Mixed Organic Cation in Chiral Two-Dimensional Organic–Inorganic Hybrid Metal Halides─An Ab Initio Study of Nonlinear Optical (NLO) Properties

The mixing of organic cations represents yet another direction to explore in the field of chiral organic-inorganic hybrid metal halides (OIHMH). Here, we perform structural optimizations, electronic structures, and nonlinear optical (NLO) studies using density functional theory of two recently synthesized chiral OIHMHs, [R-MePEA][C3A]PbBr4 and [R-MePEA][C4A]PbBr4, with mixed chiral arylammonium and achiral alkylammonium cations. We find that the noncovalent weak interactions (e.g., Br···NH interactions) play an important role in the description of these OIHMHs. Our study further indicates that the two noncentrosymmetric compounds exhibit relatively wide bandgaps (∼3.5 eV), strong second harmonic generation (SHG) responses (∼0.5–1.5 × KDP), and moderate birefringence (∼0.088 at 1064 nm), indicating possible applications for NLO materials. Atom response theory analysis reveals that the SHG responses are determined mainly by the occupied Br 3p nonbonding orbitals as well as by the unoccupied Pb 5p orbitals, which shows the important contribution of the inorganic PbBr4 layer to the nonlinear optical properties.