Reduced‐Size of Perovskite through <scp>MOF</scp> Spatial Confinement for Achieving Giant Improvement of Third‐Order <scp>NLO</scp> Performance

Comprehensive Summary The size of perovskites is a crucial factor to impact third‐order nonlinear optical (NLO) properties. The inherent channels of metal‐organic frameworks (MOFs) can be utilized for achieving the size reduction of perovskites. Confining small‐sized perovskites within the MOFs channels to form composite materials helps to regulate the NLO properties of perovskites. In this work, a self‐assembly strategy was employed to confine MBAPbBr 3 (MBA = Methylphenylethylamine) within the 1D channels of a Zn‐MOF {[Zn 1.75 L 0.625 (Pz‐NH 2 ) 0.25 ( μ 3 ‐O) 0.25 ( μ 2 ‐O) 0.25 (H 2 O) 1.25 ]·4CH 3 CN} n (L = 5,5'‐(1 H ‐2,3,5‐triazole‐1,4‐diyl)di‐isophthalic acid), obtaining small‐sized perovskites. At the excitation wavelengths of 532, 900, or 1064 nm, the NLO absorption coefficient and NLO refractive index of MBAPbBr 3 @Zn‐MOF were enhanced by an order of magnitude compared with those of intrinsic MBAPbBr 3 . Femtosecond transient absorption spectroscopy and theoretical calculations indicate that the strong charge interactions and bandgap coupling between the Zn‐MOF and perovskite enhance the light absorption cross‐section and the lifetime of photogenerated carriers, thereby facilitating the multi‐photon absorption‐induced excited‐state absorption process and enabling the materials to achieve higher energy conversion. This work explores the influence of the tiny size of perovskites and the charge interaction with MOFs on the third‐order NLO properties, providing a new perspective for the design and regulation of third‐order NLO materials.