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.

Acid⋠⋠⋠Amide Supramolecular Synthon for Tuning Amino Acid-Based Hydrogels' Properties.

Supramolecular hydrogels formed by the self-assembly of N-Fmoc-l-phenylalanine derivatives are gaining relevance for several applications in the materials and biomedical fields. In the challenging attempt to predict or tune their properties, we selected Fmoc-pentafluorophenylalanine (1) as a model efficient gelator, and studied its self-assembly in the presence of benzamide (2), a non-gelator able to form strong hydrogen bonds with the amino acid carboxylic group. Equimolar mixtures of 1 and 2 in organic solvents afforded a 1 : 1 co-crystal thanks to the formation of an acid⋅⋅⋅amide heterodimeric supramolecular synthon. The same synthon occurred in the transparent gels formed by mixing the two components in 1 : 1 ratio in aqueous media, as revealed by structural, spectroscopic, and thermal characterizations performed on both the co-crystal powder and the lyophilized hydrogel. These findings revealed the possibility of modulating the properties of amino acid-based hydrogels by involving the gelator in the formation of a co-crystal. Such a crystal engineering-based approach is shown also to be useful for the time-delayed release of suitable bioactive molecules, when involved as hydrogel coformers.

Acid⋠⋠⋠Amide Supramolecular Synthon for Tuning Amino Acid-Based Hydrogels' Properties.

Invited for the cover of this issue is the Laboratory of Supramolecular and Bio-Nanomaterials, coordinated by Pierangelo Metrangolo, at the Politecnico di Milano, Italy. The image depicts the co-crystal formed by N-Fmoc-pentafluorophenylalanine and benzamide, which is also involved in the formation of their mixed hydrogels. Read the full text of the article at 10.1002/chem.202301743.

The N,N,N-trimethylammonium moiety as tetrel bond donor site: crystallographic and computational studies.

Five structures bearing the N,N,N-trimethylammonium unit have been investigated to address the ability of the N+-CH3 unit to function as a tetrel bond donor site. Charged and neutral electron density donors display close contacts with different carbon atoms of methyl groups on the ammonium moiety. The Hirshfeld atom refinement (HAR) technique was used on selected structures to accurately and precisely determine the hydrogen atom positions and, consequently, to get better insights into the N+-C⋯Nu (Nu = nucleophile) interactions occurring in the crystals. In particular, the performed analyses highlighted specific geometrical features of the moieties involved in the interactions and allowed distinguishing between tetrel and hydrogen bonds.

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.

Tunable Linear and Nonlinear Optical Properties from Room Temperature Phosphorescent Cyclic Triimidazole-Pyrene Bio-Probe.

Organic materials with multiple emissions tunable by external stimuli represent a great challenge. TTPyr, crystallizing in different polymorphs, shows a very rich photophyisics comprising excitation-dependent fluorescence and phosphorescence at ambient conditions, and mechanochromic and thermochromic behavior. Transformation among the different species has been followed by thermal and X-ray diffraction analyses and the emissive features interpreted through structural results and DFT/TDDFT calculations. Particularly intriguing is the polymorph TTPyr(HT), serendipitously obtained at high temperature but stable also at room temperature, whose non-centrosymmetric structure guarantees an SHG efficiency 10 times higher than that of standard urea. Its crystal packing, where only the TT units are strongly rigidified by π-π stacking interactions while the Pyr moieties possess partial conformational freedom, is responsible for the observed dual fluorescence. The potentialities of TTPyr for bioimaging have been successfully established.

Anion⋠⋠⋠Anion Interactions Involving σ-Holes of Perrhenate, Pertechnetate and Permanganate Anions.

In this communication experimental and theoretical results are reported affording strong evidence that interactions between electron rich atoms and the metal of tetroxide anions of group 7 elements are a new case of attractive and σ-hole interactions. Single crystal X-ray analyses, molecular electrostatic potentials, quantum theory of atoms-in-molecules, and noncovalent interaction plot analyses show that in crystalline permanganate and perrhenate salts the metal in Mn/ReO4- anion can act as electron acceptors, the oxygen of another Mn/ReO4- anion can act as the donor and supramolecular anionic dimers or polymers are formed. The name matere bond (MaB) is proposed to categorize these noncovalent interactions and to differentiate them from the classical metal-ligand coordination bond.

A Step toward the Quantification of Noncovalent Interactions in Large Biological Systems: The Independent Gradient Model-Extremely Localized Molecular Orbital Approach.

The independent gradient model (IGM) is a recent electron density-based computational method that enables to detect and quantify covalent and noncovalent interactions. When applied to large systems, the original version of the technique still relies on promolecular electron densities given by the sum of spherically averaged atomic electron distributions, which leads to approximate evaluations of the inter- and intramolecular interactions occurring in systems of biological interest. To overcome this drawback and perform IGM analyses based on quantum mechanically rigorous electron densities also for macromolecular systems, we coupled the IGM approach with the recently constructed libraries of extremely localized molecular orbitals (ELMOs) that allow fast and reliable reconstructions of polypeptide and protein electron densities. The validation tests performed on small polypeptides and peptide dimers have shown that the novel IGM-ELMO strategy provides results that are systematically closer to the fully quantum mechanical ones and outperforms the IGM method based on the crude promolecular approximation, but still keeping a quite low computational cost. The results of the test calculations carried out on proteins have also confirmed the trends observed for the IGM analyses conducted on small systems. This makes us envisage the future application of the novel IGM-ELMO approach to unravel complicated noncovalent interaction networks (e.g., in protein-protein contacts) or to rationally design new drugs through molecular docking calculations and virtual high-throughput screenings.

Anion⋠⋠⋠Anion Coinage Bonds: The Case of Tetrachloridoaurate.

Interactions in crystalline tetrachloridoaurates of acetylcholine and dimethylpropiothetine are characterized by Au⋅⋅⋅Cl and Au⋅⋅⋅O short contacts. The former interactions assemble the AuCl4 - units into supramolecular anionic polymers, while the latter interactions append the acetylcholine and propiothetine units to the polymer. The distorted octahedral geometry of the bonding pattern around the gold center is rationalized on the basis of the anisotropic distribution of the electron density, which enables gold to behave as an electrophile (π-hole coinage-bond donor). Computational studies prove that gold atoms in negatively charged species can function as acceptors of electron density. The attractive nature of the Au⋅⋅⋅Cl/O interactions described here complement the known aurophilic bonds involved in gold-centered interactions.

Molecular Electrostatic Potential and Noncovalent Interactions in Derivatives of Group 8 Elements.

This communication reports experimental and theoretical evidences of σ-hole interactions in adducts between nitrogen or oxygen nucleophiles and tetroxides of osmium or other group 8 elements. Cocrystals between pyridine or pyridine N-oxide derivatives and osmium tetroxide are characterized through various techniques and rationalized as σ-hole interactions using DFT calculations and several other computational tools. We propose the term "osme bond" (OmB, Om=Fe, Ru, Os, (Hs)) for naming the noncovalent interactions wherein group 8 elements have the role of the electrophile. The word osme is the transcription of ὀσµÎ®, the ancient Greek word for smell that was used to name the heaviest group 8 element in relation to the smoky odor of its tetroxide.

The Chalcogen Bond in Crystalline Solids: A World Parallel to Halogen Bond.

The distribution of the electron density around covalently bonded atoms is anisotropic, and this determines the presence, on atoms surface, of areas of higher and lower electron density where the electrostatic potential is frequently negative and positive, respectively. The ability of positive areas on atoms to form attractive interactions with electron rich sites became recently the subject of a flurry of papers. The halogen bond (HaB), the attractive interaction formed by halogens with nucleophiles, emerged as a quite common and dependable tool for controlling phenomena as diverse as the binding of small molecules to proteinaceous targets or the organization of molecular functional materials. The mindset developed in relation to the halogen bond prompted the interest in the tendency of elements of groups 13-16 of the periodic table to form analogous attractive interactions with nucleophiles. This Account addresses the chalcogen bond (ChB), the attractive interaction formed by group 16 elements with nucleophiles, by adopting a crystallographic point of view. Structures of organic derivatives are considered where chalcogen atoms form close contacts with nucleophiles in the geometry typical for chalcogen bonds. It is shown how sulfur, selenium, and tellurium can all form chalcogen bonds, the tendency to give rise to close contacts with nucleophiles increasing with the polarizability of the element. Also oxygen, when conveniently substituted, can form ChBs in crystalline solids. Chalcogen bonds can be strong enough to allow for the interaction to function as an effective and robust tool in crystal engineering. It is presented how chalcogen containing heteroaromatics, sulfides, disulfides, and selenium and tellurium analogues as well as some other molecular moieties can afford dependable chalcogen bond based supramolecular synthons. Particular attention is given to chalcogen containing azoles and their derivatives due to the relevance of these moieties in biosystems and molecular materials. It is shown how the interaction pattern around electrophilic chalcogen atoms frequently recalls the pattern around analogous halogen, pnictogen, and tetrel derivatives. For instance, directionalities of chalcogen bonds around sulfur and selenium in some thiazolium and selenazolium derivatives are similar to directionalities of halogen bonds around bromine and iodine in bromonium and iodonium compounds. This gives experimental evidence that similarities in the anisotropic distribution of the electron density in covalently bonded atoms translates in similarities in their recognition and self-assembly behavior. For instance, the analogies in interaction patterns of carbonitrile substituted elements of groups 17, 16, 15, and 14 will be presented. While the extensive experimental and theoretical data available in the literature prove that HaB and ChB form twin supramolecular synthons in the solid, more experimental information has to become available before such a statement can be safely extended to interactions wherein elements of groups 14 and 15 are the electrophiles. It will nevertheless be possible to develop some general heuristic principles for crystal engineering. Being based on the groups of the periodic table, these principles offer the advantage of being systematic.

The Relevance of Size Matching in Self-assembly: Impact on Regio- and Chemoselective Cocrystallizations.

Decamethonium diiodide is reported to perform the chemo- and regioselective encapsulation of para-dihalobenzenes through the competitive formation of halogen-bonded cocrystals starting from solutions that also contain ortho and meta isomers. Selective caging in the solid occurs even when an excess ortho or meta isomers, or even a mixture of them, is present in the solution. A prime matching between the size and shape of the dication and the formed dianions plays a key role in enabling the selective self-assembly, as proven by successful encapsulation of halogen-bond donors as weak as 1,4-dichlorobenzene and by the results of cocrystallization trials involving mismatching tectons. Encapsulated para-dihalobenzenes guest molecules can be removed quantitatively by heating the cocrystals under reduced pressure and be recovered as pure materials. The residual decamethonium diiodide can be recycled with no reduction in selectivity.

Radicalradical chalcogen bonds: CSD analysis and DFT calculations.

This manuscript reports a combination of crystallographic analysis (Cambridge Structural Database) and theoretical DFT calculations in chalcogen bonding interactions involving radicals in both the Ch bond (ChB) donor and acceptor. As a radical ChB acceptor (nucleophile) we have used benzodithiazolyl radical (BDTA) and as Ch bond donors (electrophile) we have used dithiadiazolyl and diselenadiazolyl radicals of the general formula p-X-C6F4-CNChChN (Ch = S, and Se). We have evaluated how the para substituent (X) affects the interaction energy, spin density and charge/spin transfer from the electron rich BDTA radical to the electron poor dichalcogenadiazolyl ring. The ability of the latter rings to form ChBs in the solid state has been examined by a comprehensive search in the CSD; several cases are used to exemplify the preferred geometric features of the complexes and they are compared with the theory. The molecular surface electrostatic potentials calculated for these ChB donors allow for a very precise rationalization of the self-assembly motifs most frequently adopted in the crystalline state and of their relative robustness.

BODIPY Dyes Bearing Multibranched Fluorinated Chains: Synthesis, Structural, and Spectroscopic Studies.

A small series of boron-dipyrromethene (BODIPY) dyes, characterized by the presence of multibranched fluorinated residues, were designed and synthesized. The dyes differ in both the position (para-perfluoroalkoxy-substituted phenyl ring or boron functionalization) and number of magnetically equivalent fluorine atoms (27 or 54 fluorine atoms per molecule). Photophysical and crystallographic characterization of the synthesized BODIPYs was carried out to evaluate the effect of the presence of highly fluorinated moieties on the optical and morphological properties of such compounds.

Molecular Bases for Anesthetic Agents: Halothane as a Halogen- and Hydrogen-Bond Donor.

Although instrumental for optimizing their pharmacological activity, a molecular understanding of the preferential interactions given by volatile anesthetics is quite poor. This paper confirms the ability of halothane to work as a hydrogen-bond (HB) donor and gives the first experimental proof that halothane also works as a halogen-bond (HaB) donor in the solid state and in solution. A halothane/hexamethylphosphortriamide co-crystal is described and its single-crystal X-ray structure shows short HaBs between bromine, or chlorine, and the phosphoryl oxygen. New UV/Vis absorption bands appear upon addition of diazabicyclooctane and tetra(n-butyl)ammonium iodide to halothane solutions, indicating that nitrogen atoms and anions may mediate the HaB-driven binding processes involving halothane as well. The ability of halothane to work as a bidentate/tridentate tecton by acting as a HaB and HB donor gives an atomic rationale for the eudismic ratio shown by this agent.

Tight Xenon Confinement in a Crystalline Sandwich-like Hydrogen-Bonded Dimeric Capsule of a Cyclic Peptide.

A cyclic hexapeptide with three pyridyl moieties connected to its backbone forms a hydrogen-bonded dimer, which tightly encapsulates a single xenon atom, like a pearl in its shell. The dimer imprints its shape and symmetry to the captured xenon atom, as demonstrated by 129 Xe NMR spectroscopy, single-crystal X-ray diffraction, and computational studies. The dimers self-assemble hierarchically into tubular structures to form a porous supramolecular architecture, whose cavities are filled by small molecules and gases.

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.

Dicarboxylic Acid Separation by Dynamic and Size-Matched Recognition in Solution and in the Solid State.

Bis(trimethylammonium) alkane diiodides dynamically encapsulate dicarboxylic acids through intermolecular hydrogen bonds between the I- anions of the hosts and the carboxylic OH groups of the guests. A selective recognition is realized when the size of the I- ⋅⋅⋅HOOC(CH2 /CF2 )n COOH⋅⋅⋅I- superanion matches the dication alkyl chain length. Dynamic recognition is also demonstrated in solution, where the presence of the size-matching organic salt boosts the acid solubility profile, thus allowing efficient mixture separation.

Rotational Dynamics of Diazabicyclo[2.2.2]octane in Isomorphous Halogen-Bonded Co-crystals: Entropic and Enthalpic Effects.

Based on rotational dynamics measurements carried out with isomorphic co-crystals formed by halogen-bonding (XB) between tritylacetylene halides (TrX) and diazabicyclo[2.2.2]octane (dabco), we were able to distinguish the sources of the enthalpic and entropic components in the rotational free energy barrier. We describe the formation of the 1:1 co-crystals (TrX···N(R)3N) obtained from 1 equiv of dabco and 1 equiv of either TrI or TrBr, respectively, to give 4a and 4b instead of the potential 2:1 complexes. The co-crystals were prepared by solvent evaporation and mechanochemical synthesis. No co-crystal with TrCl was obtained, reflecting the weaker nature of the TrCl···NR3 interaction. Single-crystal X-ray diffraction confirmed structures that resemble a spinning top on a tripod and revealed that the two XB co-crystals are isomorphous, with slightly different C-X···NR3 (X = I, Br) distances and packing interactions. Quadrupolar-echo 2H NMR experiments with 2H-labeled samples showed that fast rotation of dabco in these co-crystals follows a six-fold potential energy surface with three lowest energy minima. Variable-temperature 1H NMR spin-lattice relaxation (VT 1H T1) data revealed rotational dynamics with indistinguishable pre-exponential factors and small but distinguishable activation energies. The activation energy of 4b (Ea = 0.71 kcal mol-1) is the lowest reported in the field of amphidynamic crystals. Using the Eyring equation, we established that their activation entropy for rotation is small but negative (ΔS⧧ = -3.0 cal mol-1 K-1), while there is almost a 2-fold difference in activation enthalpies, with 4a having a higher barrier (ΔH⧧ = 0.95 kcal mol-1) than 4b (ΔH⧧ = 0.54 kcal mol-1). Analysis of the rotator cavity in the two co-crystals revealed subtle differences in steric interactions that account for their different activation energies.