Flexible Luminescent MOF: Trapping of Less Stable Conformation of Rotational Isomers, In Situ Guest-Responsive Turn-Off and Turn-On Luminescence and Mechanistic Study

Flexible and dynamic Cu-I metal-organic framework [Cu(I)-MOF (1)] with well-defined nanoporous channel built with flexible terpyridine ligand offers a scaffold for the inclusion of different classes of guest molecules through a single-crystal-to-single-crystal (SCSC) transformation in the vapor phase at ambient conditions with visual color change. Thus, Cu(I)-MOF (1) offers a potential platform for molecular recognition and undergoes guest-responsive structural dynamism that can be triggered by interfacial interactions. Despite having the stable conformation of the rotational isomers, it selectively encapsulates the less stable conformation (eclipsed and gauche) into its cavity from their vapor phases in the SCSC process. All of the guest-exchanged processes are reversible. It shows selectivity toward less polar guest in a class. The intermediate of all of the guest-exchanged processes appeared as a black material (H2O@Cu(I)-MOF) (1z) prior to the encapsulation of each guest that happens through the SCSC manner followed by encapsulation of the guests replacing H2O in situ at ambient conditions through SCSC transformation. This confirms that the process is a two-step process leading to a common intermediate. The MOF loses its luminescence behavior with H2O removing lattice solvents in situ and appears as a black material, and it regains its luminescence property with the guests replacing H2O. Thus, the MOF displays both luminescence turn-off and turn-on before and after incorporation of the guests, respectively, leading to a common turn-off mechanism. A fluorescence titration experiment shows selectivity toward aniline among benzene and its derivatives.