Attractive acceptor-acceptor interactions in self-complementary quadruple hydrogen bonds for molecular self-assembly

Molecular self-assembly provides the means for creating large supramolecular structures, extending beyond the capability of standard chemical synthesis. To harness the power of self-assembly, it is necessary to understand its driving forces. A potent method is to exploit self-complementary hydrogen bonding, where a molecule interacts with its own copy by suitable positions of hydrogen-bond donor (D) and acceptor (A) groups. With four hydrogen bonds, there are two possible self complementary patterns: the DDAA/AADD and the DADA/ADAD motifs. Of these, the DDAA pattern is usually more stable. The traditional explanation assumes that the secondary interactions between equal groups, that is, between donors (D⋯D) or acceptors (A⋯A), are repulsive. DDAA arrays would then have two, and DADA arrays six repulsive interactions. Here, using high-end quantum chemical analysis, we show that contrary to the traditional explanation, the secondary A⋯A interactions are, in fact, attractive. We revise the model of secondary interactions accordingly.

Supramolecular Synthesis of Star Polymers

Supramolecular polymers, in which monomers are assembled via intermolecular interactions, have been extensively studied. The fusion of supramolecular polymers with conventional polymers has attracted the attention of many researchers. In this review article, the recent progress in the construction of supramolecular star polymers, including regular star polymers and miktoarm star polymers, is discussed. The initial sections briefly provide an overview of the conventional classification and synthesis methods for star polymers. Coordination-driven self-assembly was investigated for the supramolecular synthesis of star polymers. Star polymers with multiple polymer chains radiating from metal-organic polyhedra (MOPs) have also been described. Particular focus has been placed on the synthesis of star polymers featuring supramolecular cores formed through hydrogen-bonding-directed self-assembly. After describing the synthesis of star polymers based on host-guest complexes, the construction of miktoarm star polymers based on the molecular recognition of coordination capsules is detailed.

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

Direct Detection of Hydrogen Bonds in Supramolecular Systems Using 1H-15N Heteronuclear Multiple Quantum Coherence Spectroscopy

Hydrogen-bonded supramolecular systems are usually characterized in solution through analysis of NMR data such as complexation-induced shifts and nuclear Overhauser effects (nOe). Routine direct detection of hydrogen bonding particularly in multicomponent mixtures, even with the aid of 2D NMR experiments for full assignment, is more challenging. We describe an elementary rapid ¹H-¹⁵N HMQC NMR experiment which addresses these challenges without the need for complex pulse sequences. Under readily accessible conditions (243/263 K, 50 mM solutions) and natural ¹⁵N abundance, unambiguous assignment of ¹⁵N resonances facilitates direct detection of intra- and intermolecular hydrogen bonds in mechanically interlocked structures and quadruply hydrogen-bonded dimers─of dialkylaminoureidopyrimidinones, ureidopyrimidinones, and diamidonaphthyridines─in single or multicomponent mixtures to establish tautomeric configuration, conformation, and, to resolve self-sorted speciation.

Molecular duplexes featuring NH···N, CH···O and CH···π interactions in solid-state self-assembly of triazine-based compounds

Synthetic supramolecular structures constructed through the cooperative action of numerous non-covalent forces are highly desirable as models to unravel and understand the complexity of systems created in nature via self-assembly. Taking advantage of the low cost of 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride) and the sequential nucleophilic substitution reactions with almost all types of nucleophiles, a series of six structurally related novel s-triazine derivatives 1-6 were synthesized and structurally characterized based on their physical, spectral and crystallographic data. The solid-state structures of all the six compounds showed intriguing and unique molecular duplexes featuring NH···N, CH···O and CH···π interactions. Careful analysis of different geometric parameters of the involved H-bonds indicates that they are linear, significant and are therefore responsible for guiding the three-dimensional structure of these compounds in the solid state. The prevalence of sextuple hydrogen bond array-driven molecular duplexes and the possibility of structural modifications on the s-triazine ring render these novel triazine derivatives 1-6 attractive as a platform to create heteroduplex constructs and their subsequent utility in the field of supramolecular chemistry and crystal engineering.

Exploring the Role of H-Bonding in Organic Electrochemistry - From Supramolecular Applications to Mechanistic Investigations

H-bonds can exert a substantial impact on the course of organic electrode reactions due to their ability to stabilize charged intermediates and products formed during these reactions, as well as facilitate proton-coupled electron transfer (PCET) reactions. This has fundamental implications for the mechanism of organic electrode reactions, but also practical impact in supramolecular chemistry and potentially synthetic electrochemistry. My group's main focus has been on the supramolecular applications, using electron transfer to alter the strength of H-bonds to create highly redox-responsive H-bond dimers. Initially we sought to avoid proton transfer because we feared that would lead to irreversible electrochemistry. However, inevitably proton transfer did show up, but, to our surprise, did not lead to irreversible electrochemistry. To explain this, we developed a new mechanism, the "wedge scheme", that shows how H-bonding can facilitate reversible electron and proton transfer. This insight recently led us to a new PCET-based design strategy for the creation of our most highly redox-responsive H-bond dimers yet.

Self-healing gelatin-based shape memory hydrogels via quadruple hydrogen bonding and coordination crosslinking for controlled delivery of 5-fluorouracil

Gelatin-UPy based on gelatin with ureidopyrimidinone (UPy) side chains was prepared with varying content of UPy units. On increasing the UPy content, the glass transition temperature, crystallinity and swelling decreased. Gelatin-UPy demonstrated self-healing properties as the UPy units could reversibly form dimers. At the same time, the gelatin-UPy and gelatin-UPy hydrogels demonstrated thermal responsive shape memory behaviors. The introduction of coordination crosslinking by introducing Fe³⁺ in gelatin-UPy hydrogels not only enhanced the crosslinking degree of gelatin-UPy and decreased the swelling degree, but also significantly improved the self-healing properties. As a drug carrier, gelatin-UPy hydrogels could achieve controlled release of 5-fluorouracil (5-FU) drug on increasing the content of UPy and concentration of Fe³⁺. The gelatin-UPy based materials are expected to find significant use as suppository and tissue engineering materials to treat tumors.

A pH-Switchable Triple Hydrogen-Bonding Motif

A stimuli responsive linear hydrogen bonding motif, capable of in situ protonation and deprotonation, has been investigated. The interactions of the responsive hydrogen bonding motif with complementary partners were examined through a series of 1H NMR experiments, revealing that the recognition preference of the responsive hydrogen bonding motif in a mixture can be switched between two states.

Assembly of miscible supramolecular network blends using DDA·AAD hydrogen-bonding interactions of pendent side-chains

Miscible blends of poly(methyl methacrylate) and polystyrene polymers are assembled through triple hydrogen bonding between complementary ureidoimidazole and amidoisocytosine heterodimers.

AAAA-DDDD Quadruple H-Bond-Assisted Ionic Interactions: Robust Bis(guanidinium)/Dicarboxylate Heteroduplexes in Water

The use of geminal di(guanidinium) and acridin-9(10H)-one-derived di(carboxylate) derivatives (1a-c and 2a-e, respectively) allows stabilization of heterodimers characterized by high binding affinities in water (maximum ΔG < -7 kcal mol^-1, K_a > 10⁵ M^-1) as inferred from UV-vis spectroscopic titrations and ITC measurements, therefore rivaling or surpassing the interaction energy between the strongest DNA or RNA triplet pairs. These duplexes are readily accessible and are structurally modifiable, rendering them attractive as building blocks for creating heteroduplex constructs. Incorporating poly(ethylene glycol)-decorated benzyl groups into the dicarboxylate, allows formation of hydrogels in the case of 1b-2c.

Supramolecular Self-Sorting Networks using Hydrogen-Bonding Motifs

A current objective in supramolecular chemistry is to mimic the transitions between complex self-sorted systems that represent a hallmark of regulatory function in nature. In this work, a self-sorting network, comprising linear hydrogen motifs, was created. Selecting six hydrogen-bonding motifs capable of both high-fidelity and promiscuous molecular recognition gave rise to a complex self-sorting system, which included motifs capable of both narcissistic and social self-sorting. Examination of the interactions between individual components, experimentally and computationally, provided a rationale for the product distribution during each phase of a cascade. This reasoning holds through up to five sequential additions of six building blocks, resulting in the construction of a biomimetic network in which the presence or absence of different components provides multiple unique pathways to distinct self-sorted configurations.

Thieno[3,4-c]phosphole-4,6-dione: A Versatile Building Block for Phosphorus-Containing Functional π-Conjugated Systems

A versatile phosphorus-containing π-conjugated building block, thieno[3,4-c]phosphole-4,6-dione (TPHODO), has been developed. The utility of this simple but hitherto unknown building block has been demonstrated by preparing novel functional organophosphorus compounds and bandgap-tunable conjugated polymers.

Use of an electrochemically-induced proton-coupled electron transfer reaction to control dimerization in a ureidopyrimidone 4 H-bond array

Cyclic voltammetric and spectroelectrochemical evidence is presented showing that the H-bonded dimer formed from a ureidopyrimidone derivative containing a phenylenediamine redox couple can be reversibly broken apart at mM concentrations in CH2Cl2 by an electrochemically induced proton-coupled electron transfer reaction.

pH- and concentration-controlled self-assembly of spherical micelles with cavity, necklace and cylindrical micelles

A novel diblock copolymer with one block composed of hydrophobic motifs, hydrogen-bonding carbamates and pH-triggered carboxy groups was developed, which could self-assemble into spherical micelles with cavity, necklaces and cylindrical micelles.

Hirshfeld Surface Investigation of Structure-Directing Interactions within Dipicolinic Acid Derivatives

Six compounds based on dipicolinic acid esters have been synthesized and Hirshfeld surfaces used to investigate the structure-directing effects of functional groups in controlling their solid-state behavior. Compounds 1-4 are 4-bromo dipicolinic acid esters substituted with methyl, ethyl, propyl, and benzyl groups, respectively. The main structure-directing motif within 1-3 is a pairwise O···H interaction involving two carbonyl oxygen atoms and two aromatic H atoms. The introduction of bulky benzyl groups in 4 forces a significant change in the position of this interaction. Compounds 2 and 4 were used in Suzuki coupling reactions to prepare extended analogues 5 and 6, respectively, and their solid-state behavior was also studied using Hirshfeld surfaces. Extension of these dipicolinic acid esters results in the complete loss of the pairwise O···H interaction in 5, where the dominant structure-directing motifs are π-based interactions. However, the pairwise O···H interaction reappears for the more flexible 6, demonstrating control of the solid-state structure of these dipicolinic acid derivatives through the choice of functional groups.

Revealing the supramolecular nature of side-chain terpyridine-functionalized polymer networks

Nowadays, finely controlling the mechanical properties of polymeric materials is possible by incorporating supramolecular motifs into their architecture. In this context, the synthesis of a side-chain terpyridine-functionalized poly(2-(dimethylamino)ethyl methacrylate) is reported via reversible addition-fragmentation chain transfer polymerization. By addition of transition metal ions, concentrated aqueous solutions of this polymer turn into metallo-supramolecular hydrogels whose dynamic mechanical properties are investigated by rotational rheometry. Hence, the possibility for the material to relax mechanical constrains via dissociation of transient cross-links is brought into light. In addition, the complex phenomena occurring under large oscillatory shear are interpreted in the context of transient networks.

Functional carbon nanotubes for high-quality charge transfer and moisture regulation through membranes: structural and functional insights

Electrically-responsive breathable membranes through which assisted charge and moisture transport are achieved by selective donor–acceptor-site interactions involving functional carbon nanotubes.

An AAA-DDD triply hydrogen-bonded complex easily accessible for supramolecular polymers

For a complementary hydrogen-bonded complex, when every hydrogen-bond acceptor is on one side and every hydrogen-bond donor is on the other, all secondary interactions are attractive and the complex is highly stable. AAA-DDD (A=acceptor, D=donor) is considered to be the most stable among triply hydrogen-bonded sequences. The easily synthesized and further derivatized AAA-DDD system is very desirable for hydrogen-bonded functional materials. In this case, AAA and DDD, starting from 4-methoxybenzaldehyde, were synthesized with the Hantzsch pyridine synthesis and Friedländer annulation reaction. The association constant determined by fluorescence titration in chloroform at room temperature is 2.09×10(7)  M(-1) . The AAA and DDD components are not coplanar, but form a V shape in the solid state. Supramolecular polymers based on AAA-DDD triply hydrogen bonded have also been developed. This work may make AAA-DDD triply hydrogen-bonded sequences easily accessible for stimuli-responsive materials.

Supramolecular chemistry with ureido-benzoic acids

A novel, complementary quadruple hydrogen-bonding motif is presented that shows very strong dimerization and is switchable with pH.

An insight into the extraction of transition metal ions by picolinamides associated with intramolecular hydrogen bonding and rotational isomerization

Disruption of intramolecular H-bonding via N-substitution leads to rotational isomerization and much improvement in extraction of Hg²⁺.