Metal-organic frameworks (MOFs) for nonlinear optical properties: design principles, DFT insights, and future...

Metal-organic frameworks (MOFs) have emerged as a uniquely versatile platform for nonlinear optical (NLO) applications, combining the large hyperpolarizabilities of organic chromophores with the chemical robustness and structural programmability of crystalline porous materials. Although many reviews have covered various facets of MOF-NLO chemistry, no review has brought together structural design principles, family-to-family quantitative performance comparisons, and DFT-based hyperpolarizability evaluations within a unified critical framework. This review of NLO properties of MOFs is considered through three interconnected facets: the principles of structural design to achieve non-centrosymmetric, high-second and third-order response structures; computational tools based on density functional theory (DFT) to predict and rationalize the electronic and optical properties of MOFs; and the measurement tools such as Z-scan and two-photon excited fluorescence (TPEF). The particular focus is on triphenylamine (TPA)-based MOFs, with their multi-branched donor-π-acceptor structure and robust intramolecular charge transfer (ICT) features rendering them the most promising third-order NLO materials. They provide externally tunable third-order responses (β = 10^-3 to 10^-4 cm W^-1) via electric-field modulation, guest loading, and interpenetration engineering. Moreover, the MOF families lanthanide MOFs, bimetallic Zn/Cu systems, Ti-based MIL-125, Zr-based UiO-66, Cu-HHTP, bismuth-organic frameworks, zeolitic imidazolate frameworks, and porphyrin-based 2D frameworks are critically considered based on their NLO activity. Second-order NLO activity (d ₃₃ ≈ 19.86 pm V^-1, ∼12× KDP) is maximized using bimetallic Zn/Cu MOFs and electrically tunable Cu-HHTP films, which represent the state-of-the-art for actuatable switched third-order polymeric NLO materials. The DFT methods, such as hybrid functionals, dispersion-corrected methods and machine learning-accelerated screening emerging methods, are evaluated based on their ability to quantitatively predict band gaps, charge-transfer energies, and hyperpolarizability tensors. High-priority research frontiers include frequency-dependent NLO computations, chiral MOF engineering for SHG, guest@MOF switching, and 2D nanosheet architectures. This review provides not only a practical design guide but also a significant computational roadmap for emerging MOF-based photonic technologies such as optical data storage, ultra-fast all-optical switching, frequency-conversion lasers, and non-invasive bio-imaging.