From Linear to Angular Isomers: Achieving Tunable Charge Transport in Single‐Crystal Indolocarbazoles Through Delicate Synergetic CH/NH⋅⋅⋅π Interactions

Weak intermolecular interaction in organic semiconducting molecular crystals plays an important role in molecular packing and electronic properties. Here, four five‐ring‐fused isomers were rationally designed and synthesized to investigate the isomeric influence of linear and angular shapes in affecting their molecular packing and resultant electronic properties. Single‐crystal field‐effect transistors showed mobility order of 5,7‐ICZ (3.61 cm² V^?1 s^?1) >5,11‐ICZ (0.55 cm² V^?1 s^?1) >11,12‐ICZ (ca. 10^?5 cm² V^?1 s^?1) and 5,12‐ICZ (ca. 10^?6 cm² V^?1 s^?1). Theoretical calculations based on density functional theory (DFT) and polaron transport model revealed that 5,7‐ICZ can reach higher mobilities than the others thanks to relatively higher hole transfer integral that links to stronger intermolecular interaction due to the presence of multiple NH???π and CH???π(py) interactions with energy close to common NH???N hydrogen bonds, as well as overall lower hole‐vibrational coupling owing to the absence of coupling of holes to low frequency modes due to better π conjugation.     

Unusually Short Chalcogen Bonds Involving Organoselenium: Insights into the Se–N Bond Cleavage Mechanism of the Antioxidant Ebselen and Analogues

Structural studies on the polymorphs of the organoselenium antioxidant ebselen and its derivative show the potential of organic selenium to form unusually short Se???O chalcogen bonds that lead to conserved supramolecular recognition units. Se???O interactions observed in these polymorphs are the shortest such chalcogen bonds known for organoselenium compounds. The FTIR spectral evolution characteristics of this interaction from solution state to solid crystalline state further validates the robustness of this class of supramolecular recognition units. The strength and electronic nature of the Se???O chalcogen bonds were explored using high‐resolution X‐ray charge density analysis and atons‐in‐molecules (AIM) theoretical analysis. A charge density study unravels the strong electrostatic nature of Se???O chalcogen bonding and soft‐metal‐like behavior of organoselenium. An analysis of the charge density around Se?N and Se?C covalent bonds in conjunction with the Se???O chalcogen bonding modes in ebselen and its analogues provides insights into the mechanism of drug action in this class of organoselenium antioxidants. The potential role of the intermolecular Se???O chalcogen bonding in forming the intermediate supramolecular assembly that leads to the bond cleavage mechanism has been proposed in terms of electron density topological parameters in a series of molecular complexes of ebselen with reactive oxygen species (ROS).     

Influence of Intermolecular N?H⋅⋅⋅π Interactions on Molecular Packing and Field‐Effect Performance of Organic Semiconductors

Charge transport in organic semiconductors is strongly dependent on their molecular packing modes in the solid state. Therefore, understanding the relationship between molecular packing and charge transport is imperative, both experimentally and theoretically. However, so far, the fundamental effects of solid‐state packing and molecular interactions (e.g. N? H ??? π) on charge transport need further elucidation. Herein, indolo[3,2‐b]carbazole (ICZ) and a derivative thereof are used as examples to approach this scientific target. An interesting insight obtained thereby is that N? H ??? π interactions among ICZ molecules facilitate charge transport for higher mobility. Subtle changes in the of N? H ??? π interactions can significantly influence both the molecular packing and the charge‐transport properties. Therefore, a method for exploiting intermolecular N? H ??? π interactions would yield novel molecular systems with designable characteristics.     

The Nature of Activated Non‐classical Hydrogen Bonds: A Case Study on Acetylcholinesterase–Ligand Complexes

Molecular recognition events in biological systems are driven by non‐covalent interactions between interacting species. Here, we have studied hydrogen bonds of the CH???Y type involving electron‐deficient CH donors using dispersion‐corrected density functional theory (DFT) calculations applied to acetylcholinesterase–ligand complexes. The strengths of CH???Y interactions activated by a proximal cation were considerably strong; comparable to or greater than those of classical hydrogen bonds. Significant differences in the energetic components compared to classical hydrogen bonds and non‐activated CH???Y interactions were observed. Comparison between DFT and molecular mechanics calculations showed that common force fields could not reproduce the interaction energy values of the studied hydrogen bonds. The presented results highlight the importance of considering CH???Y interactions when analysing protein–ligand complexes, call for a review of current force fields, and opens up possibilities for the development of improved design tools for drug discovery.     

Supramolecular Cl⋅⋅⋅H and O⋅⋅⋅H Interactions in Self‐Assembled 1,5‐Dichloroanthraquinone Layers on Au(111)

The role of halogen bonds in self‐assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5‐dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl???H and O???H hydrogen bonds with marginal contributions from Cl‐related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br‐ and I‐related halogen bonds, Cl‐related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.     

Construction of Hetero‐Four‐Layered Tripalladium(II) Cyclophanes by Transannular π⋅⋅⋅π Interactions

A synthetic strategy for the generation of new molecular species utilizing a provision of nature is presented. Nano‐dimensional (23(2)×21(1)×16(1) ų) hetero‐four‐layered trimetallacyclophanes were constructed by proof‐of‐concept experiments that utilize a suitable combination of π???π interactions between the central aromatic rings, tailor‐made short/long spacer tridentate donors, and the combined helicity. The behavior of the unprecedented four‐layered metallacyclophane system offers a landmark in the development of new molecular systems.     

Antiparallel Self‐Association of a γ,α‐Hybrid Peptide: More Relevance of Weak Interactions

To learn how a preorganized peptide‐based molecular template, together with diverse weak non‐covalent interactions, leads to an effective self‐association, we investigated the conformational characteristics of a simple γ,α‐hybrid model peptide, Boc‐γ‐Abz‐Gly‐OMe. The single‐crystal X‐ray diffraction analysis revealed the existence of a fully extended β‐strand‐like structure stabilized by two non‐conventional C?H???O=C intramolecular H‐bonds. The 2D ¹H NMR ROESY experiment led us to propose that the flat topology of the urethane‐γ‐Abz‐amide moiety is predominantly preserved in a non‐polar environment. The self‐association of the energetically more favorable antiparallel β‐strand‐mimic in solid‐state engenders an unusual ‘flight of stairs’ fabricated through face‐to‐face and edge‐to‐edge Ar???Ar interactions. In conjunction with FT‐IR spectroscopic analysis in chloroform, we highlight that conformationally semi‐rigid γ‐Abz foldamer in appositely designed peptides may encourage unusual β‐strand or β‐sheet‐like self‐association and supramolecular organization stabilized via weak attractive forces.     

Photophysical Processes in ‘Supramolecular Balls’ Formed by Lanthanide Chloride with 2,2′‐Bipyridine

The europium complex [EuCl₂(bpy)₂(H₂O)₂]Cl?1.25 C₂H₆O?0.37 H₂O, where bpy is 2,2′‐bipyridine, was synthesized and investigated with the aim to relate its molecular geometry and crystal packing to the efficiency of energy‐transfer processes. The presence of H‐bonds between noncoordinated Cl^? ions and coordinated H₂O molecules leads to the formation of discrete trimers assembled by a number of C? H???Cl and stacking interactions into ‘supramolecular balls’ which contain Cl^? ions and solvate molecules (H₂O and EtOH). The additional stabilization of the complex is due to intramolecular N???C interactions between two bpy ligands that causes some shortening of the Eu? N bonds. Deciphering the luminescence properties of the Eu complex was performed under consideration of both the composition of the inner coordination sphere and the peculiarities of the crystal packing. The influence of the latter and the bpy orientation on the energy of the ligand→Eu charge‐transfer state (LMCT) was established, and an additional excited state induced by the π‐stacking interaction (SICT) was identified.     

Rational Modification of the Hierarchy of Intermolecular Interactions in Molecular Crystal Structures by Using Tunable Halogen Bonds

A family of 16 isomolecular salts (3‐XpyH)₂[MX′₄] (3‐XpyH=3‐halopyridinium; M=Co, Zn; X=(F), Cl, Br, (I); X′=Cl, Br, I) each containing rigid organic cations and tetrahedral halometallate anions has been prepared and characterized by X‐ray single crystal and/or powder diffraction. Their crystal structures reflect the competition and cooperation between non‐covalent interactions: N? H???X′? M hydrogen bonds, C? X???X′? M halogen bonds and π–π stacking. The latter are essentially unchanged in strength across the series, but both halogen bonds and hydrogen bonds are modified in strength upon changing the halogens involved. Changing the organic halogen (X) from F to I strengthens the C? X???X′? M halogen bonds, whereas an analogous change of the inorganic halogen (X′) weakens both halogen bonds and N? H???X′? M hydrogen bonds. By so tuning the strength of the putative halogen bonds from repulsive to weak to moderately strong attractive interactions, the hierarchy of the interactions has been modified rationally leading to systematic changes in crystal packing. Three classes of crystal structure are obtained. In type A (C? F???X′? M) halogen bonds are absent. The structure is directed by N? H???X′? M hydrogen bonds and π‐stacking interactions. In type B structures, involving small organic halogens (X) and large inorganic halogens (X′), long (weak) C? X???X′? M interactions are observed with type I halogen–halogen interaction geometries (C? X???X′ ≈ X???X′? M ≈155°), but hydrogen bonds still dominate. Thus, minor but quite significant perturbations from the type A structure arise. In type C, involving larger organic halogens (X) and smaller inorganic halogens (X′), stronger halogen bonds are formed with a type II halogen–halogen interaction geometry (C? X???X′ ≈180°; X???X′? M ≈110°) that is electrostatically attractive. The halogen bonds play a major role alongside hydrogen bonds in directing the type C structures, which as a result are quite different from type A and B.     

Hardness Equation for Ormosils

Hardness of ormosils coating on various kinds of substrates is important, and by considering recent progresses in understanding the hardness of ionic crystals or covalent crystals, new hardness equations for calculating the hardness of glasses or ormosils from chemical compositions were derived. When we applied an indenter to the surface of glass or sol-gel coatings, the surface of indenter is a declined one to the flat surface of glass or coating, thus the applied force should be analyzed by using the shear modulus, S, and Young's modulus, E. This is now well accepted for the analysis of hardness of ionic or covalent bonding inorganic materials. For example, by considering the binding energy and plastic deformation, Gilman showed that Hv of NaCl crystal can be calculated by an equation including elastic stiffness which indicated a good agreement between calculated and observed value. For covalent crystals he reported that the strength of chemical bonds could be correlated with the glide (shear) activation energy for covalent crystals quantitatively. These explanations are basically applied to the hardness of silicate glasses and ormosils. By considering both shear modulus and Young's modulus we have derived equations for calculating the hardness of glasses or ormosils from chemical composition, which includes packing density of atoms and bond energy per unit volume has been taken account. The agreements between calculated and observed hardness values for ormosils were comparatively good.     

Interplay between Metal⋅⋅⋅π Interactions and Hydrogen Bonds: Some Unusual Synergetic Effects of Coinage Metals and Substituents

The ternary systems of C₂H₄ (C₂H₂ or C₆H₆)‐MCN‐HF (M=Cu, Ag, Au) and the respective binary systems were investigated to study the interplay between metal???π interactions and hydrogen bonds. The metal???π interactions in C₂H₄‐MCN become stronger with the irregular order Ag<Cu<Au, while the hydrogen bonds in MCN‐HF become weaker following the same order. The metal???π interactions are weakened as the H atoms in the π system are replaced with electron‐withdrawing groups and enhanced by electron‐donating groups. Type 1 of these ternary systems, in which MCN acts as Lewis base and acid simultaneously, is more stable than type 2, in which C₂H₄ acts as a double Lewis base. Negative cooperativity is present in type 2 ternary systems with a weakening of the metal???π interactions and the hydrogen bonds. Positive cooperativity is found in type 1 ternary systems with an enhancement of the metal???π interactions and the hydrogen bonds, except for C₂(CN)₄‐AuCN‐HF‐1. The weaker metal???π interaction in C₆H₆‐AuCN has a greater enhancing effect on the hydrogen bond in AuCN‐HF than those in C₂H₄‐AuCN and C₂H₂‐AuCN. These synergetic effects were analyzed with the natural bond orbital and energy decomposition.     

Intramolecular Hydrogen Bonds in Low‐Molecular‐Weight Polyethylene Glycol

We used static DFT calculations to analyze, in detail, the intramolecular hydrogen bonds formed in low‐molecular‐weight polyethylene glycol (PEG) with two to five repeat subunits. Both red‐shifted O?H???O and blue‐shifting C?H???O hydrogen bonds, which control the structural flexibility of PEG, were detected. To estimate the strength of these hydrogen bonds, the quantum theory of atoms in molecules was used. Car–Parrinello molecular dynamics simulations were used to mimic the structural rearrangements and hydrogen‐bond breaking/formation in the PEG molecule at 300 K. The time evolution of the H???O bond length and valence angles of the formed hydrogen bonds were fully analyzed. The characteristic hydrogen‐bonding patterns of low‐molecular‐weight PEG were described with an estimation of their lifetime. The theoretical results obtained, in particular the presence of weak C?H???O hydrogen bonds, could serve as an explanation of the PEG structural stability in the experimental investigation.     

Self‐Complementary Dimers of Oxalamide‐Functionalized Resorcinarene Tetrabenzoxazines

Self‐complementarity is a useful concept in supramolecular chemistry, molecular biology and polymeric systems. Two resorcinarene tetrabenzoxazines decorated with four oxalamide groups were synthesized and characterized. The oxalamide groups possessed self‐complementary hydrogen bonding sites between the carbonyls and amide groups. The self‐complementary nature of the oxalamide groups resulted in self‐included dimeric assemblies. The hydrogen bonding interactions within the tetrabenzoxazines gave rise to the formation of dimers, which were confirmed by single‐crystal X‐ray diffractions analysis and supported by NMR spectroscopy and mass spectrometry. The self‐included dimers were connected by numerous and strong intermolecular N?H???O and C?H???O hydrogen bonds supplemented with C?H???π interactions, forming one‐dimensional polymers, which were then further linked into three‐dimensional networks.     

The Role of Weak Interactions in the Mechano‐induced Single‐Crystal‐to‐Single‐Crystal Phase Transition of 8‐Hydroxyquinoline‐Based Co‐crystals

Mechano‐induced single‐crystal‐to‐single‐crystal (SCSC) phase transitions in crystalline materials that change their properties have received more and more attention. However, there are still too few examples to study molecular‐level mechanisms in the mechano‐induced SCSC phase transitions, making the systematic and in‐depth understanding very difficult. We report that bis‐(8‐hydroxyquinolinato) palladium(II)‐tetracyanoquinodimethane (PdQ₂‐TCNQ) and bis‐(8‐hydroxyquinolinato) copper(II)‐tetracyanoquinodimethane (CuQ₂‐TCNQ) show very different mechano‐response behaviors during the SCSC phase transition. Phase transition in CuQ₂‐TCNQ can be triggered by pricking on the crystal surface, while in PdQ₂‐TCNQ it can only be induced by applying pressure uniformly over the whole crystal face. The crystallography data and Hirshfeld surface analysis indicate that the weak intra‐layer C?H???O, C?H???N hydrogen bonds and inter‐layer stacking interactions determine the feasibility of the SCSC phase transition by mechanical stimuli. Weaker intra‐layer interactions and looser inter‐layer stacking make the SCSC phase transition occur much more easily in the CuQ₂‐TCNQ.     

Highly Energetic,Low Sensitivity Aromatic Peroxy Acids

The synthesis, structure, and energetic materials properties of a series of aromatic peroxy acid compounds are described. Benzene‐1,3,5‐tris(carboperoxoic) acid is a highly sensitive primary energetic material, with impact and friction sensitivities similar to those of triacetone triperoxide. By contrast, benzene‐1,4‐bis(carboperoxoic) acid, 4‐nitrobenzoperoxoic acid, and 3,5‐dinitrobenzoperoxoic acid are much less sensitive, with impact and friction sensitivities close to those of the secondary energetic material 2,4,6‐trinitrotoluene. Additionally, the calculated detonation velocities of 3,5‐dinitrobenzoperoxoic acid and 2,4,6‐trinitrobenzoperoxoic acid exceed that of 2,4,6‐trinitrotoluene. The solid‐state structure of 3,5‐dinitrobenzoperoxoic acid contains intermolecular O‐H???O hydrogen bonds and numerous N???O, C???O, and O???O close contacts. These attractive lattice interactions may account for the less sensitive nature of 3,5‐dinitrobenzoperoxoic acid.     

Dynamic Properties of Molecular Tweezers with a Bis(2‐hydroxyphenyl)pyrimidine Backbone

4,6‐Bis(2‐hydroxyphenyl)‐2‐alkylpyrimidines with two anthryl or 9‐ethylnylanthryl substituents at the positions para to the OH groups prefer a U‐shaped conformation supported by two intramolecular OH ??? N hydrogen bonds in the solid state and in CDCl₃ solution. The compound with a hexyl substituent on the pyrimidine group and two 9‐ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7‐trinitrofluorenone. Its association constant K_a was estimated to be 2100 M ^?1 at 298 K, which is larger than those of other molecular tweezers (K_a<1000 M ^?1). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π–π interaction between the anthryl groups and 2,4,7‐trinitrofluorenone. Addition of nBu₄NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH ??? N hydrogen bonds, formation of new O ??? HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6‐bis(2‐hydroxyphenyl)‐2‐methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ?<0.01), owing to photoinduced electron transfer of the molecule with a U‐shaped structure. However, the O‐hexylated compound exhibits emission from the anthryl groups with ?=0.39.     

The Counterion Effect on the Assembly of Coordination Networks of Cationic (η3‐Allyl)palladium Complexes Containing 3,5‐Dimethyl‐4‐nitro‐1H‐pyrazole

Two new (η³‐allyl)palladium complexes containing the ligand 3,5‐dimethyl‐4‐nitro‐1H‐pyrazole (Hdmnpz) were synthesized and characterized as [Pd(η³‐C₃H₅)(Hdmnpz)₂]BF₄ ( 1 ) and [Pd(η³‐C₃H₅)(Hdmnpz)₂]NO₃ ( 2 ). The structures of these compounds were determined by single‐crystal X‐ray diffraction to evaluate the intermolecular assembly. Each complex exhibits similar coordination behavior consistent with cationic entities comprised of two pyrazole ligands coordinated with the [Pd(η³‐C₃H₅)]⁺ fragment in an almost square‐planar coordination geometry. In 1 , the cationic entities are propagated through strong intermolecular H‐bonds formed between the pyrazole NH groups and BF ions in one‐dimensional polymer chains along the a axis. These chains are extended into two‐dimensional sheet networks via bifurcated H‐bonds. New intermolecular interactions established between NO₂ and Me substituents at the pyrazole ligand of neighboring sheets give rise to a three‐dimensional network. By contrast, compound 2 presents molecular cyclic dimers formed through N? H???O H‐bonds between two NO counterions and the pyrazole NH groups of two cationic entities. The dimers are also connected to each other through C? H???O H‐bonds between the remaining O‐atom of each NO ion and the allyl CH₂ H‐atom. Those interactions expand in a layer which lies parallel to the face (101).     

Influence of the Li⋅⋅⋅π Interaction on the H/X⋅⋅⋅π Interactions in HOLi⋅⋅⋅C6H6⋅⋅⋅HOX/XOH (X=F,Cl, Br,I) Complexes

The influences of the Li???π interaction of C₆H₆???LiOH on the H???π interaction of C₆H₆???HOX (X=F, Cl, Br, I) and the X???π interaction of C₆H₆???XOH (X=Cl, Br, I) are investigated by means of full electronic second‐order Møller–Plesset perturbation theory calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The binding energies, binding distances, infrared vibrational frequencies, and electron densities at the bond critical points (BCPs) of the hydrogen bonds and halogen bonds prove that the addition of the Li???π interaction to benzene weakens the H???π and X???π interactions. The influences of the Li???π interaction on H???π interactions are greater than those on X???π interactions; the influences of the H???π interactions on the Li???π interaction are greater than X???π interactions on Li???π interaction. The greater the influence of Li???π interaction on H/X???π interactions, the greater the influences of H/X???π interactions on Li???π interaction. QTAIM studies show that the intermolecular interactions of C₆H₆???HOX and C₆H₆???XOH are mainly of the π type. The electron densities at the BCPs of hydrogen bonds and halogen bonds decrease on going from bimolecular complexes to termolecular complexes, and the π‐electron densities at the BCPs show the same pattern. Natural bond orbital analyses show that the Li???π interaction reduces electron transfer from C₆H₆ to HOX and XOH.     

Self‐Assembly and (Hydro)gelation Triggered by Cooperative π–π and Unconventional CH⋅⋅⋅X Hydrogen Bonding Interactions

Weak C? H???X hydrogen bonds are important stabilizing forces in crystal engineering and anion recognition in solution. In contrast, their quantitative influence on the stabilization of supramolecular polymers or gels has thus far remained unexplored. Herein, we report an oligophenyleneethynylene (OPE)‐based amphiphilic Pt^II complex that forms supramolecular polymeric structures in aqueous and polar media driven by π–π and different weak C‐H???X (X=Cl, O) interactions involving chlorine atoms attached to the Pt^II centers as well as oxygen atoms and polarized methylene groups belonging to the peripheral glycol chains. A collection of experimental techniques (UV/Vis, 1D and 2D NMR, DLS, AFM, SEM, and X‐Ray diffraction) demonstrate that the interplay between different weak noncovalent interactions leads to the cooperative formation of self‐assembled structures of high aspect ratio and gels in which the molecular arrangement is maintained in the crystalline state.