Molecular spur gears comprising triptycene rotators and bibenzimidazole-based stators

Dynamic gearing of molecular spur gears, the most common type of mechanical gear, is elucidated. Molecular design and conformational analysis show that derivatives of 4,4-bis(triptycen-9-ylethynyl)bibenzimidazole represent suitable constructs to investigate gearing behavior of collateral triptycene (Tp) groups. To test this design, DFT calculations (B97-D/Def2-TZVP) were employed and the results suggest that these molecules undergo geared rotation preferentially to gear slippage. Synthesis of derivatives was carried out, providing a series of molecular spur gears, including the first desymmetrized spur gear molecules, which were subsequently subjected to stereochemical analysis.     

Methoxy and Methyl Group Rotation: Solid‐State NMR 1H Spin‐Lattice Relaxation,Electronic Structure Calculations,X‐ray Diffractometry,and Scanning Electron Microscopy

We report solid‐state ¹H nuclear magnetic resonance (NMR) spin‐lattice relaxation experiments, X‐ray diffractometry, field‐emission scanning electron microscopy, and both single‐molecule and cluster ab initio electronic structure calculations on 1‐methoxyphenanthrene ( 1 ) and 3‐methoxyphenanthrene ( 2 ) to investigate the rotation of the methoxy groups and their constituent methyl groups. The electronic structure calculations and the ¹H NMR relaxation measurements can be used together to determine barriers for the rotation of a methoxy group and its constituent methyl group and to develop models for the two coupled motions.     

Circumambulatory rearrangement with characteristics of a 2:1 covalent molecular bevel gear

anti-W(CO)(5)-complexed 9-methyl-9-phosphabicyclo[6.1.0]nonatriene represents a covalently interlocked molecular bevel gear. Correlated movement of the phosphorus atom and the eight-membered ring by way of a "walk" rearrangement makes gear slippage impossible. The gearing motion is transferred to the four-toothed W(CO)(5) propeller connected to the rotating phosphorus atom, enabling a gearing ratio of 2:1 according to B3LYP and Car-Parrinello Molecular Dynamics calculations. Methyl substitution of the eight-membered ring tempers the gearing process, with the PMeW(CO)(5) entity passing the substituted carbon atom only at temperatures above 50 degrees C.     

Rotationally resolved electronic spectra of 2- and 3-methylanisole in the gas phase: a study of methyl group internal rotation

Rotationally resolved fluorescence excitation spectra of several torsional bands in the S1 <-- S0 electronic spectra of 2-methylanisole (2MA) and 3-methylanisole (3MA) have been recorded in the collision-free environment of a molecular beam. Some of the bands can be fit with rigid rotor Hamiltonians; others exhibit perturbations produced by the coupling between the internal rotation of the methyl group and the overall rotation of the entire molecule. Analyses of these data show that 2MA and 3MA both have planar heavy-atom structures; 2MA has trans-disposed methyl and methoxy groups, whereas 3MA has both cis- and trans-disposed substituents. The preferred orientations (staggered or eclipsed) in two of the conformers and the internal rotation barriers of the methyl groups in all three conformers change when they are excited by light. Additionally, the values of the barriers opposing their motion depend on the relative positions of the substituent groups, in both electronic states. In contrast, no torsional motions of the attached methoxy groups were detected. Possible reasons for these behaviors are discussed.     

Conformational behaviour, hydrogen bond competition and intramolecular dynamics in vanillin derivatives: acetovanillone and 6-hydroxy-3-methoxyacetophenone

The conformational landscape of the structural isomers acetovanillone (apocynin, AV) and 6-hydroxy-3-methoxyacetophenone (HMAP) has been investigated in a supersonic jet using Fourier transform microwave spectroscopy. Two conformers have been detected in the jet-cooled expansion for each molecule (s-cis and s-trans in AV; s-trans and a-trans for HMAP), differing in the relative orientation of the acetyl and methoxy groups. Both molecules are stabilized by O-H···O or O-H···O=C hydroxyl intramolecular hydrogen bonds, either constraining the local conformations of the methoxy group in AV, or that of the acetyl group in HMAP. Internal rotation splittings have been observed in both conformers of each molecule, originated by the acetyl group, that yield information on the influence of the intramolecular hydrogen bonds on the methyl torsion. The similar internal rotation barriers in both molecules (6.6 and 7.4 kJ mol(-1) in AV; 7.3 and 7.0 kJ mol(-1) in HMAP) suggest that the acetyl torsion is only slightly affected by intramolecular hydrogen bonding. The absence of torsional tunnellings due to the methoxy group indicates torsional barriers above 10.2 and 8.9 kJ mol(-1) for AV conformers, 10.1 and 10.4 kJ mol(-1) for HMAP. Conformational ratios and relative free energies have been estimated from relative intensity measurements of the spectral lines. Ab initio (MP2) and density functional calculations using the recent M05-2X empirical functional have been used to aid the experimental work in describing the structures, internal rotation barriers and isomerization potentials.     

Stereodynamics of bond rotation in tertiary aromatic amides

The degree to which the rotations about the C-N and Ar-CO bonds of aromatic amides occur in a concerted manner was investigated by a variety of NMR and kinetic techniques. Otherwise complex kinetic analyses were simplified by exploiting symmetry and asymmetry in the N-substituents of amides. In 2-unsubstituted 1-naphthamides bearing branched N-substituents, most conformational changes about the amide group were by correlated rotation, though uncorrelated Ar-CO rotation also occurred to some extent. In 2-substituted 1-naphthamides, correlated rotation accounted for all of the Ar-CO rotations, though a significant amount of uncorrelated C-N rotation also occurred. Naphthamides bearing branched N-substituents thus turn out to be efficient molecular gears: Compound 12 showed almost no gear slippage.     

The microwave spectrum of a two-top peptide mimetic: the N-acetyl alanine methyl ester molecule

The rotational spectrum of N-acetyl alanine methyl ester, a derivative of the biomimetic, N-acetyl alanine N'-methyl amide or alanine dipeptide, has been measured using a mini Fourier transform spectrometer between 9 and 25 GHz as part of a project undertaken to determine the conformational structures of various peptide mimetics from the torsion-rotation parameters of low-barrier methyl tops. Torsion-rotation splittings from two of the three methyl tops capping the acetyl end of the -NH-C(=O)- and the methoxy end of -C(=O)-O- groups account for most of the observed lines. In addition to the AA state, two E states have been assigned and include an AE state having a torsional barrier of 396.45(7) cm(-1) (methoxy rotor) and an EA state having a barrier of 64.96(4) cm(-1) (acetyl rotor). The observed torsional barriers and rotational constants of alanine dipeptide and its methyl ester are compared with predictions from M?ller-Plesset second-order perturbation theory (MP2) and density functional theory (DFT) in an effort to explore systematic errors at the two levels of theory. After accounting for zero-point energy differences, the torsional barriers at the MP2/cc-pVTZ level are in excellent agreement with experiment for the acetyl and methoxy groups while DFT predictions range from 8% to 80% too high or low. DFT is found to consistently overestimate the overall molecular size while MP2 methods give structures that are undersized. Structural discrepancies of similar magnitude are evident in previous DFT results of crystalline peptides.     

o-Amino conjugation effect on the photochemistry of trans-aminostilbenes

The excited-state behavior of a series of trans-2-(N-arylamino)stilbenes (aryl = phenyl (o1H), 4-methylphenyl (o1Me), 4-methoxyphenyl (o1OM), and 4-cyanophenyl (o1CN)) and trans-2-(N,N-diphenylamino)stilbene (o2) in both nonpolar and polar solvents is reported and compared to that of the parent trans-2-aminostilbene and the corresponding meta- and para-isomers (m1R and p1R, where R = H, Me, OM, and CN, and m2 and p2). Two types of torsional motions, the D-A torsion that results in a nonfluorescent twisted intramolecular charge transfer (TICT) state and the C═C torsion that leads to the cis isomers, account for the radiationless decays of o1R and o2. The relative efficiencies of these torsions can be readily evaluated from their quantum yields for fluorescence (Φ(f)) and trans → cis isomerization (Φ(tc)). The propensities of the D-A torsion are similar for the ortho and meta isomers, which is 1OM > 1Me and negligible for 1H, 1CN, and 2. The activation parameters determined from temperature-dependent fluorescence lifetimes suggest that the C═C torsion occurs mainly via the triplet state for the ortho systems, a behavior again similar to that of the meta isomers. Whereas the intersystem crossing in o1R, m1R, and m2 is essentially a nonactivated process, it encounters a barrier of 2.7-3.8 kcal mol(-1) in o2. As a result of the barriers that decelerate the radiationless decays and the slow fluorescence rate for o2 in acetonitrile, the observed long fluorescence lifetime 24.5 ns at room temperature reaches a new record for unconstrained trans-stilbenes.     

Introduction of clutch function into a molecular gear system by silane-silicate interconversion

Introduction of the clutch-declutch mechanism into a new gear system, bis(4-methyl-9-triptycyl)difluorosilane 1, is achieved by the reversible attachment of fluoride ion giving the corresponding fluorosilicate 2. Although the phase isomers of 1 (1(dl) and 1(meso)) cannot be separated because of the equilibrium via a slow gear slippage process (DeltaH(double dagger) = 17.2 +/- 0.2 kcal x mol(-1) and DeltaS(double dagger) = 0.9 +/- 0.9 cal x mol(-1) x K(-1)), 1 works as meshed molecular gears in solution at room temperature. On the other hand, silicate 2 in the solid state has quite an unusual TBP structure having two organic triptycyl groups at the apical positions and three electronegative fluorine atoms at the equatorial positions against the Muetterties rule. Rotation of the two triptycyl groups around Si-C bonds in 2 is facile and independent to each other in solution. Silicate 2 is reverted to the corresponding silane mixture by treating with excess water.     

Meta conjugation effect on the torsional motion of aminostilbenes in the photoinduced intramolecular charge-transfer state

The photochemical behavior of a series of trans-3-(N-arylamino)stilbenes (m1, aryl = 4-substituted phenyl with a substituent of cyano (CN), hydrogen (H), methyl (Me), or methoxy (OM)) in both nonpolar and polar solvents is reported and compared to that of the corresponding para isomers (p1CN, p1H, p1Me, and p1OM). The distinct propensity of torsional motion toward a low-lying twisted intramolecular charge-transfer (TICT) state from the planar ICT (PICT) precursor between the meta and para isomers of 1CN and 1Me reveals the intriguing meta conjugation effect and the importance of the reaction kinetics. Whereas the poor charge-redistribution (delocalization) ability through the meta-phenylene bridge accounts for the unfavorable TICT-forming process for m1CN, it is such a property that slows down the decay processes of fluorescence and photoisomerization for m1Me, facilitating the competition of the single-bond torsional reaction. In contrast, the quinoidal character for p1Me in the PICT state kinetically favors both fluorescence and photoisomerization but disfavors the single-bond torsion. The resulting concept of thermodynamically allowed but kinetically inhibited TICT formation could also apply to understanding the other D-A systems, including trans-4-cyano-4'-(N,N-dimethylamino)stilbene (DCS) and 3-(N,N-dimethylamino)benzonitrile (3DMABN).     

Limitations of Global Hybrids in Predicting the Geometries and Torsional Energy Barriers of Dimeric Systems and the Role of Hartree Fock and DFT Exchange

Prediction of accurate geometries is a prerequisite for accurate prediction of molecular properties. Impact of Hartree Fock (HF) exchange (a₀) on geometry in the framework of DFT is investigated by monitoring dihedral angles, bond length alternations, and torsional energy barriers of 10 dimeric systems against CCSD (ADZ/ATZ) benchmarks. A strong correlation is observed between the fraction of HF exchange, equilibrium dihedral angles, and the potential energy barriers in global hybrids. Full HF exchange is critical to accurately predict the nonplanarity. Lower fractions of (a₀)/larger DFT exchange (1-a₀) results in overestimation of torsional energy barriers at 90⁰ and underestimation at 0⁰. Large contributions of (1-a₀) in global hybrid functionals tend to overestimate torsional energy barriers (90⁰) and are biased toward planar geometries. However, inclusion of larger fractions of (a₀)/lower (1-a₀) also overestimate the torsional energy barriers in syn-conformations due to the localization errors associated with HF exchange in global hybrids. Hence, irrespective of the fraction of HF/DFT exchange incorporated, global hybrids fail to accurately predict torsional energy barriers at 0⁰ and 90⁰ simultaneously. Long-range corrected (LC) functionals, which employ full HF exchange at longer regions, outperform global hybrid functionals in predicting geometries and torsional energy barriers of the dimeric molecules. The distance dependence of (a₀) thus provides a balanced fraction of HF exchange as the dihedral torsion varies. Impact of range separation parameter on geometries is marginal in altering the planarity/nonplanarity. However, range separation parameter within 0.20–0.40 bohr⁻¹ predicts more reliable torsional energies and geometries. © 2019 Wiley Periodicals, Inc.     

Investigation on poly[(methylsilylene ethynylene phenylene ethynylene)-co-(tetramethyldisiloxane ethynylene phenylene ethynylene)]

<正>Poly[(methylsilylene ethynylene phenylene ethynylene)-co-(tetramethyldisiloxane ethynylene phenylene ethynylene)]was synthesized by polycondensation reaction of m-diethynylbenzene magnesium reagent with 1,3-dichlorotetramethyldisiloxane and dichloromethylsilane.The copolymer was characterized by FT-IR,~1H NMR,differential scanning calorimetry and thermogravimetric analysis.The results show that the copolymer exhibits good processability and cures at low temperatures.The cured copolymer shows high thermal stability.     

Synthesis,Electrochemical, and Optical Properties of a Novel PPV/PPE Block‐Copolymer

Summary: A novel poly(p‐phenylene vinylene) (PPV)/poly(p‐phenylene ethynylene) (PPE) block‐copolymer was synthesized by a cross‐coupling polycondensation with Pd(PPh₃)₂Cl₂ and a phase‐transfer catalyst, and was confirmed by ¹H NMR and IR spectroscopy and elemental analysis. The thermal, electrochemical, and photoluminescent properties of the new copolymer have been investigated. The incorporation of triple bonds into the cyano‐substituted PPV (CN‐PPV) backbone leads to higher oxidation and reduction potentials than poly(2‐methoxy‐5‐(2‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) and CN‐PPV, potentially making the copolymer a good electron‐transporting material for use in a light‐emitting‐diode device.

The cyclic voltammogram of the novel poly(p‐phenylene vinylene) (PPV)/poly(p‐phenylene ethynylene) (PPE) block‐copolymer synthesized here.     

Practical methods for including torsional anharmonicity in thermochemical calculations on complex molecules: the internal-coordinate multi-structural approximation

Many methods for correcting harmonic partition functions for the presence of torsional motions employ some form of one-dimensional torsional treatment to replace the harmonic contribution of a specific normal mode. However, torsions are often strongly coupled to other degrees of freedom, especially other torsions and low-frequency bending motions, and this coupling can make assigning torsions to specific normal modes problematic. Here, we present a new class of methods, called multi-structural (MS) methods, that circumvents the need for such assignments by instead adjusting the harmonic results by torsional correction factors that are determined using internal coordinates. We present three versions of the MS method: (i) MS-AS based on including all structures (AS), i.e., all conformers generated by internal rotations; (ii) MS-ASCB based on all structures augmented with explicit conformational barrier (CB) information, i.e., including explicit calculations of all barrier heights for internal-rotation barriers between the conformers; and (iii) MS-RS based on including all conformers generated from a reference structure (RS) by independent torsions. In the MS-AS scheme, one has two options for obtaining the local periodicity parameters, one based on consideration of the nearly separable limit and one based on strongly coupled torsions. The latter involves assigning the local periodicities on the basis of Voronoi volumes. The methods are illustrated with calculations for ethanol, 1-butanol, and 1-pentyl radical as well as two one-dimensional torsional potentials. The MS-AS method is particularly interesting because it does not require any information about conformational barriers or about the paths that connect the various structures.     

Silylanions: inversion barriers and NMR chemical shifts

Alpha-substituent effects on inversion barriers and NMR chemical shifts have been studied on a set of silyl anions, [X(3-n)Y(n)Si](-) (X, Y=H, CH(3), and SiH(3)). The MP2/6-31+G* optimized structures show a pattern of increasing inversion barriers with augmenting numbers of methyl substituents. The highest barrier of 48.5 kcalmol(-1) is obtained for the (CH(3))(3)Si(-) ion. The silyl group displays the opposite effect by decreasing the inversion barrier to a minimum of 16.3 kcalmol(-1) in (SiH(3))(3)Si(-). The influence of counterions on these barriers is probed by addition of a lithium or potassium cation. In most cases, a decrease of the energy barriers with respect to the bare anions is observed. The (29)Si NMR chemical shifts calculated at the IGLO-DFT and GIAO-MP2 level of theory are also analyzed in view of the substituents and counterions.     

Atropisomerism in the 2-arylimino-N-(2-hydroxyphenyl)thiazoline series: influence of hydrogen bonding on the racemization process

A series of original atropisomeric iminothiazolines 1 in which X = OH or (and) Y = OH were prepared from the corresponding methoxy precursors. The resolution of the atropisomeric enantiomers on chiral support is reported, and the barriers to enantiomerization are given. These barriers were determined either by off-line racemization studies or by treatment of the plateau-shape chromatogram during chromatography on chiral support. When X = OH, the barriers are quite low due to the development of a hydrogen bond between the proton of the OH group and the nitrogen of the imino group. For these compounds, plateau shape chromatograms were obtained during HPLC on chiral support. DFT calculations confirmed the occurrence of hydrogen bonding all along the rotation process and produced calculated barriers in close agreement with the experimental data. Compound 1i (OH, OH) in which both X and Y are hydroxy groups was particularly easy to prepare by demethylation with BBr3 of the dimethoxy precursor. Since the above-mentioned precursor is readily available from N,N'-bis(2-methoxyphenyl)thiourea and 1-chloropropan-2-one, 1i (OH, OH) is a good candidate for further functionalization. Atropisomerism in a 12-membered bridged bisether prepared from 1i (OH, OH) is reported as an illustrating example.     

含二茂铁齐聚苯乙炔分子导线的合成

通过二碘代二茂铁与不同炔化物进行Sonogashira偶联反应,合成了3种新颖的含有二茂铁单元的苯乙炔齐聚物,用1H NMR、13C NMR和MS测试技术分别对其结构进行了表征。 并借助量子化学计算对3种分子的电子结构进行了预测,结果显示,该类型分子导线具有不同于其它类型苯乙炔齐聚物的LUMO能级持续衰减特征,因此极有可能具备极为优良的电子传输性能。     

Energetic preferences for alpha,beta versus beta,gamma unsaturation

Density functional theory has been applied at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level to examine the energetics of alpha,beta- versus beta,gamma-unsaturation for some common organic functional groups. Specifically, the relative stabilities of allyl-X (H2C=CHCH2X) and 1-propenyl-X (H3CCH=CHX) isomers have been computed for X = methyl, vinyl, phenyl, formyl, acetyl, methoxy, methylthio, methylsulfinyl, methylsulfonyl, sulfamoyl, and methoxysulfonyl, and the results are compared to available experimental data. The intrinsic preference of 3 kcal/mol for the 1-propenyl isomer when X = CH3 is exceeded by 2-4 kcal/mol for first-row conjugating groups, but it is not met for the sulfur-containing groups. In particular, alpha,beta-unsaturation is favored by less than 1 kcal/mol for the sulfone and sulfonamide analogues, while it is preferred by 8 kcal/mol for the vinyl-substituted case. Detailed structural results and torsional energy profiles are also reported.     

Synthesis of 9,10-bis(9-triptycyloxy)triptycenes : Molecular design of a system with doubly correlated internal rotation

Torsional motions around the two C—C and C—O bonds in di(9-triptycyl)methanes, Tp₂CH₂, and di(9-triptycyl) ethers, Tp₂O, respectively, have a high barrier to uncorrelated rotation and a very low barrier to coupled disrotation. As a result, new stereoisomerism is generated due to different phase relationships between appropriately labeled benzene rings, at least one on each Tp unit. To extend the concept and further demonstrate the high correlation in the torsional motions for these systems, a doubly geared molecule, 9,10-bis(3-chlorotriptycyloxy)triptycene (1), was conceived and constructed. Bis(3-chloro-9-triptycyl) 9,10-triptycenebis(peroxycarboxylate) was prepared. The meso and dl isomers of 1 were separated by HPLC on microsilica. The structures were confirmed by high resolution ¹³C-NMR spectra which revealed an interesting stercochemical feature : one benzene ring of the middle unsubstituted triptycene moiety is diastereotopic to the other two. The rates of isomerization were measured in diphenylmethane solution to give the activation parameters for the gear slipping process: ΔH³ = 42.1±1.3 kcal mol^-1 and ΔS³ = -3.2±2.3 e.u. The significance of these findings as an extreme case for the dynamics of molecular chains is discussed.     

Synthetic Methodology for Structurally Defined and Insulated Molecular Wires Bearing Non‐centrosymmetric Conjugated Axle Components via Iterative Intramolecular Slippage

Insulated molecular wires (IMWs) bearing non‐centrosymmetric conjugated axle components were precisely synthesized via iterative cross‐coupling reactions in organic solvents and subsequent intramolecular slippage transformation in aqueous solvents. This programmable synthetic procedure selectively afforded both insulated and uninsulated molecular wires bearing oligo(phenylene ethynylene) and permethylated α‐cyclodextrins with well‐defined conjugation lengths and supramolecular structures. High selectivity of this method was confirmed by NMR and mass spectroscopic analyses. The resultant IMWs exhibited distinct optical properties because of different conjugation lengths and insulated structures. This synthetic strategy for structurally defined IMWs bearing non‐centrosymmetric conjugated axle components could provide a platform for obtaining diverse functionalized materials useful in the fields of non‐centrosymmetric molecular machines and molecular electronics.