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