Low-Pressure CO₂ and Aromatic Recognition by Interlayer Adaptive Crystal of π-Hole-Functionalized Zn(II) Coordination...

A layer-type host, an interlayer adaptive crystal (LAC) constructed from paddle-wheel Zn(II) nodes, tetrafluoroisophthalate (L) linkers, and pyridine (Py) axial ligands, [Zn(L)Py]_2n, exhibits guest-induced, electrostatically driven breathing. The crystal contains 2.6 Å π-hole-defined ultramicropores that act not as classical channels but as electrostatic triggers for guest-induced single-crystal-to-single-crystal expansion. Dynamic modulation of interlayer spacing and surface contacts yields exceptional flexibility and molecular selectivity. Tetrafluorophenyl π-holes generate positive quadrupole fields that drive adsorption through host-host repulsion and strong recognition of guests with negative quadrupole moments. Uniaxial voids occupy 12% of the volume, allowing uptake of molecules larger than the ultramicropore. N₂, CO₂, CH₄, and benzene show type-I adsorption from very low pressure, incorporating 1.2, 2.0, 1.5, and 1.0 molecules per [Zn(L)Py]₂ unit. CO₂ uptake is accompanied by fully reversible interlayer expansion, while benzene inclusion expands the void space to 186% of that of the guest-free crystal. CO₂ is preferentially adsorbed over N₂ and CH₄, corroborated by recognition trends among benzene, trifluorobenzene, and hexafluorobenzene. Guest molecules form one-dimensional arrays within the expanded lattice, and the breathing two-dimensional sheet combines the uptake capability of metal-organic frameworks (MOFs) with the molecular precision of nonporous adaptive crystals (NACs), demonstrating an interaction-programmed mode of selective molecular inclusion.