Extensive Hydrogen Bonds Research Articles

Crystal structure of a maltooligosaccharide-forming amylase from Bacillus stearothermophilus STB04

To better understand structure-function relationships, an X-ray crystal structure of the maltooligosaccharide-forming amylase from Bacillus stearothermophilus STB04 (Bst-MFA) with bound acarbose has been determined at 2.2 Å. The structure revealed a classical three-domain fold stabilized by four calcium ions, in which CaI–CaIII form an unprecedented linear metal triad in the interior of domain B. Catalytic residues are deduced to be two aspartic acids and one glutamic acid (Asp234, Glu264, Asp331), and the acarbose is bound to surrounding amino acid residues, mainly through extensive hydrogen bonds. Furthermore, analysis of the structure indicates the existence of at least 8 subsites in Bst-MFA, six glycone sites (−6, −5, −4, −3, −2, −1) and two aglycone sites (+1, +2). Subsite +3 remains to be further explored. Sugar-binding subsites contribute to further presentation of the oligosaccharide-binding mode, which explains the product specificity of Bst-MFA to some extent. In addition, we propose a mechanism by which maltooligosaccharide-forming amylases produce particular maltooligosaccharide products, a result different from that seen with typical α-amylases. Finally, the three-dimensional structure of Bst-MFA complexed with acarbose provides the basis for further studies, designed to increase product specificity.

Effect of high pressure on the typical 2D hydrogen-bonded crystal azodicarbonamide

We have performed in situ Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) investigations to explore high-pressure behaviors of azodicarbonamide (C2N4O2H4, ADC) to 21.6 and 23.8 GPa, respectively. ADC exhibits the representative two-dimensional (2D) hydrogen-bonded networks, and is the most hugely used foaming agent in industry both under ambient and high pressures. Careful identification of external modes and indexation of Bragg diffraction peaks under different pressures demonstrate ADC crystal remains the P21/c symmetry in this study. The bulk modulus (B0) and its pressure derivative (B0’) are determined to be 13.2(7) GPa and 8.2(2) by fitting the isothermal third Vinet equation of state (EOS). The conformation change of ADC molecule has been observed at 12.7 GPa. And this is evidenced by the splitting of skeleton atoms vibrations and the discontinuous evolutions with respect to hydrogen bond donor (NH) vibrations. First principle calculation reveals the observed confirmation change arises from the discontinuous variation of the torsion angel between H2NC=O and CN=NC skeleton groups. Hirshfeld surface analysis indicates that the extensive hydrogen bonds within 2D networks dominate the intermolecular close contacts even under high pressures. The intermolecular interaction energy calculation also implies that the energy between neighboring molecules in the 2D layer dominates the networks stability, whereas the energy between neighboring molecules in the second nearest adjacent layer is the primary factor for crystal instability. The cooperativity of the molecular flexibility and directional hydrogen bonds is responsible for structural stability under high pressures.

Ecological toxicity reduction of dinotefuran to honeybee: New perspective from an enantiomeric level

In last decade, there has been a concerted effort to reduce the potential threats of honeybees' population due to exposure to neonicotinoid pesticides. A new perspective was put forward to reduce the potential ecological toxicity of neonicotinoid dinotefuran to honeybee in terms of an enantiomeric level in the study. Toxicity of dinotefuran was enantioselective, and S-dinotefuran was 41.1- to 128.4-fold more toxic than R-dinotefuran to honeybee Apis mellifera (Apis mellifera Linnaeus), whereas R-dinotefuran exhibited comparative insecticidal activities (1.7–2.4 times) to typical sucking pests Aphis gossypii and Apolygus lucorum compared to racemic mixtures. Our data suggested that use of R-dinotefuran could have a good efficacy in controlling target pests while minimizing hazard to honeybees. The mechanism for chiral specific toxicity to honeybee was further characterized by electrophysiological studies and molecular docking. S-dinotefuran appears to be more toxic by binding to α8 subunit of nAChR of Apis mellifera. The α8 also have a more stable, functional binding cavity to S-dinotefuran with a higher binding score of 7.15, primarily due to an extensive hydrogen bond network. Therefore, new chiral products with a high proportion of or an enantiomeric pure R-dinotefuran are recommended to achieve effective pests control reducing hazard to honeybee populations.

Ammelinium Sulfate Monohydrate and Ammelinium Sulfate Cyanuric Acid – Synthesis and Structural Characterization

Two ionic carbon nitride type compounds containing the ammelinium cation, ammelinium sulfate cyanuric acid (6C3N5H6O+·3SO42–·1⅔C3N3H3O3·H2O) (1) and ammelinium sulfate monohydrate (2C3N5H6O+·SO42–·H2O) (2) were synthesized through hydrolysis of melam (C6N11H9) in diluted sulfuric acid. 1 crystallizes in hexagonal space group P63 (no. 173) with lattice parameters of a = 14.642(3), c = 13.113(4), and Z = 2. The structure is comprised of protonated ammelinium ions and neutral cyanuric acid molecules, which form a layered structure, as well as sulfate ions that span through these layers. 2 crystallizes in the triclinic space group P1 with lattice parameters of a = 7.404(3), b = 9.673(4), c = 10.040(4), α = 91.098(15), β = 109.884(10), γ = 92.567(13), and Z = 2. As for 1, the ammelinium rings form layers with the sulfate ions located in between. In both structures, no extended hydrogen bond networks between the respective triazine‐based molecules are formed. Instead, single molecules or small building blocks occur isolated and interact primarily with sulfate anions. Compound 1, which was obtained phase pure, was further investigated by FTIR spectroscopy, solid‐state NMR spectroscopy and powder X‐ray diffractometry.

Design-properties relationships of polyurethanes elastomers depending on different chain extenders structures

In this study, the effect of the structure and amount of the chain extenders on the morphological image and physico-chemical properties of some new polyurethane elastomers has been investigated. To achieve this, three series of polyurethane elastomers based on poly(tetramethylene ether) glycol, hexamethylene diisocyanate and chain extenders with different structures (triethylene glycol, 3,6-dithia-1,8-octanediol, 1,6-hexanediol) were synthesized. The chain-extenders which introduce oxygen or sulfur atoms into the polyurethane backbone chains (hard domains) change the behavior of the properties compared to the corresponding polyurethanes chain-extended with aliphatic diols. The structures of the new polyurethane elastomers were examined by FTIR, X-ray diffraction analysis and by atomic force microscopy (AFM). They were also characterized for thermal and tensile properties. The polyurethanes with sulfur into their hard segment structure were found to exhibit improved thermal stability properties and equivalent mechanical properties with polyurethanes obtained with aliphatic diols. This is due to the extensive and many fold hydrogen bond network that characterizes polyurethanes with sulfur in their hard segment structure.

Structural Features of Two Pyridyl Compounds of 1,5-Bis-(2′-pyridyl)pentane-1,3,5-trione and a New Salt of Doubly Protonated Hydroxyterpyridinium

A keto-enol equilibrium between two tautomers of the 1,5-bis(2′-pyridyl)pentane-1,3,5-trione (trione) (1) was observed in solution based on 1H and 13C NMR spectroscopy. NMR data shows that the trione exists as a mixture of mono-(minor product) and bis-enolic (major product) forms in solution. However, the crystal structure of trione (1) indicates that the bis-enolic form is the predominant species in the solid state, which is probably due to the C=O⋯H–O intramolecular hydrogen bonding. The crystal structure of 1 was determined by X-ray diffraction analysis. Trione (1) is monoclinic and crystallizes in the space group P21/n with a = 3.7449(3) A, b = 29.594(3) A, c = 12.5148(12) A, s = 92.701(2)°. Notably, the chloride salt of doubly protonated 4′-hydroxy-2,2′:6′,2″-terpyridinium [H2(tpyOH)]Cl2·H2O (tpyOH = 4′-hydroxy-2,2′:6′,2″-terpyridine) (2) obtained during the reaction of tpyOH in the presence of CrCl3·6H2O as a Lewis acid in methanol rather than complexation. The crystal structure of [H2(tpyOH)]Cl2·H2O (2) is triclinic and crystallizes in the space group P$$\bar {1}$$ with a = 5.1169(15) A, b = 12.288(4) A, c = 13.212(4) A, α = 109.667(7)°, s = 100.321(7)°, γ = 94.111(7)°. The crystal structure of 2 reveals the protonation of two nitrogen atoms of outer pyridines in tpyOH to form the [H2(tpyOH)]2+ cation. The packing of [H2tpyOH]Cl2·H2O features weak interactions including three N–H⋯Cl, O–H⋯Cl and O–H⋯O hydrogen bonds and extensive hydrogen bonds due to the cis–cis conformation of the doubly protonated salt of [H2tpyOH]2+ cation. Crystal structures of two pyridyl compounds of 1,5-bis-(2′-pyridyl)pentane-1,3,5-trione and doubly protonated 4′-hydroxy-2,2′:6′,2″-terpyridinium are reported which reveal the presence of hydrogen bonding interactions.

Chiral Superbases with Extended Hydrogen Bond Networks.

Extended hydrogen bond networks can substantially increase basicity through stabilization of the resultant conjugate acid. In this study, the chiral superbases (9-11) having anilinyl substituents attached to the prototype superbase 1,8-bis(dimethylamino)naphthalene 1 are examined using DFT methods (PBE1PBE/6-311+G(d,p). While in the gas phase, these are more basic than 1, in THF solution they are slightly weaker bases than 1. While 9-11 are chiral, their rotational barrier about the Caryl-Caryl bond is about 20 kcal mol-1. Aminonaphthyl substituents (18-20) increase the rotational barrier by about 10 kcal mol-1. As a proof-of-concept, deprotonation of propanal by the chiral bases 9-11 is predicted to have ee's ranging from 58% (with 9) to 95% (with 11). The deprotonation of 4- tert-butylcyclohexanone and cyclohexanoxide by 9 are predicted to have modest (23%) and excellent (97%) ee's, respectively.

Structure and magnetic properties of two new lanthanide complexes with the 1-((E)-2-pyridinylmethylidene)semicarbazone ligand

Two novel semicarbazone-lanthanide(III) complexes were prepared and structurally characterized as [Ln (Hscpy)2 (NO3)2]NO3·MeOH (Ln = Gd and Tb; Hscpy = 1-((E)-2-pyridinylmethylidene)semicarbazone). The 4f metal ions experience deca-coordination geometry. Each molecular formula contains two neutral Hscpy molecules in the keto form coordinated through two nitrogen atoms and one oxygen atom, while two nitrate ligands are both coordinated in a chelate mode. The 1 + charge of the cation-complex is balanced by a nitrate anion. Extensive intermolecular hydrogen bonds are formed through the methanol solvate molecule, which acts both as a donor and an acceptor molecule. The chemical composition of the compounds was confirmed by high resolution mass spectra (ESI-MS); peaks at m/z = 122.07 and 148.05, assigned to the fragments C6H8N3+ and C7H6N3O+, respectively, are in agreement with the coordination of Hscpy. Alternating current magnetic susceptibility analysis was performed in the 10–10000 Hz range, and the terbium-complex showed slow relaxation of the magnetization when immersed in a static magnetic field of 1 kOe and 1.5 kOe, with an activation barrier to the relaxation (21.9(4) cm−1) among the highest found for ten-coordinated Tb(III) complexes. This behavior of slow relaxation of the magnetization is relevant as a memory effect regarding the development of Single Molecule Magnets (SMM).

Molecular dynamics involving proton exchange of a protic ionic liquid-water mixture studied by NMR spectroscopy.

Protic ionic liquids (PILs) are proposed as alternative anhydrous proton conducting electrolytes for intermediate temperature fuel cells. One of the key factors in their performance as electrolytes, as far as charge transport is concerned, is their proton conductivity. Noting the success of water-containing electrolytes and recognising faster proton mobility than structural relaxation (via mechanisms such as Grotthuss) as their advantage, such an advantage is envisaged for PILs and in some cases deduced. As extended hydrogen bond networks and proton exchange are at the heart of these mechanisms, here we report our results on a prototypical characterisation of proton exchange in a PIL (C2HimNTf2)-water mixture. NMR lineshape analysis and exchange spectroscopy (EXSY) are used to quantify the proton exchange rate. The obtained exchange rate is then used to explain the diffusion behaviour of the exchangeable proton as measured by pulse field gradient NMR methods; a marginal anomaly in the translational dynamics of the exchangeable proton in the form of a faster NH proton is observed. As far as we know this is the first report on systematic characterisation of proton exchange in PILs with the aim of understanding its effect on translational motion as a way of discerning exchange related mobility anomalies.

Self-hybridized coralloid graphitic carbon nitride deriving from deep eutectic solvent as effective visible light photocatalysts

The room temperature deep eutectic solvent (DES) is demonstrated as an ideal precursor to prepare self-hybridized graphitic carbon nitride (g-C3N4) for the first time, for its special components, randomly reorganized molecules and extensive hydrogen bonds. In this work, by adjusting the proportion of urea and ammonium thiocyanate in the precursor DES, the self-hybridized coralloid g-C3N4, stacked by a mass of double-layer g-C3N4/g-C3N4 nanocages, were prepared without any templates. Compared to conventional g-C3N4 prepared using melamine, urea and AT as precursors, the improved charge transfer between the interfaces of the self-hybridized g-C3N4 has been demonstrated, which is attributed to the well-matched electronic band structures of the two g-C3N4 species, the abundant 2D/2D interfacial contact and increased photocatalytic active sites. As expected, it exhibits highly enhanced photocatalytic activity, which is among the best g-C3N4 photocatalysts without doping any other elements.

Structure-Based Drug Design Studies Toward the Discovery of Novel Chalcone Derivatives as Potential Epidermal Growth Factor Receptor (EGFR) Inhibitors.

Human Epidermal Growth Factor Receptor-1 (EGFR), a transmembrane tyrosine kinase receptor (RTK), has been associated with several types of cancer, including breast, lung, ovarian, and anal cancers. Thus, the receptor was targeted by a variety of therapeutic approaches for cancer treatments. A series of chalcone derivatives are among the most highly potent and selective inhibitors of EGFR described to date. A series of chalcone derivatives were proposed in this study to investigate the intermolecular interactions in the active site utilizing molecular docking and molecular dynamics simulations. After a careful analysis of docking results, compounds 1a and 1d were chosen for molecular dynamics simulation study. Extensive hydrogen bond analysis throughout 7 ns molecular dynamics simulation revealed the ability of compounds 1a and 1d to retain the essential interactions needed for the inhibition, especially MET 93. Finally, MM-GBSA calculations highlight on the capability of the ligands to bind strongly within the active site with binding energies of −44.04 and −56.6 kcal/mol for compounds 1a and 1d, respectively. Compound 1d showed to have a close binding energy with TAK-285 (−66.17 kcal/mol), which indicates a high chance for compound 1d to exhibit inhibitory activity, thus recommending to synthesis it to test its biological activity. It is anticipated that the findings reported here may provide very useful information for designing effective drugs for the treatment of EGFR-related cancer disease.

(μ-Methylenediphosphonato-κ4 O,O':O'',O''')bis[(ethyl-ene-diamine-κ2 N,N')palladium(II)] tetra-hydrate.

The title compound, [Pd2(C2H8N2)2(CH2O6P2)]·4H2O, comprises of a binuclear mol-ecule (point group symmetry 2), with a twofold rotation axis running through the central C atom of the methyl-enedi-phospho-nate (MDP) anion. The PdII atom has a square-planar coordination environment defined by the N atoms of a bidentate ethyl-enedi-amine (en) ligand and two O atoms of the bridging MDP anion. In the crystal structure, metal complexes are arranged in layers parallel (001) and are sandwiched between layers containing disordered water mol-ecules of crystallization. Extensive intra-layer hydrogen bonds of the type N-H⋯O in the metal complex layer and O-H⋯O in the water layer, as well as O-H⋯O hydrogen bonds between the two types of layers, lead to the formation a three-dimensional network structure. The two lattice water mol-ecules are each equally disordered over two positions.

Distinct Photoinduced Wetting Transitions of Differently Doped GaN: An Indication of Collective Behavior

Water is a unique molecule made up of only three atoms. At room temperature, water molecules are linked together by hydrogen bonds; this arrangement is responsible for many unusual properties distinct from those of other liquids. We explore here the wetting behavior of water on gallium nitride (GaN) and reveal that UV irradiation promotes a wetting transition on n-type GaN but not on p-type GaN. A comprehensive investigation reveals that the oxygen impurities present in n-type GaN are responsible for the wetting transition. The photoluminescence (PL) and humidity-dependent behaviors also support the proposed explanation. In particular, an individual oxygen atom could affect ∼105 water molecules through the extended hydrogen bond network, which provides direct evidence that wettability by water molecules is a collective behavior.

Crystal structure of vardenafil hydrochloride trihydrate, C23H33N6O4SCl (H2O)3

The crystal structure of vardenafil hydrochloride trihydrate has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. Vardenafil hydrochloride trihydrate crystallizes in space group C2/c (#15) with a = 34.78347(16), b = 11.56752(4), c = 14.69308(5) Å, β = 93.3410(4), V = 5901.839(30) Å3, and Z = 8. The fused ring system and the phenyl ring are nearly co-planar; the interplanar angle between them is 6.0°. Two intramolecular hydrogen bonds help determine this conformation. These planes stack along the c-axis. The side chains of these ring systems have a large Uiso and are neighbors in the stacks. Along the a-axis, these stacks are separated by hydrophilic layers of chloride, water molecules, and the positively charged nitrogen atoms of the vardenafil cation. Hydrogen bonds are prominent in the crystal structure. The protonated nitrogen atom forms a strong hydrogen bond to the chloride anion. The water molecules form a hexagon, making hydrogen bonds with themselves, as well as the C1 and a ring nitrogen atom. These discrete hydrogen bonds form a cluster, and there is no extended hydrogen bond network. There are many C–H⋯Cl, C–H⋯O, and C–H⋯N hydrogen bonds, which (although individually weak) contribute significantly to the crystal energy. The powder pattern is included in the Powder Diffraction File™ as entry 00-066-1620.

GC–MS Profiling and In Vitro Tyrosinase Inhibitory Activities of Bauhinia racemosa Lamk. and Bauhinia tomentosa Linn.

Bauhinia racemosa Lamk. and Bauhinia tomentosa linn. (Ceasalpiniaceae) leaves are used as holistic medicines mainly for inflammations and various skin disorders by the tribal sovereignty in Odisha ecoregion. This research work presents a study on chemical composition and tyrosinase inhibitory property of the methanol extract of leaves of Bauhinia racemosa (MBR) and Bauhinia tomentosa (MBT) with computational approaches. GC–MS technique, in-silico approach and in vitro tyrosinase inhibitory assay models were used for analysis of skin-whitening efficacies of these plant extracts. The major chemotype of the MBR was found to be myo-Inositol 4-C-methyl- (14.50%), whereas 3-O-Methyl-d-glucose (91.89%) was present in MBT as major compound. Tyrosinase inhibition was investigated against monophenolase (l-tyrosine). IC50 values of MBR and MBT were found to be 280 ± 0.9 and 610 ± 1.7 μg/mL, respectively. Molecular docking analysis of the binding of myo-Inositol, 4-C-methyl- and 3-O-Methyl-d-glucose with protein (PDB ID: 1wx2) showed an extensive hydrogen bond network with amino acid residues like ASN 191, ALA 202 and SER 206, THR 203, ASN 191, respectively. Both these compounds showed good binding affinities with target receptors and can be used as lead molecules for inhibition of tyrosinase. MBR and MBT might be suggested as a better source of skin-whitening agents for cosmetic and medicinal applications.

Impact of Intermolecular Interactions, Dimeric Structures on the Glass Forming Ability of Naproxen, and a Series of Its Derivatives.

In this article, thermal properties, molecular dynamics, crystallization kinetics, and intermolecular interactions in pure naproxen (NAP), its amide (NH2-NAP), and four esters (methyl, Met-NAP; isopropyl, Iso-NAP; hexyl, Hex-NAP; and benzyl, Ben-NAP) have been investigated using differential scanning calorimetry as well as broadband dielectric and Fourier transform infrared spectroscopies. We found that the modification of the NAP molecule by substituting a hydrogen atom from the hydroxyl group strongly inhibits the crystallization tendency of this active pharmaceutical ingredient (API) and simultaneously increases its glass forming ability (GFA). In this context, it is worthwhile to stress that pure naproxen and its amide crystallized very quickly, regardless of the cooling rate. Therefore, these compounds cannot be classified as good glass-formers. On the other hand, ester derivatives of API can be easily vitrified. Moreover, dielectric measurements revealed that with an increasing molecular weight of the substituent, the rate of crystallization process slows down significantly. Consequently, Ben-NAP was characterized by the highest GFA among all investigated API esters. Comprehensive FTIR studies clearly indicated that the strong tendency to create dimeric structures in the nonmodified NAP and NH2-NAP is responsible for their enhanced crystallization. At the first sight, our results stay in contrast to most literature data, showing that H-bonds favor the glass formation ability. However, this effect is usually observed for the materials, which form extensive multidirectional hydrogen bonds and associates. In NAP and NH2-NAP, the situation is much different, since both compounds exist mainly as dimers. Therefore, one can postulate that specific intermolecular interactions are an important parameter determining the GFA of different materials, including APIs.

High Z′ Crystal Structure of a New Polymorph of a Thioimidazole Disulfide: Importance of Conformational Flexibility

A new polymorph of a thioimidazole disulfide (2-[(1H-imidazol-2-yl) disulfanyl]-1H-imidazole; IDI) was prepared, which exhibits a rare crystal structure with 4 symmetry independent molecules (Z′ = 4; Z = 16) and all possible conformers predicted for the isolated molecule within the crystal unit cell. The crystal structure is non-centrosymmetric, Ia monoclinic space group, with cell parameters: a = 7.45910(10) A, b = 44.1680(8) A, c = 11.3522(2) A, and β = 103.0240(10)°. The four symmetry independent molecules establish an extensive hydrogen bond network involving NH groups as proton donors and N atoms of neighboring molecules as acceptors. A computational structural investigation of the isolated molecule of the compound was performed at the DFT(B3LYP)/6-311++G(d,p) level of theory in order to characterize in detail its conformational space, and these results were taken into account to evaluate the role of conformational flexibility and intermolecular interactions in the observed crystal packing. The excep...

Towards thermoplastic hemicellulose: Chemistry and characteristics of poly-(ε-caprolactone) grafting onto hemicellulose backbones

Hemicellulose is a highly available polysaccharide but with poor industrial applications attributed mainly to difficulties in processability, owing to extensive intermolecular hydrogen bonds. In the current study, hemicellulose was subjected to a chemical modification by ring-opening graft polymerization of ε-caprolactone (CL) to improve its processability for value-added applications. Hemicellulose-graft-poly-(ε-caprolactone) (HCgPCL) copolymers were synthesized using 1,5,7-triazabicyclodecene [4.4.0] (TBD) as an organic catalyst. The extent and length of grafted PCL sidechains in HCgPCL copolymers were controlled by adjusting the molar ratios of CL monomer to anhydroxylose residues. The various characterization analysis of the physicochemical and mechanical properties of HCgPCL materials revealed a successful grafting. The NMR analyses indicated that the degree of polymerization (DP) of the grafted PCL can range between 1.82 and 4.26 based on the changes in the molar ratio of the reactants. Furthermore, results indicated that the mechanical and the hydrophobic properties of the materials were enhanced by PCL grafting onto hemicellulose. Finally, biodegradability measurements indicated a remarkable (95.3–99.7%) materials biodegradation. We anticipate that the HCgPCL copolymers will have great potential to be an eco-friendly part of the bioplastic industry.

Synthesis of chromophores and polyimides with a green chemistry approach for second‐order nonlinear optical applications

This article presents the synthesis of nonlinear optical responsive chromophores by adopting a green chemistry approach by coupling N‐methyl‐N‐(2‐hydroxyethyl)‐4‐amino benzaldehyde with barbituric acid, 1,3‐indanedione, and 1,3‐diethyl‐2‐thiobarbituric acid as the acceptors through stilbene linkage. We performed the synthesis in less than 10 minutes at room temperature with water as a solvent without catalyst. Two different side‐chain polyimides were synthesized from poly(hydroxy‐imide)s with chromophores by Mitsunobu reaction. The chromophores were characterized by Fourier transform infrared, 1H NMR, 13C NMR, and elemental analysis. However, the polyimides were characterized by Fourier transform infrared and 1H NMR. The inherent viscosities (ηinh) of polyimides were determined by Ubbelohde viscometer, which ranged between 0.1793 and 0.1890 dL/g. The molecular weights of the polyimides were determined using gel permeation chromatography and were in range of 23 000 to 26 000. Polyimides demonstrated an excellent solubility in polar aprotic solvents, indicating good processability. Thermal behavior of these polyimides was studied by differential scanning calorimetry and thermogravimetric analysis. The Tg's were in the range of 185°C to 255°C. The change in the molecular orientation in the polymer films after electrical poling was ascertained using ultraviolet‐visible spectrophotometer and atomic force microscopy. The thicknesses and refractive indices of the thin films were determined by an ellipsometer. The second harmonic generation coefficients of the corona‐poled polymer films at Topt's, determined by the Maker fringe technique, ranged between 59.33 and 77.82 pm/V. High thermal endurance observed for the polyimides is attributed to the extensive hydrogen bonds in the matrix. The developed polyimides showed no decay in second harmonic generation signals below 110°C, indicating the acceptance for nonlinear optical devices.

Novel photo- and/or hydrochromic organometallics derived from methyl viologen cations

By the reaction of TM(NO3)2 (TM = Co, Ni, Cd) with 1,1′-bis(methyl)-4,4′-bipyridinium dibromide (MV·2Br) and 1,4-benzenedicarboxylic acid (p-H2BDC), three novel organometallic compounds have been obtained, (MV)[Co(H2O)6](p-BDC)2·4H2O (1), (MV)[Ni(H2O)6](p-BDC)2·4H2O (2), {(MV)[Cd(p-BDC)(Br)2]}n (3). Compounds 1 and 2 are isostructural ionic salts made up of one MV2+ cation, one [TM(H2O)6]2+ cation, two uncoordinated p-BDC2− anions, and four crystallization water molecules, and linked by extensive hydrogen bonds to afford the 3D networks; Compound 3 is constituted by MV2+ cations and zigzag chains of Cd(II) ions bridged by the deprotonated p-BDC2- species through four carboxylate oxygen atoms to form a 1D coordination polymer. Compound 3 exhibits electron-transfer photochromism similar to other viologen-based materials, while 1 and 2 are photoinert. However, the thermal stimulation not only makes compounds 1 and 2 exhibit hydrochromic behaviors by means of dehydration and rehydration of the crystals, but also can induce a phase transition from photochemically inert to active states, demonstrating a positive role in improving photoinduced electron transfer.