In Situ Coordination and Confinement of Two-Photon Active Unit Within Metal-Organic Frameworks for High-Order Multiphoton-Excited Fluorescent Performance.

Exploration of high-order multiphoton-excited fluorescent (H-MPEF) material for bioimaging with deeper tissue penetration and higher spatial resolution has attracted great interest but remains a challenge. Herein, a H-MPEF-responsive metal-organic framework-based material, , was rationally fabricated by the pore confinement of a two-photon active unit () using the "ship-in-bottle" strategy. It demonstrated that the judicious selection effectively avoided the annoying weakened/quenched H-MPEF behavior due to the instability of in the molecular state. Moreover, the obtained material, , exhibited bright four-photon-excited fluorescence (4PEF) under the excitation of a 1550 nm femtosecond laser. In view of deep-tissue biological applications, it is envisioned that this work provides a promising platform for bright H-MPEF imaging.