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 CuI 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.