Thermally Activated Delayed Fluorescence in Bridged Dinuclear Copper(I) Cluster-Based Scintillators for High-Resolution X-ray Imaging

Copper(I) halide-based complexes are promising alternatives scintillators due to their excellent optoelectronic properties, structural diversity, and good solution processability. However, achieving an optimal balance among X-ray absorption, radiative-nonradiative competition, exciton utilization, and structural stability remains challenging. Herein, a compact, π-electron-rich, and rigid bidentate phosphine ligand, diphenyl-2-pyridylphosphine (N̂P), is introduced to construct <b>Cu</b>_<b>2</b><b>X</b>_<b>2</b><b>(N̂P)</b>_<b>3</b> <b>(X = Cl, Br, I)</b> complexes. The small-volume rigid ligand reduces the organic fraction, enhances X-ray absorption, and suppresses excited-state Jahn-Teller distortion and nonradiative relaxation through rigid bridging and intramolecular π-π interactions. The resulting <b>Cu</b>_<b>2</b><b>X</b>_<b>2</b><b>(N̂P)</b>_<b>3</b> <b>(X = Cl, Br, I)</b> complexes exhibit thermally activated delayed fluorescence (TADF) with a small singlet-triplet energy gap (Δ<i>E</i>_ST), enabling a high photoluminescence quantum yield of 97.6%. Under X-ray excitation, the <b>Cu</b>_<b>2</b><b>I</b>_<b>2</b><b>(N̂P)</b>_<b>3</b> complex achieves a light yield 2.87 times that of commercial scintillator (Bi₄Ge₃O₁₂, BGO) and an ultralow detection limit of 172.9 nGy_air s^-1. Benefiting from their nonmechanical photochromism and excellent processability, uniform scintillator films fabricated via microelectronic printing in a blade-coating configuration exhibit outstanding irradiation and humidity stability, as well as high X-ray imaging resolution (>20 lp mm^-1). This work highlights the crucial role of ligand design in realizing efficient TADF Cu₂X₂ nanocluster scintillators for high-resolution X-ray imaging.