Silver clusters of less than 20 atoms in sizes wrapped in single-strand DNA provide tunable and bright fluorescence, making them promising materials in bio-imaging and bio-labeling. However, the mechanisms of formations of Ag clusters and control over their optical properties are not fully understood yet. We focus on neutral and charged Ag5 and Ag6 clusters capped by cytosine and guanine bases and their dimers interacting via pi-pi stacked guanines. Using density functional theory (DFT) for optimizing ground state geometry and time-dependent DFT (TD-DFT) for calculation of absorption spectra, we study the formation mechanisms of dimers and their effects on the optical transitions. Calculated absorption spectra of all clusters show that first singlet peak has d-d* and d-π* character. The energy and nature of these transitions not only depends on the DNA base type but also on the conformation of Ag cluster. When two cationic Ag6 clusters are connected through pi-stacked guanines, the dimer is decomposed into the 1-D chain of Ag2 clusters bound via DNA bases, which significantly increases the oscillator strength of the lowest energy optical transition at ~2 eV. In contrast, dimers formed from Guanine-stacked cationic Ag5 clusters stay stable, while their lower energy band has significantly lower intensity with several optically inactive red-shifted transitions. Overall, our computational results provide insights into the photophysical changes during the dimer formation of the small silver clusters.

Effects of charge and interactions with DNA bases on the structure and optical response of dimers of silver clusters

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.