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.

Singlet Biradical Versus Triplet Biradical/Zwitterion Characteristics in Isomers of C6 -C5 -C6 -C7 -C6 -Fused Pentacyclic Aromatic Hydrocarbons Revealed through Reactivity Patterns.

Among various polycyclic aromatic hydrocarbons, C6 -C5 -C6 -C7 -C6 fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6, where the radicaloid carbons are linked through a para-phenylene, forms a head-to-head dimer like a singlet biradical. In contrast, isomer 7, where the para-linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C6 -C5 -C6 -C6 -C6 ring contraction product 13. Cation 14, the protonated form of 7, was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules.

Charge transport properties of open-shell graphene fragments: a computational study of the phenalenyl tilings.

Thinking outside the box of the phenalenyl radical: a systematic structure design strategy, phenalenyl tiling, is found to benefit the electron transport properties of open-shell graphene fragments with one free radical. Compared with the closed-shell species, phenalenyl-based π-radicals exhibit smaller intramolecular reorganization energies and larger intermolecular electronic couplings. However, the on-site Coulomb repulsion can be too strong and impedes the charge transport efficiency of such materials. The repulsion can be weakened in radical species by spin delocalization. In this paper, the extended π-radicals we studied are categorized into three types of open-shell structures: the zigzag, the armchair and the discotic odd alternant hydrocarbons. The latter two belong to phenalenyl tilings. We found that the phenalenyl tilings fully inherit the desirable features of the singly occupied molecular orbital of the phenalenyl radical in a predictable and delocalized fashion, and their on-site Coulomb repulsion is effectively reduced. The zigzag π-radicals are less satisfactory. Therefore, the phenalenyl tilings are favorable candidates for charge transporting materials.

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.

A design strategy for motion control systems with identical binding sites.

Molecular motion control systems such as bistable rotaxanes or molecular elevators are mostly designed with different binding sites. In this theoretical study, it is demonstrated that, with the use of a redox reaction center and its position effect, it is possible to build a motion control system with identical binding sites.

Cooperative effect of unsheltered amide groups on CO2 adsorption inside open-ended channels of a zinc(II)-organic framework.

A unique spatial arrangement of amide groups for CO2 adsorption is found in the open-ended channels of a zinc(II)-organic framework {[Zn4(BDC)4(BPDA)4]·5DMF·3H2O}n (1, BDC = 1,4-benzyl dicarboxylate, BPDA = N,N'-bis(4-pyridinyl)-1,4-benzenedicarboxamide). Compound 1 consists of 4(4)-sql [Zn4(BDC)4] sheets that are further pillared by a long linker of BPDA and forms a 3D porous framework with an α-Po 4(12)·6(3) topology. Remarkably, the unsheltered amide groups in 1 provide a positive cooperative effect on the adsorption of CO2 molecules, as shown by the significant increase in the CO2 adsorption enthalpy with increasing CO2 uptake. At ambient condition, a 1:1 ratio of active amide sites to CO2 molecules was observed. In addition, compound 1 favors capture of CO2 over N2. DFT calculations provided rationale for the intriguing 1:1 ratio of amide sorption sites to CO2 molecules and revealed that the nanochamber of compound 1 permits the slipped-parallel arrangement of CO2 molecules, an arrangement found in crystal and gas-phase CO2 dimer.

Aggregation-induced emission enhancement (AIEE) of tetrarhenium(I) metallacycles and their application as luminescent sensors for nitroaromatics and antibiotics.

The self-assembly of tetrarhenium metallacycles [{Re(CO)3}2(µ-dhaq)(µ-N-N)]2 (3a, N-N = 1,3-bis(1-butylbenzimidazol-2-yl)benzene; 3b, N-N = 1,3-bis(1-octylbenzimidazol-2-yl)benzene), (H2-dhaq = 1,4-dihydroxy-9,10-anthraquinone) and [{Re(CO)3}2(µ-thaq)(µ-N-N)]2 (4, N-N = 1,3-bis(1-butylbenzimidazol-2-yl)benzene), (H2-thaq = 1,2,4-trihydroxy-9,10-anthraquinone) under solvothermal conditions is described. The metallacycles 3a,b and 4 underwent aggregation-induced emission enhancement (AIEE) in THF upon the incremental addition of water. TEM images revealed that metallacycle 3a in a 60% aqueous THF solution formed rectangular aggregates with a wide size distribution, while a 90% aqueous THF solution resulted in the formation of a mixture of nanorods and amorphous aggregates due to rapid and abrupt aggregation. UV-vis and emission spectral profiles supported the formation of nanoaggregates of metallacycles 3a,b and 4 upon the gradual addition of water to a THF solution containing metallacycles. Further studies indicated that these nanoaggregates were excellent probes for the sensitive and selective detection of nitro group containing picric acid (PA) derivatives as well as antibiotics.

Intriguing electrochemical behavior of free base porphyrins: effect of porphyrin-meso-phenyl interaction controlled by position of substituents on meso-phenyls.

Electrochemical properties of substituted free base meso-tetraphenylporphyrins (H(2)T(o,o'-X)PP, H(2)T(o-X)PP, and H(2)T(p-X)PP, where X = OCH(3), CH(3), H, F, or Cl on the phenyl rings) are examined by cyclic voltammetry. When a substituent is located only at the para position of the meso-phenyl group, the difference between the first and second oxidation potentials (ΔE(ox), i.e., E(2)(ox) - E(1)(ox)), is generally significantly smaller than those of the H(2)TPPs with bulky o,o'-substituents on the phenyl group. This trend is elucidated with density functional theory calculations and attributed mainly to the sterically controlled π-conjugation of the meso-phenyl groups to the central porphyrin ring, rather than the often discussed deformation of porphyrin.

Factors that regulate the conformation of m-benziporphodimethene complexes: agostic metal-arene interaction, hydrogen bonding, and η2,π coordination.

Group 12 and silver(I) tetramethyl-m-benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric silver(I) complex displays an unusual η(2),π coordination from the ß-pyrrolic C=C bond to the silver ion. All of the complexes displayed a close contact between the metal ion and the inner C(22)-H(22) on the m-phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd(II) and H(22) strongly support the presence of an agostic interaction between the metal ion and inner C(22)-H(22). Crystal structures revealed that the syn form is the predominant conformation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m-benziporphyrin and tetraphenyl-m-benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen-bonding interactions between axial chloride and methyl groups stabilize syn conformations. Unlike the merely syn conformation observed in the solid-state structures of TMBPDM complexes that contain an axial chloride, in solution these complexes display highly solvent- and temperature-dependent syn/anti ratio changes. The observation of dynamic (1)H NMR spectroscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations revealed that intermolecular hydrogen-bonding interactions between the axial chloride and CHCl(3) stabilizes the anti conformation, which explains the increased ratio for the anti form when dichloromethane or chloroform was used as the solvent.

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.

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.

Dual-action acid/base- and base/acid-controllable molecular switch.

[Structure: see text] We report a molecular switch that not only can be switched between its complexed and decomplexed states through the sequential addition of an acid and a base (NH4+ and Proton Sponge, respectively) but also can be operated equally through the sequential addition of basic and acidic reagents (Et2NH and TFA, respectively).