Cold photo-carving of halogen-bonded co-crystals of a dye and a volatile co-former using visible light.

The formation of co-crystals by the assembly of molecules with complementary molecular recognition functionalities is a popular strategy to design or improve a range of solid-state properties, including those relevant for pharmaceuticals, photo- or thermoresponsive materials and organic electronics. Here, we report halogen-bonded co-crystals of a fluorinated azobenzene derivative with a volatile component-either dioxane or pyrazine-that can be cut, carved or engraved with low-power visible light. This cold photo-carving process is enabled by the co-crystallization of a light-absorbing azo dye with a volatile component, which gives rise to materials that can be selectively disassembled with micrometre precision using low-power, non-burning laser irradiation or a commercial confocal microscope. The ability to shape co-crystals in three dimensions using laser powers of 0.5-20 mW-substantially lower than those used for metals, ceramics or polymers-is rationalized by photo-carving that targets the disruption of weak supramolecular interactions, rather than the covalent bonds or ionic structures targeted by conventional laser beam or focused ion beam machining processes.

Hypergolic zeolitic imidazolate frameworks (ZIFs) as next-generation solid fuels: Unlocking the latent energetic behavior of ZIFs.

Hypergolic materials, capable of spontaneous ignition upon contact with an external oxidizer, are of critical importance as fuels and propellants in aerospace applications (e.g., rockets and spacecraft). Currently used hypergolic fuels are highly energetic, toxic, and carcinogenic hydrazine derivatives, inspiring the search for cleaner and safer hypergols. Here, we demonstrate the first strategy to design hypergolic behavior within a metal-organic framework (MOF) platform, by using simple "trigger" functionalities to unlock the latent and generally not recognized energetic properties of zeolitic imidazolate frameworks, a popular class of MOFs. The herein presented six hypergolic MOFs, based on zinc, cobalt, and cadmium, illustrate a uniquely modular platform to develop hypergols free of highly energetic or carcinogenic components, in which varying the metal and linker components enables the modulation of ignition and combustion properties, resulting in excellent hypergolic response evident by ultrashort ignition delays as low as 2 ms.

A mechanochemical strategy for IRMOF assembly based on pre-designed oxo-zinc precursors.

We demonstrate a mechanochemical strategy that allowed the first successful mechanosynthesis of IRMOFs based on an oxo-centred secondary building unit (SBU). The presented study indicates that controlling the acid-base relationship between reagents is key to mechanochemical synthesis of IRMOFs, revealing a pre-assembled oxo-zinc amidate cluster as an efficient precursor for IRMOF mechanosynthesis.

Tunable recognition of the steroid alpha-face by adjacent pi-electron density.

We report a previously unknown recognition motif between the alpha-face of the steroid hydrocarbon backbone and pi-electron-rich aromatic substrates. Our study is based on a systematic and comparative analysis of the solid-state complexation of four steroids with 24 aromatic molecules. By using the solid state as a medium for complexation, we circumvent solubility and solvent competition problems that are inherent to the liquid phase. Characterization is performed using powder and single crystal X-ray diffraction, infrared solid-state spectroscopy and is complemented by a comprehensive cocrystal structure prediction methodology that surpasses earlier computational approaches in terms of realism and complexity. Our combined experimental and theoretical approach reveals that the alpha...pi stacking is of electrostatic origin and is highly dependent on the steroid backbone's unsaturated and conjugated character. We demonstrate that the alpha...pi stacking interaction can drive the assembly of molecules, in particular progesterone, into solid-state complexes without the need for additional strong interactions. It results in a marked difference in the solid-state complexation propensities of different steroids with aromatic molecules, suggesting a strong dependence of the steroid-binding affinity and even physicochemical properties on the steroid's A-ring structure. Hence, the hydrocarbon part of the steroid is a potentially important variable in structure-activity relationships for establishing the binding and signaling properties of steroids, and in the manufacture of pharmaceutical cocrystals.