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 Cu 2 X 2 (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 Cu 2 X 2 (N̂P) 3 (X = Cl, Br, I) complexes exhibit thermally activated delayed fluorescence (TADF) with a small singlet–triplet energy gap (Δ E ST ), enabling a high photoluminescence quantum yield of 97.6%. Under X-ray excitation, the Cu 2 I 2 (N̂P) 3 complex achieves a light yield 2.87 times that of commercial scintillator (Bi 4 Ge 3 O 12, 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 Cu 2 X 2 nanocluster scintillators for high-resolution X-ray imaging.