Thermally Activated Delayed Fluorescence-Assisted Near-Infrared Emission from Cationic-Ligand Cuprous Iodide Hybrids for Near-Infrared Imaging

Abstract Developing efficient near-infrared (NIR) emitters based on metal halide hybrids requires both narrow bandgaps and precise control of radiative excited-state decay. Herein, we report two low-bandgap, all-in-one (AIO) CuI hybrid emitters synthesized using cationic ligands. Both exhibit broad NIR emission centered around 900 nm but display markedly different photoluminescence quantum yields (PLQYs). Compound 1 achieves the highest reported PLQY of 32.0% among metal-halide hybrid NIR emitters with emission wavelengths longer than 800 nm, whereas compound 2 shows a much lower PLQY of 8.3%. Structural, temperature-dependent photophysical, and TD-DFT studies reveal that the ligand-controlled coordination geometry plays a key role in regulating the emissive excited states. The more regular coordination environment in compound 1 favors a charge-transfer excited state with a small singlet–triplet energy gap, enabling thermally activated delayed fluorescence-assisted NIR emission. Conversely, compound 2 is dominated by low-lying triplet states with stronger cluster-centered character. Furthermore, compound 1 can be easily processed into nanoparticles and polymer composite films, enabling proof-of-concept phosphor-converted NIR imaging. This work expands the cationic-ligand strategy for the development of NIR-emissive CuI hybrids and provides key insights into leveraging excited-state dynamics to achieve efficient low-energy emission.