Optically Activated Delayed Fluorescence.

We harness the photophysics of few-atom silver nanoclusters to create the first fluorophores capable of optically activated delayed fluorescence (OADF). In analogy with thermally activated delayed fluorescence, often resulting from oxygen- or collision-activated reverse intersystem crossing from triplet levels, this optically controllable/reactivated visible emission occurs with the same 2.2 ns fluorescence lifetime as that produced with primary excitation alone but is excited with near-infrared light from either of two distinct, long-lived photopopulated dark states. In addition to faster ground-state recovery under long-wavelength co-illumination, this "repumped" visible fluorescence occurs many microsceconds after visible excitation and only when gated by secondary near-IR excitation of ∼1-100 μs-lived dark excited states. By deciphering the Ag nanocluster photophysics, we demonstrate that OADF improves upon previous optical modulation schemes for near-complete background rejection in fluorescence detection. Likely extensible to other fluorophores with photopopulatable excited dark states, OADF holds potential for drastically improving fluorescence signal recovery from high backgrounds.