Heterogeneous Solvatochromism of Fluorescent DNA-Stabilized Silver Clusters Precludes Use of Simple Onsager-Based Stokes Shift Models

The diverse optical and chemical properties of DNA stabilized silver clusters (Ag-N-DNAs) have challenged the development of a common model for these sequence-tunable fluorophores. Although correlations between cluster geometry and fluorescence color have begun to shed light on how the optical properties of Ag-N- DNAs are selected, the exact mechanisms responsible for fluorescence remain unknown. To explore these mechanisms, we study four distinct purified Ag-N-DNAs in ethanol water and methanol water mixtures and find that the solvatochromic behavior of Ag-N-DNAs varies widely among different cluster species and differs markedly from prior results on impure material. Placing Ag-N-DNAs within the context of standard Lippert-Mataga solvatochromism models based on the Onsager reaction field, we show that such nonspecific solvent models are not universally applicable to Ag-N-DNAs. Instead, alcohol-induced solvatochromism of Ag-N-DNAs may be governed by changes in hydration of the DNA template, with spectral shifts resulting from cluster shape changes and/or dielectric changes in the local vicinity of the cluster.