Facile fabrication of recyclable robust noncovalent porous crystals from low-symmetry helicene derivative

Abstract

Porous frameworks constructed via noncovalent interactions show wide potential in molecular separation and gas adsorption. However, it remains a major challenge to prepare these materials from low-symmetry molecular building blocks. Herein, we report a facile strategy to fabricate noncovalent porous crystals through modular self-assembly of a low-symmetry helicene racemate. The P and M enantiomers in the racemate first stack into right- and left-handed triangular prisms, respectively, and subsequently the two types of prisms alternatively stack together into a hexagonal network with one-dimensional channels with a diameter of 14.5 Å. Remarkably, the framework reveals high stability upon heating to 275 °C, majorly due to the abundant π-interactions between the complementarily engaged helicene building blocks. Such porous framework can be readily prepared by fast rotary evaporation, and is easy to recycle and repeatedly reform. The refined porous structure and enriched π-conjugation also favor the selective adsorption of a series of small molecules.

Fig. 1. Molecular building blocks for noncovalent porous frameworks.

Noncovalent porous frameworks formed by various organic molecules including tris-o-phenylenedioxycyclotriphosphazene (TTP), tris(3,5-dipyridylphenyl)mesitylene (Py6Mes), benzene-1,3,5-triyltris(9H-carbazol-9-yl)methanone (PhTCz), 3,3′,4,4′-tetra(trimethylsilylethynyl)biphenyl (TMSBP), 2,8-di(10H-phenothiazin-10-yl)dibenzofuran (PBO), tetra(9-anthracyl-p-phenyl)methane (TAPM), 4,7-di(10-phenyl-10H-phenothiazin-3-yl)[1,2,5]thiadiazolo[3,4-c]pyridine (DPBT), N-(4-(9H-carbazol-9-yl)phenyl)phthalimide (PAICz). The abbreviations are adopted from the literatures^–.

Fig. 2. Single crystal structure of racemic D6H.

a Space-filling representation of the crystal. Carbon atoms of M- and P-D6H are marked in blue and red, respectively. CH₂Cl₂ and n-pentane molecules possibly existing in a disordered form were removed by solvent mask using Olex2. b Analysis on the modular self-assembly of racemic D6H into a hexagonal porous framework with 1D channels.

Fig. 3. Thermodynamic properties of D6H porous crystals.

a DSC curves of the crystals of racemic D6H obtained by solvent diffusion (scan rate = 10 °C/min). b Variable-temperature PXRD (Co Kα radiation, λ = 1.79021 Å) patterns of the crystals of racemic D6H obtained by solvent diffusion. c Space-filling representation of the crystals of racemic D6H obtained after melting and PXRD (Cu Kα radiation, λ = 1.54178 Å) patterns of the two types of crystals of racemic D6H. d Crystal diagrams and NCI maps showing the intermolecular interactions (denoted by arrows) between D6H molecules in a crystal of racemic D6H obtained by solvent diffusion.

Fig. 4. Facile preparation of powder of D6H porous crystals.

a Schematic illustration showing the preparation of single crystals of racemic D6H via slow solvent diffusion and the facile preparation of powder of small crystals of racemic D6H via fast rotary evaporation. b, c Corresponding SEM images and PXRD (Cu Kα radiation, λ = 1.54178 Å) patterns. d TGA curves of the powder of racemic D6H dried from CH₂Cl₂ before and after activation (scan rate = 10 °C/min) and SEM image of the activated powder.

Fig. 5. Adsorption performance of the activated powder of racemic D6H.

a Adsorption isotherms of N₂, CO₂ and THF vapor. b Schematic illustration of an adsorption experiment where the activated powder of racemic D6H is exposed to various solvent vapors for 3 days. c TGA curves of the activated powder of racemic D6H before and after the adsorption of various solvents (scan rate = 10 °C/min). The adsorption capacities are listed in the inserted table. d Crystal diagrams of THF@D6H and NCI map for the intermolecular interactions between THF and D6H molecules. e Adsorption of THF by the activated powder of racemic D6H upon multiple adsorption-desorption cycles. Insets are the corresponding SEM images of the powder sampled from the adsorption-desorption cycles. f ¹H NMR (500 MHz, CDCl₃) spectra of the activated powder of racemic D6H after adsorption of DOX, CYH and a blend vapor of DOX with CYH.