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Mohammed Jabed1 Svetlana Kilina1

1, North Dakota State Univ, Fargo, North Dakota, United States

Silver clusters ‘grown’ in single-stranded DNA are promising new fluorophores for applications in bio-labels and bio-imaging, due to their highly emissive properties from visible to the near-infrared range and good biocompatibility. However, their photophysics is sensitive to their geometry, size, and interactions with the DNA nucleotides. The small silver cluster could also form the dimer and its photophysical properties largely depend on dimer formation mechanism. We have performed Density Functional Theory (DFT) calculations to study Ag clusters of 5 and 6 atoms in sizes passivated by three different nucleotides- cytosine, guanine, and thymine bases. Our calculations show that the geometry of clusters, the interaction between the cluster and bases depend on the oxidation state of the system, while different base-cluster binding trends are observed for mixed capping instead of only cytosine or only guanine passivation. It is also reported that dimer also could form, which has distinct photophysical properties. Time-dependent DFT (TD-DFT) calculations predict the red-shifted lower intensity peak in absorption spectra of both clusters, which originates from doublet transitions. The second more intensive absorption band appears nearly at the same energies for both singlet and doublet transitions but its intensity is largely ligand dependent. In case of the dimer, energy of the absorption peaks is almost same, but intensity depends on the dimer formation mechanism. It is shown that first peak is almost independent of dimer size and bridging but 2nd peak intensity is decreased if pi-pi stacking was formed during the dimer formation.

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