Dyes encapsulated in a novel flexible metal−organic framework show tunable and stimuli-responsive phosphorescence

Abstract

A novel flexible metal-organic framework (MOF), [ZnL]∙H2O∙DMA (USTC-1), with interesting topology has been constructed based on a new T-shaped ligand, 5-(2-chloroimidazo[1,2-a]pyridine-3-carboxamido)isophthalic acid (H2L). This MOF was characterized by IR spectroscopy, thermogravimetry, single-crystal, and powder X-ray diffraction methods. The single-crystal analysis reveals four carboxylate groups connect two zinc ions to generate a paddlewheel secondary building unit (SBU). Each paddlewheel SBU is six-connected to six L2− ligands, and each L2− is three-connected to three SBUs, thus, the framework can be described as a binodal (3,6)-connected three-dimensional (3D) flexible network. The low N2 and CO2 gas uptake should be attributed to the flexible vibration of amide chains at ligands that close the pore window, however, the closed pore window can be opened by dye molecules, such as Rhodamine B (RB), Methyl Orange (MO) or Methylene Blue (MB). Two methods for dye encapsulation were investigated: soaking and in situ encapsulation. PXRD data showed that the framework in USTC-1 was unchanged after encapsulation of dyes. Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue two-color luminescence with intensity ratio of 4.5 and tunable phosphorescence color. Moreover, RB@USTC-1 and MO@USTC-1 could undergo protonation upon exposure to HCl vapor, and reveal reversible phosphorescent color switching in the response to acid-base vapor stimuli. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF materials via encapsulation of various guests.