In the recent years, the interest in chiral hybrid organic-inorganic metal halides (HOIMHs) for applications such as optoelectronics, spintronics, photodetection, energy harvesting and beyond has significantly increased, due to the promising absorption and subsequent emission of polarized light with enhanced tunability across the electromagnetic spectrum.[1,2] In these compounds, the insertion of chiral molecules inducing crystallization in noncentrosymmetric crystal structures allows for intriguing nonlinear optical properties in addition to the features mentioned above. Despite the significant scientific interest on such topic, several key questions still need to be addressed. On one hand, the research has widely centred on low dimensional systems only, i.e. from 2D to 0D, due to the steric hindrance usually displayed by the chiral organic cations, posing challenges in applications where a isotropic charge transport is demanded since the organic layers usually act as dielectrics.[3] On the other hand, the impact of octahedral distortions on the chiroptical behaviour is not well understood, although it is a crucial parameter for optimizing practical devices.

In this scenario, we have developed novel series of chiral HOIMHs by wisely tuning composition and dimensionality with the aim of unveiling the relationships between crystal structure and optical behaviour. By integrating the relatively narrow ditopic cation R/S-3-aminoquinuclidine (R/S-3AQ), the 3D corner-sharing (R/S-3AQ)Pb₂Br₆ compound has been attained along with its 2D counterpart (R/S-3AQ)₂PbBr₄·2Br, disclosing a delocalized photoexcitation resembling that of 3D materials and promising for charge transport along the three dimensions. Moreover, an increase of the chiral dissymmetry factor has been unveiled by lowering the dimensionality, in line with other corner-sharing 1D and 2D materials obtained in our laboratory. Starting from the 2D compound, the series (R/S-3AQ)₂MBr₄·2Br (M: Pb, Sn, Ge) has been synthesized, unveiling a Ruddlesden-Popper structure in all cases and demonstrating a significant octahedra distortion increase with the trend Pb^II < Sn^II < Ge^II. The same trend holds true for the series (R/S-3APD)PbX₄ - (R/S-3APD)SnX₄ - (R/S-3APD)₂GeX₄·2X (R/S-3APD : R/S-3-aminopiperidine; X: Br, I), where changes in the crystal structure have been observed by tuning the metal center but a corner-sharing connectivity was disclosed in the highly distorted tin- and germanium-based compounds. In contrast, a small distortion index change was reported for the two metal centers in the face-sharing (R/S-AMOL)MI₃ (R/S-AMOL: (2-R/S,2'-R/S)-1,1'-azanediylbis(butan-2-ol); M: Pb^II, Sn^II), indicating the role of octahedra connectivity in the octahedral tilting. With these examples, we aim to rationally investigate the structural factors concurring to optimal chiro-optical responses, providing new tools for the development of next-generation nonlinear functional materials.