A new coordination polymer exhibiting unique 2D hydrogen-bonded (H2O)16 ring formation and water-dependent luminescence properties

A new coordination polymer, [Zn(dpe)(bdc)]·4H(2)O (ZndB; dpe=1,2-bis(4-pyridyl)ethane, bdc(2-)=dianion of benzenedicarboxylic acid), which possesses a 3D metal-organic framework (MOF) has been synthesized and structurally characterized. This 3D MOF is constructed by the assembly of helical channels filled with guest water molecules in both inner and outer regions of the channel. The resulting network also creates a 2D water layer containing hydrogen-bonded (H(2)O)(16) rings as the basic building units. Thermogravimetric and powder X-ray diffraction measurements of ZndB revealed a two-step weight loss of water molecules with a reversible water adsorption/desorption process in the inner channel for the first stage, and irreversible water desorption in the outer channel for the second stage. This spongelike property is manifested by the excimer emission originating from interaction between dpe (π*) and the other dpe (π) of the proximal helical channel, which is highly sensitive to the environmental perturbation. Powder X-ray analyses reveal that the dehydration process induces the readjustment of dpe π-π stacking distance/orientation, which results in dramatic luminescence changes from dim pale blue (λ(em)≈470 nm) upon hydration to bright white-light generation (broad, λ(em)≈500-550 nm) upon water depletion, accompanied by a ≈100-fold increase in the emission intensity.