Structure control and crystal-to-crystal transformation for two series of lanthanide–organic coordination polymers†
Two series of lanthanide-based coordination polymers, [Ln(HCPOB)(CPOB)(H2O)2]n [Ln = Eu(a1), Gd(a2), Tb(a3), Dy(a4), Ho(a5), Er(a6)] and [Ln(HCPOB)(CPOB)]n [Ln = Eu(b1), Gd(b2), Tb(b3), Dy(b4), Ho(b5), Er(b6)], have been synthesized using an asymmetric semi-rigid V-shape multicarboxylate ligand [H4CPOB = 2-(4-carboxyphenoxy)benzoic acid] by a hydrothermal method at different temperatures. Series a were obtained at 130 °C and have one-dimensional (1D) chain architectures, while series b were synthesized at 180 °C and exhibit three-dimensional (3D) structures. Interestingly, temperature-dependent XRD patterns for 1D compounds indicate that the 1D structure changes to form the 3D coordination polymers on heating the crystals. Single crystals of series a can transform into crystals of series b without any significant change in crystal quality. The variations from 1D to 3D coordination structures are attributed to variable coordination modes of multicarboxylate ligand H2CPOB. In addition, we have studied the luminescent properties of Eu3+, Tb3+, and Dy3+ compounds of series a and series b, and found that series a give very weak luminescence because of coordinated water molecules existing in the complexes. Due to the “antenna effect” or “luminescence sensitization”, series b exhibit much stronger fluorescent intensity than that of series a.