Pressure-Induced Single-Crystal-to-Single-Crystal Dimerization and Irreversible Optical Reconstruction in a 2D Hybrid Lead Halide

External pressure is a powerful tool for tuning hybrid metal halides (HMHs), yet the resulting structural and optical changes are usually reversible upon decompression. Locking pressure-accessed states through covalent bond formation, particularly while preserving single-crystallinity and optical functionality, remains challenging. Here, we report a pressure-induced single-crystal-to-single-crystal (SC-SC) [2 + 2] dimerization in a two-dimensional lead halide, (S-MAMBA)2PbBr4, which contains acrylate-functionalized S-MAMBA+ cations. This process yields a stable dimerized single-crystal phase upon decompression. The topochemically preorganized arrangement of S-MAMBA+ enables irreversible cyclobutane formation while preserving the crystallinity. High-pressure vibrational and diffraction measurements, together with recovered single-crystal structure determination, establish the pressure-induced bond-forming transformation. During compression, the emission evolves from self-trapped exciton (STE)-dominated to free-exciton (FE)-dominated recombination near the dimerization threshold, indicating a reconstructed excitonic pathway. After decompression, the dimerized phase shows irreversible optical reconstruction, characterized by opposite absorption and emission shifts. Density functional theory (DFT) reveals that this chemical transformation perturbs the inorganic-framework band-edge electronic structure through changes in organic–inorganic interactions and interlayer coupling. This work demonstrates that pressure can be used as a novel and powerful trigger for permanent bond and electronic structure reconstruction in hybrid materials.