Chiral Nonlinear Optical Response in Achiral Quasi‐2D Perovskite Ferroelectric Featuring Unusually Large A‐Site Cation

The exploration of 2D perovskites and chiral nonlinear optical materials has drawn significant interest due to their structural tunability and strong optical responses. However, the development of quasi‐2D perovskites has long been constrained by the limited choice of A‐site cations imposed by the traditional tolerance factor. In this study, we present a new 2D tri‐layered lead bromide perovskite (4AM) 4 Pb 3 Br 10 ( n = 3) with the general formula (A ′ ) 2 (A) n −1 M n X 3 n +1 , in which 4‐aminomonpholine cations (4AM) that act simultaneously as A‐site and spacer components, exhibiting an ultrahigh tolerance factor of 1.74—far beyond conventional limits. Crystallizing in the polar space group Cc , the compound shows an order‐disorder ferroelectric phase transition at 418 K. Interestingly it also exhibits notable chiral nonlinear optical properties, including strong second‐harmonic generation circular dichroism ( g SHG‐CD ≈ 0.63). This work breaks the classic tolerance factor constraint and opens a pathway for designing new chiral nonlinear optical materials in low‐dimensional perovskites.