Perovskite-Like Liquid-Crystalline Materials Based on Polyfluorinated Imidazolium Cations.

Hybrid Organic-Inorganic Halide Perovskites (HOIHPs) represent an emerging class of semiconducting materials, widely employed in a variety of optoelectronic applications. Despite their skyrocket growth in the last decade, a detailed understanding on their structure-property relationships is still missing. In this communication, we report two unprecedented perovskite-like materials based on polyfluorinated imidazolium cations. The two materials show thermotropic liquid crystalline behavior resulting in the emergence of stable mesophases. The manifold intermolecular F ⋅ ⋅ ⋅ F interactions are shown to be meaningful for the stabilization of both the solid- and liquid-crystalline orders of these perovskite-like materials. Moreover, the structure of the incorporated imidazolium cation was found to tune the properties of the liquid crystalline phase. Collectively, these results may pave the way for the design of a new class of halide perovskite-based soft materials.

Halogen Bonding in Perovskite Solar Cells: A New Tool for Improving Solar Energy Conversion.

Hybrid organic-inorganic halide perovskites (HOIHPs) have recently emerged as a flourishing area of research. Their easy and low-cost production and their unique optoelectronic properties make them promising materials for many applications. In particular, HOIHPs hold great potential for next-generation solar cells. However, their practical implementation is still hindered by their poor stability in air and moisture, which is responsible for their short lifetime. Optimizing the chemical composition of materials and exploiting non-covalent interactions for interfacial and defects engineering, as well as defect passivation, are efficient routes towards enhancing the overall efficiency and stability of perovskite solar cells (PSCs). Due to the rich halogen chemistry of HOIHPs, exploiting halogen bonding, in particular, may pave the way towards the development of highly stable PSCs. Improved crystallization and stability, reduction of the surface trap states, and the possibility of forming ordered structures have already been preliminarily demonstrated.

Comparing the Halogen Bond to the Hydrogen Bond by Solid-State NMR Spectroscopy: Anion Coordinated Dimers from 2- and 3-Iodoethynylpyridine Salts.

Halogen bonding is an increasingly important tool in crystal engineering, and measuring its influence on the local chemical and electronic environment is necessary to fully understand this interaction. Here, we present a systematic crystallographic and solid-state NMR study of self-complementary halogen-bonded frameworks built from the halide salts (HCl, HBr, HI, HI3 ) of 2-iodoethynylpyridine and 3-iodoethynylpyridine. A series of single crystal X-ray structures reveals the formation of discrete charged dimers in the solid state, directed by simultaneous X- ⋅⋅⋅H-N+ hydrogen bonds and C-I⋅⋅⋅X- halogen bonds (X=Cl, Br, I). Each compound was studied using multinuclear solid-state magnetic resonance spectroscopy, observing 1 H to investigate the hydrogen bonds and 13 C, 35 Cl, and 79/81 Br to investigate the halogen bonds. A natural localized molecular orbital analysis was employed to help interpret the experimental results. 1 H SSNMR spectroscopy reveals a decrease in the chemical shift of the proton participating in the hydrogen bond as the halogen increases in size, whereas the 13 C SSNMR reveals an increased 13 C chemical shift of the C-I carbon for C-I⋅⋅⋅X- relative to C-I⋅⋅⋅N halogen bonds. Additionally, 35 Cl and 79/81 Br SSNMR, along with computational results, have allowed us to compare the C-I⋅⋅⋅X- halogen bond involving each halide in terms of NMR observables. Due to the isostructural nature of these compounds, they are ideal cases for experimentally assessing the impact of different halogen bond acceptors on the solid-state NMR response.

The Halogen Bond.

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Crystal Structure of the DFNKF Segment of Human Calcitonin Unveils Aromatic Interactions between Phenylalanines.

Although intensively studied, the high-resolution crystal structure of the peptide DFNKF, the core-segment of human calcitonin, has never been described. Here we report how the use of iodination as a strategy to promote crystallisation and facilitate phase determination, allowed us to solve, for the first time, the single-crystal X-ray structure of a DFNKF derivative. Computational studies suggest that both the iodinated and the wild-type peptides populate very similar conformations. Furthermore, the conformer found in the solid-state structure is one of the most populated in solution, making the crystal structure a reliable model for the peptide in solution. The crystal structure of DFNKF(I) confirms the overall features of the amyloid cross-ß spine and highlights how aromatic-aromatic interactions are important structural factors in the self-assembly of this peptide. A detailed analysis of such interactions is reported.

Photoresponsive ionic liquid crystals assembled via halogen bond: en route towards light-controllable ion transporters.

We demonstrate that halogen bonding (XB) can offer a novel approach for the construction of photoresponsive ionic liquid crystals. In particular, we assembled two new supramolecular complexes based on 1-ethyl-3-methylimidazolium iodides and azobenzene derivatives containing an iodotetrafluoro-benzene ring as XB donor, where the iodide anion acted as an XB acceptor. DSC and X-ray diffraction analyses revealed that the preferred stoichiometry between the XB donors and acceptors is 2 : 1, and that the iodide anions act as bidentate XB-acceptors, binding two azobenzene derivatives. Due to the high directionality of the XB, calamitic superanions are obtained, while the segregation occurring between the charged and uncharged parts of the molecules gives rise to a layered structure in the crystal lattice. Despite the fact that the starting materials are non-mesomorphic, the halogen-bonded supramolecular complexes exhibited monotropic lamellar liquid-crystalline phases over broad temperature ranges, as confirmed with polarized optical microscopy. Due to the presence of the azobenzene moieties, the LCs were photoresponsive, and a LC-to-isotropic phase transition could be obtained by irradiation with UV light. We envisage that the light-induced phase transition, in combination with the ionic nature of the LC, provides a route towards light-induced control over ion transport and conductance in these supramolecular complexes.

Halogen Bonding in Hypervalent Iodine Compounds.

Halogen bonds occur when electrophilic halogens (Lewis acids) attractively interact with donors of electron density (Lewis bases). This term is commonly used for interactions undertaken by monovalent halogen derivatives. The aim of this chapter is to show that the geometric features of the bonding pattern around iodine in its hypervalent derivatives justify the understanding of some of the longer bonds as halogen bonds. We suggest that interactions directionality in ionic and neutral λ(3)-iodane derivatives is evidence that the electron density distribution around iodine atoms is anisotropic, a region of most positive electrostatic potential exists on the extensions of the covalent bonds formed by iodine, and these positive caps affect, or even determine, the crystal packing of these derivatives. For instance, the short cation-anion contacts in ionic λ(3)-iodane and λ(5)-iodane derivatives fully match the halogen bond definition and geometrical prerequisites. The same holds for the short contacts the cation of ionic λ(3)-iodanes forms with lone-pair donors or the short contacts given by neutral λ(3)-iodanes with incoming nucleophiles. The longer and weaker bonds formed by iodine in hypervalent compounds are usually called secondary bondings and we propose that the term halogen bond can also be used. Compared to the term secondary bond, halogen bond may possibly be more descriptive of some bonding features, e.g., its directionality and the relationships between structure of interacting groups and interaction strength.

Superfluorinated Ionic Liquid Crystals Based on Supramolecular, Halogen-Bonded Anions.

Unconventional ionic liquid crystals in which the liquid crystallinity is enabled by halogen-bonded supramolecular anions [Cn F2 n+1 -I⋅⋅⋅I⋅⋅⋅I-Cn F2 n+1 ](-) are reported. The material system is unique in many ways, demonstrating for the first time 1) ionic, halogen-bonded liquid crystals, and 2) imidazolium-based ionic liquid crystals in which the occurrence of liquid crystallinity is not driven by the alkyl chains of the cation.

Halogen bond: a long overlooked interaction.

Because of their high electronegativity, halogen atoms are typically considered, in most of their derivatives, as sites of high electron density and it is commonly accepted that they can form attractive interactions by functioning as the electron donor site (nucleophilic site). This is the case when they work as hydrogen bond acceptor sites. However, the electron density in covalently bound halogens is anisotropically distributed. There is a region of higher electron density, accounting for the ability of halogens to function as electron donor sites in attractive interactions, and a region of lower electron density where the electrostatic potential is frequently positive (mainly in the heavier halogens). This latter region is responsible for the ability of halogen atoms to function as the electron-acceptor site (electrophilic site) in attractive interactions formed with a variety of lone pair-possessing atoms, anions, and π-systems. This ability is quite general and is shown by a wide diversity of halogenated compounds (e.g., organohalogen derivatives and dihalogens). According to the definition proposed by the International Union of Pure and Applied Chemistry, any attractive interactions wherein the halogen atom is the electrophile is named halogen bond (XB). In this chapter, it is discussed how the practice and the concept of XB developed and a brief history of the interaction is presented. Papers (either from the primary or secondary literature) which have reported major experimental findings in the field or which have given important theoretical contributions for the development of the concept are recollected in order to trace how a unifying and comprehensive categorization emerged encompassing all interactions wherein halogen atoms function as the electrophilic site.

Halogen-bonded photoresponsive materials.

The aim of the present review is to illustrate to the reader the state of the art on the construction of supramolecular azobenzene-containing materials formed by halogen bonding. These materials include several examples of polymeric, liquid crystalline or crystalline species whose performances are either superior to the corresponding performances of their hydrogen-bonded analogues or simply distinctive of the halogen-bonded species.

A superfluorinated molecular probe for highly sensitive in vivo(19)F-MRI.

(19)F-MRI offers unique opportunities to image diseases and track cells and therapeutic agents in vivo. Herein we report a superfluorinated molecular probe, herein called PERFECTA, possessing excellent cellular compatibility, and whose spectral properties, relaxation times, and sensitivity are promising for in vivo (19)F-MRI applications. The molecule, which bears 36 equivalent (19)F atoms and shows a single intense resonance peak, is easily synthesized via a simple one-step reaction and is formulated in water with high stability using trivial reagents and methods.

The halogen bond in the design of functional supramolecular materials: recent advances.

Halogen bonding is an emerging noncovalent interaction for constructing supramolecular assemblies. Though similar to the more familiar hydrogen bonding, four primary differences between these two interactions make halogen bonding a unique tool for molecular recognition and the design of functional materials. First, halogen bonds tend to be much more directional than (single) hydrogen bonds. Second, the interaction strength scales with the polarizability of the bond-donor atom, a feature that researchers can tune through single-atom mutation. In addition, halogen bonds are hydrophobic whereas hydrogen bonds are hydrophilic. Lastly, the size of the bond-donor atom (halogen) is significantly larger than hydrogen. As a result, halogen bonding provides supramolecular chemists with design tools that cannot be easily met with other types of noncovalent interactions and opens up unprecedented possibilities in the design of smart functional materials. This Account highlights the recent advances in the design of halogen-bond-based functional materials. Each of the unique features of halogen bonding, directionality, tunable interaction strength, hydrophobicity, and large donor atom size, makes a difference. Taking advantage of the hydrophobicity, researchers have designed small-size ion transporters. The large halogen atom size provided a platform for constructing all-organic light-emitting crystals that efficiently generate triplet electrons and have a high phosphorescence quantum yield. The tunable interaction strengths provide tools for understanding light-induced macroscopic motions in photoresponsive azobenzene-containing polymers, and the directionality renders halogen bonding useful in the design on functional supramolecular liquid crystals and gel-phase materials. Although halogen bond based functional materials design is still in its infancy, we foresee a bright future for this field. We expect that materials designed based on halogen bonding could lead to applications in biomimetics, optics/photonics, functional surfaces, and photoswitchable supramolecules.

Orthogonal halogen and hydrogen bonds involving a peptide bond model†Electronic supplementary information (ESI) available: Experimental part, DSC, IR spectroscopic and crystallographic data. CCDC 899779-899785. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ce01514bClick here for additional data file.Click here for additional data file.

The peptide bond model N-methylacetamide self-assembles with a range of dihalotetrafluorobenzenes forming co-crystals that all show the occurrence of orthogonal hydrogen and halogen bonds.

The 1:1 co-crystal of triphen-yl(2,3,5,6-tetra-fluoro-benz-yl)phospho-nium bromide and 1,1,2,2-tetra-fluoro-1,2-di-iodo-ethane.

The title compound, C25H18F4P(+)·Br(-)·C2F4I2, is a 1:1 co-crystal of triphen-yl(2,3,5,6-tetra-fluoro-benz-yl)phospho-nium (TTPB) bromide and 1,1,2,2-tetra-fluoro-1,2-di-iodo-ethane (TFDIE). The crystal structure consists of a framework of TTPB cations held together by C-H⋯Br inter-actions. In this framework, infinite channels along [100] are filled by TFDIE mol-ecules held together in infinite ribbons by short F⋯F [2.863 (2)-2.901 (2)Å] inter-actions. The structure contains halogen bonds (XB) and hydrogen bonds (HB) in the bromide coordination sphere. TFDIE functions as a monodentate XB donor as only one I atom is linked to the Br(-) anion and forms a short and directional inter-action [I⋯Br(-) 3.1798 (7) Šand C-I⋯Br(-) 177.76 (5)°]. The coordination sphere of the bromide anion is completed by two short HBs of about 2.8 Š(for H⋯Br) with the acidic methyl-ene H atoms and two longer HBs of about 3.0 Šwith H atoms of the phenyl rings. Surprisingly neither the second iodine atom of TFDIE nor the H atom on the tetra-fluoro-phenyl group make any short contacts.

Multinuclear Solid-State Magnetic Resonance as a Sensitive Probe of Structural Changes upon the Occurrence of Halogen Bonding in Co-crystals.

Although the understanding of intermolecular interactions, such as hydrogen bonding, is relatively well-developed, many additional weak interactions work both in tandem and competitively to stabilize a given crystal structure. Due to a wide array of potential applications, a substantial effort has been invested in understanding the halogen bond. Here, we explore the utility of multinuclear ((13)C, (14/15)N, (19)F, and (127)I) solid-state magnetic resonance experiments in characterizing the electronic and structural changes which take place upon the formation of five halogen-bonded co-crystalline product materials. Single-crystal X-ray diffraction (XRD) structures of three novel co-crystals which exhibit a 1:1 stoichiometry between decamethonium diiodide (i.e., [(CH3)3N(+)(CH2)10N(+)(CH3)3][2 I(-)]) and different para-dihalogen-substituted benzene moieties (i.e., p-C6X2Y4, X=Br, I; Y=H, F) are presented. (13)C and (15)N NMR experiments carried out on these and related systems validate sample purity, but also serve as indirect probes of the formation of a halogen bond in the co-crystal complexes in the solid state. Long-range changes in the electronic environment, which manifest through changes in the electric field gradient (EFG) tensor, are quantitatively measured using (14)N NMR spectroscopy, with a systematic decrease in the (14)N quadrupolar coupling constant (CQ) observed upon halogen bond formation. Attempts at (127)I solid-state NMR spectroscopy experiments are presented and variable-temperature (19)F NMR experiments are used to distinguish between dynamic and static disorder in selected product materials, which could not be conclusively established using solely XRD. Quantum chemical calculations using the gauge-including projector augmented-wave (GIPAW) or relativistic zeroth-order regular approximation (ZORA) density functional theory (DFT) approaches complement the experimental NMR measurements and provide theoretical corroboration for the changes in NMR parameters observed upon the formation of a halogen bond.

Halogen bonding and pharmaceutical cocrystals: the case of a widely used preservative.

3-Iodo-2-propynyl-N-butylcarbamate (IPBC) is an iodinated antimicrobial product used globally as a preservative, fungicide, and algaecide. IPBC is difficult to obtain in pure form as well as to handle in industrial products because it tends to be sticky and clumpy. Here, we describe the preparation of four pharmaceutical cocrystals involving IPBC. The obtained cocrystals have been characterized by X-ray diffraction, solution and solid-state NMR, IR, and DSC analyses. In all the described cases the halogen bond (XB) is the key interaction responsible for the self-assembly of the pharmaceutical cocrystals thanks to the involvement of the 1-iodoalkyne moiety of IPBC, which functions as a very reliable XB-donor, with both neutral and anionic XB-acceptors. Most of the obtained cocrystals have improved properties with respect to the source API, in terms, e.g., of thermal stability. The cocrystal involving the GRAS excipient CaCl2 has superior powder flow characteristics compared to the pure IPBC, representing a promising solution to the handling issues related to the manufacturing of products containing IPBC.

The halogen-bonded adduct 1,4-bis(pyri-din-4-yl)buta-1,3-diyne-1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-hexa-deca-fluoro-1,8-diiodo-octane (1/1).

In the crystal structure of the title compound, C8F16I2·C14H8N2, the mol-ecules form infinite chains parallel to [2-11] through two symmetry-independent C-I⋯N halogen bonds (XBs). As commonly found, the perfluoro-alkyl mol-ecules segregate from the hydro-carbon ones, forming a layered structure. Apart from the XBs, the only contact below the sum of van der Waals radii is a weak H⋯F contact. The topology of the network is a nice example of the paradigm of the expansion of ditopic starting modules; the XB leads to the construction of infinite supramolecular chains along [2-11] formed by alternating XB donors and acceptors.

Tetra-phenyl-phospho-nium iodide-1,3,5-tri-fluoro-2,4,6-tri-iodo-benzene-methanol (3/4/1).

The crystallization of a 1:1 molar solution of 1,3,5-tri-fluoro-2,4,6-di-iodo-benzene (TFTIB) and tetra-phenyl-phosponium iodide (TPPI) from methanol produced tetra-gonal needles of pure TPPI and tabular pseudo-hexa-gonal truncated bipyramids of the title compound, 3C24H20P(+)·3I(-)·4C6F3I3·CH4O or (TPPI)3(TFTIB)4·MeOH. The asymmetric unit is composed of six TPPI mol-ecules, eight TFTIB mol-ecules and two methanol mol-ecules, overall 16 constituents. The formation of the architecture is essentially guided by a number of C-I⋯I(-) halogen bonds (XB), whose lengths are in the range 3.276 (1)-3.625 (1) Å. Layers of supra-molecular polyanions are formed parallel to (10-1) wherein iodide anions function as penta-, tetra- or bidentate XB acceptors. The structure is not far from being P21/n, but the centrosymmetry is lost due to a different conformation of a single couple of cations and the small asymmetry in the formed supra-molecular anion. One methanol mol-ecule is hydrogen bonded to an iodide anion, while the second is linked to the first one via an O-H⋯O contact. This second methanol mol-ecule is more loosely pinned in its position than the first and presents very high anisotropic displacement parameters and a seeming shortening of the C-O bond length. The crystal studied was refined as a perfect inversion twin.

(Tris{2-[2-(2,3,5,6-tetra-fluoro-4-iodo-phen-oxy)eth-oxy]eth-yl}amine)-potassium iodide.

The title adduct, [K(C30H24F12I3NO6)]I, gives an extended tape of cations linked through I⋯I(-) halogen bonds (XBs), two of them being quite short and one quite long. In the structure, the cation is hosted in a cavity formed by the arms of the podand which presents a closed conformation wherein two tetra-fluoro-iodo-benzene rings are near parallel [dihedral angle = 15.8 (4)°; centroid-centroid distance = 3.908 (5) Å] and the third ring is closer to orthogonal [dihedral angles = 66.28 (14) and 75.20 (19)°] to the other two rings. The coordination sphere of the K(+) cation is composed of the six O atoms, the N atom and an F atom in the ortho position of one of the rings.

(4,7,13,16,21,24-Hexaoxa-1,10-di-aza-bicyclo-[8.8.8]hexa-cosa-ne)sodium iodide-1,1,2,2,tetra-fluoro-1,2-diiodo-ethane (2/3).

The title complex (CX1), [Na(C18H36N2O6)]I·1.5C2F4I2, is a three-component adduct containing a [2.2.2]-cryptand, sodium iodide and 1,1,2,2-tetra-fluoro-1,2-di-iodo-ethane. The di-iodo-ethane works as a bidentate halogen-bonding (XB) donor, the [2.2.2]-cryptand chelates the sodium cation, and the iodide counter-ion acts as a tridentate XB acceptor. A (6,3) network is formed in which iodide anions are the nodes and halocarbons the sides. The network symmetry is C 3i and the I⋯I(-) XB distance is 3.4492 (5) Å. This network is strongly deformed and wrinkled. It forms a layer 9.6686 (18) Šhigh and the inter-layer distance is 4.4889 (10) Å. The cations, inter-acting with each other via weak O⋯H hydrogen bonds, are confined between two anionic layers and also form a (6,3) net. The structure of CX1 is closely related to that of the KI homologue (CX2). The 1,1,2,2,-tetrafluoro-1,2-diiodoethane molecule is rotationally disordered around the I⋯I axis, resulting in an 1:1 disorder of the C2F4 moiety.