EnhancingTwo-Photon Excited Fluorescence of Metal–OrganicFramework Single Crystals through Modulation of Inorganic Nodes

Abstract

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.