A single nucleobase tunes nonradiative decay in a DNA-bound silver cluster

DNA strands are polymeric ligands that both protect and tune molecular-sized silver cluster chromophores. We studied single-stranded DNA C(4)AC(4)TC(3)XT(4) with X = guanosine and inosine that form a green fluorescent Ag-10(6+) cluster, but these two hosts are distinguished by their binding sites and the brightness of their Ag-10(6+) adducts. The nucleobase subunits in these oligomers collectively coordinate this cluster, and fs time-resolved infrared spectra previously identified one point of contact between the C2-NH2 of the X = guanosine, an interaction that is precluded for inosine. Furthermore, this single nucleobase controls the cluster fluorescence as the X = guanosine complex is similar to 2.5x dimmer. We discuss the electronic relaxation in these two complexes using transient absorption spectroscopy in the time window 200 fs-400 mu s. Three prominent features emerged: a ground state bleach, an excited state absorption, and a stimulated emission. Stimulated emission at the earliest delay time (200 fs) suggests that the emissive state is populated promptly following photoexcitation. Concurrently, the excited state decays and the ground state recovers, and these changes are similar to 2x faster for the X = guanosine compared to the X = inosine cluster, paralleling their brightness difference. In contrast to similar radiative decay rates, the nonradiative decay rate is 7x higher with the X = guanosine vs inosine strand. A minor decay channel via a dark state is discussed. The possible correlation between the nonradiative decay and selective coordination with the X = guanosine/inosine suggests that specific nucleobase subunits within a DNA strand can modulate cluster-ligand interactions and, in turn, cluster brightness. Published under an exclusive license by AIP Publishing.

Automated summary

The specific nucleobase subunits within a DNA strand modulate cluster-ligand interactions and brightness, with guanosine and inosine affecting the fluorescence brightness and decay rate differently. Nonradiative decay is 7x higher with guanosine than inosine, indicating a potential correlation between nonradiative decay and selective coordination with specific nucleobase subunits.