Photoresponsive AA/BB supramolecular polymers comprising stiff-stilbene based guests and bispillar[5]arenes

We report a novel photoresponsive AA/BB supramolecular polymer comprising stiff-stilbene bridged guests and disulfide-bridged bispillar[5]arenes.

Supramolecular control of liquid crystals by doping with halogen-bonding dyes

Halogen bonding between photoactive guest dopants and liquid-crystal host enhances the optical performance of doped liquid crystals.

Polyimides with Heavy Halogens Exhibiting Room-Temperature Phosphorescence with Very Large Stokes Shifts

Semiaromatic polyimides (PIs) and imide compounds containing heavy halogens (Br, I) in pyromellitic moieties were designed and synthesized to examine their photoluminescence properties. Solutions of imides and PI films exhibited reddish-color room-temperature phosphorescence (RTP) with very large Stokes shifts (ca. 10000 cm^-1). In addition, the PI films showed small-Stokes-shifted fluorescence emissions at around 540 nm with absorption bands arising from aggregated PI chains at 400-500 nm. Enhanced phosphorescence observed for the PI films under vacuum indicates that the RTP lifetime is significantly influenced by the triplet-triplet energy transfer to atmospheric oxygen. These PIs with very-large-Stokes-shifted RTP are applicable as spectral conversion materials in displays, photovoltaic devices, and crop cultivators, as well as to oxygen/air sensors.

Quantitative Assessment of Halogen Bonding Utilizing Vibrational Spectroscopy

A total of 202 halogen-bonded complexes have been studied using a dual-level approach: ωB97XD/aug-cc-pVTZ was used to determine geometries, natural bond order charges, charge transfer, dipole moments, electron and energy density distributions, vibrational frequencies, local stretching force constants, and relative bond strength orders n. The accuracy of these calculations was checked for a subset of complexes at the CCSD(T)/aug-cc-pVTZ level of theory. Apart from this, all binding energies were verified at the CCSD(T) level. A total of 10 different electronic effects have been identified that contribute to halogen bonding and explain the variation in its intrinsic strength. Strong halogen bonds are found for systems with three-center-four-electron (3c-4e) bonding such as chlorine donors in interaction with substituted phosphines. If halogen bonding is supported by hydrogen bonding, genuine 3c-4e bonding can be realized. Perfluorinated diiodobenzenes form relatively strong halogen bonds with alkylamines as they gain stability due to increased electrostatic interactions.

Efficient Light-Induced Phase Transitions in Halogen-Bonded Liquid Crystals

Here, we present a new family of light-responsive, fluorinated supramolecular liquid crystals (LCs) showing efficient and reversible light-induced LC-to-isotropic phase transitions. Our materials design is based on fluorinated azobenzenes, where the fluorination serves to strengthen the noncovalent interaction with bond-accepting stilbazole molecules, and increase the lifetime of the cis-form of the azobenzene units. The halogen-bonded LCs were characterized by means of X-ray diffraction, hot-stage polarized optical microscopy, and differential scanning calorimetry. Simultaneous analysis of light-induced changes in birefringence, absorption, and optical scattering allowed us to estimate that <4% of the mesogenic units in the cis-form suffices to trigger the full LC-to-isotropic phase transition. We also report a light-induced and reversible crystal-to-isotropic phase transition, which has not been previously observed in supramolecular complexes. In addition to fundamental understanding of light-responsive supramolecular complexes, we foresee this study to be important in the development of bistable photonic devices and supramolecular actuators.

Substituent Effects on the [N-I-N](+) Halogen Bond

We have investigated the influence of electron density on the three-center [N–I–N]⁺ halogen bond. A series of [bis(pyridine)iodine]⁺ and [1,2-bis((pyridine-2-ylethynyl)benzene)iodine]⁺ BF₄^– complexes substituted with electron withdrawing and donating functionalities in the para-position of their pyridine nitrogen were synthesized and studied by spectroscopic and computational methods. The systematic change of electron density of the pyridine nitrogens upon alteration of the para-substituent (NO₂, CF₃, H, F, Me, OMe, NMe₂) was confirmed by ¹⁵N NMR and by computation of the natural atomic population and the π electron population of the nitrogen atoms. Formation of the [N–I–N]⁺ halogen bond resulted in >100 ppm ¹⁵N NMR coordination shifts. Substituent effects on the ¹⁵N NMR chemical shift are governed by the π population rather than the total electron population at the nitrogens. Isotopic perturbation of equilibrium NMR studies along with computation on the DFT level indicate that all studied systems possess static, symmetric [N–I–N]⁺ halogen bonds, independent of their electron density. This was further confirmed by single crystal X-ray diffraction data of 4-substituted [bis(pyridine)iodine]⁺ complexes. An increased electron density of the halogen bond acceptor stabilizes the [N···I···N]⁺ bond, whereas electron deficiency reduces the stability of the complexes, as demonstrated by UV-kinetics and computation. In contrast, the N–I bond length is virtually unaffected by changes of the electron density. The understanding of electronic effects on the [N–X–N]⁺ halogen bond is expected to provide a useful handle for the modulation of the reactivity of [bis(pyridine)halogen]⁺-type synthetic reagents.

Remarkable Role of C-I···N Halogen Bonding in Thixotropic 'Halo'gel Formation

Halogen-bonding-induced self-assembly in the solution of equimolar mixtures of certain pyridyl-ended oligo p-phenylenevinylene (OPV)-derivatives with 1,4-diiodotetrafluorobenzene is reported. The mode of self-assembly, that is, cocrystallization, thixotropic gelation, or precipitation, depends strongly on the nature of chains (n-alkyl chains as a function of length or short oxyethylene chain) appended to the OPV-backbone as well as on the cooling rate of the corresponding hot solution. Single-crystal X-ray diffraction studies of the cocrystals reveal the "infinite" chain formation via C-I···N halogen-bonding interactions between the two components. In addition, multiple noncovalent interactions induce cross-links among these halogen-bonded "infinite" chains. Interestingly, the molecular packing in the "Cogel" bearing OPV-derivative with oxyethylene chains is found to be very similar to that of the cocrystal of the same.

σ-Hole Bond vs π-Hole Bond: A Comparison Based on Halogen Bond

The σ-hole and π-hole are the regions with positive surface electrostatic potential on the molecule entity; the former specifically refers to the positive region of a molecular entity along extension of the Y-Ge/P/Se/X covalent σ-bond (Y = electron-rich group; Ge/P/Se/X = Groups IV-VII), while the latter refers to the positive region in the direction perpendicular to the σ-framework of the molecular entity. The directional noncovalent interactions between the σ-hole or π-hole and the negative or electron-rich sites are named σ-hole bond or π-hole bond, respectively. The contributions from electrostatic, charge transfer, and other terms or Coulombic interaction to the σ-hole bond and π-hole bond were reviewed first followed by a brief discussion on the interplay between the σ-hole bond and the π-hole bond as well as application of the two types of noncovalent interactions in the field of anion recognition. It is expected that this review could stimulate further development of the σ-hole bond and π-hole bond in theoretical exploration and practical application in the future.

Can an entirely negative fluorine in a molecule, viz. perfluorobenzene, interact attractively with the entirely negative site(s) on another molecule(s)? Like liking like!

The study presents the possibility for the formation of attractive intermolecular interactions between various entirely negative Lewis bases, leading to the formation of the thirteen isolated binary complexes examined.

Trifluoromethyl nitrogen heterocycles: synthetic aspects and potential biological targets

The synthetic methodologies and the potential biological targets of α-trifluoromethylated nitrogen heterocycles are presented.

Electrochemical activation of a tetrathiafulvalene halogen bond donor in solution

Electrochemistry: a powerful tool for probing and controlling halogen bonding in solution.

A PGSE NMR approach to the characterization of single and multi-site halogen-bonded adducts in solution

The diffusional PGSE NMR technique allows a detailed characterization of single- and multi-site halogen-bonded systems.

Halogen bonding in a multi-connected 1,2,2-triiodo-alkene involving geminal and/or vicinal iodines: a crystallographic and DFT study

Four halogen-bonded organizations of a 1,2,2-triiodo-alkene involving geminal and/or vicinal iodine atoms were studied both by X-ray diffraction and density functional theory (DFT).

One “Click” access to self-complementary molecular modules for halogen bonding

The application of “click chemistry” has proved its efficacy for the construction of novel molecular modules for halogen bonding driven self-assembly.

A new water-soluble aromatic polyamide hydrogelator with thixotropic properties

The water-soluble aromatic polyamide poly(3-sodium sulfo-p-phenylene terephthalamide) forms a hydrogel with anisotropy, which exhibits good thixotropic behaviour, even at the critical gel concentration of the gelator (1.0 wt%).

Halogen bonding-enhanced electrochemical halide anion sensing by redox-active ferrocene receptors

The first redox-active halogen bonding anion receptors display larger electrochemical voltammetric responses to halide binding compared to their hydrogen bonding analogues.