Controlling the Orientation of Polar Axis in Chiral Metal-Halides to Reveal Their Intrinsic Circularly Polarized Luminescence

Abstract

Finely modulating the structure of the organic cation in metal-halide semiconductors is a powerful strategy to influence the symmetry and polarity of the hybrid compound as well as its optoelectronic properties. In this study, chiral organic S/R-hydroxybutylammonium (S/R-HBA) cations are used to prepare a series of 1D metal-halide networks. The resulting chiral and polar (S/R-HBA)₂Pb₃X₈ (X = Br or I) compounds show both second harmonic generation (SHG), circular dichroism (CD), and circularly polarized luminescence (CPL). Contrary to the 1D networks previously prepared from S/R-2-hydroxypropyl-1-ammonium (S/R-HP1A) cations, the new compounds show no alteration of their CD signal upon sample flipping, highlighting the suitable orientation of the unique polar axis in both bulk crystal and thin film states. Since no macroscopic effect can interfere with the true chiroptical response when the optical axis of the compound is normal to the incident light beam, (S/R-HBA)₂Pb₃Br₈ presents a true CPL without sign modulation after flipping or rotating the sample, despite the inherent optical anisotropy in this family of crystalline materials.