The Interplay between Luminescence Dissymmetry Factor and Quantum Yield: Symmetric and Asymmetric Hydrogen Bonding

Unraveling the luminescence-structure relationship in metal-free chiral perovskites remains challenging, particularly for achieving both a strong circularly polarized luminescence (CPL) and high photoluminescence quantum yield (PLQY). Here, we attempt to decode this hidden linkage by constructing the isomorphic polymorph-based metal-free chiral perovskite through the rational design of the A-site cation. The distinct steric hindrance between meta- and ortho-ammonium sites in 2-methylpiperazine-1,4-diium provides two possibilities for forming the hydrogen bond, which could control the crystallization pathways. Therefore, two different metal-free chiral perovskite polymorphs (A and B) with symmetric or asymmetric hydrogen bonding were obtained, which exhibit a remarkable difference in PLQY (36.51% vs 1.85%) and photoluminescence dissymmetry factor (|g_lum|, 0.032 vs 0.066). The highly luminescent polymorph A demonstrates stronger intermolecular coupling mediated by the symmetric hydrogen bonding, which effectively suppresses the nonradiative decay through rigidification of the octahedral framework, thereby enhancing the PLQY. In contrast, the asymmetric hydrogen bonding in polymorph B induces a greater structural distortion, which enhances the electron-phonon coupling and consequently reduces the PLQY. However, this asymmetric hydrogen bonding promotes efficient chirality transfer, leading to an increase of |g_lum|. Our work provides a molecular-level understanding of symmetric and asymmetric hydrogen bonding to determine the crystal packings and exciton dynamics, and thus the luminescence dissymmetry factor and quantum yield.