Controllable Synthesis and Pressure-Induced Luminescence Response of Copper Bromide Hybrids for Latent Fingerprint Recognition, Writable Ink, and White Light-Emitting Diode

Copper-based hybrid halides have attracted growing interest as tunable luminescent materials with diverse coordination architectures. Herein, a series of copper bromide-based hybrids were synthesized featuring distinct architectures: (MEP)(Cu 2 Br 4 ) 0.5 (M1), (MEP)(Cu 4 Br 6 ) 0.5 (M2), and (MEP) 8 (Cu 4 Br 6 ) 4 (M3, MEP = Methyltriphenylphosphonium). All compounds exhibit broad-band emissions at room temperature, spanning green to orange. Systematic structural and photophysical analyses demonstrate that variations in Cu–Br coordination geometry, cluster distortion, and crystal packing critically regulate electron–phonon coupling, exciton localization, and metallophilic interactions, thereby leading to markedly different photoluminescence quantum efficiencies (PLQYs) and environmental stabilities. Among them, M2 possesses the highest PLQY and superior stability, which is attributed to its higher lattice rigidity, symmetric cluster geometry, and compact organic–inorganic framework. In addition, in situ high-pressure measurements suggest M2 is highly sensitive to pressure. As the pressure increased to 4.1 GPa, M2 shows a three-stage fluorescence response, and no phase transition was observed, demonstrating its excellent structural stability and high-pressure tunable optical response. Moreover, M2 was further applied in multifunctional demonstrations, including latent fingerprint visualization, fluorescent writing, and white light-emitting diode (WLED). This study uncovers the correlation between structure and photophysical behavior, providing a basis for developing efficient, lead-free, and multifunctional luminescent materials.