Metal node engineering in metal–organic frameworks for enhanced high-order multiphoton excited fluorescence

Abstract

High-order multiphoton excited fluorescence (H-MPEF) probes operating in the second near-infrared window (NIR-II) are highly desirable for deep-tissue imaging with minimal photodamage. However, their development is often constrained by material limitations and challenges in structural tunability, necessitating alternative design strategies. Herein, we report a systematic investigation into enhancing H-MPEF in metal–organic frameworks (MOFs) through metal node engineering. By synthesizing a series of MOFs (Zr_xHf_1−xTc) with varying Zr⁴⁺/Hf⁴⁺ ratios, we demonstrate that partial substitution with Hf significantly modulates the multiphoton excited fluorescence behavior. Remarkably, both two-photon excited fluorescence (2PEF) and H-MPEF intensities exhibit a volcano-shaped trend, reaching a maximum at an optimal Hf doping level (Zr_0.4Hf_0.6Tc). This work establishes metal node doping as an effective strategy for tuning H-MPEF in MOFs, opening new avenues for the design of advanced optical materials in bioimaging applications.