An exceptionally flexible hydrogen-bonded organic framework with large-scale void regulation and adaptive guest accommodation abilities

Abstract

Flexible hydrogen-bonded organic frameworks (FHOFs) are quite rare but promising for applications in separation, sensing and host-guest chemistry. They are difficult to stabilize, making their constructions a major challenge. Here, a flexible HOF (named 8PN) with permanent porosity has been successfully constructed. Nine single crystals of 8PN with different pore structures are obtained, achieving a large-scale void regulation from 4.4% to 33.2% of total cell volume. In response to external stimuli, multimode reversible structural transformations of 8PN accompanied by changes in luminescence properties have been realized. Furthermore, a series of high-quality co-crystals containing guests of varying shapes, sizes, aggregation states and even amounts are obtained, showing that 8PN can adapt to different guests by regulating the molecular conformations and assembling forms of its building blocks. The unexpected flexibility of 8PN makes it a promising material for enriching the applications of existing porous materials.

Fig. 1

The design strategy for 8PN. The flexible framework is constructed from TPE-4pn through hydrogen-bonding interactions

Fig. 2

Investigation of single-crystal structures of 8PN. a Packing diagram of 8PN-EA along the [010] direction, with the solvent-accessible void space visualized by gray/yellow (inner/outer) curved planes generated with a probe of 1.2 Å. Color code: green, C; yellow, N; Orange, O. H atoms are omitted for clarity. b Single-crystal X-ray structures of eight 8PN frameworks with varying void space. H atoms and solvent molecules in voids are omitted for clarity. Abbreviations for different solvents: ACT acetone, DMF N,N-dimethylformamide, EA ethyl acetate, TCM chloroform, THF tetrahydrofuran, DCM dichloromethane, TOL toluene. c Visualization of the S-shaped channel surface of 8PN-THF. d Void spaces, cell volumes, void ratios of the eight 8PN frameworks. e Illustration of large-scale regulation of the pore size by dihedral angles among different types of 8PN-200, 8PN-400, and 8PN-600. f Illustration of small-scale regulation of the pore size by hydrogen bond distances in the two frameworks within the same type, that is 8PN-ACT and 8PN-DMF in 8PN-200 type, 8PN-EA, and 8PN-2ACT in 8PN-400 type or 8PN-TCM and 8PN-THF in 8PN-600 type

Fig. 3

Single-crystal-to-single-crystal (SCSC) transformation determined by the single-crystal X-ray diffraction. a Structural transformations of the large pore (LP) form in 8PN-EA to the narrow pore (NP) form in 8PN-Heated accompanied by removal of solvent molecules. b Illustration of the significant changes in the dihedral angles between the plane of the ethylene core and the planes of two different phenyl rings in one arm of TPE-4pn in 8PN-EA and 8PN-Heated due to phenyl ring-rotation

Fig. 4

Characterization and multimode reversible transformations of porous structures of 8PN. a CO₂ (195 K) and N₂ (77 K) adsorption/desorption isotherms for 8PN-THF. b Differential scanning calorimetry (DSC) curve of 8PN-THF. c Variable-temperature powder X-ray diffraction (PXRD) patterns of 8PN-THF. d Electrostatic potential (ESP) diagram of the pore constructed by four TPE-4pn molecules in 8PN-THF indicating the nitro groups inside the pore present the less electron density (in yellow color) than the nitro groups outside the pore (in red color). The potential energy range is −4.8 × 10⁻² to 4.8 × 10⁻². e Emission spectra of 8PN excited at 365 nm. f Photographs of 8PN taken under UV-light irradiation (365 nm). g Illustration of the multimode reversible structural transformations realized in 8PN including transitions between various porous phases as well as transitions between nonporous and porous states in response to different kinds of external stimuli. Source data are provided as a Source Data file

Fig. 5

Summary of the adaptive accommodations of different guests by 8PN in various host−guest co-crystal structures. a Molecular size of diphenylamine and single-crystal X-ray structure of 8PN-NDP. b Molecular size of N-phenylnaphthalen-2-amine and single-crystal X-ray structure of 8PN-NPNA. c Molecular size of 3-hexylthiophene and single-crystal X-ray structure of 8PN-SC6. d Molecular size of pyrene and single-crystal X-ray structure of 8PN-PY. e single-crystal X-ray structure of 8PN-3PY. Color code for TPE-4pn: green, C; yellow, N; orange, O; gray, H. Color code for guests: pink, C; yellow, H; blue, N; light green, S, except that C atoms of the three pyrene molecules in the unit cell of 8PN-3PY are colored pink, light green, and light blue, respectively, for clarity