Selective probing of one molecule from one class of similar molecules is highly challenging due to their similar chemical and physical properties. Herein, a novel metal-organic framework FJI-H15 with flexible porous cages has been designed and synthesized, which can specifically recognize ethylbenzene with ultrahigh enhancement efficiency from a series of alkyl-aromatics, in which an unusual size-dependent interaction has been found and proved. It can also selectively detect phenolic-nitroaromatics among a series of nitroaromatics based on energy transfer and electrostatic interactions. Such unusual specificity and variable mechanisms responding to different types of molecules have not yet been reported, which will provide a new strategy for the development of more effective chemosensors based on MOFs for probing small structural differences in molecules.

Excessive CO2 in the air can not only lead to serious climate problems but also cause serious damage to humans in confined spaces. Here, a novel metal–organic framework (FJI‐H38) with adaptive ultramicropores and multiple active sites is prepared. It can sieve CO2 from air with the very high adsorption capacity/selectivity but the lowest adsorption enthalpy among the reported physical adsorbents. Such excellent adsorption performances can be retained even at high humidity. Mechanistic studies show that the polar ultramicropore is very suitable for molecular sieving of CO2 from N2, and the distinguishable adsorption sites for H2O and CO2 enable them to be co‐adsorbed. Notably, the adsorbed‐CO2‐driven pore shrinkage can further promote CO2 capture while the adsorbed‐H2O‐induced phase transitions in turn inhibit H2O adsorption. Moreover, FJI‐H38 has excellent stability and recyclability and can be synthesized on a large scale, making it a practical trace CO2 adsorbent. This will provide a new strategy for developing practical adsorbents for CO2 capture from the air.