Exploration of Two‐photon/Multi‐photon Excited Luminescence in Metal−Organic Frameworks ( MOFs ) †

Comprehensive Summary In recent years, two‐photon excited luminescence (TPEL) and multi‐photon excited luminescence (MPEL) materials have attracted increasing attention due to their unique nonlinear optical (NLO) properties, particularly in the realm of metal−organic frameworks (MOFs). MOFs, as a type of flourishing framework materials linked by coordination bonds, have distinguished themselves with their outstanding TPEL/MPEL performances, providing innovative tools for the exploration of mysterious nonlinear optics and promising applications. This review systematically introduces the basic mechanisms of TPEL/MPEL materials, emphasizing the role of photoluminescence quantum yield (PLQY), two‐photon absorption (TPA)/multi‐photon absorption (MPA) cross‐sections, and photostability in material design. Then, recent progresses in the rational construction of MOFs with tailored NLO properties is highlighted, including strategies such as linear/tripodal/quadrupodal ligand engineering, guest@MOF host‐guest systems, and post‐synthetic modifications. These advancements have unlocked diverse applications, such as anti‐counterfeiting via 3D coding and patterning, bioimaging with deep‐tissue penetration and high spatial resolution, stimulated emission for low‐threshold lasing, and optical data storage. Furthermore, the potential challenges in enhancing MOFs’ NLO efficiency, structural stability, and biocompatibility are addressed, and perspectives in the forthcoming development of this field are proposed. The insights presented herein aim to inspire innovative approaches in the design and application of MOF‐based NLO materials across disciplines, fostering advancements in photonics, biomedical engineering, and materials science. Key Scientists The field of TPEL/MPEL and MOF research has been significantly advanced by a group of outstanding scientists. In 1931, Göppert‐Mayer made the theoretical prediction of the two‐photon absorption (TPA) phenomenon, laying the cornerstone for future research in this area. [1] Decades later, in 1990, Denk and co‐workers developed the first two‐photon excitation microscope, which revolutionized imaging techniques in biological and materials science. [2] In 2012, the Pan group contributed to the development of functional MOF materials by working on lanthanide MOFs with TPEL. [3] In 2013, the Qian group pushed the boundaries of MOF applications in optoelectronics by developing a two‐photon‐pumped micro‐laser using dye‐encapsulated MOFs. [4] In 2014, the Zhou group achieved a remarkable feat by synthesizing a MOF with a photoluminescent quantum yield (PLQY) of 99.9%. [5] In 2015, Vittal and co‐workers further enhanced the optical properties of MOFs by improving 4PEL via Förster resonance energy transfer (FRET). [6] In 2017, Fischer and co‐workers deepened the understanding of photophysical properties of MOFs by studying intrinsic stimulated emission (STE) and MPEL. [7‐8] In 2022, the Jiang group set a new record by achieving the highest TPA action cross‐section value of MOFs to date. [9] In the same year, Wang and co‐workers introduced interpretable machine learning techniques to TPA research, opening up new avenues for data analysis in this field. [10] More recently, in 2024, the Bu group provided new insights into the TPA process by investigating its mechanism under low‐power density non‐coherent excitation. [11] Collectively, these contributions have significantly propelled the field forward.