Mechanism of Multiple Emission in Gold Quantum Boxes and Design Rules for Strong Near-Infrared Luminescence.

Ligand-protected gold nanoclusters (Au NCs) hold great promise for near-infrared photonics and bioimaging, but their low photoluminescence quantum yields (PLQYs) and unclear emission mechanisms have limited the rational design of highly luminescent NCs. Here, we report a comprehensive theoretical investigation of the representative Au8n+4(SR)4n+8 quantum-box series (n = 3, 4, 5, 6) using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to elucidate the structure-photophysics relationship. We reveal that high structural symmetry and small aspect ratios (AR) severely limit transition dipole moments and radiative decay rates in these face-centered-cubic (FCC)-type NCs, in contrast to highly emissive hexagonal close-packed (HCP) quantum-rod analogues. On the other hand, the quantum-box NCs exhibit unusually small singlet-triplet energy gaps (ΔEST) and strong spin-orbit coupling, enabling ultrafast intersystem crossing (ISC) and reverse intersystem crossing (RISC), giving rise to simultaneous prompt fluorescence (PF), thermally activated delayed fluorescence (TADF), and phosphorescence. The small gaps arise from spatially decoupled electron-hole distributions distinct from those in organic TADF materials. Moreover, low-frequency ligand (the -R group) torsional and librational motions dominate internal conversion via large reorganization energies, while one-dimensional cluster growth modulates staple-motif packing and induces nonmonotonic, ligand-dominated nonradiative relaxation. Collectively, weak radiative transitions due to symmetry constraints and efficient nonradiative decay via unconstrained ligand vibrations account for the universally low PLQY of quantum-box Au NCs. This work clarifies the fundamental roles of symmetry, anisotropy, and ligand dynamics in luminescence, offering critical design rules for developing high-efficiency near-infrared-emissive Au NCs.