Toward fluorinated spacers for MAPI-derived hybrid perovskites: Synthesis, characterization, and phase transitions of (FC₂H₄NH₃)₂PbCl₄

The intrinsic moisture sensitivity of the hybrid perovskite methylammonium lead iodide (MAPI) calls for new synthetic strategies to enhance moisture resistance and, thus, long-term stability. Here, we combine two strategies: (i) transitioning from 3D to 2D hybrid perovskites by inserting larger A-site cations as spacers and (ii) using fluorinated linkers to enhance the hydrophobicity of the material - and identify two new hybrid perovskite-type compounds, (FC₂H₄NH₃)₂PbCl₄ and (FC₂H₄NH₃)PbBr₃·DMF, carrying 2-fluoroethylammonium (FC₂H₄NH₃)⁺ as a promising organic cation for the synthesis of moisture-resistant hybrid perovskites. (FC₂H₄NH₃)₂PbCl₄ features a two-dimensional structure and pronounced long-term stability as confirmed by single-crystal and powder X-ray diffraction. The observed reversible phase transitions at 87 and 107 °C investigated with thermal analysis, temperature-dependent powder X-ray diffraction measurements, and ¹H, ¹³C, and ²⁰⁷Pb solid-state NMR spectroscopy can be assigned to changes in the inorganic lead chloride and organic sublattices, respectively, both having clearly observable fingerprints in the solid-state NMR spectra. DFT calculations trace the origin of the observed severe distortion of the inorganic sublattice in (FC₂H₄NH₃)₂PbCl₄ back to structural features including the formation of hydrogen bonds. The optical properties of (FC₂H₄NH₃)₂PbCl₄ were characterized by optical absorption spectroscopy and time-resolved photoluminescence measurements with a view toward the interaction between the organic and inorganic sublattices. The broad photoluminescence spectrum as well as specific absorption characteristics are assigned to exciton self-trapping due to a strong coupling of the excited states to lattice distortions.