Understanding the folding process of synthetic polymers by small-molecule folding agents

Two acceptor containing polyimides PDI and NDI carrying pyromellitic diimide units and 1,4,5,8-naphthalene tetracarboxy diimide units, respectively, along with hexa(oxyethylene) (EO6) segments as linkers, were prepared from the corresponding dianhydrides and diamines. These polyimides were made to fold by interaction with specifically designed folding agents containing a dialkoxynaphtha-lene (DAN) donor linked to a carboxylic acid group. The alkali-metal counter-ion of the donor carboxylic acid upon complexation with the EO6 segment brings the DAN unit in the right location to induce a charge-transfer complex formation with acceptor units in the polymer backbone. This two-point interaction between the folding agent and the polymer backbone leads to a folding of the polymer chain, which was readily monitored by NMR titrations. The effect of various parameters, such as structures of the folding agent and polymer, and the solvent composition, on the folding propensities of the polymer was studied.     

Hydrogen‐Bonding Induced Alternate Stacking of Donor (D) and Acceptor (A) Chromophores and their Supramolecular Switching to Segregated States

This paper reports comprehensive studies on the mixed assembly of bis‐(trialkoxybenzamide)‐functionalized dialkoxynaphthalene (DAN) donors and naphthalene‐diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge‐transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure‐dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self‐assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self‐assembly propensity, the mode of co‐assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen‐bonding interaction instead of a weak CT interaction. Consequently, in a highly non‐polar solvent like methylcyclohexane (MCH), although kinetically controlled CT‐gelation was initially noticed, within a few hours the system sacrificed the CT‐interaction and switched over to the more stable self‐sorted gel to maximize the gain in enthalpy from the hydrogen‐bonding interaction. In contrast, in a relatively less non‐polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT‐state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D–A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co‐assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co‐assembly, morphology, and the unprecedented supramolecular‐switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.     

Charge Transfer Induces Formation of Stimuli‐Responsive,Chiral, Cohesive Vesicles‐on‐a‐String that Eventually Turn into a Hydrogel

A charge transfer (CT) mediated two‐component, multistimuli responsive supergelation involving a L ‐histidine‐appended pyrenyl derivative (PyHisOMe) as a donor and an asymmetric bolaamphiphilic naphthalene‐diimide (Asym‐NDI) derivative as an acceptor in a 2:1 mixture of H₂O/MeOH was investigated. Asym‐NDI alone self‐assembled into pH‐responsive vesicular nanostructures in water. Excellent selectivity in CT gel formation was achieved in terms of choosing amino acid appended pyrenyl donor scaffolds. Circular dichroism and morphological studies suggested formation of chiral, interconnected vesicular assemblies resembling “pearls‐on‐a‐string” from these CT mixed stacks. XRD studies revealed the formation of monolayer lipid membranes from these CT mixed stacks that eventually led to the formation of individual vesicles. Strong cohesive forces among the interconnected vesicles originate from the protrusion of the oxyethylene chains from the surfaces of the chiral vesicles.     

Autoresolution of Segregated and Mixed p‐n Stacks by Stereoselective Supramolecular Polymerization in Solution

A “chirality driven self‐sorting” strategy is introduced for the controlled supramolecular organization of donor (D) and acceptor (A) molecules in multicomponent assemblies. The trans‐1,2‐bis(amido)cyclohexane (trans‐BAC) has been identified as a supramolecular motif with strong homochiral recognition to direct this chirality controlled assembly process of enantiomers in solution. Stereoselective supramolecular polymerization of trans‐BAC appended naphthalene diimide monomers (NDIs) has been probed in detail by spectroscopic and mechanistic investigations. This chirality‐driven self‐sorting design of enantiomeric components also offers to realize mixed and segregated D‐A stacks by supramolecular co‐assembly of the NDI acceptors with trans‐BAC appended dialkoxynaphthalene (DAN) donor monomers. Such an unprecedented chirality control on D‐A organization paves the way for the creation of supramolecular p‐n nanostructures with controlled molecular‐level organization.     

Progress in charge transfer systems utilizing porphyrin donors and simple aromatic diimide acceptor units

An overview of the evolution of artificial photosynthetic charge transfer systems containing porphyrin donors and pyromellitic or naphthalene diimide acceptor units is presented. Progression in this area of research is highlighted by the complexity of the systems, the nature of the medium separating donor and acceptor as well as the progression in the lifetime of the charge-separated state upon photoexcitation. A number of supramolecular systems that utilize hydrogen bonding or axial ligation of zinc porphyrins as a means for spatial orientation are highlighted.     

Zipper-featured delta-peptide foldamers driven by donor--acceptor interaction. Design, synthesis, and characterization

Donor-acceptor interaction between electron-rich 1,5-dioxynaphthalene (DAN) and electron-deficient pyromellitic diimide (PDI) has been utilized to induce the formation of a new kind of zipper-featured delta-peptide foldamers. Seven l-ornithine-based delta-peptides 1a-g, in which one to three DNA and PDI units are incorporated to the two ends of the peptide backbones, respectively, have been designed and prepared by the standard liquid-phase synthetic method. (1)H NMR, UV-vis, and fluorescent quenching studies reveal that all the delta-peptides adopt folding conformations in nonpolar chloroform and polar DMF as a result of intramolecular donor-acceptor interaction between the DAN and PDI units. The folding states become more compact for the peptide skeletons possessing more donor-acceptor interacting sites. Variable-temperature UV-vis experiments indicate that, although the folding is a dynamic process, the folding state can remain even at 150 degrees C in DMF. Circular dichroism (CD) investigations reveal that the new generation of delta-peptides have similar folding patterns. A zipper-featured folding motif has been proposed for the new generation of delta-peptide foldamers. Molecular modeling has generated two most stable folding states for the longest delta-peptide 1g, with an energy difference of 26.80 kcal/mol.     

Hydrogen‐Bonding‐Mediated Vesicular Assembly of Functionalized Naphthalene–Diimide‐Based Bolaamphiphile and Guest‐Induced Gelation in Water

This paper describes the spontaneous vesicular assembly of a naphthalene–diimide (NDI)‐based non‐ionic bolaamphiphile in aqueous medium by using the synergistic effects of π‐stacking and hydrogen bonding. Site isolation of the hydrogen‐bonding functionality (hydrazide), a strategy that has been adopted so elegantly in nature, has been executed in this system to protect these moieties from the bulk water so that the distinct role of hydrogen bonding in the self‐assembly of hydrazide‐functionalized NDI building blocks could be realized, even in aqueous solution. Furthermore, the electron‐deficient NDI‐based bolaamphiphile could engage in donor–acceptor (D–A) charge‐transfer (CT) interactions with a water‐insoluble electron‐rich pyrene donor by virtue of intercalation of the latter chromophore in between two NDI building blocks. Remarkably, even when pyrene was located between two NDI blocks, intermolecular hydrogen‐bonding networks between the NDI‐linked hydrazide groups could be retained. However, time‐dependent AFM studies revealed that the radius of curvature of the alternately stacked D–A assembly increased significantly, thereby leading to intervesicular fusion, which eventually resulted in rupturing of the membrane to form 1D fibers. Such 2D‐to‐1D morphological transition produced CT‐mediated hydrogels at relatively higher concentrations. Instead of pyrene, when a water‐soluble carboxylate‐functionalized pyrene derivative was used as the intercalator, non‐covalent tunable in‐situ surface‐functionalization could be achieved, as evidenced by the zeta‐potential measurements.     

Effect of Side Chains on Charge Transfer in Quaterthiophene-Naphthalene Diimide Based Donor-Bridge-Acceptor Dyads

We have probed the effect of side chains on the charge transfer dynamics in dyads containing quaterthiophene (QT) donor and naphthalene diimide (NDI) acceptor. The donor and the acceptor are covalently linked using a flexible linker. Four dyads (1–4) were synthesized with the quaterthiophene bearing hexyl side chain and the naphthalene diimide bearing hydrocarbon, fluorocarbon, branched or polar side chains. The UV-Vis spectra for these dyads showed the existence of a donor-acceptor complex. The time-resolved fluorescence (TRF) decay studies show a rapid quenching of fluorescence in all the dyads upon excitation of the donor. We found that the side chains on the NDI did not alter the quenching rates in solution.     

Photophysical and electrochemical characterisation of the interactions between components in neutral pi-associated

The electrochemical and photophysical properties of a variety of neutral pi-associated [2]catenanes have been explored by using cyclic voltammetry, absorption and luminescence spectrophotometry and the measurement of exited-state lifetimes. Several trends that could be correlated with interactions between the mechanically linked components of the structures were revealed from comparative study of catenane precursors, model compounds and the [2]catenanes themselves. Throughout, emphasis is placed on pair-wise comparisons between systems which differ in a single structural feature. Substitution in a catenane of a pyromellitic diimide unit by a naphthalene diimide yields more readily reduced derivatives, whose absorption spectra reveal charge transfer within the catenane to be a lower energy process of reduced intensity. Conversion of the butadiyne links within the diimide macrocycle of the catenanes to saturated chains results in an increase in both the energy and intensity of their charge-transfer bands; electrochemically these derivatives are all harder to reduce than the parent systems. Replacement of one of the electron-donating components of the catenanes with a less effective aromatic donor bearing a carboxy group also decreases the energy and intensity of the charge-transfer feature and is accompanied by a slightly more ready reduction. A sequence of reduction and translational events is proposed to explain the intriguing electrochemical behaviour of a catenane that contains one pyromellitic and one naphthalene diimide. For some systems the photophysical and electrochemical techniques, whilst exploring distinct physical phenomena, are shown to be in good agreement by comparison of shifts of electrochemical reduction waves with those of charge-transfer absorption features.     

Photoinduced ionic polymerization. III. Cationic polymerization and copolymerization of cyclohexene oxide and α-methylstyrene in the presence of pyromellitic dianhydride

The photoinduced ionic polymerization of cyclohexene oxide was studied in the presence of pyromellitic dianhydride. The polymerization is initiated by the excited chargetransfer complex between cyclohexene oxide and the electron-acceptor and proceeds by a cationic mechanism. Photoinduced cationic polymerization of α-methylstyrene was also observed in the presence of pyromellitic dianhydride. The initiation mechanism of the polymerization was elucidated by means of electron spin resonance measurements. The concentration of pyromellitic dianhydride anion-radicals measured in this way was found to be proportional to the rate of polymerization. This result shows clearly that the photopolymerization is initiated by cation-radicals formed from photoexcited donoracceptor complexes. The attempted photocopolymerization of cyclohexene oxide and α-methylstyrene gave a mixture of homopolymers. The composition of the product depends on the wavelength of the light used.     

Hydrogen-Bonding Induced Alternate Stacking of Donor (D) and Acceptor (A) Chromophores and their Supramolecular Switching to Segregated States

This paper reports comprehensive studies on the mixed assembly of bis-(trialkoxybenzamide)-functionalized dialkoxynaphthalene (DAN) donors and naphthalene-diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge-transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure-dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self-assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self-assembly propensity, the mode of co-assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen-bonding interaction instead of a weak CT interaction. Consequently, in a highly non-polar solvent like methylcyclohexane (MCH), although kinetically controlled CT-gelation was initially noticed, within a few hours the system sacrificed the CT-interaction and switched over to the more stable self-sorted gel to maximize the gain in enthalpy from the hydrogen-bonding interaction. In contrast, in a relatively less non-polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT-state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D-A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co-assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co-assembly, morphology, and the unprecedented supramolecular-switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.     

From Bola‐amphiphiles to Supra‐amphiphiles: The Transformation from Two‐Dimensional Nanosheets into One‐Dimensional Nanofibers with Tunable‐Packing Fashion of n‐Type Chromophores

With a rational design of the supra‐amphiphiles, we have successfully demonstrated that not only the dimension of the self‐assembled nanostructures, but also the packing fashion of the functional naphthalene diimide (a typical n‐type chromophore), can be tuned in a noncovalent way in aqueous solution. Naphthalene diimide is incorporated into a bola‐amphiphile as the rigid core, whereas viologen derivatives are used as the hydrophilic head. The bola‐amphiphile self‐assembles into two‐dimensional nanosheets, in which naphthalene diimide adopts a “J‐type” aggregation. Water‐soluble supramolecular complexation between viologen derivatives and the 8‐hydroxypyrene‐1, 3, 6‐trisulfonic acid trisodium salt is used as a driving force for the formation of the supra‐amphiphiles. Upon formation of the supra‐amphiphiles, the nanosheets transform into ultralong nanofibers with a close packing of naphthalene diimide. Notably, just by mixing the two building blocks of the supra‐amphiphiles in aqueous solution, a dimension‐controlled evolution of the nanostructures is formed that leads to a different packing fashion of the n‐type functional chromophores, which is facile and environmental friendly.     

Visualization of Stereoselective Supramolecular Polymers by Chirality‐Controlled Energy Transfer

Chirality‐driven self‐sorting is envisaged to efficiently control functional properties in supramolecular materials. However, the challenge arises because of a lack of analytical methods to directly monitor the enantioselectivity of the resulting supramolecular assemblies. Presented herein are two fluorescent core‐substituted naphthalene‐diimide‐based donor and acceptor molecules with minimal structural mismatch and they comprise strong self‐recognizing chiral motifs to determine the self‐sorting process. As a consequence, stereoselective supramolecular polymerization with an unprecedented chirality control over energy transfer has been achieved. This chirality‐controlled energy transfer has been further exploited as an efficient probe to visualize microscopically the chirality driven self‐sorting.     

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

Infrared laser action spectroscopy in a Fourier‐transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second solvation shell. Spectral features indicate the presence of multiple structures of Ca(H₂O)₇²⁺ in which outer‐shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner‐shell water molecules. Double‐acceptor water molecules are predominately observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second‐shell water molecules, particularly the appearance of inner‐shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.     

One donor–two acceptor (D–A1)‐(D–A2) random terpolymers containing perylene diimide,naphthalene diimide,and carbazole units

A series of one donor–two acceptor (D–A₁)‐(D–A₂) random terpolymers containing a 2,7‐carbazole donor and varying compositions of perylene diimide (PDI) and naphthalene diimide (NDI) acceptors was synthesized via Suzuki coupling polymerization. The optical properties of the terpolymers are weighted sums of the constituent parent copolymers and all show strong absorption over the 400 to 700 nm range with optical bandgaps ranging from 1.77 to 1.87 eV, depending on acceptor composition. The copolymers were tested as acceptor materials in bulk heterojunction all‐polymer solar cells using poly[(4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b;4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(4‐(2‐ethylhexanoyl)‐thieno[3,4‐b]thiophene)‐2,6‐diyl] (PBDTTT‐C) as the donor material. In contrast to the optoelectronic properties, the measured device parameters are not composition dependent, and rather depend solely on the presence of the NDI unit, where the devices containing any amount of NDI perform half as well as those using the parent polymer containing only carbazole and PDI. Overall this is the first example of a one donor–two acceptor random terpolymer system containing perylene diimide (PDI) and naphthalene diimide (NDI) acceptor units, and demonstrates a facile method of tuning polymer optoelectronic properties while minimizing the need for complicated synthetic and purification steps. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3337–3345     

Ring opening reactions of pyromellitic dianhydride for the synthesis of first row transition metal dicarboxylate complexes

The ring opening reaction of pyromellitic dianhydride by methanol is an effective method to prepare first row transition metal dicarboxylate complexes. The reactions of different first row transition metal salts with pyromellitic dianhydride in the presence of nitrogen donating bidentate ligands such as 1,10-phenanthroline and 2,2′-bipyridine gives different compositions depending on the ligand and the metal salts used. For example, the reaction of nickel(II) acetate with pyromellitic dianhydride in the presence of 1,10-phenanthroline results in the formation of a carboxylato bridged nickel(II) metallacycle through the ring opening reaction of pyromellitic dianhydride (PAH) at the 1 and 3-positions, whereas a mononuclear tetra-aqua 2,2′-bipyridine nickel(II) complex is formed in a similar reaction of nickel(II) acetate through ring opening at the 1,4-position of PAH. Mononuclear cobalt(II) dicarboxylate complexes are formed from the ring opening reaction of pyromellitic dianhydride in methanol in the presence of the nitrogen donor ligands 1,10-phenanthroline or 2,2′-bipyridine. Copper(II) chloride on reaction with PAH and 2,2′-bipyridine gives a mononuclear complex via ring opening at the 1 and 4-positions; having chlorides inside and outside the coordination sphere. Whereas, the reaction of copper(II)acetate gives dinuclear copper complexes having a monodentate carboxylato bridge arising from the carboxylato groups at the 1 and 4-positions on the aromatic ring. The crystal structures of all the complexes have been determined.     

Donor-sigma-acceptor molecules incorporating a nonadecyl-swallowtailed perylenediimide acceptor

Donor-sigma-acceptor-lipid molecules were prepared by using perylenetetracarboxylic diimide as the acceptor, starting from perylenetetracarboxylic dianhydride. One imide nitrogen was attached to a "swallowtail" lipid (a long alkyl tail connected at midchain), which imparts enough solubility to make the system tractable and provides a lipophilic region suitable for promoting Langmuir-Blodgett monolayer formation. The other imide link was to a donor group (pyrene, ferrocene, tetramethylphenylenediamine, phenyl) through a short alkyl sigma bridge. Features of the 1H and 13C NMR spectra of swallowtailed perylenediimides are interpreted as resulting from restricted rotation about the imide C-N bond; the 13C NMR spectra and stereochemistry of these molecules are contrasted with the case of the related bis-(2,5-di-tert-butylphenyl)perylenetetracarboxylic diimide.     

Rigid-rod polymers with polysiloxane side chains. I. Synthesis and characterization of rigid-rod polyimides with polysiloxane side chains and their compatibility with linear polysiloxanes

Rigid-rod aromatic polyimdies having polydimethylsiloxane side chains were prepared for the purpose of dispersing rigid-rod molecules in silicone matrices for molecular reinforcement. The polyimides were obtained by imidizing the polyamide-acids bearing the side chains either thermally or chemically, which were synthesized by reacting 4,4′-diaminobiphenyl-terminated polysiloxane macromonomers with pyromellitic dianhydride in THF. The polyamide-acid films obtained by removing the solvent were soluble in THF, but the polyimides were insoluble in any common solvent. The polyimides showed no melting transition below 350°C on DSC analysis, at which temperature the side chain started decomposing. Although all the polyimides were anisotropic as observed by a polarizing microscope, x-ray diffraction analysis suggested the presence of only limited chain organization. Blends with linear polydimethylsiloxanes were prepared by casting the THF solution containing the polyamide-acid and the polydimethylsiloxane, followed by solvent evaporation and thermal imidization. With relatively longer side chains, the polyimides were proved to be compatibilized with polydimethylsiloxanes. © 1994 John Wiley & Sons, Inc.     

Solvophobic Acceleration of a Diels–Alder Reaction in True Solutions in Organic Solvents

The rate of Diels–Alder reaction of diene 9,10‐bis(hydroxymethyl)anthracene with dienophile N‐ethylmaleimide was studied in a series of solvents with different polarity and hydrogen‐bonding ability. Enthalpies and entropies of activation were determined from the temperature dependences of the rate constants. Rate acceleration in nonaqueous protic solvents such as glycerol, propylene, and ethylene glycols was observed. In addition, enthalpy versus entropy of activation plots show a compensation pattern different from the other considered solvents, which can be linked with the solvophobic effects observed in polyhydric alcohols. However, the solvophobic acceleration was not as strong as the hydrophobic acceleration in water. Hydrogen bonding of the reactants and transition state with solvent also influences the reaction rate. The studied reaction is slightly promoted in hydrocarbon solvents in comparison with aprotic polar solvents. This was explained by hydrogen bonding of the hydroxyl groups of diene with dienophile in transition state, which requires prior breaking of the hydrogen bonds of these groups with polar solvent molecules.     

Preparation and properties of novel polyimides derived from 4‐aryl‐2,6 bis(4‐amino phenyl)pyridine

To prepare novel polyimides with enhanced thermal stability and high solubility in common organic solvents, diamine monomers, 4‐aryl‐2,6 bis‐(4‐amino phenyl)pyridine, were introduced. The diamines were reacted with three different conventional aromatic dianhydrides including pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, and hexafluoroisopropylidene‐2,2‐bis(phthalic‐dianhydride) (6FDA) in dimethylacetamide solvent to obtain the corresponding polyimides via the polyamic acid precursors and chemical imidization. The monomers and polymers were characterized by Fourier transform infrared spectroscopy, ¹H NMR, mass spectroscopy, and elemental analysis; and the best condition of polymerization and imidization were obtained via the study of model compound. The polyimides showed little or no weight loss by thermogravimetric analysis up to 500 °C, and those derived from 6FDA exhibited good solubility in various polar solvents. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3826–3831, 2001