Structure Directing Role of Non-Primary Ammonium Cations in Two-Dimensional Hybrid Perovskites*

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are semiconductors with alternating layers of organic ammonium molecules and inorganic metal halide frameworks. The non-covalent interactions between these sublattices direct the structure and optoelectronic properties. Here, we show how systematic functionalization of the organic cations can influence the symmetry and dynamics of these structures. Based on HOIP structures with primary ammonium (-NH₃⁺) cations obtained from single-crystal X-ray diffraction (SCXRD), we modeled structures where the -NH₃⁺ is substituted by secondary or tertiary ammoniums (-NH₂(CH₃)⁺ or NH(CH₃)₂⁺).  These cations tend to favor structures with reduced symmetry more than their primary ammonium counterparts. The same could then be verified for five different compositions that could be experimentally synthesized. Using temperature-dependent SCXRD experiments and Ab-initio molecular dynamics simulations, we show that such symmetry reduction originates from the reduced rotational freedom of the ammonium group. This restricted motion of the non-primary ammonium also leads to a dynamically more stable inorganic framework compared to primary ammonium-based compositions.