Modulating Two-Photon Absorption in a Pyrene-Based MOF Series: An In-Depth Investigation of Structure–Property Relationships

Metal–organic frameworks (MOFs) are emerging as promising materials for multiphoton absorption (MPA), a nonlinear optical phenomenon relevant for applications, such as bioimaging, phototherapy, and photonic devices. However, the structural features that govern their exceptional MPA activity remain poorly understood. To address this, a family of pyrene-based MOFs (NU-1000, NU-901, SrTBAPy, and BaTBAPy), that vary in topology and secondary building units (SBUs), is systematically investigated. Significant differences are observed in the two-photon absorption (2PA) cross-section σ⁽²⁾, with BaTBAPy exhibiting the highest activity (8.2 × 10⁴ GM). Quantum mechanical calculations reveal that intermolecular chromophore interactions and SBU-induced effects contribute strongly to enhanced σ⁽²⁾ values, particularly in NU-901 and BaTBAPy. The findings demonstrate that both framework topology and metal coordination environments are critical to modulating MPA behavior. These insights into the structure–property relationships of MOFs pave the way for rational design of next-generation nonlinear optical materials.