Optically enhanced, near-IR, silver cluster emission altered by single base changes in the DNA template

Few-atom silver clusters harbored by DNA are promising fluorophores due to their high molecular brightness along with their long- and short-term photostability. Furthermore, their emission rate can be enhanced when co-illuminated with low-energy light that optically depopulates the fluorescence-limiting dark state. The photophysical basis for this effect is evaluated for two near-infrared-emitting clusters. Clusters emitting at ∼800 nm form with C(3)AC(3)AC(3)TC(3)A and C(3)AC(3)AC(3)GC(3)A, and both exhibit a trap state with λ(max) ∼ 840 nm and an absorption cross section of (5-6) × 10(-16) cm(2)/molecule that can be optically depopulated. Transient absorption spectra, complemented by fluorescence correlation spectroscopy studies, show that the dark state has an inherent lifetime of 3-4 micros and that absorption from this state is accompanied by photoinduced crossover back to the emissive manifold of states with an action cross section of ∼2 × 10(-18) cm(2)/molecule. Relative to C(3)AC(3)AC(3)TC(3)A, C(3)AC(3)AC(3)GC(3)A produces a longer-lived trap state and permits more facile passage back to the emissive manifold. With the C(3)AC(3)AC(3)AC(3)G template, a spectrally distinct cluster forms having emission at ∼900 nm, and its trap state has a ∼4-fold shorter lifetime. These studies of optically gated fluorescence bolster the critical role of the nucleobases in both the formation and excited state dynamics of these highly emissive metallic clusters.