Raman Spectroscopic Signatures of Piezochromism in Organic-Inorganic Hybrid Compound (p-bromoanilinium)₂CuCl₄

Organic-inorganic hybrid compounds (OIHCs) of the A₂CuCl₄ type exhibit tunable structural and functional properties, making them attractive candidates for multifunctional materials. These compounds are 2D layered perovskites, with corner-sharing [CuCl₄]^2- unit, separated by organic cations (A⁺). We report the synthesis and high-pressure Raman investigation (0-28.4 GPa) of (p-bromoanilinium)₂CuCl₄ (PBrAnCuCl). At ambient conditions, this compound shows an unusual out-of-phase layer arrangement parallel to the ab-plane, distinct from the nearly flat 2D structure of its chlorine analogue, (p-chloroanilinium)₂CuCl₄. Raman spectra revealed not only the expected red shift but also new Cu─Cl vibrations: at 0.2 and 7.2 GPa for long bonds, and at 14.7 GPa for short bonds, persisting to 26.3 GPa. Remarkably, pressure induced sequential color changes: yellow-green (0-1.4 GPa) → orange-yellow (2.0-3.5 GPa) → red-orange (4.2-4.9 GPa) → deep red (6.4-7.2 GPa) → red-black (8.2-10.2 GPa) → opaque black (10.9-15.5 GPa) → surprisingly, partially transparent black (17.1-28.4 GPa), returning to transparent yellow-green upon decompression. Thus, bromine substitution in the organic moiety modifies polarizability, Jahn-Teller distortion, and antiferrodistortive (AFD) ordering, with Raman spectroscopy revealing tunable optical responses that establish halogen substitution as a strategy for designing piezochromic OIHC perovskites.