Raman Spectroscopic Signatures of Piezochromism in Organic‐Inorganic Hybrid Compound (p‐bromoanilinium) 2 CuCl 4

Organic–inorganic hybrid compounds (OIHCs) of the A 2 CuCl 4 type exhibit tunable structural and functional properties, making them attractive candidates for multifunctional materials. These compounds are 2D layered perovskites, with corner‐sharing [CuCl 4 ] 2− unit, separated by organic cations (A⁺). We report the synthesis and high‐pressure Raman investigation (0–28.4 GPa) of (p‐bromoanilinium) 2 CuCl 4 (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) 2 CuCl 4 . 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.