(757f) Dynamic Motion of Organic Spacer Cations in 2D Ruddlesden-Popper Perovskites Probed By Solid-State NMR

Layered hybrid organic-inorganic perovskites, such as the lead halide Ruddlesden-Popper (RP) series, are solution-processable 2D materials with tunable quantum confinement and long carrier lifetimes. 2D hybrid perovskites are formed from an ionic perovskite sub-phase (e.g. methylammonium lead iodide) and cation ‘spacer’ molecules that separate the material into layers. The soft ionic structure of layered perovskites can be influenced by static and dynamic interactions with spacer molecules, with impacts on optoelectronic properties relevant for device applications. In this work, dynamic motions of spacer molecules in lead iodide RP phases (n = 1) are characterized using multinuclear solid-state NMR spectroscopy at room-temperature. The site-specific rigidity of different prototypical spacers is compared using dynamic cross-polarization measurements. We find that short spacers (e.g. butylammonium and phenethylammonium) are relatively rigid near the octahedral surface, and more flexible in the interlayer. However, longer alkylammonium spacers (e.g. octylammonium and dodecylammonium) show an opposite trend, with greater rigidity deeper in the interlayer. Spacer motions appear to be too slow to couple with photogenerated carrier dynamics, but correlate with time-averaged octahedral tilting observed by XRD. These ssNMR results indicate that the interplay between spacer interactions with lead iodide octahedra (Coulombic and hydrogen-bonding) and van der Waals forces between spacers dictates site-specific dynamics and local distortions at intermediate timescales (µs-ms). This study demonstrates a general method to characterize nanoscale structures and site-specific dynamics that contribute to structural and electronic disorder in low-dimensional perovskites.