Azobenzene-based difunctional halogen-bond donor: towards the engineering of photoresponsive co-crystals

Modulating photoswitch performance with halogen, coordinative and hydrogen bonding: a comparison of relative bond strengths

The behavior of an enediyne photoswitch is modulated with halogen bonding, coordinative bonding and hydrogen bonding. Through NMR and computational studies we demonstrate that the relative strength of the secondary bonding directly influences the rate of photoisomerization and the photostationary state.

Visible light-triggered gel-to-sol transition in halogen-bond-based supramolecules

Photoresponsive supramolecular gels have aroused continuous attention because of their extensive applications; however, most studies utilize UV light, which inevitably brings about some health and environmental issues. The halogen bond is an important driving force for constructing supramolecules due to its high directionality, tunable strength, good hydrophobicity, and large size of the halogen atoms. Yet, it still remains a formidable challenge to utilize halogen bonds as a driving force to fabricate a visible light responsive gel. In this work, to fabricate such a gel, azopyridine-containing Azopy-C_n (n = 8, 10, 12) was selected as a halogen bond acceptor, while 1,2-bis(2,3,5,6-tetrafluoro-4-iodophenyl)diazene (BTFIPD) was chosen as both the halogen bond donor and visible light responsive moiety. The visible light response of BTFIPD resulted from the significant separation of n-π* energy levels between trans and cis isomers due to the introduction of an electron-withdrawing group (fluorine) to azobenzene at the ortho-position. Interestingly, the gel exhibited a good gel-to-sol transition behavior upon green light irradiation. At the same time, the morphologies varied from uniform narrow flakes to broad sheets with increasing illumination time. We provide an environmentally-friendly visible light-triggered method to regulate the phase transition of supramolecular materials in applications ranging from energy conversion to information storage.

[2+2] Halogen-bonded boxes employing azobenzenes

Herein, we report the synthesis and crystal structures of three [2+2] supramolecular boxes assembled by halogen bonding.

Crystal structures of the solvent-free and ethanol disolvate forms of 4,4'-(diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid) exemplifying self-stabilized azo-benzene cis-configurations

cis-4,4'-(Diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid), C14H2F8N2O4, and its ethanol disolvate, C14H2F8N2O4·2C2H5OH, represent new examples of self-stabilized cis-configured azo-benzenes obtained by a common crystallization procedure at room temperature under normal laboratory lighting conditions. The target structure constitutes of two 2,3,5,6-tetra-fluoro-benzoic acid residues linked to each other by a cis-configured azo group and was confirmed for two isolated specimens extracted from the same sample, corresponding to a solvent-free form and an ethanol disolvate. In the solvent-free form, the mol-ecule is characterized by rotational symmetry around a twofold rotation axis bis-ecting its central N=N bond while this symmetry is not present in the solvated form. The values of the inclination angles of the terminal carboxyl groups towards the corresponding benzene rings vary from 5.2 (4) to 45.7 (2)°, depending on the crystal composition. In the unsolvated form, the mol-ecules are linked through identical hydrogen bonds with a classical R 2 2(8) graph-set ring motif of carb-oxy-lic acids, by generating supra-molecular chains running approximately parallel to [101]. The presence of ethanol in the solvated form also leads to changes in the short-contact pattern to produce both the R 4 4(12) ring and open-chain motifs with alternating alcohol and di-carb-oxy-lic acid mol-ecules.

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.

Azo⋯phenyl stacking: a persistent self-assembly motif guides the assembly of fluorinated cis-azobenzenes into photo-mechanical needle crystals

We describe a novel, persistent motif of molecular assembly in photo-mechanical crystals and cocrystals of fluorinated cis-azobenzenes.

Coordination networks incorporating halogen-bond donor sites and azobenzene groups

A coordination network decorated with halogen-bond donor sites for specific guest binding.

Photo-induced motion of azo dyes in organized media: from single and liquid crystals, to MOFs and machines

Incorporation of photo-reversible azo dyes into molecular assemblies provides a new family of advanced optical and photo-mechanical materials that enable the direct transformation of light energy into mechanical motion.

Photoresponsive liquid crystals based on halogen bonding of azopyridines

A series of photoresponsive halogen-bonded liquid crystals (LCs) were successfully constructed using molecular halogen and azopyridine compounds, which show interesting properties of photoinduced phase transition upon UV irradiation. In addition, bromine-bonded LCs were first obtained with high mesophase stability.

Halogen bonding in polymer science: from crystal engineering to functional supramolecular polymers and materials

The applications of halogen bonding in surface functionalization, soft, luminescent and magnetic materials, interpenetrated networks, synthetic methods, and separation and inclusion techniques are reviewed.

Photo-mechanical azobenzene cocrystals and in situ X-ray diffraction monitoring of their optically-induced crystal-to-crystal isomerisation

We demonstrate the first supramolecular cocrystallisation strategy for generating crystalline azobenzene materials with a range of photo-mechanical and thermochemical properties: from those that exhibit isomerisation without any change in crystal shape to those that undergo a crystal-to-crystal cis–trans isomerisation accompanied by large scale bending.