Embedding Multiphoton Active Units within Metal-Organic Frameworks for Turning on High-Order Multiphoton Excited Fluorescence for Bioimaging.

The fabrication of high-order multiphoton excited fluorescent materials for second near-infrared (NIR-II, 1000-1700 nm) light-induced fluorescence imaging has always been an intractable challenge. In this study, a reasonable strategy guided by theoretical calculations was employed to fabricate a novel high-order multiphoton excited fluorescence (H-MPEF)-responsive UiO-type metal-organic framework (MOF, ZrTc). Strategically, the functionalization of the photonic-responsive thiazolothiazole-based organic ligand gave rise to amplified H-MPEF (four-photon activity) performance and prolonged excitation wavelength (1550 nm) of ZrTcI. The results reveal that the extended π-electron system, enhanced charge transfer, improved dipole moment, and weakened π-π stacking interactions within the MOFs endow them with excellent H-MPEF performance for NIR-II light-induced fluorescence imaging. It is an extremely rare report on H-MPEF bioimaging using MOFs and provide a universal strategy for the fabrication of H-MPEF-responsive materials.