Reduction in Crystal Size of Flexible Porous Coordination Polymers Built from Luminescent Ru(II)-Metalloligands

Abstract

In this study, we examined the reduction in crystal size of the porous coordination polymers (PCPs) {Sr4(H2O)9][4Ru]2·9H2O]} (Sr2[4Ru]) and [Mg(H2O)6]{[Mg2(H2O)3[4Ru]·4H2O} (Mg2[4Ru]) composed of a luminescent metalloligand [Ru(4,4′-dcbpy)]4–([4Ru]; 4,4′-dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) using a coordination modulation method. Scanning electron microscopy measurements clearly show that the sizes of crystals of Sr2[4Ru] and Mg2[4Ru] were successfully reduced to the mesoscale (about 500 nm width and 10 nm thickness for Sr2[4Ru] (abbreviated as m-Sr2[4Ru]) and about 1 μm width and 30 nm thickness for Mg2[4Ru] (abbreviated as m-Mg2[4Ru])) using lauric acid as a coordination modulator. Interestingly, the nanocrystals of m-Sr2[4Ru] formed flower-like aggregates with diameters of 1 μm, whereas flower-like aggregates were not formed in m-Mg2[4Ru]. Water vapor adsorption isotherms of these nanocrystals suggest that the water adsorption behavior of m-Sr2[4Ru], which has a three-dimensional lattice structure containing small pores, is significantly different from that of the bulk Sr2[4Ru] crystal, as shown by the vapor adsorption isotherm. In contrast, m-Mg2[4Ru], which has a two-dimensional sheet structure, had an adsorption isotherm very similar to that of the bulk sample. These contrasting results suggest that the dimensionality of the coordination framework is an important factor for the guest adsorption behavior of nanocrystalline PCPs.