Hydrogen-bonded Supramolecular Assemblies Research Articles

Systematic Exploration of Structural Topologies in Hydrogen-Bonded Supramolecular Assemblies of Citric Acid with Different Heterocyclic Compounds.

Hydrogen-bonded supramolecular assemblies of citric acid, CA, with some heterocyclic compounds (N-donor and N-oxide)-acridine (acr), phenazine (phenz), 1,10-phenanthroline (110phen), 1,7-phenanthroline (17phen), 4,7-phenanthroline (47phen), 1,4-diazabicyclo[2.2.2]octane (dabco), and 4,4'-bipyridyl-N,N'-dioxide (bpydo)-have been reported. Among these, only the N-donors phenz and N-oxide (bpydo) form neutral co-crystals, while the others form salts owing to the deprotonation of -COOH. Thus, depending on the nature of the aggregate (salt/co-crystal), recognition between the co-formers is established through O-H···N/N+-H···O/N+H···O-heteromeric hydrogen bonding. Additionally, CA molecules establish homomeric interactions mediated by O-H···O hydrogen bonds. Moreover, CA forms a cyclic network with the co-formers or on its own, with a noteworthy feature of formation of host-guest networks in the assemblies with acr and phenz (solvated). In the assembly of acr, the CA molecules form a host network and captivate acr molecules as guest species, while in the case of phenz assembly, both the co-formers together encapsulate the solvent in the channels. However, the observed cyclic networks in the other structures form three-dimensional topologies in the form of ladders, a sandwich, lamellar layers, and interpenetrated networks. The structural features of the ensembles are evaluated unequivocally by the single-crystal X-ray diffraction method, while the homogeneity and phase purity are evaluated by using the powder X-ray diffraction method and differential scanning calorimetry. Further, conformational analysis of CA molecules reveals three types of conformations-T-shape (type I), syn-anti (type II), and syn (type III) as also observed in the literature for other CA co-crystals. In addition, the strength of the intermolecular interactions is quantified by performing Hirshfeld analysis.

Pronounced Chirality Effect on the Ferroelectricity of Hydrogen-Bonded Supramolecular Assemblies of Ambipolar Chromophoric Systems

Pronounced Chirality Effect on the Ferroelectricity of Hydrogen-Bonded Supramolecular Assemblies of Ambipolar Chromophoric Systems

Stable room temperature ferroelectricity in hydrogen-bonded supramolecular assemblies of ambipolar π-systems.

This article reports H-bonding driven supramolecular polymerization of naphthalimide (A)–thiophene (D)–naphthalimide (A) (ADnA, n = 1–4) conjugated ambipolar π-systems and its remarkable impact on room temperature ferroelectricity. Electrochemical studies confirm the ambipolar nature of these ADnA molecules with the HOMO–LUMO gap varying between 2.05 and 2.29 eV. Electron density mapping from ESP calculations reveals intra-molecular charge separation as typically observed in ambipolar systems. In the aggregated state, AD1A and AD2A exhibit bathochromically shifted absorption bands while AD3A and AD4A show typical H-aggregation with a hypsochromic shift. Polarization vs. electric field (P–E) measurements reveal stable room temperature ferroelectricity for these supramolecular assemblies, most prominent for the AD2A system, with a Curie temperature (Tc) ≈ 361 K and saturation polarization (Ps) of ∼2 μC cm−2 at a rather low coercive field of ∼2 kV cm−1. Control molecules, lacking either the ambipolar chromophore or the amide functionality, do not show any ferroelectricity, vindicating the present molecular and supramolecular design. Computational studies enable structural optimization of the stacked oligomer(s) of AD2A molecules and reveal a significant increase in the macro-dipole moment (in the range of 10–12 Debye) going from the monomer to the oligomer(s), which provides the rationale for the origin of ferroelectricity in these supramolecular polymers.

Hydrogen-bonded supramolecular assemblies of folic acid with simple hexoses

• Folic acid (FA) forms 1:1 supramolecular assemblies with D-hexoses. • The stabilities of FA/D-hexoses assemblies were identified by UV-Vis, MS, and DFT. • PTN subunit of FA was found to be essential for the molecular assembly with hexoses. • PABA and GLN subunits of FA have no effect on assembly formation. • In solution, the most stable complexes were formed with hexoses having unprotected hydroxyls 3, 4, and 6. Simple D-hexoses such as D-glucose, D-mannose, and D-galactose were found to form supramolecular complexes with folic acid (FA) in phosphate-buffered saline solution (PBS, pH 7.4) and in the gas phase. Pterin subunit of FA was found to be essential for the molecular assembly with hexoses, whereas PABA (p-amino benzoic acid) and GLN (glutamic acid) subunits of FA have virtually no effect on assembly formation. Experimental measurements along with DFT theoretical calculations reveal that hexose-FA assemblies are stabilized by three complementary hydrogen bonds between pterin subunit of FA and 3,4,6-trihydroxyl moiety of hexose.

Design and synthesis of methyl 2-{[4-phenyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-3-yl]sulfanyl}acetate (phpy2NS) as a ligand for complexes of Group 12 elements: structural assessment and hydrogen-bonded supramolecular assembly analysis.

The reaction of 2-cyanopyridine with N-phenylthiosemicarbazide afforded 2-[amino(pyridin-2-yl)methylidene]-N-phenylhydrazine-1-carbothioamide (Ham4ph) and crystals of 4-phenyl-5-(pyridin-2-yl)-2,4-dihydro-3H-1,2,4-triazole-3-thione (pyph3NS, 1, C13H10N4S). Crystals of methyl 2-{[4-phenyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-3-yl]sulfanyl}acetate (phpy2NS, 2, C16H14N4O2S), derived from 1, were obtained by the reaction of Ham4ph with chloroacetic acid, followed by the acid-catalyzed esterification of the carboxylic acid with methyl alcohol. Crystals of bis(methanol-κO)bis(methyl 2-{[4-phenyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-3-yl-κ2N1,N5]sulfanyl}acetato)zinc(II)/cadmium(II) hexabromidocadmate(II), [Zn0.76Cd0.24(C16H14N4O2S)2(CH3OH)2][Cd2Br6] or [Zn0.76Cd0.24(phpy2NS)2(MeOH)2][Cd2Br6], 3, and dichlorido(methyl 2-{[4-phenyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-3-yl-κ2N1,N5]sulfanyl}acetato)mercury(II), [HgCl2(C16H14N4O2S)] or [Hg(phpy2NS)Cl2], 4, were synthesized using ligand 2 and CdBr2 or HgCl2, respectively. The molecular and supramolecular structures of the compounds were studied by X-ray diffractometry. The asymmetric unit of 3 is formed from CdBr3 and M(phpy2NS)(MeOH) units, where the metal centre M has a 76% occupancy of ZnII and 24% of CdII. The M2+ centre of the cation, located on a crystallographic inversion centre, is hexacoordinated and appears as a slightly distorted octahedral [MN4O2]2+ cation. The Cd centre of the anion is coordinated by two terminal bromide ligands and two bridging bromide ligands that generate [Cd2Br6]2- cadmium-bromide clusters. These clusters display crystallographic inversion symmetry forming two edge-shared tetrahedra and serve as agents that direct the structure in the formation of supramolecular assemblies. In mononuclear complex 4, the coordination geometry around the Hg2+ ion is distorted tetrahedral and comprises two chloride ligands and two N-atom donors from the phpy2NS ligand, viz. one pyridine N atom and the other from triazole. In the crystal packing, all four compounds exhibit weak intermolecular interactions, which facilitate the formation of three-dimensional architectures. Along with the noncovalent interactions, the structural diversity in the complexes can be attributed to the metal centre and to the coordination geometry, as well as to its ionic or neutral character.

A Diprotonated Porphyrin as an Electron Mediator in Photoinduced Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies

We have successfully constructed hydrogen-bonded supramolecular assemblies based on a diprotonated saddle-distorted porphyrin (H4DPP2+) and redox-active molecules bearing a carboxylate group, such ...

Effect of Cavity Size of Mesoporous Silica on Short DNA Duplex Stability.

We studied the stabilities of short (4- and 3-bp) DNA duplexes within silica mesopores modified with a positively charged trimethyl aminopropyl (TMAP) monolayer (BJH pore diameter 1.6-7.4 nm). The DNA fragments with fluorescent dye were introduced into the pores, and their fluorescence resonance energy transfer (FRET) response was measured to estimate the structuring energies of the short DNA duplexes under cryogenic conditions (temperature 233-323 K). The results confirmed the enthalpic stability gain of the duplex within size-matched pores (1.6 and 2.3 nm). The hybridization equilibrium constants found for the size-matched pores were 2 orders of magnitude larger than those for large pores (≥3.5 nm), and this size-matching effect for the enhanced duplex stability was explained by a tight electrostatic interaction between the duplex and the surface TMAP groups. These results indicate the requirement of the precise regulation of mesopore size to ensure the stabilization of hydrogen-bonded supramolecular assemblies.

Formation of Hydrogen-Bonded Supramolecular Hetero-Triads with a Diprotonated Porphyrin

Supramolecular assemblies based on intermolecular hydrogen bonding (H bonding) have been intensively investigated to develop functional materials. Along this line, we have developed supramolecular assemblies using protonated and saddle-distorted porphyrins as H-bonding electron acceptors to gain unique photofunctionality of the assemblies.1 In order to expand the possibility of structures and components of the assemblies, it is required to develop a strategy to construct H-bonded supramolecular assemblies involving different components. Herein, we present selective formation of unusual H-bonded hetero-triads. We examined formation of hydrogen-bonded supramolecular assemblies using dodecaphenylporphyrin (H­2DPP) with various acids including carboxylic acids, sulfonic acids, and carboxylic acids with positively charged moieties. It was revealed that the stability of homo-assembly (H4DPPX2; X = conjugate base of an acid) can be controlled by the strength of the H bonding depending on the acidity of an acid used to protonate H2DPP and the electrostatic repulsion between the H4DPP2+ cation and the positively charged moiety of an acid used.2 By employing a carboxylic acid with a pyridinium moiety, the PF6 - salt of N-benzyl-4-carboxyphenylpyridinium (BnPy+-COOH•PF6 -) to react with H4DPPCl2 in chloroform, we succeeded in the formation and crystal structure determination of a supramolecular hetero-triad, [H4DPP2+(Cl–)(BnPy+-PhCOO–)](PF6 –). S. Fukuzumi, T. Honda, T. Kojima, Coord. Chem. Rev. 2012, 256, 2488-2502.W. Suzuki, H. Kotani, T. Ishizuka, Y. Shiota, K. Yoshizawa, T. Kojima, Chem. Commun. 2017, 53, 6359.

Characterization of the supramolecular assembly in 1,4-butanediol

Abstract Experimental small- and wide-angle X-ray scattering (SWAXS) data, molecular dynamics simulations, and the complemented-system approach for the calculation of the SWAXS intensities from simulation data were used to study the hydrogen-bonded supramolecular assemblies formed in liquid 1,4-butanediol and to check the structural performance of the well-established force-field models TraPPE-UA, GROMOS96-54a7, CHARMM27, OPLS-AA, and AMBER03 for this diol. We found that all the models were able to reproduce the main scattering peak in the experimental SWAXS curve, though the all-atom models, rather surprisingly, failed to convincingly interpret its scattering shoulder stretching towards smaller values of the scattering vector. The united-atom TraPPE-UA force field showed the best performance in terms of the position of the peak and the absolute values of the scattering intensity. Therefore, its results were further analysed to reveal the various contributions to the total scattering intensity and to visualize the obtained typical molecular conformations and other supramolecular structural details in 1,4-butanediol, reaching up to the colloidal scale. Thus, the importance of the presented results is two-fold. First, they reveal the details of the molecular structure in a model diol that is, due to an additional –OH group per alcohol molecule, rather different in comparison to typical simple alcohols, and second, they nicely illustrate the importance of a careful structural consideration and the necessity for an experimental validation of the molecular models – a feature that is often missing in theoretical studies, thus yielding the structural studies of the models themselves and not the structural studies of the real systems.

Two closely related 2-(benzo-furan-2-yl)-2-oxoethyl benzoates: structural differences and C-H⋯O hydrogen-bonded supra-molecular assemblies.

The compounds 2-(1-benzo-furan-2-yl)-2-oxoethyl 2-nitro-benzoate, C17H11NO6 (I), and 2-(1-benzo-furan-2-yl)-2-oxoethyl 2-amino-benzoate, C17H13NO4 (II), were synthesized under mild conditions. Their mol-ecular structures were characterized by both spectroscopic and single-crystal X-ray diffraction analysis. The mol-ecular conformations of both title compounds are generally similar. However, different ortho-substituted moieties at the phenyl ring of the two compounds cause deviations in the torsion angles between the carbonyl group and the attached phenyl ring. In compound (I), the ortho-nitro-phenyl ring is twisted away from the adjacent carbonyl group whereas in compound (II), the ortho-amino-phenyl ring is almost co-planar with the carbonyl group. In the crystal of compound (I), two C-H⋯O hydrogen bonds link the mol-ecules into chains propagating along the c-axis direction and the chains are inter-digitated, forming sheets parallel to [20-1]. Conversely, pairs of N-H⋯O hydrogen bonds in compound (II) link inversion-related mol-ecules into dimers, which are further extended by C-H⋯O hydrogen bonds into dimer chains. These chains are inter-connected by π-π inter-actions involving the furan rings, forming sheets parallel to the ac plane.

(Invited) Stabilization of a Saddle-Distorted Porphyrin Monoacid

Porphyrins have been known to undergo diprotonation to form porphyrin diacids (diprotonated porphyrins) under acidic conditions without forming porphyrin monoacids (monoprotonated porphyrins). So far, chemistry of monoprotonated porphyrins has not yet to be sufficiently pursued due to the difficulty of the formation. It should be required to establish a solid preparation procedure of monoprotonated porphyrins in solution and in solid for the development of supramolecular assemblies based on monoprotonated porphyrins. Previously, saddle-distorted dodecaphenylporphyrin (H2DPP) allowed us to observe stepwise protonation in benzonitrile and isolation of monoprotonated H3DPP+ with use of 2-anthrathene sulfonic acid (2-AnSO3H) as a hydrogen-bonded supramolecular assembly, [H3DPP](2-AnSO3)(CH3OH), which was obtained by crystallization from dichloromethane/CH3OH [1]. In order to establish the requirements for the formation of monoprotonated porphyrin, we examined reactions of H2DPP with carboxylic acids, including trifluoroacetic acid (TFA), in acetone in the presence of methanol. In the course of titration of H2DPP using TFA in a mixture of acetone/methanol (3%), we could observe two-step spectral change showing different isosbestic points, indicating step-wise protonation of H2DPP. Upon addition of 1 eq of TFA to the solution of H2DPP in acetone-d6/methanol-d4 (3%), the 1H NMR spectrum of the mixture allowed us to observe signals assigned to the o-protons of the meso-phenyl groups of H3DPP+ at 8.1 ppm, those of H2DPP and H4DPP2+ at 7.7 and 8.3 ppm, respectively. We found that the amount of methanol added had a great impact on the yield of the monoacid relative to the initial concentration of H2DPP: The increase of the portion of methanol up to 10% gained the yield, reaching to 90%. These results indicate that the formation of monoprotonated porphyrin selectively occurs in the presence of a protic solvent such as methanol even using TFA, which affords only diprotonated species in acetone. The importance of methanol for the formation of H3DPP+ was demonstrated by the crystal structure of [H3DPP](CF3COO)(MeOH). As depicted in Figure 1, the crystal structure involves hydrogen bonding of both a methanol molecule and CF3COO– with H3DPP+. Thus, together with the impact of the amount of methanol on the formation yield of H3DPP+, the hydrogen bonding between MeOH and H3DPP+ should be important for the stabilization of the monoprotonated porphyrin. Addition of other protic solvents (3%) in acetone, such as water and 2,2,2-trifluoroethanol, also allowed us to observe the H3DPP+ formation with TFA; however, we just observed the formation of H4DPP2+ in the presence of acetonitrile (5%) as a polar aprotic solvent in place of methanol with comparable permittivity. Thus, we conclude that the hydrogen bond formation of a protic solvent molecule with H3DPP+ plays a crucial role to stabilize the monoacid. We also revealed that the acidity (pK a) of an acid used for the protonation is an important factor for the selective formation of H3DPP+: The smaller pK a value of an acid, the higher formation yield of H3DPP+. In the case of p-toluenesulfonic acid as a strong acid, the formation yield of the monoacid was 100%, indicating the complete selectivity in the monoacid formation. This result is consistent with the stepwise protonation of H2DPP in benzonitrile even in the absence of methanol [1]. The higher selectivity for a strong acid should stem from weak hydrogen bonding of the corresponding conjugate base with protonated species of H2DPP, allowing competitive interaction of the protic solvent molecule with H3DPP+. We will also report the thermodynamics of the equilibrium of the protonation of H2DPP in acetone-methanol mixed solvents. Figure 1. A side view of the crystal structure of [H3DPP](CF3COO)(CH3OH). Reference Honda, T.; Kojima, T.; Fukuzumi, S. Chem. Commun. 2009, 4994-4996. Figure 1

Photoindued Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies Using a Diprotonated Saddle-Distorted Porphyrin

Photoinduced electron transfer is an essential reaction to gain charge separation for photosynthesis in bacteria and plants, solar cells, and photoconductive materials. Among light-absorbing molecules and materials for photofunctionality, porphyrins showing strong absorption bands in the visible region are attractive molecules and have been utilized to envision the charge separation toward energy conversion. A number of photofunctional systems performing charge separation have been developed using porphyrins and the derivatives as electron donors in photoinduced electron transfer.1 On the other hand, we have developed a new concept by using saddle-distorted dodecaphenylporphyrin (H2DPP) and its derivatives, which can be easily protonated to afford stable diprotonated species, such as H4DPP2+.2 The diprotonated species can act as electron acceptors3 and also form strong intermolecular hydrogen bonding with molecular entities having carboxylate groups to afford stable supramolecular assemblies.4 Based on the hydrogen bonding, we can construct photofunctional supramolecular assemblies composed of H4DPP2+ as a electron acceptor and electron donors with a carboxylate group.5 For example, a hydrogen-bonded supramolecular assembly can be formed using H2DPP and a Zn(II) complex of a saddle-distorted phthalocyanine as an electron donor with nitrogen-bound 4-carboxyl pyridine (PyCOOH) as an axial ligand of the Zn(II) center.6 The usage of PyCOOH may expand the possibility to attain a variety of hydrogen-bonded supramolecules using stable metal complexes, which can hold N-bound PyCOOH as a ligand and also act as electron donors. Herein, we will report construction of a supramolecular assembly using H2DPP and a Ru(II)-pyridylamine complex with N-bound PyCOOH as a ligand and photodynamics of photochemical reactions to afford an electron-transfer (ET) state. Dimeric [RuCl(TPA)]2(ClO4)2 (TPA = tris(2-pyridylmethyl)amine)7 reacted with PyCOOH in MeOH to give [RuCl(TPA)(PyCOOH)]ClO4 (1) in 38% yield. The complex 1 was reacted with H2DPP in acetone to form a 2:1 assembly, RuIITPA•••H4DPP2+•••RuTPA (Scheme 1). Addition of MeOH (3%) to the acetone solution allowed us to obtain a unique 1:1 hydrogen-bonded supramolecular assembly of the monoprotonated H2DPP, H3DPP+, with 1(Scheme 1). (insert Image) Scheme 1. Formation of supramolecular assemblies composed of 1 and H­2DPP in acetone. The association constant for the 2:1 assembly was determined to be (1.6 ± 0.7) × 1012 M–2 in acetone at 298 K. In the presence of MeOH (3%) in acetone, we could observe two-step association to form 1:1 and 2:1 assemblies and the binding constants were determined to be (3.2 ± 0.5) × 106 M–1 for the first step forming the 1:1 assembly and (5.0 ± 0.2) × 104 M–1 for the second step affording the 2:1 assembly. Since the MeOH molecule has been demonstrated to form hydrogen bonding with H3DPP+,4 it is plausible that the MeOH binding on the opposite side of the H3DPP+can stabilize the 1:1 assembly. The redox potentials of the H4DPP2+ moiety of the 2:1 assembly in acetone was determined to be –0.53 V (vs SCE) and the RuII/RuIII couple of the RuII-TPA moiety was done to be +0.59 V (vs SCE). Thus, the energy level of an electron-transfer state of the assembly was estimated to be 1.12 eV. Femtosecond laser flash photolysis of the 2:1 assembly with photoexcitation at 500 nm in acetone/MeOH (3%) allowed us to observe the formation of H4DPP•+, which should be derived from an ET State generated by photoinduced ET from the RuII-TPA component to the H4DPP2+ moiety. The lifetime of the ET state was determined to be 172 ps (k BET = 5.8 × 106 s–1). In this presentation, details of formation of the supramolecular assemblies and the photodynamics upon visible light excitation will be described. References S. Fukuzumi, T. Kojima, J. Mater. Chem. 2008, 18, 1427.R. Harada, T. Kojima, Chem. Commun. 2005, 716.(a) T. Kojima et al., Chem.–Eur. J. 2007, 13, 8714. (b) T. Nakanishi et al., J. Am. Chem. Soc. 2009, 131, 577. (c) T. Honda et al. J. Phys. Chem. C 2010, 114, 14290. (d) S. Fukuzumi, T. Honda, T. Kojima, Coord. Chem. Rev. 2012, 256, 2488.T. Honda, T. Kojima, S. Fukuzumi, Chem. Commun. 2009, 4994.T. Honda et al., J. Am. Chem. Soc. 2010, 132, 10155.T. Kojima et al., Angew. Chem. Int. Ed. 2008, 47, 6712.T. Kojima et al., Inorg. Chem. 1998, 37, 4076. Figure 1

Formation of Hydrogen-Bonded Supramolecular Assemblies Based on Functionalized Saddle-Distorted Porphyrins

Supramolecular assemblies consisting of porphyrins have been intensively investigated in the light of promising photo-functionality including photoinduced electron transfer, photo-conducting materials, dye-sensitized solar cells, and guest inclusion toward the emergence of novel functions on the basis of unique spatial host-guest arrangements and interactions. Along this line, we have developed a series of supramolecular assemblies using the diprotonated form of saddle-distorted dodecaphenylporphyrin (H2DPP) through intermolecular π-π interaction, such as Porphyrin Nanochannels, which include electron-donating guest molecules.1 In this presentation, we present formation and characterization of supramolecular assemblies based on H2DPP derivatives having hydrogen-bonding functional groups and their metal complexes. Introduction of carboxyl groups or hydroxyl groups to the meso-phenyl groups of H2DPP as hydrogen-bonding sites allows us to construct robust and well-organized supramolecular assemblies. A diprotonated tetracarboxyl-DPP derivative (1, Fig. 1a), for example, formed two-dimensional hydrogen-bonding networks (Fig. 1b) to form a porphyrin nanochannel structure, which were reminiscent of that formed by [H4DPP2+]Cl2 and was persistent up to 150°C, in contrast to that of [H4DPP2+]Cl2.2 To expand the size of the nanochannels, we also introduced 4-carboxylphenyl groups at the 4-positions of the meso-phenyl groups of H4DPP2+. A Zn(II) complex of H2DPP derivatives having hydroxyl groups at the 4-positions of the meso-phenyl groups (2) was prepared to form a dimer bridged by 4,4’-bypyridine.3 The dimer was revealed to form intermolecular hydrogen-bonding network among the hydroxyl groups in the crystal. In addition to the crystal structures, we will also mention adsorption of gaseous molecules such as N2 and CO2 with porous supramolecular assemblies of functionalized H2DPP derivatives in the solid state. References (a) R. Harada et al., Chem. Commun. 2005, 716. (b) T. Kojima et al., Chem.–Eur. J. 2007, 13, 8714. (c) T. Nakanishi et al., Chem. Mater. 2008, 21, 7492. (d) S. Fukuzumi et al., Coord. Chem. Rev. 2012, 256, 2488.T. Ishizuka et al. Chem. Commun. 2012, 48, 6481.T. Ishizuka et al. Dalton Trans. 2013, 42, 16073. Figure 1

Salts and Co-crystals of Theobromine and their phase transformations in water

Theobromine, a xanthine derivative analogous to caffeine and theophylline, is an effective central nervous system stimulant. It has lower aqueous solubility than caffeine and theophylline. Salts of theobromine with hydrochloric acid, phosphoric acid, methanesulfonic acid, benzenesulfonic acid and p-toluenesulfonic acid were prepared using liquid-assisted grinding (LAG). Proton transfer from the strong acid to the weak base imidazole N resulted in N+–H ⋯O− hydrogen-bonded supramolecular assemblies of theobromine salts. The mesylate salt is polymorphic with amide N–H ⋯O dimer and catemer synthons for the theobromine cations. A variable stoichiometry for phosphate salts (1:3 and 1:2.5) were observed with the latter being more stable. All new salts were characterized by FT-IR, PXRD, DSC and finally single crystal X-ray diffraction. In terms of stability, these salts transformed to theobromine within 1 h of dissolution in water. Remarkably, the besylate and tosylate salts are 88 and 58 times more soluble than theobromine, but they dissociated within 1 h. In contrast, theobromine co-crystals with gallic acid, anthranilic acid and 5-chlorosalicylic acid were found to be stable for more than 24 h in the aqueous slurry conditions, except malonic co-crystal which transformed to theobromine within 1 h. Water mediated phase transformation of theobromine salts and co-crystal may be due to the incongruency (high solubility difference) between the components. These results suggest that even though traditional salts are highly soluble compared to co-crystals, co-crystals can be superior in terms of stability. Mesylate salt polymorphs of theobromine containing amide dimer and catemer synthons of TBRH+ represent a rare category of synthon polymorphs for salts. TBR salts are shown to be more soluble than the corresponding cocrystals, but in terms of stability the salts dissociated within 1 h in aqueous medium.

Nine closely related tetrahydro-1,4-epoxy-1-benzazepines carrying pendant heterocyclic substituents: hydrogen-bonded supramolecular assembly in zero, one and two dimensions

The structures are reported of nine closely related tetrahydro-1,4-epoxy-1-benzazepines carrying pendant heterocyclic substituents, namely: 2-exo-(5-nitrofuran-2-yl)-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(14)H(12)N(2)O(4), (I), 7-fluoro-2-exo-(1-methyl-1H-pyrrol-2-yl)-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(15)H(15)FN(2)O, (II), 7-fluoro-2-exo-(5-methylfuran-2-yl)-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(15)H(14)FNO(2), (III), 7-fluoro-2-exo-(3-methylthiophen-2-yl)-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(15)H(14)FNOS, (IV), 7-fluoro-2-exo-(5-methylthiophen-2-yl)-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(15)H(14)FNOS, (V), 7-chloro-2-exo-(5-methylfuran-2-yl)-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(15)H(14)ClNO(2), (VI), 2-exo-(5-methylfuran-2-yl)-7-trifluoromethoxy-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(16)H(14)F(3)NO(3), (VII), 2-exo-(3-methylthiophen-2-yl)-7-trifluoromethoxy-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(16)H(14)F(3)NO(2)S, (VIII), and 2-exo-(5-nitrofuran-2-yl)-7-trifluoromethoxy-2,3,4,5-tetrahydro-1,4-epoxy-1H-1-benzazepine, C(15)H(11)F(3)N(2)O(5), (IX). All nine compounds crystallize in centrosymmetric space groups as racemic mixtures with configuration (2RS,4SR). There are no direction-specific interactions between the molecules in (V). The molecules in (III), (IV), (VI) and (VII) are linked into simple chains, by means of a single C-H...O hydrogen bond in each of (III), (VI) and (VII), and by means of a single C-H...π(arene) hydrogen bond in (IV), while the molecules in (VIII) are linked into a chain of rings. In each of (I) and (II), a combination of one C-H...O hydrogen bond and one C-H...π(arene) hydrogen bond links the molecules into sheets, albeit of completely different construction in the two compounds. In (IX), the sheet structure is built from a combination of four independent C-H...O hydrogen bonds and one C-H...π(arene) hydrogen bond. Comparisons are made with some related compounds.

Photoluminescence quenching effects of surface-modified gold nanoparticles on side-chain polymers containing pyridyl H-acceptors with various lateral polarities

In this study, we used photoluminescence (PL) quenching and transmission electron microscopy (TEM) to study the morphological behavior of hydrogen-bonded (H-bonded) supramolecular assemblies of luminescent H-acceptor polymers and H-donor gold nanoparticles (Au NPs). In fluorescence titration experiments, the lateral Me and MeO substituents on the fluorescent H-acceptor side-chain polymers PBOT1–PBOT3 and PBT1–PBT3 exhibited different electron-donating capabilities, thereby inducing different degrees of H-bonding and dipole–dipole interactions, as evidenced by effective fluorescence quenching upon the addition of surface-modified Au NPs bearing acid and acid-free surfactants (AuSCOOH and AuSC10, respectively). Among all of our tested nanocomposites, the highest Stern–Volmer quenching constant (KSV) was that obtained from the assembly of AuSCOOH with the homopolymer PBOT1. In addition, we developed fittable exponential equations to predict the values of KSV of other fluorescent polymers (containing various molar ratios of pyridyl conjugated units) when titrated with these NP quenchers. The morphologies observed in the TEM images confirmed that fluorescence quenching resulted from the self-assembly of the supramolecular nanocomposites, mediated by H-bonds between the fluorescent H-acceptors of the polymers and the H-donors of the Au NPs presenting acid-modified surfactants.

The variability of hydrogen-bonded supramolecular assemblies in crystalline picrates prepared from ferrocenyl-substituted β-aminoalcohols

Ferrocene-based β-aminoalcohols FcCH 2 NHCR 2 CH 2 OH (R = H, 1a ; R = Me, 1b ) and ( S )-FcCH 2 NHCH(CHMe 2 )CH 2 OH ( 1c ; Fc = ferrocenyl) react with 2,4,6-trinitrophenol (Hpic) under proton transfer to afford the corresponding ammonium picrates 2a – c . In the crystal, these picrates associate predominantly via N–H⋯O and O–H⋯O bifurcated hydrogen bonds between the NH 2 + and OH groups in the aminoalcohol chain as the donors and the phenoxide and NO 2 oxygen atoms of the picrate anion as the acceptors. Compounds 2a and 2b form closed dimeric assemblies [ 1n H] 2 [pic] 2 ( n = a , b ) around the crystallographic inversion centres. By contrast, their chiral analogue 2c gives rise to monomeric units [ 1c H][pic] (albeit through similar interactions), that further aggregate into infinite linear chains via N–H⋯O hydrogen bonds. The formed assemblies are interconnected by the soft C–H⋯O hydrogen bonds and via π⋯π stacking interactions of the picrate ions.

The influence of ethylene glycol chains on the thermodynamics of hydrogen-bonded supramolecular assemblies in apolar solvents

Substitution of hydrogen bond directed supramolecular assemblies with ethylene glycol chains leads to a reduction in the association constant in apolar solvents, where the reduction of the association constant is dependent on the length of the aliphatic spacer connecting the hydrogen bonds and the ethylene glycol chain.

Tris(biimidazolato)chromium(III) as a building block for hydrogen-bonded supramolecular assemblies

The trischelate [Cr(H2biim)3](NO3)3complex of 2,2′-biimidazole (H2biim) was obtained by reacting CrCl3·3THF with [Ag(H2biim)](NO3) in methanol. In the solvent-free material, each ligand forms two N-H···O bonds to a nitrate ion and generates locally neutral [Cr(H2biim)3](NO3)3units. A methanol solvate was also obtained in which intermolecular interactions involve optimal use of the hydrogen-bonding ability of the [Cr(H2biim)3]3+cations, NO3–anions, and methanol molecules. In both cases, there is no long-range regular organization of the complex units. Deprotonation of [Cr(H2biim)3](NO3)3with NaOCH3yielded neutral Cr(Hbiim)3. Its powder pattern is similar to that of Ru(Hbiim)3, suggesting that it also consists of mutually perpendicular interlocked honeycomb sheets. Recrystallization by slow diffusion of diisopropyl ether into a methanol solution yielded a porous material of composition Cr(Hbiim)3·2.6C6H14O in which superposed honeycomb sheets create infinite channels (~13 Å diameter) filled with disordered solvent molecules. A totally different structure is adopted by the solvate Cr(Hbiim)3·C6H6·2H2O, where the benzene molecule is encapsulated in a cavity created by five complex molecules.Key words: chromium, biimidazole, supramolecular, crystal structure, hydrogen bonding.

Tunable interchromophore electronic interaction of a merocyanine dye in hydrogen-bonded supramolecular assemblies scaffolded by bismelamine receptors

The absorption and fluorescence properties of a barbiturate-type donor-pi-acceptor (D-pi-A) merocyanine dye are controlled by complexation with dimeric melamine receptors featuring different tether lengths.