Preparation and conformation of octaethylbiphenylene

Dimerization of tetraethylbenzyne (generated by reaction of 1, 2-dibromo-3,4,5,6-tetraethylbenzene (8) with 1 equiv of BuLi) afforded in low yield octaethylbiphenylene (3), together with a major product which was characterized as 2,3,4,5,3',4', 5'-heptaethyl-2'-vinylbiphenyl (9). X-ray diffraction indicates that biphenylene 3 adopts in the crystal a conformation of approximate C(2)(h )()symmetry with the ethyl groups within each phenylene ring arranged in an alternated up-down fashion. Notably, pairs of vicinal ethyl groups located at peri positions are oriented in a syn arrangement in the crystal. Low temperature NMR spectroscopy is consistent with the presence in solution of either the crystal conformation or a fully alternated conformation lacking any syn interaction. Molecular mechanics (MM3), semiempirical (AM1, PM3), and ab initio calculations indicate that the crystal conformation is a high energy form, and that the lowest energy conformation is the fully alternated form. The topomerization barrier of the methylene protons of the ethyl groups of 3 is 9.4 +/- 0.1 kcal mol(-)(1), which is between the rotational barriers of 8 and 1,2,3, 4-tetraethylbenzene 7 (9.9 +/- 0.1 and 8.2 +/- 0.1 kcal mol(-)(1), respectively). The similarity in rotational barriers suggests that a given tetraethylphenylene subunit does not markedly affect the rotational barrier of the ethyl groups of the other subunit.     

Stereodynamics and characterization of the hexa(4-n-dodecylbiphenylyl)benzene hexaanion that includes a twisted benzene core

Hexa(4-n-dodecylbiphenylyl)benzene (HDBB) was reduced by a series of alkali metals in THF under high vacuum. Three reduction states were identified by NMR spectroscopy, namely the dianion, tetraanion and hexaanion. The NMR spectra of HDBB(6-) revealed a remarkable distortion of symmetry, which is interpreted by adoption of a twisted conformation of the central benzene ring and a slow rotation of the inner phenylene rings of the biphenyl units. Due to the surprising thermal stability of the hexaanion, a dynamic NMR investigation revealed the pseudorotation of the twisted conformation and the phenylene rotation mentioned above.     

The preparation and characterization of [M∗N(Pr2PS)2∗ (M = Pt, Pd) and [Pd∗N(Pr2 PS)2∗ ∗HN(Pr2 PS)2∗]Cl

Reaction of PtCl₂(cod) with Pr₂ⁱ P(S)NHP(S)Pr₂ⁱ gives [M*N(Pr₂ⁱ PS)₂*₂] M = Pt1, Pd 2). Repeated crystallization of the palladium complex from CH₂Cl₂ resulted in the protonation of one ligand to give [Pd*HN(Pr₂ⁱ PS)₂**N(Pr₂ⁱ PS)₂*]Cl. The X-ray structures of 1 and 2 reveal a dramatic change from flattened chair to pseudo-boat conformation for the MS₂P₂N rings. Protonation of the ring does not appear to have major effects on ring conformation. VT ³¹P NMR studies on 1 suggest the presence of two conformers at low temperature.     

3,3,4,4,5,5‐Hexafluoro‐1‐(2‐methoxyphenyl)‐2‐[5‐(4‐methoxyphenyl)‐2‐methyl‐3‐thienyl]cyclopent‐1‐ene: a photochromic compound

The photochromic title compound, C₂₄H₁₈F₆O₂S, has thienyl and aryl substituents on the C=C double bond of the shallow half‐chair‐shaped cyclopentene ring. The planes of the two substituent rings are inclined to that of the cyclopentene ring, with dihedral angles between the mean plane of the cyclopentene ring and those of the phenylene and thienyl rings of 51.2 (1) and 51.3 (1)°, respectively. The molecule adopts an antiparallel conformation, with a distance between the two photoreactive C atoms of 3.717 (2) Å.     

Glassy poly(p-phenylene sulphide): determination of the local structure

Methods are described for obtaining conformational and packing information about para-linked phenylene polymers from wide-angle X-ray scattering. On the basis of these procedures, together with conformational energy calculations, a random chain conformation is determined for non-crystalline poly(p-phenylene sulphide), in which the phenylene groups are rotated away from the coplanar position by 40° and the conformation persists for ～15 Å. Whilst there is no evidence for parallel chain packing at any level, it is necessary to invoke local pairwise correlation of phenylene groups within an otherwise random chain packing system, to obtain satisfactory agreement with the experimental results.     

2H NMR characterization of phenylene ring flip motion in poly(p-phenylene vinylene) films

Solid-state ²H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of ²H spin lattice relaxation times have been obtained for films of poly(p-phenylene vinylene) deuterated in phenylene ring positions (PPV-d₄). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the ²H line shapes show that the jump motion is thermally activated, with a median activation energy, E_a = 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T₂ relaxation associated with ²H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. ²H NMR provides a method for determining the phenylene-vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.     

Surface analysis of polymers electrically improved by plasma-source ion-implantation

Inert-gas (Ar, Xe) and reactive-gas (O(2), N(2)) plasma-source ion-implantation (PSII) treatment of PI, PET, PS-BD, and MPPO surfaces was performed at an ion energy of 30 keV to improve the electrical properties of the polymers. The effect of ion energy, treatment time, rf frequency, and power on the surface resistivity of polymer was investigated. Depending on ion energy, dose, and ion species, the surface resistivity of the film was reduced by several orders of magnitude. XPS, TOF-SIMS, and SEM were used to characterize MPPO surfaces treated by Ar-PSII and Xe-PSII. From these measurements it was found that the improvement in surface resistivity after PSII treatment was related to graphite carbon or cross-linked carbon-double-bond species formed on the surface.     

Correlating Pressure‐Induced Emission Modulation with Linker Rotation in a Photoluminescent MOF

Conformational changes of linker units in metal‐organic frameworks (MOFs) are often responsible for gate‐opening phenomena in selective gas adsorption and stimuli‐responsive optical and electrical sensing behaviour. Herein, we show that pressure‐induced bathochromic shifts in both fluorescence emission and UV/Vis absorption spectra of a two‐fold interpenetrated Hf MOF, linked by 1,4‐phenylene‐bis(4‐ethynylbenzoate) ligands ( Hf‐peb ), are induced by rotation of the central phenyl ring of the linker, from a coplanar arrangement to a twisted, previously unseen conformer. Single‐crystal X‐ray diffraction, alongside in situ fluorescence and UV/Vis absorption spectroscopies, measured up to 2.1 GPa in a diamond anvil cell on single crystals, are in excellent agreement, correlating linker rotation with modulation of emission. Topologically isolating the 1,4‐phenylene‐bis(4‐ethynylbenzoate) units within a MOF facilitates concurrent structural and spectroscopic studies in the absence of intermolecular perturbation, allowing characterisation of the luminescence properties of a high‐energy, twisted conformation of the previously well‐studied chromophore. We expect the unique environment provided by network solids, and the capability of combining crystallographic and spectroscopic analysis, will greatly enhance understanding of luminescent molecules and lead to the development of novel sensors and adsorbents.     

Chain conformation change upon heating for pauci‐chain polystyrene microsphere made by microemulsion polymerization

The conformation change of pauci‐chain polystyrene microsphere (micro‐PS) upon heating was investigated by in‐situ FTIR. For the peaks at 1492 and 1452 cm⁻¹ due to phenyl ring semicircle stretch, there are two discontinuities in the plots of peak height versus temperature. The first discontinuity at around 100 °C corresponds to the glass transition. The second discontinuity at about 155 °C is in good agreement with the second exotherm in the first DSC scan, which clearly suggests a non‐negligible change of the chain conformation at this temperature. This finding may be of help in further illustrating the two exotherms of micro‐PS in its first DSC scan.     

Coordination to metal centers: a tool to fix high energy conformations in organic molecules. Application to 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene and related macrocycles

The solid state conformational preferences of ligand 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (L1) and its 9-methyl derivative (L2) in transition metal complexes have been determined by a probabilistic method using data retrieved from the Cambridge Structural Database. These macrocyclic compounds, as ligands, tend to adopt a preferential conformation (85% of cases). The ring containing the C=N bond adopts a distorted half-chair conformation, the ring defined by both the N-sp(3) shows a distorted envelope conformation, and the remaining ring exhibits a chair conformation. This conformation corresponds to the enantiomer pair R(N5)S(N9)S(P)/S(N5)R(N9)R(P). Molecular mechanics calculations demonstrate that this is a high energy conformation for the organic molecule, far from the energy minimum. Two other enantiomer pairs are observed in experimental structures. The influence of the coordination on the conformation of the organic ligands has been studied by DFT calculations, and a clear correlation with the geometry of the coordination sphere has been found.     

Influence of protonation on the conformations of rigid-chain polymers

The conformations, internal rotation barriers, and the energies of bending deformation of molecules that model chain segments of the rigid-chain polymer poly(p-phenylenebenzobisoxazole) were calculated with the use of the AM1 semiempirical quantum-chemical method. The model molecules included several heterocycles and had a size larger than the repeat unit of the polymer, which includes one heterocyclic ring and one phenylene ring. For molecules in which all heterocyclic rings are diprotonated (at nitrogen atoms), the planar conformation is optimal (as in the case of uncharged molecules). At the same time, the internal rotation barriers in such molecules are reduced relative to the neutral molecules. However, when not all heterocycles are protonated in the molecule, the barriers turn out to be substantially higher than in the neutral molecule. For molecules in which all heterocycles are tetraprotonated, ab initio calculations of the optimal conformation were also performed. For these molecules, the conformation in which the phenyl rings and the heterocyclic rings are turned by almost 50° with respect to one another appeared to be optimal. In this case, the height of the rotation barriers is even lower than in molecules with diprotonated heterocycles. The protonation was found to have a weak effect on the bending rigidity of the poly(p-phenylenebezobisoxazole) chain.     

Turning the Tap: Conformational Control of Quantum Interference to Modulate Single‐Molecule Conductance

Together with the more intuitive and commonly recognized conductance mechanisms of charge‐hopping and tunneling, quantum‐interference (QI) phenomena have been identified as important factors affecting charge transport through molecules. Consequently, establishing simple and flexible molecular‐design strategies to understand, control, and exploit QI in molecular junctions poses an exciting challenge. Here we demonstrate that destructive quantum interference (DQI) in meta‐substituted phenylene ethylene‐type oligomers (m‐OPE) can be tuned by changing the position and conformation of methoxy (OMe) substituents at the central phenylene ring. These substituents play the role of molecular‐scale taps, which can be switched on or off to control the current flow through a molecule. Our experimental results conclusively verify recently postulated magic‐ratio and orbital‐product rules, and highlight a novel chemical design strategy for tuning and gating DQI features to create single‐molecule devices with desirable electronic functions.     

Low-temperature mechanical relaxations in polymers containing aromatic groups

Studies have been made of the secondary relaxation processes in the solid state of a number of polymers containing aromatic groups in the polymer chain. The polymers investigated include one, polystyrene, with the aromatic group in side-chain positions, and six high polymers in which phenylene rings lie in the main backbone chain. In polystyrene, wagging and torsional motions of the side chain phenyl rings give rise to a low-temperature δ-relaxation which is centered at 33°K (1.7 Hz) and which has an activation energy of about 2.3 kcal/mol. Most of the polymers with phenylene rings in the main chain exhibit a low-temperature relaxation in the temperature region from 100°–200°K. This relaxation process is centered at 159°K (0.54 Hz) in poly-p-xylylene, at 162°K (0.67 Hz) in polysulfone, and at 165°K (1.24 Hz) in poly(diancarbonate). In poly(2,6-dimethyl-p-phenylene oxide), two overlapping low-temperature relaxations are found, one in the range 125–140°K and the other near 277°K (ca. 1 Hz). The low-temperature secondary relaxation process in all of these polymers is believed to be associated with local reorientational motion of the phenylene, or substituted phenylene, rings or with combined motion of the phenylene rings and nearby chain units. For these low temperature relaxation processes, the activation energy is about 10 kcal/-mole. The temperature location of the relaxation appears to depend on the specific units to which the phenylene rings are attached and on steric and polar effects caused by substituents on the ring. In the poly-p-xylylenes the relaxation is shifted to much higher temperatures by a single Cl substitution on the ring but remains at essentially the same temperature position when dichlorosubstitution is made. The effects of water on the magnitude and temperature location of the observed low temperature relaxations, as well as the implications of the study for other polymers containing aromatic groups in their backbone chains, are discussed.     

Solvent- and pH-induced self-assembly of cationic meta-linked poly(phenylene ethynylene): effects of helix formation on amplified fluorescence quenching and Förster resonance energy transfer

We reported here the synthesis and characterization of a novel water-soluble, meta-linked poly(phenylene ethynylene) (m-PPE-NEt(2)Me(+)) featuring quaternized side groups. We studied the solvent-induced self-assembly of m-PPE-NEt(2)Me(+) in MeOH/H(2)O solvent mixtures by using UV-vis absorption and fluorescence spectroscopies. The results showed that the polymer folded into a helical conformation and that the extent of helical folding increased with the volume % water in the solvent. This cationic polymer also exhibited unique pH-induced helix formation, which was attributed to the partial neutralization of quaternized side groups at high pH and the meta-links in the main chain of the polymer. Studies on the fluorescence quenching of m-PPE-NEt(2)Me(+) by anthraquinone-2,6-disulfonate (AQS) and Fe(CN)(6)(4-), two small-molecule anionic quenchers with different typical structures, revealed more efficient quenching of helical conformation by AQS than by Fe(CN)(6)(4-). We proposed that the two quenchers most likely interacted with the polymer helix in two different modes; that was, AQS featuring large planar aromatic ring could intercalate within adjacent π-stacked phenylene ethynylene units in the polymer helix, whereas Fe(CN)(6)(4-) mainly bound to the periphery of polymer helix through ion-pair formation. Finally, the results of FRET from the helical polymer to the fluorescein (C*)-labeled polyanions, ssDNA-C* (ssDNA: single-stranded DNA) and dsDNA-C* (dsDNA: double-stranded DNA) also suggested two different modes of interactions. As compared with the FRET to dsDNA-C*, the FRET to ssDNA-C* was slightly more efficient, which was believed to arise from the additional binding of ssDNA-C* with the polymer via intercalation of its exposed hydrophobic bases into the π stack of adjacent phenylene ethynylene units in the polymer helix.     

Impact of position of electron withdrawing cyano groups on nonlinear optical properties of centrosymmetric donor‐π‐acceptor system

Two symmetrically substituted phenylenevinylene D‐A‐D′‐A‐D type siblings, (2Z,2′Z)‐2,2′‐(2,5‐dimethoxy‐1,4‐phenylene)bis(3‐(4‐(dimethylamino)phenyl)acrylonitrile) (↑‐dscn) and (2Z,2′Z)‐3,3′‐(2,5‐dimethoxy‐1,4‐phenylene)bis(2‐(4‐(dimethylamino)phenyl)acrylonitrile) (↓‐dscn), are prepared. We investigate the effect of different but symmetrical location of these cyano groups in vinylene bridges on the 1‐photon and 2‐photon absorption behaviors. We report that the closeness of CN group on the vinyl group to the central phenyl ring in ↑‐dscn induces an intramolecular geometric distortion between the central phenyl ring and vinylene group, preventing the effective π‐conjugation length in ground and excited states. Thus, the transition energy that is observed in 1‐photon absorption and fluorescence is larger in ↑‐dscn than in ↓‐dscn. This leads to a different intramolecular charge distribution, as a result of which the linear and nonlinear optical properties strongly depend on the position of acceptors. These results are theoretically unraveled in terms of charge transfer pathways, charge distribution, and charge distribution differences on transition.     

Benzyl 5‐{2‐[2‐(diethoxy­phosphinoyl)­acetyl]‐3‐oxo‐7‐thia‐2,4‐di­aza­bi­cyclo­[3.3.0]­oct‐6‐yl}pentanoate,a novel biotin derivative

The title compound, C₂₃H₃₃N₂O₇PS, has its phospho­no­acetate carbonyl group rotated slightly out of the plane of the ureido ring, with a C—N—C—O torsion angle of ?6.9 (4)°. The sulfur‐containing ring has an envelope conformation, while the ureido ring is nearly planar.     

Para‐linked and meta‐linked cationic water‐soluble fluorene‐containing poly(aryleneethynylene)s: Conformational changes and their effects on iron–sulfur protein detection

Four para‐linked or meta‐linked cationic water‐soluble fluorene‐containing poly(aryleneethynylene)s (PAEs) were synthesized to investigate the solvent‐induced π‐stacked self‐assembly. These PAE backbones are composed of fluorenylene and phenylene units, which are alternatively linked by ethynylene bonds. UV–vis absorption and photoluminescence spectra were used to study their conformational changes as solvent was gradually changed from MeOH to H₂O. In pure water, with gradually increased meta‐phenylene content (0, 50, and 100%), they underwent a gradual transition process of conformation from disordered aggregate structure to helix structure, which was not compactly folded. Moreover, the polymer with an ammonium‐functionalized side chain on the meta‐phenylene unit appeared to adopt a more incompact or extended helix conformation than the corresponding one without this side chain. Furthermore, the conformational changes of these cationic PAEs in H₂O were used to study their effects on biological detection. Rubredoxin (Rd), a type of anionic iron–sulfur‐based electron transfer protein, was chosen to act as biological analyte in the fluorescence quenching experiments of these polymers. Preliminary results suggest that they all exhibit amplified fluorescence quenching, and that the polymer with more features of helix conformation tends to be quenched by Rd more efficiently. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5424–5437, 2006     

Molecular Dynamics Simulation of Sub‐Transition for Polyethersulfone

Molecular dynamics (MD) simulations of a polyethersulfone (PES) chain are carried out in the amorphous state by using the Dreiding 2.21 force field at four temperatures. Two types of molecular motion, i.e. rotations of phenylene rings and torsions of large segments containing two oxygen atoms, two sulfur atoms, and five phenylene rings on the backbone, are simulated. The modeling results show that the successive phenylene rings should be in‐phase cooperative rotations, whereas the successive large segments should be out‐of‐phase cooperative torsions. By calculating the diffusion coefficient for the phenylene ring rotations, it is found that this rotation contributes to the β‐transition of PES.     

Synthesis and crystal structure of core-modified benziporphyrin: thia-p-benziporphyrin

The core-modified 5,20-phenyl-10,15-tolyl-thia-p-benziporphyrin (SBzP) can be prepared from the condensation of 1,4-bis(α-hydroxyl-benzyl)benzene with 5,10-ditolyl-16-thia-5,10,15,17-tetrahydrotripyrrin using BF₃·OEt₂ as catalyst. Spectroscopic studies suggest an aromatic macrocycle with a rapid flipping phenylene ring. SBzP exhibits a tilted phenylene ring and crystal packing shows dimeric structure with two SBzP rings linked by hydrogen bonding and π-π interaction. TFA acidified SBzPH₂²⁺ has a saddle-shaped dication porphyrin ring with two solvated trifluoroacetate and two solvated trifluoroacetic acid linked by hydrogen bondings.     

Controlling coffee ring structure on hydrophobic polymer surface by manipulating wettability with O2 plasma

A simple and novel method is firstly reported for controlling coffee ring structure on polystyrene (PS) film surface by O₂ plasma. O₂ plasma treatment leads to the wettability change of PS surface from hydrophobic to hydrophilic. For hydrophilic PS surface the coffee ring structure is avoided relying on the motion of contact line (CL) while SiO₂ microspheres are left. The motion of the CL is produced based on the viscosity and Marangoni effect with the addition of polymer additives. For hydrophobic PS surface coffee ring structure still persists even with polymer additives because SiO₂ microspheres transfer with the motion of the CL at the beginning of droplet evaporation and accumulate at the droplet edge at late stage with the pinning of the CL. As a result, uniform and macroscale SiO₂ microspheres deposition without coffee ring structure and SiO₂ microspheres deposition with coffee ring structure are controlled by O₂ plasma. This method provides a new way to tune coffee ring structure with smart surface and may be potentially useful for a range of application at material deposition and diagnosing diseases.