Enhancing\nTwo-Photon Excited Fluorescence of Metal–Organic\nFramework Single Crystals through Modulation of Inorganic Nodes

Regulation of the two-photon excited fluorescence (TPEF)\nemission\nintensity and wavelength of metal–organic framework (MOF) crystals\nwith similar constitutions presents a significant challenge. In this\nstudy, two MOFs, Zn–BTPPA and Cd<sub>3</sub>–BTPPA,\nwere constructed using tetrakis­(1,1′-biphenyl-4-carboxylic\nacid)-1,4-benzenediamine (H<sub>4</sub>BTPPA) as the organic ligand\nand mononuclear Zn and trinuclear Cd<sub>3</sub> inorganic nodes,\nrespectively. The incorporation of H<sub>4</sub>BTPPA within the MOF\nstructures enables effective TPEF emission in both Zn–BTPPA\nand Cd<sub>3</sub>–BTPPA. The TPEF results show that Zn–BTPPA\nand Cd<sub>3</sub>–BTPPA exhibited strong emissions at 523\nand 463 nm, respectively, when excited with a 780 nm laser. Moreover,\nZn–BTPPA and Cd<sub>3</sub>–BTPPA exhibited much higher\ntwo-photon absorption cross sections, approximately 4.9 and 5.2 times\nhigher than that of the reported dinuclear MOF, Cd<sub>2</sub>–BTPPA,\nwith a similar composition, respectively. With different inorganic\nnodes, the stacking of chromophores, π···π\ninteractions, and ligand geometry were found to correlate with the\nenhanced TPEF in Cd<sub>3</sub>–BTPPA and the blue-shifted\nTPEF in Zn–BTPPA. This work serves as an inspiration for designing\nefficient TPEF MOF materials based on the structure–property\nrelationship.