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 Cd3inorganic 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.