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 Cu2X2(N̂P)3 (X = Cl, Br, I) 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 Cu2X2(N̂P)3 (X = Cl, Br, I) complexes exhibit thermally activated delayed fluorescence (TADF) with a small singlet–triplet energy gap (ΔEST), enabling a high photoluminescence quantum yield of 97.6%. Under X-ray excitation, the Cu2I2(N̂P)3 complex achieves a light yield 2.87 times that of commercial scintillator (Bi4Ge3O12, BGO) and an ultralow detection limit of 172.9 nGyair 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.