Enhancing Two-Photon Excited Fluorescence of Metal–Organic Framework Single Crystals through Modulation of Inorganic Nodes

Regulation of the two-photon excited fluorescence (TPEF) emission intensity and wavelength of metal-organic framework (MOF) crystals with similar constitutions presents a significant challenge. In this study, two MOFs, Zn-BTPPA and Cd₃-BTPPA, were constructed using tetrakis(1,1'-biphenyl-4-carboxylic acid)-1,4-benzenediamine (H₄BTPPA) as the organic ligand and mononuclear Zn and trinuclear Cd₃ inorganic nodes, respectively. The incorporation of H₄BTPPA within the MOF structures enables effective TPEF emission in both Zn-BTPPA and Cd₃-BTPPA. The TPEF results show that Zn-BTPPA and Cd₃-BTPPA exhibited strong emissions at 523 and 463 nm, respectively, when excited with a 780 nm laser. Moreover, Zn-BTPPA and Cd₃-BTPPA exhibited much higher two-photon absorption cross sections, approximately 4.9 and 5.2 times higher than that of the reported dinuclear MOF, Cd₂-BTPPA, with a similar composition, respectively. With different inorganic nodes, the stacking of chromophores, π···π interactions, and ligand geometry were found to correlate with the enhanced TPEF in Cd₃-BTPPA and the blue-shifted TPEF in Zn-BTPPA. This work serves as an inspiration for designing efficient TPEF MOF materials based on the structure-property relationship.