Dynamics of Chiral Cations in Two-Dimensional CuX₄ and PbX₄ Perovskites (X = Cl and Br)

Chiral organic ammonium cations ((R)-2-methylphenethylammonium (R-MPhA) and (R)-3,7-dimethyloctylammonium (R-DMOA)) cations were combined with [MX₄]^2- anions (M = Cu and Pb, X = Cl and Br) to form two-dimensional (2D) perovskites: (R-MPhA)₂CuCl₄ (1a), (R-MPhA)₂CuBr₄ (1b), (R-DMOA)₂CuCl₄ (2a), (R-DMOA)₂CuBr₄ (2b), (R-DMOA)₂PbCl₄ (2c), and (R-DMOA)₂PbBr₄ (2d). The point shearing of the MX₄ octahedron formed 2D perovskite layers, which were sandwiched by the bilayer molecular assembly of chiral organic ammonium cations. We found that the flexible and polar organic R-MPhA and R-DMOA cations in the 2D perovskites played an important role in the phase transition behavior and dielectric responses. Salts 2a-2d showed similar solid-solid (S1-S2) phase transitions, for which the temperatures decreased in the order of CuCl₄ (2a) > PbCl₄ (2c) > CuBr₄ (2b) > PbBr₄ (2d). The occupation volume of one R-DMOA per MX₄ octahedron determined the dynamic crystalline space for the motional freedom of chiral ammonium in the 2D perovskite layer. Although thermally activated dielectric fluctuations were observed in salts 2a, 2b, and 2c, only an order-disorder-type dielectric phase transition was observed in salt 2d. Interband optical transitions were observed in the CuCl₄ and CuBr₄ 2D perovskites, whereas sharp exciton absorptions were observed in the 2D PbCl₄ and PbBr₄ layers in perovskite salts 2c and 2d.