Vapochromic Luminescence and Flexibility Control of Porous Coordination Polymers by Substitution of Luminescent Multinuclear Cu(I) Cluster Nodes

Two luminescent porous coordination
polymers (PCPs), i.e., [Cu2(μ2-I)2ctpyz]n and [Cu4(μ3-I)4ctpyz]n (Cu2 and Cu4, respectively; ctpyz = cis-1,3,5-cyclohexanetriyl-2,2′,2″-tripyrazine), were
successfully synthesized and characterized by single-crystal X-ray
diffraction and luminescence spectroscopic measurements. Cu2 consists of rhombus-type dinuclear {Cu2I2}
cores bridged by ctpyz ligands, while Cu4 is constructed
of cubane-type tetranuclear {Cu4I4} cores bridged
by ctpyz ligands. The void fraction of Cu4 is estimated
to be 48.0%, which is significantly larger than that of Cu2 (19.9%). Under UV irradiation, both PCPs exhibit red luminescence
at room temperature in the solid state (λem values
of 660 and 614 nm for Cu2 and Cu4, respectively).
Although the phosphorescence of Cu2 does not change upon
removal and/or adsorption of EtOH solvent molecules in the porous
channels, the solid-state emission maximum of Cu4 red-shifts
by 36 nm (λem = 650 nm) upon the removal of the adsorbed
benzonitrile (PhCN) molecules from the porous channels (and vice versa).
This large difference in the vapochromic behavior of Cu2 and Cu4 is closely related to the framework flexibility.
The framework of Cu2 is sufficiently rigid to retain
the porous structure without solvated EtOH molecules, whereas the
porous structure of Cu4 collapses easily after removal
of the adsorbed PhCN molecules to form a nonporous amorphous phase.
The original vapor-adsorbed porous structure of Cu4 is
regenerated by exposure of the amorphous solid to not only PhCN vapor
but also tetrahydrofuran, acetone, ethyl acetate, and N,N-dimethylformamide vapors. The Cu4 structures with the various adsorbed solvents showed almost the
same emission maxima as the original PhCN-adsorbed Cu4, except for DMF-adsorbed Cu4, which showed no luminescence
probably because of weak coordination of the DMF vapor molecules to
the Cu­(I) centers of the tetranuclear {Cu4I4} core.