Dynamics of Chiral Cations in Two-Dimensional CuX and PbX Perovskites (X = Cl and Br).

Chiral organic ammonium cations (()-2-methylphenethylammonium (-MPhA) and ()-3,7-dimethyloctylammonium (-DMOA)) cations were combined with [MX] anions (M = Cu and Pb, X = Cl and Br) to form two-dimensional (2D) perovskites: (-MPhA)CuCl (), (-MPhA)CuBr (), (-DMOA)CuCl (), (-DMOA)CuBr (), (-DMOA)PbCl (), and (-DMOA)PbBr (). The point shearing of the MX 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 -MPhA and -DMOA cations in the 2D perovskites played an important role in the phase transition behavior and dielectric responses. Salts - showed similar solid-solid (S1-S2) phase transitions, for which the temperatures decreased in the order of CuCl () > PbCl () > CuBr () > PbBr (). The occupation volume of one -DMOA per MX 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 , , and , only an order-disorder-type dielectric phase transition was observed in salt . Interband optical transitions were observed in the CuCl and CuBr 2D perovskites, whereas sharp exciton absorptions were observed in the 2D PbCl and PbBr layers in perovskite salts and .