H-bonding Motifs Research Articles

An experimental and computational investigation of the cyclopentene-containing peptide-derived compounds: focus on pseudo-cyclic motifs via intramolecular interactions.

Conformational flexibility is one of the main disadvantages of peptide-based compounds. We focus on their molecular 'chameleonicity' related to forming pseudo-cyclic motifs via modulation of weak intramolecular interactions. It is an appealing strategy for controlling equilibrium between the polar open and the nonpolar closed conformations. Within this context, we report here the crystal structure of the (R)-(2-tert-butoxycarbonyl)amino-1-oxo-3-phenyl)propyl)-1-cyclopentene (1), synthesis of which in high yield was achieved by a facile multi-step protocol. Our Cambridge Structural Database (CSD) overview for the peptide-based crystals revealed the exclusivity of this compound from the viewpoint of the unusual pseudo-bicyclic system via C-H…O and C-O…π interactions, in which cyclopentene shields the amide bond. Notably, cyclopentene as a bioisostere of proline is an appealing scaffold in medicinal chemistry. An extensive combined experimental and computational study provided more profound insight into the supramolecular landscape of 1 with respect to similar derivatives deposited in the CSD, including the tendency of cyclopentene for the generation of pseudo-cyclic motifs through weak H-bonding and π-based intramolecular interactions. These weak interactions have been examined by either the quantum theory of 'atoms-in-molecules' (QTAIM) or complex Hirshfeld surface methodology, including enrichment ratios, molecular electrostatic potential surfaces and energy frameworks. In all analysed crystals, all types of H-bonded motifs involving cyclopentene are formed at all levels of supramolecular architecture. A library of cyclopentene-based H-bonding synthons is provided. A molecular docking study depicted vital interactions of cyclopentene with key amino acid residues inside the active sites of two prominent protein kinases, uncovering the therapeutic potential of 1 against breast cancer. To a large extent, dispersion forces have significance in stabilizing the supramolecular structure of both ligand and bio-complex ligand-protein. Finally, the satisfactory in silico bio-pharmacokinetic profile of 1 related to drug-likeness and blood-brain barrier permeation was also revealed.

Recurrent Supramolecular Patterns in a Series of Salts of Heterocyclic Polyamines and Heterocyclic Dicarboxylic Acids: Synthesis, Single-Crystal X-ray Structure, Hirshfeld Surface Analysis, Energy Framework, and Quantum Chemical Calculations

A series of novel salts based on aromatic polyamines and 2,3-pyrazinedicarboxylic acid, such as C10H12N6O5 (1), C10H9ClN6O4 (2), C11H10N8O4 (3), and C14H17N16O5.5 (4) or 3,4-thiophenedicarboxylic acid, such as C10H10N4O4S (5), C10H9ClN4O4S (6), and C10H10N4O4S2 (7), were synthesized and characterized by single-crystal X-ray diffraction. All compounds crystallize in a monoclinic space group. The structure was subjected to complex Hirshfeld surface analysis, molecular electrostatic potential, enrichment ratio, and energy framework calculations. The influence of different cations on the packing of 3-carboxypyrazine-2-carboxylate and 4-carboxythiophene-3-carboxylate anions in the crystal lattice was studied. O…H/H…O interactions are the main contributor in all crystals. In addition, in a series of pyrazine-containing structures, N(C)…H/H…N(C) interactions have relevance, while in a series of thiophene-based compounds, C…H/H…C and S…H(O)/H(O)…S. In addition, Cl-based interactions are observed in compound 2. According to the enrichment ratio calculations, O…H/H…O and C…C are the most preferable interactions in all structures. The energy frameworks are dominated by the dispersive contribution, only in compound 3 is the electrostatic term dominant. The analyzed structures reveal intra- and intermolecular recurrent supramolecular synthons. In both series of crystals, the robust H-bonded centrosymmetric dimer R22(8) as homo- or as heterosynthon (in compounds 2, 3, 6, and 7) and the intramolecular synthon S(7) generated by O-H…O interactions (in compounds 2, 6, and 7) are present. The supramolecular patterns formed by π…π (C…C) and C-O(Cl,S)…C are also noticeable. Notably, a dual synthon linking the supramolecular chain via π…π interactions and the homosynthon R22(8) via N-H…N interactions is visible in both series of new salts. A library of H-bonding motifs at diverse levels of supramolecular architecture is provided. We extended the analysis of intramolecular H-bonding motifs to similar structures deposited in the Cambridge Structural Database. Another important feature is the existence of an intramolecular O…H…O bridge between two neighboring carboxylic groups as substituents in anions in compounds 3 and 5. In this context, we performed quantum theory of atoms-in-molecule calculations to reveal more details.

Hydrogen-Bonding-Regulated Morphology Control and the Impact on the Antibacterial Activity of Cationic π-Amphiphiles.

This manuscript describes the synthesis, self-assembly, and antibacterial properties of naphthalene-diimide (NDI)-derived cationic π-amphiphiles. Three such asymmetric NDI derivatives with a nonionic hydrophilic wedge and a guanidine group in the two opposite sides of the NDI chromophore were considered. They differ by a single functional group (hydrazide, amide, and ester for NDI-1, NDI-2, and NDI-3, respectively), located in the linker between the NDI and the hydrophilic wedge. For NDI-1, the H-bonding among the hydrazides regulated unilateral stacking and a preferential direction of curvature of the resulting supramolecular polymer, producing an unsymmetric polymersome with the guanidinium groups displayed at the outer surface. NDI-3, lacking any H-bonding group, exhibits π-stacking without any preferential orientation and generates spherical particles with a relatively poor display of the guanidium groups. In sharp contrast to NDI-1, NDI-2 exhibits an entangled one-dimensional (1D) fibrillar morphology, indicating the prominent role of the H-bonding motif of the amide group and flexibility of the linker. The antibacterial activity of these assemblies was probed against Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). NDI-1 showed the most promising antibacterial activity with a minimum inhibitory concentration (MIC) of ∼7.8 μg/mL against S. aureus and moderate activity (MIC ∼ 125 μg/mL) against E. coli. In sharp contrast, NDI-3 did not show any significant activity against the bacteria, suggesting a strong impact of the H-bonding-regulated directional assembly. NDI-2, forming a fibrillar network, showed moderate activity against S. aureus and negligible activity against E. coli, highlighting a significant impact of the morphology. All of these three molecules were found to be compatible with mammalian cells from the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT) and hemolysis assay. The mechanistic investigation by membrane polarization assay, live/dead fluorescence assay, and microscopy studies confirmed the membrane disruption mechanism of cell killing for the lead candidate NDI-1.

Imparting Ultrahigh Strength to Polymers via a New Concept Strategy of Construction of up to Duodecuple Hydrogen Bonding among Macromolecular Chains.

Interconnecting macromolecules via multiple hydrogen bonds (H-bonds) can simultaneously strengthen and toughen polymers, but material synthesis becomes extremely difficult with increasing number of H-bonding donors and acceptors; therefore, most reports are limited to triple and quadruple H-bonds. Herein, this bottleneck is overcome by adopting a quartet-wise approach of constructing H-bonds instead of the traditional pairwise method. Thus, large multiple hydrogen bonds can be easily established, and the supramolecular interactions are further reinforced. Especially, when such multiple H-bond motifs are embedded in polymers, four macromolecular chains-rather than two as usual-are tied, distributing the applied stress over a larger volume and more significantly improving the overall mechanical properties. Proof-of-concept studies indicate that the proposed intermolecular multiple H-bonds (up to duodecuple) are readily introduced in polyurethane. A record-high tensile strength (105.2MPa) is achieved alongside outstanding toughness (352.1 MJ m-3), fracture energy (480.7 kJ m-2), and fatigue threshold (2978.4 J m-2). Meantime, the polyurethane has acquired excellent self-healability and recyclability. This strategy is also applicable to nonpolar polymers, such as polydimethylsiloxane, whose strength (15.3MPa) and toughness (50.3 MJ m-3) are among the highest reported to date for silicones. This new technique has good expandability and can be used to develop even more and stronger polymers.

Spectroscopic Evidence for Doubly Hydrogen-Bonded Cationic Dimers in the Solid, Liquid, and Gaseous Phases of Carboxyl-Functionalized Ionic Liquids.

Intermolecular interactions determine whether matter sticks together, gases condense into liquids, or liquids freeze into solids. The most prominent example is hydrogen bonding in water, responsible for the anomalous properties in the liquid phase and polymorphism in ice. The physical properties are also exceptional for ionic liquids (ILs), wherein a delicate balance of Coulomb interactions, hydrogen bonds, and dispersion interactions results in a broad liquid range and the vaporization of ILs as ion pairs. In this study, we show that strong, local, and directional hydrogen bonds govern the structures and arrangements in the solid, liquid, and gaseous phases of carboxyl-functionalized ILs. For that purpose, we explored the H-bonded motifs by X-ray diffraction and attenuated total reflection (ATR) infrared (IR) spectroscopy in the solid state, by ATR and transmission IR spectroscopy in the liquid phase, and by cryogenic ion vibrational predissociation spectroscopy (CIVPS) in the gaseous phase at low temperature. The analysis of the CO stretching bands reveals doubly hydrogen-bonded cationic dimers (c═c), resembling the archetype H-bond motif known for carboxylic acids. The like-charge doubly hydrogen-bonded ion pairs are present in the crystal structure of the IL, survive phase transition into the liquid state, and are still present in the gaseous phase even in (2,1) complexes wherein one counterion is removed and repulsive Coulomb interaction increased. The interpretation of the vibrational spectra is supported by quantum chemical methods. These observations have implications for the fundamental nature of the hydrogen bond between ions of like charge.

Comparison and investigation of H-bond assisted reusable PU adhesives with high shear strength

Hot-melt adhesive (HMA) is commonly used in industrial manufacturing due to its ease of use. However, the direct factors that affect shear strength have been unclear in recent years, making it difficult to prepare hot melt adhesives with high shear strength, which are mostly lower than 6 MPa. In this study, three types of polyurethane HMAs, differing in the position of H-bond motifs, ureidopyrimidinone (UPy), were synthesized and characterized, the most central factor for shear strength, Young's modulus, was firstly found out and proved. Side-chain UPy provides better flexibility and more chain entanglement. The lap shear strength of SPU was about 13.48 MPa, which was the highest shear strength of PU HMA, and remained 73.4 % even after 4 cycles. Compared to the other two samples, the side-chain motifs improved the possibility of forming hydrogen bonds with adherends and increased chain mobility. This led to a more efficient wetting process and strengthened Young's modulus directly, enhancing adhesive property incomparably. This work provides a satisfactory method for achieving high shear strength in HMA.

Hydroxy-Functionalized Ionic Liquids under Pressure: The Influence on Hydrogen Bonding between Ions of Opposite and Like Charges.

Hydroxy functionalization of cations in ionic liquids (ILs) can lead to formation of hydrogen bonds between their OH groups, resulting in so-called (c-c) H-bonds. Thereby, the (c-c) H-bonds compete with regular H-bonds (c-a) between the OH groups and the anions. Polarizable cations, weakly interacting anions, and long alkyl chains at the cation support the propensity for the formation of (c-c) H-bonds. At low temperatures, the equilibrium between (c-c) and (c-a) H-bonds is strongly shifted in favor of the cation-cation interaction. Herein, we clarify the pressure dependence on (c-c) and (c-a) H-bond distributions in the IL 1-(2-hydroxyethyl)-3-methylimidazolium hexafluorophosphate [HOC2C1Im][PF6], in mixtures of [HOC2C1Im][PF6] with the nonhydroxy-functionalized IL 1-propyl-3-methylimidazolium hexafluorophosphate [C3C1Im][PF6] and in [HOC2C1Im][PF6] including trace amounts of water. The infrared (IR) spectra provide clear evidence that the (c-c) H-bonds diminish with increasing pressure in favor of the (c-a) H-bonds. Adding trace amounts of water results in enhanced (c-c) clustering due to cooperative effects. At ambient pressure, the water molecules are involved in the (c-c) H-bond motifs. Increasing pressure leads to squeezing them out of H-bond clusters, finally resulting in demixing of water and the IL at the microscopic level.

A Semiflexible Tetrahydrazone Macrocycle for Binding of Pyrophosphate and Smaller Anions.

Macrocyclization has proven to be a useful design strategy in the development of efficient anion receptors. In addition to the ring size, the overall preorganization due to structural rigidity is key. To explore this in the context of developing an efficient pyrophosphate receptor, three macrocycles featuring a 26-membered interior ring size and similar H-bonding motifs have been synthesized, and their anion binding ability has been investigated. Computational studies and nuclear magnetic resonance (NMR) data showed different degrees of preorganization as a result of differences in flexibility. The interaction of the three macrocycles with chloride, dihydrogen phosphate, and dihydrogen pyrophosphate was investigated in solution by NMR and ultraviolet-visible spectroscopy and in the solid state by X-ray crystallography. The tetrahydrazone-based macrocycle featuring intermediate flexibility exhibited the best affinity for all three anions investigated. Our results suggest that in addition to the proper preorganization of binding groups in a macrocycle a certain degree of flexibility is also required for an optimal affinity with the target guest.

Highly Stretchable, Self-Healing, and Sensitive E-Skins at -78 °C for Polar Exploration.

Ultralow temperature-tolerant electronic skins (e-skins) can endow polar robots with tactile feedback for exploring in extremely cold polar environments. However, it remains a challenge to develop e-skins that enable sensitive touch sensation and self-healing at ultralow temperatures. Herein, we describe the development of a sensitive robotic hand e-skin that can stretch, self-heal, and sense at temperatures as low as -78 °C. The elastomeric substrate of this e-skin is based on poly(dimethylsiloxane) supramolecular polymers and multistrength dynamic H-bonds, in particular with quadruple H-bonding motifs (UPy). The structure-performance relationship of the elastomer at ultralow temperatures is investigated. The results show that elastomers with side-chain UPy units exhibit higher stretchability (∼3257%) and self-healing efficiency compared to those with main-chain UPy units. This is attributed to the lower binding energy variation and lower potential well. Based on the elastomer with side-chain UPy and man-made electric ink, a sensitive robotic hand e-skin for usage at -78 °C is constructed to precisely sense the shape of objects and specific symbols, and its sensation can completely self-recover after being damaged. The findings of this study contribute to the concept of using robotic hands with e-skins in polar environments that make human involvement limited, dangerous, or impossible.

Carbamate-Protected (BOC and O-NB) 2-Aminopyrimidinedione-Based Janus G-C Nucleobase Motifs as Building Blocks for Supramolecular Assembly and Smart Polymers.

In this paper, we report "carbamate protected" 2-aminopyrimidinedione-based Janus G-C nucleobases (2-APD Janus G-C nucleobases) featuring polymerizable groups and self-complementary triple H-bonding motifs DDA and AAD as building blocks for developing smart polymers and self-healing materials. The carbamate-masked H-bonding motif, cleavable under acidic (BOC group) or photocleavable (O-nitrobenzyl group) conditions, would facilitate the synthesis of smart polymers by alleviating aggregation during polymerization which in turn would exclude self-assembly-assisted solubility issues. Ready accessibility in excellent yields coupled with the possibility for facile introduction of polymerizable groups would make these building blocks excellent candidates for diverse polymerization applications.

Stability of Quadruple Hydrogen Bonds in an Ionic Liquid Environment.

Hydrogen bonds (H-bonds) are highly sensitive to the surrounding environments owing to their dipolar nature, with polar solvents kown to significantly weaken H-bonds. Herein, the stability of the H-bonding motif ureidopyrimidinone (UPy) is investigated, embedded into a highly polar polymeric ionic liquid (PIL) consisting of pendant pyrrolidinium bis(trifluoromethylsulfonyl)imide (IL) moieties, to study the influence of such ionic environments on the UPy H-bonds. The content of the surrounding IL is changed by addition of an additional low molecular weight IL to further boost the IL content around the UPy moieties in molar ratios of UPy/IL ranging from 1/4 up to 1/113, thereby promoting the polar microenvironment around the UPy-H-bonds. Variable-temperature solid-state MAS NMR spectroscopy and FT-IR spectroscopy demonstrate that the UPy H-bonds are largely present as (UPy-) dimers, but sensitive to elevated temperatures (>70°C). Subsequent rheology and DSC studies reveal that the ILs only solvate the polymeric chains but do not interfere with the UPy-dimer H-bonds, thus accounting for their high stability and applicability in many material systems.

Core-level spectroscopic probing of the heteromolecular H-bonding interaction in Cyanuric Acid/Melamine 2D networks

The preparation of complex molecular architectures routinely exploits the directionality and cooperative strength of hydrogen bonding interactions, which can be used for a reversible control of molecular self-assembly of simple organic molecules. Cyanuric acid (CA) and Melamine (M) are two such examples of simple organic molecules which can assemble in well-ordered two-dimensional networks on solid surfaces. Their CA*M adduct is a prototypical molecular system exhibiting two complementary NH••O and NH••N hydrogen bonds. In this work we exploit the initial state locality of K-shell core-electron excitation/ionization process to get a detailed information on the electronic structure modification which characterizes the long-range order of the heteromolecular H-bonding motif of the CA*M adduct as compared to the homomolecular CA and M extended structures. Both of N 1s X-Ray Photoelectron spectroscopy (XPS) and N K-shell Near Edge X-Ray Absorption Fine-Structure (NEXAFS) spectroscopy and are used to get complementary information on the prototypical CA*M heteromolecular system characterizing both the H-donor/H-acceptor nature of the molecular constituents and the extent of heteromolecular H-bond formation, due to the quenching of σ*(N–H) resonances.

Water on ceria{111}: Comparison between 23 experimental vibrational studies in the literature and new modeling.

Theoretical and experimental vibrational signatures of H2O and OH- (dissociated water) adsorbed on stoichiometric ceria{111} surfaces are compared. The experimental ones were collected from low-coverage experiments in the literature, and the theoretical anharmonic frequencies were generated using density functional theory calculations employing the optPBE-vdW functional for coverages from 0.5 to a few monolayers. It is found that (i) the experiments and our calculations overall agree well, lending credibility to both; (ii) the calculations manage to resolve the large class of H-bonded motifs into frequency classes that can guide experimental assignments; (iii) it is possible to find a geometrical H-bond definition that also captures the OH vibrational frequency downshifts well: R(H⋯O) ≤2.5 Å and the O-H⋯O angle θ ≥ 100°; and (iv) the frequency vs electric field relations for water and hydroxides (i.e., dissociated water) follow different and well-separated curves.

Completion of Crystallographic Data for the Series of 4-Halogenated-1H-pyrazoles: Crystal Structure Determination of 4-Iodo-1H-pyrazole and Spectroscopic Comparison

Prior to 2021, 4-bromo-1H-pyrazole (published in 1999) was the only structurally characterized 4-halogenated-1H-pyrazole in the Cambridge Crystallographic Data Center (CCDC). The structures of 4-chloro-1H-pyrazole and 4-fluoro-1H-pyrazole were published in 2021 and 2023, respectively. Herein, we report the crystal structure for 4-iodo-1H-pyrazole, completing the crystallographic data for the series of 4-halogenated-1H-pyrazoles. The bromo and chloro analogs are isostructural, forming trimeric H-bonding motifs, whereas the fluoro and iodo analogs form non-isostructural catemers. We also compare the experimental and theoretical (by DFT calculations) IR and 1H NMR spectroscopic data of the four halogenated 4-X-pzH compounds and unsubstituted pyrazole (pzH). An explanation is offered for some counterintuitive structural, infrared, and 1H-NMR spectroscopic data.

Structural (XRD) Characterization and an Analysis of H-Bonding Motifs in Some Tetrahydroxidohexaoxidopentaborate(1-) Salts of N-Substituted Guanidinium Cations.

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH2)2(NHMe)][B5O6(OH)4]·H2O (1), [C(NH2)2(NH{NH2})][B5O6(OH)4] (2), [C(NH2)2(NMe2)][B5O6(OH)4] (3), [C(NH2)(NMe2)2][B5O6(OH)4] (4), [C(NHMe)(NMe2)2][B5O6(OH)4]·B(OH)3 (5), and [TBDH][B5O6(OH)4] (6) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1-6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH)3 and the appropriate substituted cation. Compounds 1-6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1-3 and 6 demonstrated that they thermally decomposed via a multistage process to B2O3 at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H2O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH2)3]2[B4O5(OH)4]·2H2O, [C(NH2)3][B5O6(OH)4]·H2O, and [C(NH2)3]3[B9O12(OH)6] and in compounds 1-6 revealed that two important H-bonding R22(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation-anion H-bonded rings as a major motif in [C(NH2)3]2[B4O5(OH)4]·2H2O and [C(NH2)3]3[B9O12(OH)6], whereas the anion-anion ring motif was dominant in [C(NH2)3][B5O6(OH)4]·H2O and in compounds 1-6. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.

Construction of photoswitchable urea-based multiple H-bonding motifs

In this paper, a series of photoactive urea-based multiple H-bonding motifs were rationally constructed by tethering the photoisomerizable azobenzene units aside to the urea groups of several typical urea-constituted H-bonding arrays. Taking the integrated azo units as photoswitches, their phototriggered E/Z isomerization can be used to reversibly manipulate the locking (with intramolecular H-bonds) and unlocking (without intramolecular H-bonds) of the urea H-bonding sites of these motifs, respectively. This feature enables the E/Z photoisomeric urea derivatives to exhibit switchable intermolecular H-bonding affinities, thus achieving unique photocontrolled self/hetero complementary multiple H-bonded dimerization upon irradiation by different light sources. Benefiting from the wide application of urea group, the development of such urea-based multiple H-bonding system with photoswitchable dimerization affinity is promising in providing a generally applicable approach for the construction of copious urea-based H-bonded supramolecular assembly systems and materials with photoregulable properties and functions.

Charge-Assisted Hydrogen-Bonded Organic Frameworks with Inorganic Ammonium Regulated Switchable Open Polar Sites.

Surface open polar sites within the voids of porous molecular crystals define the localized physicochemical environment for critical functions such as gas separation and molecular recognition. This study presents a new charge-assisted hydrogen bonding (H-bonding) motif, by exploiting inorganic ammonium (NH4 + ) cations as H-bond donors, to regulate the assembly of C2 -symmetric carboxylic tectons for building robust H-bonded frameworks with permanent ultra-micropores and open oxygen sites. Diverse building blocks are bridged by tetrahedral NH4 + to expand distinctive H-bonded networks with varied pore architectures. Particularly, the open polar oxygen sites can be switched by altering NH4 + sources to tune the deprotonation of carboxyl-containing tectons. The activated porous PTBA·NH4 ·DMF preserves the pore architecture and open polar oxygen sites, exhibiting remarkably selective sorption of CO2 (107.8 cm3 g-1 ,195 K) over N2 (11.2 cm3 g-1 , 77 K) and H2 (1.4 cm3 g-1 , 77 K).

Self-healing polymers through hydrogen-bond cross-linking: synthesis and electronic applications.

Recently, polymers capable of repeatedly self-healing physical damage and restoring mechanical properties have attracted extensive attention. Among the various supramolecular chemistry, hydrogen-bonding (H-bonding) featuring reversibility, directionality and high per-volume concentration has become one of the most attractive directions for the development of self-healing polymers (SHPs). Herein, we review the recent advances in the design of high-performance SHPs based on different H-bonding types, for example, H-bonding motifs and excessive H-bonding. In particular, the effects of the structural design of SHPs on their mechanical performance and healing efficiency are discussed in detail. Moreover, we also summarize how to employ H-bonding-based SHPs for the preparation of self-healable electronic devices, focusing on promising topics, including energy harvesting devices, energy storage devices, and flexible sensing devices. Finally, the current challenges and possible strategies for the development of H-bonding-based SHPs and their smart electronic applications are highlighted.

Kinetic versus thermodynamic polymorph stabilization of a tri-carboxylic acid derivative at the solid-liquid interface.

The carboxylic acid moiety gives rise to structural variability in surface-supported self-assembly due to the common expression of various H-bonding motifs. Self-assembly of 3-fold symmetric tricarboxylic acid derivatives on surfaces typically results in monolayer structures that feature the common 2-fold cyclic R22(8) H-bond motif for at least one of the carboxylic acid groups. Polymorphs that are exclusively based on 3-fold cyclic R33(12) H-bonds were predicted but remained elusive. Here, we show the emergence of such a superflower (SF) structure purely based on R33(12) H-bonds for L-benzene-1,3,5-tricarbonyl phenylalanine (L-BTA), a molecule derived from the well-studied trimesic acid (TMA). In contrast to TMA, L-BTA is not completely planar and is also equipped with additional functional groups for the formation of secondary intermolecular bonds. At the heptanoic acid-graphite interface we transiently observe a SF structure, which is dynamically converted into a chicken-wire structure that only exhibits R22(8) H-bonds. Interestingly, when using nonanoic acid as a solvent the initially formed SF structure remained stable. This unexpected behaviour is rationalized by accompanying force field simulations and experimental determination of solvent-dependent L-BTA solubility.

High-yielding synthesis of cyclometallated iridium complexes with hydrogen bond-rich ligands.

A library of cyclometallated iridium(III) complexes with a strong H-bonding motif in their ancillary ligand was synthesized, characterized and their photophysical properties measured. Demonstrated herein is a general synthetic high yield procedure for these compounds. We ascribe these yields to the use of an intermediary primer ligand. This de novo strategy circumnavigates the standard synthetic issues of H-bond rich ligand precursors (self-aggregation and poor solubility in organic solvents).