Distinct Excitonic Emissions in 2D (C7 H7 N2 )2 PbX4 (X = Cl, Br) under Compression.

Two dimensional (2D) hybrid metal halides (HMHs) usually exhibit free excitonic (FE) emission, and self-trapped excitonic (STE) emission can also be achieved by adopting appropriate halogens and organic cations. Recently, significant efforts have been made to modulate and then clarify the transformation and connection between these two types of excitonic emissions in 2D HMHs. In this study, intriguing pressure-tuned transitions between FE emission and STE emission are observed in 2D (C7 H7 N2 )2 PbCl4 . In contrast, only FE emissions with tunable emission energies are observed in 2D (C7 H7 N2 )2 PbBr4 which possesses a similar structure with (C7 H7 N2 )2 PbCl4 under compression. Such distinct halide-dependent optical responses under pressure are experimentally revealed to arise from the intricate interplay among several factors in these HMHs, including the stiffness of the structure, the Coulomb force between the organic cations and the inorganic octahedra, and the magnitude of inorganic octahedral distortion. These high-pressure optical explorations can unravel the underlying interrelationship between the crystal structure and excitonic emission in 2D HMHs.