Excitation Spectra Argue for Threadlike Shape of DNA-Stabilized Silver Fluorescent Clusters

We calculate geometry and electronic excitation spectra of silver clusters containing 1–6 atoms bound to single DNA bases and also to dC3 cytosine oligomer using density functional theory method. We show that planar shaped silver clusters complexed with the bases exhibit nearly forbidden the lowest transitions and weak fluorescence ability. Their calculated spectra do not appear to match experimentally observed fluorescence excitation spectra of silver clusters on DNA and other polymers. On the contrary, threadlike-shaped silver clusters reveal intense the lowest transition, energy of which depends on the chain bend. We show the equilibrium structures of Ag3 + cluster stabilized by oxygen atoms on phosphates and in the minor groove of dC3 oligomer. The calculated excitation spectrum of Ag3 + cluster in the minor groove appears to be close to the experimental spectrum of green emitting clusters on cytosine oligomers.