Charge-Assisted Hydrogen-Bonded Organic Frameworks with Inorganic Ammonium Regulated Switchable Open Polar Sites

Surface open polar sites within the voids of porous molecular crystals define the localized physicochemical environment for critical functions such as gas separation and molecular recognition. This study presents a new charge-assisted hydrogen bonding (H-bonding) motif, by exploiting inorganic ammonium (NH₄ ⁺ ) cations as H-bond donors, to regulate the assembly of C₂ -symmetric carboxylic tectons for building robust H-bonded frameworks with permanent ultra-micropores and open oxygen sites. Diverse building blocks are bridged by tetrahedral NH₄ ⁺ to expand distinctive H-bonded networks with varied pore architectures. Particularly, the open polar oxygen sites can be switched by altering NH₄ ⁺ sources to tune the deprotonation of carboxyl-containing tectons. The activated porous PTBA·NH₄ ·DMF preserves the pore architecture and open polar oxygen sites, exhibiting remarkably selective sorption of CO₂ (107.8 cm³ g^-1 ,195 K) over N₂ (11.2 cm³ g^-1 , 77 K) and H₂ (1.4 cm³ g^-1 , 77 K).

Selective and Reversible Solvent Uptake in Tetra-4-(4-pyridyl)phenylmethane-based Supramolecular Organic Frameworks

The dynamic behavior of supramolecular organic frameworks (SOFs) based on the rigid tetra-4-(4-pyridyl)phenylmethane (TPPM) organic tecton has been elucidated through 3D electron diffraction, X-ray powder diffraction and differential scanning calorimetry (DSC) analysis. The SOF undergoes a reversible single-crystal-to-single-crystal transformation when exposed to vapours of selected organic solvents, moving from a closed structure with isolated small voids to an expanded structure with solvated channels along the b axis. The observed selectivity is dictated by the fitting of the guest in the expanded SOF, following the degree of packing coefficient. The effect of solvent uptake on TPPM solid-state fluorescence was investigated, evidencing a significant variation in the emission profile only in the presence of chloroform.

Tunable Fluorescence in Two-Component Hydrogen-Bonded Organic Frameworks Based on Energy Transfer

A dumbbell-shaped compound (TPAD) with four 2,4-diaminotriazine moieties as H-bond units and a benzene ring as a bridge group was found to form hydrogen-bonded organic frameworks (HOFs) with strong cyan fluorescence. An energy acceptor, 6,6',6″,6‴-(((benzo[c][1,2,5]thiadiazole-4,7-diylbis-(4,1-phenylene))bis(azanetriyl))tetrakis(benzene-4,1-diyl))tetrakis(1,3,5-triazine-2,4-diamine) (BTAD), with the same molecular skeleton as TPAD and a longer emission wavelength could homogeneously distribute within the framework of TPAD through occupying the locations of TPAD. As a result, two-component HOFs (TC-HOFs) were formed. The nonradiative energy transfer from TPAD as the donor to BTAD as the acceptor happens within frameworks owing to the efficient spectral overlap between the emission of TPAD and the absorption of BTAD. Moreover, the emission wavelengths and colors of TC-HOFs could be easily and continuously modulated by the content of the acceptor. The fluorescence color changed from cyan to orange when the content of BTAD gradually increased. This finding affirms that TC-HOFs with continuously adjustable composition can be constructed from two molecules with the same molecular skeleton, and highly efficient nonradiative energy transfer may happen in porous TC-HOFs. To the best of our knowledge, these TC-HOFs are the first example of TC-HOFs involved in energy transfer.

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pK_a shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO₂ via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO₂ .

Azine Steric Hindrances Switch Halogen Bonding to N-Arylation upon Interplay with σ-Hole Donating Haloarenenitriles

An interplay between 4-bromo- and 4-iodo-5-nitrophthalonitriles (XNPN, X=Br or I) and any one of the azines (pyridine 1, 4-dimethylaminopyridine 2, isoquinoline 3, 4-cyanopyridine 4, 2-methylpyridine 5, 2-aminopyridine 6, quinoline 7, 1-methylisoquinoline 8, and 2,2'-bipyridine 9) proceeds differently depending on steric and electronic effects of the heterocycles. Sterically unhindered azines 1-3 underwent N-arylation to give the corresponding azinium salts (characterized by ¹ H and ¹³ C{H} NMR and high-resolution ESI-MS). In contrast, azines 4-9 with sterically hindered N atoms or bearing an electron-withdrawing substituent, form stable co-crystals with XNPN, where two interacting molecules are bound by halogen bonding. In all obtained co-crystals, X⋅⋅⋅N structure-directed halogen bonds were recognized and theoretically evaluated including DFT calculations (PBE0-D3/def2-TZVP level of theory), QTAIM analysis, molecular electrostatic potential surfaces, and noncovalent interaction plot index. Estimated energies of halogen bonding vary from -7.6 kcal/mol (for 6 ⋅ INPN) to -11.4 kcal/mol (5 ⋅ INPN).

Two Orthogonal Halogen-Bonding Interactions Directed 2D Crystalline Supramolecular J-Dimer Lamellae

Dye assemblies exhibit fascinating properties and performances, both of which depend critically on the mutual packing arrangement of dyes and on the supramolecular architecture. Herein, we engineered, for the first time, an intriguing chlorosome-mimetic 2D crystalline J-dimer lamellar structure based on halogenated dyes in aqueous media by employing two distinct orthogonal halogen-bonding (XB) interactions. As the only building motif, antiparallel J-dimer was formed and stabilized by single π-stacking and dual halogen⋅⋅⋅π interactions. With two substituted halogen atoms acting as XB donors and the other two acting as acceptors, the constituent J-dimer units were linked by quadruple highly-directional halogen⋅⋅⋅halogen interactions in a staggered manner, resulting in unique 2D lamellar dye assemblies. This work champions and advances halogen-bonding as a remarkably potent tool for engineering dye aggregates with a controlled molecular packing arrangement and supramolecular architecture.