Tailoring Photonic Transport in Cluster‐Assembled Crystals via Atomic‐Level Coordination Engineering

The development of efficient optical waveguides requires precise control over photonic transport properties, yet linking atomic‐level structure to device performance remains challenging. This study demonstrates that atomic‐level coordination engineering of metal nanoclusters enables tailored photonic transport in cluster‐assembled crystals. Using bidentate phosphine ligands as molecular scalpels, we constructed a series of Pt 1 Ag x ( x = 18–37) nanoclusters with identical icosahedral kernels but systematically varied peripheral structures. The resulting crystals exhibit exceptionally low optical loss coefficient, with the Pt 1 Ag 18 SR 8 Cl 2 (DPPP) 4 (SR = 1‐adamantanethiol, DPPP = 1,3‐bis(diphenylphosphino)propane. Pt 1 Ag 18 ‐I for short.) crystal achieving a record‐low among cluster‐based active optical waveguides value of 6.4 × 10 −4 dB µm −1 . We establish quantitative positive correlations between waveguide performance and four key photophysical parameters: photoluminescence quantum yield, lifetime, refractive index, and polarization degree. This work provides a quantitative empirical structure–activity relationship (SAR) model for designing advanced photonic materials, bridging atomic‐scale precision with macroscopic optical device functionality for next‐generation integrated photonics.