Stacy Copp1,Rweetuparna Guha1,Anna Gonzalez Rosell1,Malak Rafik1
University of California, Irvine1
Stacy Copp1,Rweetuparna Guha1,Anna Gonzalez Rosell1,Malak Rafik1
University of California, Irvine1
DNA can template the chemical synthesis of few-atom silver nanoclusters with defined chiralities and strong electronic circular dichroism (CD) response. These "Ag<sub>N</sub>-DNAs" have a diversity of sizes, shapes, and optical properties that are selected by the nucleobase sequence of their DNA ligands. Composed of just 10-30 Ag atoms and 1-3 short DNA oligomers, most studies of Ag<sub>N</sub>-DNAs have focused on their bright visible to near-infrared fluorescence. The DNA sequence-selected chiroptical properties of Ag<sub>N</sub>-DNAs are also of both fundamental and applied interest and have been far less studied. As an inherently chiral ligand, DNA selects for nanocluster chirality without the need for intensive separation techniques that are typically required for metal nanocluster enantiomer separation, such as chiral chromatography. The chiral properties of Ag<sub>N</sub>-DNAs hold promise for biomolecular sensing and nanophotonics applications. However, studies of the chirality of Ag<sub>N</sub>-DNAs prepared to atomic precision are limited to date, and few X-ray crystallographic structures have been reported.<br/><br/>To understand the chiral properties of Ag<sub>N</sub>-DNAs, we investigated a large set of 19 different atomically precise Ag<sub>N</sub>-DNA species with emission at the far red/NIR spectral border [1]. The molecular formula of each purified species is determined by high-resolution mass spectrometry and correlated to its optical absorbance, emission, and CD spectra. We find that these Ag<sub>N</sub>-DNAs are either 6-electron clusters or 8-electron clusters (corresponding to the oxidation state of the silver nanocluster), with distinct absorbance and CD spectral features for these two classes of nanoclusters. 8-electron Ag<sub>N</sub>-DNAs are stabilized by two DNA oligomer copies and display complex CD spectra with multiple distinct UV and visible transitions. The electron count of these "magic-sized" 8-electron nanoclusters suggests quasispherical geometries. 6-electron Ag<sub>N</sub>-DNAs, which past studies support have rod-like shapes, exhibit three different ligand compositions, each with its own characteristic CD signatures. Ag<sub>N</sub>-DNAs stabilized by two oligomer copies exhibit a distinct positive or negative monosignate CD signature aligned with the longest visible absorbance peak of the nanocluster, suggesting a chiral nanocluster core whose handedness is selected by the specific DNA template sequence. Ag<sub>N</sub>-DNAs stabilized by three oligomer copies exhibit the same visible CD signature but distinct UV CD signatures, suggesting that DNA ligands adopt similar conformations around nanoclusters with three DNA ligands. Finally, Ag<sub>N</sub>-DNAs stabilized by two oligomer copies and additional chlorido ligands exhibit significantly suppressed visible CD signatures, supporting that the additional atomic ligands suppresses nanocluster chirality. The distinct chiroptical signatures and compositions of these four classes of Ag<sub>N</sub>-DNAs also correlate to distinct excited state emission properties, suggesting that nanocluster electron count and chirality influences both ground and excited state processes in these emitters. This study illustrates the diversity of structure-property relationships for NIR-emissive Ag<sub>N</sub>-DNAs, which could be harnessed to precisely tune these emitters for applications that exploit their DNA sequence-selected chiral properties. Moreover, the results show that the chiroptical properties of Ag<sub>N</sub>-DNAs are highly sensitive to nanocluster composition and ligand conformation.<br/><br/>References: [1] Guha, Rweetuparna, et al. "Electron count and ligand composition influence the optical and chiroptical signatures of NIR-emissive DNA-stabilized silver nanoclusters." (2023). DOI: 10.26434/chemrxiv-2023-hftx9