Breaching the Fortress: Photochemistry of DNA-Caged Ag₁₀⁶

A DNA strand can encapsulate a silver molecule to create a nanoscale, aqueous stable chromophore. A protected cluster that strongly fluoresces can also be weakly photolabile, and we describe the laser-driven photochemistry of the green fluorophore C₄AC₄TC₃GT₄/Ag₁₀⁶⁺. The embedded cluster is selectively photoexcited at 490 nm and then bleached, and we describe how the efficiency, products, and route of this photochemical reaction are controlled by the DNA cage. With irradiation at 496.5 nm, the cluster absorption progressively drops to give a photodestruction quantum yield of 1.5 (±0.2) × 10^-4, ∼10³× less efficient than fluorescence. A new λ_abs = 335 nm chromophore develops because the precursor with 4 Ag⁰ is converted into a group of clusters with 2 Ag⁰ - Ag₆⁴⁺, Ag₇⁵⁺, Ag₈⁶⁺, and Ag₉⁷⁺. The 4-7 Ag⁺ in this series are chemically distinct from the 2 Ag⁰ because they are selectively etched by iodide. This halide precipitates silver to favor only the smallest Ag₆⁴⁺ cluster, but the larger clusters re-develop when the precipitated Ag⁺ ions are replenished. DNA-bound Ag₁₀⁶⁺ decomposes because it is electronically excited and then reacts with oxygen. This two-step process may be state-specific because O₂ quenches the red luminescence from Ag₁₀⁶⁺. However, the rate constant of 2.3 (±0.2) × 10⁶ M^-1 s^-1 is relatively small, which suggests that the surrounding DNA matrix hinders O₂ diffusion. On the basis of analogous photoproducts with methylene blue, we propose that a reactive oxygen species is produced and then oxidizes Ag₁₀⁶⁺ to leave behind a loose Ag⁺-DNA skeleton. These findings underscore the ability of DNA scaffolds to not only tune the spectra but also guide the reactions of their molecular silver adducts.