Dynamic Motion of Organic Spacer Cations in Ruddlesden–Popper Lead Iodide Perovskites Probed by Solid-State NMR Spectroscopy

Layered hybrid organic–inorganic perovskites such as the lead halide Ruddlesden–Popper (RP) series are solution-processable two-dimensional (2D) materials with tunable optoelectronic properties. Dynamic interactions between the ionic perovskite substructure and organic spacer cations impact optoelectronic properties relevant for device applications. Here, the static and dynamic structures of linear alkylammonium and aromatic spacers in lead iodide RP phases (n = 1) are characterized at ambient temperatures using solid-state NMR (ssNMR) spectroscopy and compared with previously reported crystal structures derived from X-ray diffraction. Rigid and flexible sites of spacers are distinguished by examining 13C{1H} and 15N{1H} cross-polarization magic-angle spinning (CP-MAS) signal intensity build-up. Different trends in site-specific rigidity are observed for short and long alkylammonium spacers. Short spacers (e.g., butylammonium) are attached by strong affinity interactions to lead iodide octahedra, whereas longer spacers (e.g., dodecylammonium) are more rigid within the RP interlayer than near the octahedral surface. Phenethylammonium and butylammonium spacers are similarly rigid, and we estimate that the local reorientation time scale of phenyl rings is 10–100 μs by 2D 13C CP-variable contact (CP-VC) experiments. 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 is responsible for a variety of site-specific dynamics and local structural distortions at intermediate time scales (microsecond to millisecond). This study demonstrates a general method to characterize nanoscale structures and site-specific dynamics that contribute to structural and electronic disorder in functional optoelectronic RP phases.