radical. The Cα-Cß bond of the alkoxyl alanine peptide radical is more labile than the peptide bond. CONCLUSION: the rate-determining step of oxidation in protein backbone is the generation of hydroperoxy peptide radical via the reaction of alkylperoxy peptide radical with HO2. The stabilities of alkylperoxy peptide radical and complex of alkylperoxy peptide radical with HO2 are crucial in this O-base oxidation reaction.

High performance single-crystal field-effect transistors based on twisted polyaromatic semiconductor pyreno[4,5-a]coronene.

A novel p-channel semiconductor pyreno[4,5-a]coronene has been synthesized and characterized. The highly fused π-conjugated framework has a twisted geometry with an excellent on-top cofacial π-π stacking in the crystal structure and with a centroid-to-centroid distance of 3.808 Å. Single-crystal field effect transistors based on the molecule exhibit a high mobility of ~0.89 cm(2) V(-1) s(-1) and an on/off ratio of ~6 × 10(4).

Site specificity of alpha-H abstraction reaction among secondary structure motif--an ab initio study.

The initial step of protein oxidation is studied through alpha-H abstraction by an OH radical with various secondary structure motifs of proteins. It is found that there exist preferential alpha-Hs in this kind of abstractions. The typical abstraction mechanism involves three steps: forming a pre-reactive complex before abstraction, the abstraction reaction, and the H(2)O detachment from a post-reactive complex to form the product, C(alpha)-center radical. Using the stability of the pre-reactive complex and the reaction barrier, we provide some explanation for this site preference. The feasibility of alpha-H abstraction by OH radical depends not only on the types of secondary structure, but also on the reaction condition, such as in aqueous or in gas phase. Moreover, the reactivity of the abstraction also depends on the location of alpha-H in the secondary structure motifs. The preferential alpha-Hs to be abstracted in beta-sheet are those immediate to the amide or carbonyl group, and without involving hydrogen bonding, whereas in reverse turns, the preferential alpha-Hs are near the C-terminal of type I and near the N-terminal of type II. In general, the alpha-Hs in alpha-helix are more difficult to be abstracted than those in beta-sheet and polypeptide in linear form. It is consistent with the trend of their bond dissociation energies. Our theoretical rate constant of N-acetyldiglycin-methylamide (Ac(Gly)(2)NHCH(3)) in aqueous solution (6.75 x 10(8) M(-1) s(-1)) is close to the experimental observation of N-acetyldiglycinamide (Ac(Gly)(2)NH(2)) (8.6 x 10(8) M(-1) s(-1)).

Self-complementarity of oligo-2-aminopyridines: a new class of hydrogen-bonded ladders.

A new class of hydrogen-bonded ladders based on hydrogen-bonded dimerization of oligo-alpha-aminopryidines has been demonstrated. Jorgensen's model can be successfully applied to this hydrogen-bonding system in nonpolar solvents. The results show the competitive enthalpy/entropy compensation relationship upon dimerization. Although increasing the number of hydrogen-bonding interactions would enhance the hydrogen-bonding stabilization enthalpy, this stabilization enthalpy per unit would be partially sacrificed to compensate for the entropy loss due to dimerization. These results clearly support the importance of preorganization in designing hydrogen-bonding guest-host molecules.