Flexible ligand for Metal-Organic frameworks with simultaneous Large-Pore and antenna effect emission

Polycyclic aromatic ligands improve the pore size of metal–organic frameworks (MOFs), but their big π-conjugation structure forbids energy transfer for antenna effect emission. Thus, large-pore and antenna effect are a dilemma even their combination could improve the efficiency of MOFs. Herein, we construct a flexible ligand, 1,3,5-tris(5-methoxy-1,3-benzene dicarboxylic acid)benzene (L1) by attaching three isophthalic acids (L2) to a benzene ring through flexible –CH2-O- bonds. L1 retains the optical properties of L2 for antenna effect and expands the molecular size for MOFs with large pore. Single-crystal result of the MOFs obtained with L1 and Eu3+, Tb3+, or Gd3+ reveals the aperture of 18 Å and the density of 1.2 g/mL. L1 excites Eu3+ and Tb3+ ions for their antenna effect emission. The –CH2-O- group tunes the energy transfer for dual-emission at 360 and 623 nm from L1-Eu MOF under single excitation of 290 nm. The dual-emission is used for ratiometric fluorescence detection of phosphate (PO43-) and adenosine-5′-triphosphate (ATP). We reveal the effect of porous structure of MOFs on the response speed and range. The pore size affects the target entrance as L1-Eu MOF shows faster response to PO43- and ATP than L2-Eu MOF. Moreover, fast diffusion dynamics of a target is observed in large-pore MOFs for good linear response. The relative specific surface area affects the linear range as a narrower range was observed for ATP than PO43-. Thus, flexible ligand is effective to expand the aperture, improve optical properties, and study the response mechanism for the extensive applications of MOFs.