A Well-defined Hierarchical Hydrogen Bonding Strategy to Polyureas with Simultaneously Improved Strength and Toughness

A well-defined quadruple hydrogen bonding strategy involving dimerization of 2-ureido-4[1H]-pyrimidone(UPy) units is innovatively designed to prepare polyureas with high overall mechanical properties. Three polyureas containing different amounts of UPy units were synthesized by replacing a portion of isophorone diisocyanate(IPDI) with a UPy-derived diisocyanate. The formation of quadruple hydrogen bonds in hard segments via UPy dimers was confirmed by nuclear magnetic resonance(NMR) and Fourier transform infrared spectroscopy(FTIR). The mechanical properties of the polyureas were evaluated by uniaxial tensile testing. Compared to the polyurea without UPy units, remarkable improvements in Young's modulus, tensile strength, and toughness were simultaneously achieved when UPy units were incorporated. The mechanism behind the strong strengthening effect rooted in the stronger intermolecular forces among hard segments brought by the quadruple hydrogen bonds, which were stronger than the inherent bidentate and monodentate hydrogen bonds among urea groups, and the slower soft segmental dynamics reaveled by both increased Tg and relaxation time of the soft segments. The mechanism behind the strong toughening effect was ascribed to more effective energy dissipation brought by the quadruple hydrogen bonds that served as stronger sacrificial bonds upon deformation. This work may offer new insight into the design of polyurea elastomers with comprehensively improved mechanical properties.     

Light-fueled dissipative self-assembly at molecular and macro-scale enabled by a visible-light-responsive transient hetero-complementary quadruple hydrogen bond

The development of out-of-equilibrium self-assembly systems using light as input fuel is highly desirable and promising for the fabrication of smart supramolecular materials. Herein, we report the construction of new artificial light-fueled dissipative molecular and macroscopic self-assembly systems based on a visible-light-responsive transient quadruple H-bonding array, which consists of an azobenzene-modified ureidopyrimidinone (UPy) module (Azo-O-UPy) and a nonphotoactive diamidonaphthyridine (DAN) derived competitive binder (Napy-1). The visible light (410 nm) irradiation can induce the E to Z isomerization of the azobenzene unit of E-Azo-O-UPy to produce Z-Azo-O-UPy with an opened UPy binding site, which can complex with Napy-1 to form a quadruply H-bonded heterodimer. The heterodimer is metastable and can be quickly disassembled in dark, owing to the fast thermal relaxation of Z-Azo-O-UPy to E-Azo-O-UPy. While introducing such transient quadruple H-bonding interaction into a linear polymer system or a polymeric gel system, light-fueled out-of-equilibrium polymeric assembly both at molecular and macro-scale can be achieved.     

Supramolecular Macrostructures of UPy‐Functionalized Carbon Nanotubes

Carbon nanotubes (CNTs) are considered excellent materials for the construction of flexible displays due to their nanoscale dimensions and unique physical and chemical properties. By using the recognition properties of 2‐ureido‐4[1H]pyrimidinone (UPy), a versatile and simple methodology was demonstrated for the construction of macroscopic structures based on UPy‐CNT/polymer composites prepared by a combination of two functionalization approaches: 1) covalent attachment of UPy pendants on the multiwalled CNT surface ( UPy‐MWCNTs ) and 2) directed self‐assembly of UPy‐MWCNTs within polymers bearing UPy pendants ( Bis‐UPy 1 and Bis‐UPy 2 ) by quadruple complementary DDAA–AADD hydrogen‐bond recognition (D=donor, A=acceptor).     

Conformational control in the cyclization of hydrogen-bonded supramolecular polymers

Bifunctional 2-ureido-4[1H]-pyrimidinone (UPy) derivatives can form small cyclic oligomers as well as long supramolecular polymers in chloroform solutions using the quadruple hydrogen-bonding motif. Ring-chain equilibria of a set of supramolecular monomers containing methyl-substituted alkyl linkers between the hydrogen-bonding UPy moieties were investigated by (1)H NMR spectroscopy and viscometry. The data were characterized in terms of critical concentration (CC, denoting the onset of polymerization) and equilibrium cyclic dimer concentration (EDC, representing preorganization of the monomer toward selective formation of cyclic dimer). Methyl substituents in the monomer were found to promote conformations favorable for cyclic dimerization, leading to an increase in both the EDC and the CC with respect to unsubstituted monomer. Furthermore, we observed an odd-even effect in the CC and EDC with increasing length of the linker between the hydrogen-bonding units. The combined results allow tuning of the critical concentration over a broad range and offer detailed information on the correlation between monomer structure, conformation, and polymerizability which may provide new insights for the study and design of other ring-chain equilibria or helix-random coil transitions.     

A Supramolecular Polymer Network of Graphene Quantum Dots

Graphene quantum dot (GQD)–organic hybrid compounds (GQD‐ 2 b – e ) were prepared by introducing 3,4,5‐tri(hexadecyloxy)benzyl groups (C16) and linear chains terminated with a 2‐ureido‐4‐[1H]‐pyrimidinone (UPy) moiety onto the periphery of GQD‐ 1 . GQD‐ 2 b – e formed supramolecular assemblies through hydrogen bonding between the UPy units. GPC analysis showed that GQDs with high loadings of the UPy group formed larger assemblies, and this trend was confirmed by DOSY and viscosity measurements. AFM images showed the polymeric network structures of GQD‐ 2 e on mica with flat structures (ca. 1.1 nm in height), but no such structures were observed in GQD‐ 2 a , which only carries the C16 group. GQD‐ 2 c and GQD‐ 2 d formed organogels in n‐decanol, and the gelation properties can be altered by replacing the alkyl chains in the UPy group with ethylene glycol chains (GQD‐ 3 ). GQD can thus be used as a platform for supramolecular polymers and organogelators by suitable chemical functionalization.     

Dimensionally confined nanosheets self-assembled through self-shielding multiple hydrogen bonding interactions in aqueous media

Herein, we adopt a simple supramolecular strategy to effectively control the tautomerism of ureidopyrimidinone (UPy) moiety and ultimately realize the complete arrangement of enol configuration. The obtained UPy derivatives containing self-complementary quadruple hydrogen bonding interactions can spontaneously self-assemble towards the formation of well-controlled, self-organized supramolecular nanostructure morphologies in both chloroform and water. The resulting aggregates had been fully characterized by various spectroscopy (absorption, emission) and microscopy (TEM, SEM and AFM) studies. It is anticipated that this study can provide an exact and excellent monomeric unit for controllable and precise supramolecular polymerization. The results achieved here also demonstrate the utility and feasibility of multiple hydrogen bonds to direct the self-assembly of small-molecule building blocks in aqueous media, which provides a strategy for the construction of well-defined and stable supramolecular architectures with chemical functionalities and physical properties as advanced materials for biological applications.     

Generation-independent dimerization behavior of quadruple hydrogen-bond-containing oligoether dendrons

[reaction: see text] A new series of self-assembling G1-G3 dendronized dimers bearing oligoether dendrons and a dimeric 2-ureido-4-pyrimidinone (UPy) quadruple hydrogen-bonding core were prepared and characterized. It was found that the nonpolar microenvironment created by the dendrons preserved the UPy unit in its DDAA tautomeric form. As a result, the stabilities of the dimers were exceptionally strong for all three generations (K(dim) > 2 x 10(7) M(-)(1) in CDCl(3) at 25 degrees C). Furthermore, the steric size of the dendrons did not exhibit a significant effect on their dimerization behavior.     

Photo-Controlled Macroscopic Self-Assembly Based on Photo-Switchable Hetero-Complementary Quadruple Hydrogen Bonds

A photo-switchable hetero-complementary quadruple H-bonding array, which consists of an azobenzene-derived ureidopyrimidinone (UPy) module ( Azo-UPy ) and a nonphotoactive diamidonaphthyridine (DAN) derivative ( Napy-1 ), is constructed based on a reversible photo-locking approach. Upon UV (390 nm)/Vis (460 nm) light irradiations, photo-switchable quadruple H-bonded dimerization between Azo-UPy and Napy-1 can be achieved with exhibiting 4.8×10⁴-fold differences in binding strength (ON/OFF ratios). Furthermore, smart polymeric gels with unique photo-controlled macroscopic self-assembly behavior can be fabricated by introducing such quadruple H-bonding array as photo-regulable noncovalent interfacial connections.     

Transient spectra,kinetics and mechanism of Rhodamine 700 dye precursor photoreaction

A new dye precursor (DP) molecule was synthesized and its photochromic, spectroscopic and kinetic properties were determined. This dye precursor yields the strongly fluorescent Rhodamine 700 dye when it is exposed to UV light. The ability to form a highly fluorescent dye was utilized in the fabrication of two-photon volumetric optical memory materials. The spectral characteristics, kinetics and photochemistry were measured and the photoreactions mechanism is proposed.     

Synthesis and spectral investigations of a new dyad with spiropyran and fluorescein units: toward information processing at the single molecular level

A new dyad 1 with two spiropyran units as the photochromic acceptors and one fluorescein unit as the fluorescent donor was synthesized and characterized. External inputs (ultraviolet light, visible light, and proton) induce the reversible changes of the structure and, concomitantly, the absorption spectrum of dyad 1 due to the presence of two spiropyran units. Only the absorption spectrum of the ME form of the spiropyran units in dyad 1 has large spectral overlap with the fluorescence spectrum of the fluorescein unit. Thus, the fluorescence intensity of dyad 1 is modulated by reversible conversion among the three states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the ME form and the fluorescein unit. Based on the fact that dyad 1 could "read out" three external input signals (ultraviolet light, visible ligh,t and proton) and "write" a compatible specific output signal (fluorescence intensity), dyad 1 described here can be considered to perform an integrated circuit function with one OR and one AND interconnected logic gates. The present results demonstrate an efficient strategy for elaborating and transmitting information at the single molecular level.     

Two Orthogonal Halogen-Bonding Interactions Directed 2D Crystalline Supramolecular J-Dimer Lamellae

Dye assemblies exhibit fascinating properties and performances, both of which depend critically on the mutual packing arrangement of dyes and on the supramolecular architecture. Herein, we engineered, for the first time, an intriguing chlorosome-mimetic 2D crystalline J-dimer lamellar structure based on halogenated dyes in aqueous media by employing two distinct orthogonal halogen-bonding (XB) interactions. As the only building motif, antiparallel J-dimer was formed and stabilized by single π-stacking and dual halogen⋅⋅⋅π interactions. With two substituted halogen atoms acting as XB donors and the other two acting as acceptors, the constituent J-dimer units were linked by quadruple highly-directional halogen⋅⋅⋅halogen interactions in a staggered manner, resulting in unique 2D lamellar dye assemblies. This work champions and advances halogen-bonding as a remarkably potent tool for engineering dye aggregates with a controlled molecular packing arrangement and supramolecular architecture.     

Luminescence properties of the mixed J-aggregate of oxacyanine dye and thiacyanine dye. Formation of a persistence-type aggregate

The monolayer assemblies incorporating the J-aggregates of oxacyanine dye, N,N'-dioctadecyloxacyanine perchlorate (S9), and thiacyanine dye, N,N'-dioctadecylthiacyanine perchlorate (S11), S9(J) + S11(J), have been fabricated by the Langmuir-Blodgett (LB) technique. The mole fraction X of S11, X = [S11]/([S9] + [S11]), was varied from 0 to 1. Steady-state absorption spectra, fluorescence spectra, and picosecond fluorescence decay curves of the monolayer assemblies have been measured. Spectroscopic properties of the monolayer assemblies incorporating the individual dye aggregates, S9 J-aggregate (S9(J), X = 0) or S11 J-aggregate (S11(J), X = 1), are characterized by a distinct J-band and resonance fluorescence at lambda(ab) = 403 nm and lambda(em) = 403 nm for S9(J) and lambda(ab) = 456 nm and lambda(em) = 463 nm for S11(J). On the other hand, absorption spectra of the S9(J) + S11(J) assemblies for X = 0.1-0.9 display two absorption bands, a shorter wavelength one and a longer wavelength one, whose peak positions are blue-shifted from those of the corresponding J-bands of the S9 J-aggregate and the S11 J-aggregate, respectively. Furthermore, fluorescence spectra are characterized by a single band (longer wavelength fluorescence) which is somewhat blue-shifted from the resonance fluorescence of the S11 J-aggregate. The fluorescence lifetimes of the S11 J-aggregate and isolated S11 molecules in LB films appear to be tau = 110 and 1900 ps, respectively, while the fluorescence lifetime of the longer wavelength fluorescence of the S9(J) + S11(J) assemblies takes practically a constant value of tau = 170-180 ps for X = 0.2-0.8. These observations would indicate that S9 and S11 molecules in the S9(J) + S11(J) assembly can form a specific mixed aggregate distinct from the individual S9 and S11 J-aggregates. From detailed considerations of the former works on luminescence properties of the S9 J-aggregate doped with isolated S11 molecules, as well as the mosaic-type mixed J-aggregate (M-aggregate) composed of a certain thiacyanine dye, 3,3'-disulfopropyl- 5,5'-dichlorothiacyanine sodium salt, and thiacarbocyanine dye, meso-substituted 3,3'-disulfopropyl-5,5'-dichlorothiacarbocyanine potassium salt, it is suggested that S9 and S11 can form a homogeneous aggregate of the persistence type (HP-aggregate). The HP-aggregate is distinguished from the M-aggregate because it is characterized by homogeneous mixing of two component dyes and persistence of two absorption bands.     

Dynamics of entangled supramolecular polymer networks in presence of high‐order associations of strong hydrogen bonding groups

Dynamics of entangled polymer chains in the melt state are deliberately excluded in most of the studies on supramolecular polymer networks by utilizing nonentangled precursor chains. Relaxation of the system mainly depends on the dissociation of the associative groups in latter case and nonentangled chains deliver nothing to resist afterward. Conversely, in an entangled system, relaxation of polymer chains and dissociation of associative groups can occurred parallel. Supramolecular networks based on an entangled precursor polymer with different levels of strong associating ureidopyrimidinone (UPy) groups are synthesized to screen the corresponding effects on the dynamics of the system. Binary‐associated UPy groups phase separate into collective assemblies by stacking and form high‐order, needle‐like domains at higher UPy contents. Relaxation of polymer chains is significantly hindered due to the trapping of polymer segments between UPy stacks. Above a certain threshold of UPy content (~4 mol%), the plateau level and final relaxation time of networks show a significant jump, which is attributed to the onset of high‐order association of UPy groups.     

Fabrication of Novel Reversible Photoswitchable Fluorescent Nanoparticles

Novel photoswitchable fluorescent nanoparticles were fabricated by a facile one-step mini-emulsion polymerization, in which fluorescence resonance energy transfer (FRET) donor, 9,10-diphenylanthracene (DPA), and photoswitchable acceptor, spiropyran derivate, were simultaneously embedded in polymer matrix during the polymerization process. The prepared fluorescent nanoparticles exhibit the typical absorption properties of both DPA dye and spiropyran moiety, indicating that the two chromophores have been incorporated into the polymer nanoparticles. The obtained fluorescent nanoparticles exhibited superior photoswitchable fluorescent performance due to the effective photoinduced interparticle FRET. Moreover, the novel photoswitchable fluorescent nanoparticles also revealed small size (ca. 60 nm), high intensity, relatively fast photoresponsive property and good photoreversibility in aqueous media.     

Super-resolution RESOLFT microscopy of lipid bilayers using a fluorophore-switch dyad

Dyads consisting of a photochromic switch covalently linked to a fluorescent dye allow the emission from the dye to be controlled by reversible photoisomerization of the switch; one form of the switch quenches fluorescence by accepting energy from the dye. Here we investigate the use of dyads of this type for super-resolution imaging of lipid bilayers. Giant unilamellar vesicles stained with the dyads were imaged with about a two-fold resolution-enhancement compared with conventional confocal microscopy. This was achieved by exciting the fluorophore at 594 nm, using a switch activated by violet and red light (405/640 nm).

A photoswitchable quencher can be used to reversibly turn off the emission from a fluorescent dye, generating a small molecule dyad that is effective for super-resolution RESOLFT microscopy.     

Promoting Co-Crystallization in Poly(butylene succinate) and Poly(butylene fumarate) Blends via End-Group Functionalization

Co-crystallization plays a crucial role in the integration and regulation of thermal and mechanical properties in polymer blends, but the poor compatibility of the components in the crystal phase has always been a major obstacle to co-crystallization, which puts forward stricter requests for linkage and interaction between different entities. On the basis of the hydrogen-bonding interaction that can promote chain stacking and thus improve miscibility, we propose that crystalline/crystalline blends of 2-ureido-4[1H]-pyrimidinone (UPy)-functionalized poly(butylene succinate) and poly(butylene fumarate) (PBS-UPy/PBF-UPy) where UPy groups with quadruple hydrogen-bonding interaction are employed to connect different chain ends, could inhibit phase separation and improve co-crystallization. PBS-UPy/PBF-UPy blends exhibit complex component-dependent and cooling-rate-dependent co-crystallization behavior. A high level of co-crystallization occurs in the range of PBS-UPy-rich fractions, and the proportion could approach over 98% under optimized conditions with the aid of UPy quadruple hydrogen bonds interaction. This work enriches the understanding of co-crystallization in crystalline/crystalline polymer blends and provides more possibility for the design of structures and properties of polymer materials.     

A Combination of Selective Light Reflection and Fluorescence Modulation in a Cholesteric Polymer Matrix

Summary: The phase behavior and optical properties of a cholesteric ternary copolymer, containing nematogenic phenylbenzoate, cholesteric, and photochromic diarylethene side groups, and its mixture with 2 wt.‐% fluorescent dopant were studied. The investigation of the kinetics of a photochemical opening‐cycle process of the photochromic groups in the cholesteric mixture proved the energy transfer from the fluorescent dopant to the photochromic diarylethene groups. It was shown that the fluorescence intensity of the fluorescent dopant could be controlled by the portion of the “closed” form of the diarylethene groups. During the photocyclization of the photochromic groups a “degeneration” of the selective light reflection of the cholesteric matrix is observed.

Fluorescence‐resonance energy transfer makes possible the process of photosensitization of the back ring‐opening photoreaction of the photochromic diarylethene groups in the cholesteric polymer matrix.     

Influence of selectivity on the supramolecular polymerization of AB-type polymers capable of Both A x A and A x B interactions

The supramolecular polymerization of two AB-type monomers capable of hydrogen-bond-mediated A x B heterocoupling and A x A homocoupling is discussed. The AB-type supramolecular polymerization is based on the strong interaction between self-dimerizing 2-ureido-pyrimidinone (UPy) and 2,7-diamido-1,8-naphthyridine (NaPy). In an effort to reduce the "self-stoppered" effect that is inherently present in these supramolecular polymerizations we used a novel ureido-pyrimidinone substituted with a dibutylamino group at the pyrimidinone ring. As a result of the substitution, the dimerization constant of the novel UPy unit is lowered compared to the previous UPy unit while the heterodimerization strength is retained. Unexpectedly, the increased selectivity toward heteroassociation not only influences the concentration-dependent degree of polymerization due to reduction of the "self-stoppered" effect but also has a pronounced effect on the ring-chain equilibrium by increasing the tendency to cyclize. In order to quantitatively explain our results, a model was developed that accurately predicts the degree of polymerization by taking into account homo- and heterodimerization as well as cyclization. Finally, molecular weight distributions for noncyclizing AB supramolecular polymerizations with and without a reversible A x A interaction are calculated. It is found that the molecular weight distribution becomes narrower when A x A interactions are present.     

The effect of irradiation by ultraviolet light on ureido‐pyrimidinone based biomaterials

Supramolecular polymers based on ureido‐pyrimidinone (UPy) represent a promising class of biocompatible materials for medical applications. Here, the chemical modification effect of UV irradiation, used to sterilize these materials, is studied. Besides anticipated crosslinking effects, UV irradiation causes telechelic UPy‐polymers to become fluorescent. UPy‐model compounds confirm a relation between UV‐induced changes and the UPy‐moiety. UV‐induced fluorescence and IR‐spectral changes are (partially) reversible by heat and/or solvent treatment. The results indicate the presence of at least two distinct UV‐induced molecular species. UPy‐model compounds with specific tautomeric forms directly relate fluorescence to UPy‐enol tautomers. Photo‐enolization is hypothesized to occur via an excited‐state intermolecular double proton transfer. Changes in UPy‐tautomeric equilibrium and crosslinking are factors that influence the dynamics of UPy‐based materials. Identification and understanding of such factors will aid in the successful application of these materials, for example as biomaterial in tissue engineering applications. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 81–90     

Molecular Conductance through a Quadruple‐Hydrogen‐Bond‐Bridged Supramolecular Junction

A series of self‐complementary ureido pyrimidinedione (UPy) derivatives modified with different aurophilic anchoring groups were synthesized. Their electron transport properties through the quadruple hydrogen bonds in apolar solvent were probed employing the scanning tunneling microscopy break junction (STMBJ) technique. The molecule terminated with a thiol shows the optimal electron transport properties, with a statistical conductance value that approaches 10^?3 G₀. The ¹H NMR spectra and control experiments verify the formation of quadruple hydrogen bonds, which can be effectively modulated by the polarity of the solvent environment. These findings provide a new design strategy for supramolecular circuit elements in molecular electronics.