Electron count and ligand composition influence the optical and chiroptical signatures of NIR-emissive DNA-stabilized silver nanoclusters

Near-infrared (NIR) emissive DNA-stabilized silver nanoclusters (AgN-DNAs) are promising fluorophores in the biological tissue transparency windows. Hundreds of NIR-emissive AgN-DNAs have been recently discovered, but their structure-property relationships remain poorly understood. Here, we investigate 19 different far-red and NIR emissive AgN-DNA species stabilized by 10-base DNA templates, including several well-studied emitters whose compositions and chiroptical properties have never been reported before. The molecular formula of each purified species is determined by high-resolution mass spectrometry and correlated to its optical absorbance, emission, and circular dichroism (CD) spectra. We find that AgN-DNAs emissive at the far red/NIR spectral border are either 6-electron clusters or 8-electron clusters. 8-electron AgN-DNAs are stabilized by two DNA oligomer copies, while three different ligand compositions are observed for 6-electron AgN-DNAs: two oligomer copies, three oligomer copies, or two oligomer copies with additional chlorido ligands. Distinct optical and chiroptical signatures correlate with ligand composition of 6-electron AgN-DNAs. AgN-DNAs with three oligomer ligands exhibit shorter Stokes shifts than AgN-DNAs with two oligomers, and the presence of chlorido ligands is correlated with increased Stokes shifts and significantly suppressed visible CD transitions. Nanocluster electron count also significantly influences electronic structure and optical properties, with 6-electron and 8-electron AgN-DNAs exhibiting distinct absorbance and CD spectral features. This study shows that the optical and chiroptical properties of NIR-emissive AgN-DNAs are highly sensitive to nanocluster composition and illustrates the diversity of structure-property relationships for NIR-emissive AgN-DNAs, which could be harnessed to precisely tune these emitters for bioimaging applications.