Optically Activated Delayed Fluorescence

Abstract

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 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 microsecond 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.

Anti-Stokes fluorescence microscopy using direct and indirect dark state formation

“…[22][23][24][25] According to literature and in compliance with the phenomenological electronic state diagram in Fig. 1A, the dark state (D 1 ) can be optically depopulated by absorbing a photon from a secondary laser, which can bring the DNA-AgNC to the emissive state, as was recently demonstrated by Fleicher et al 30 We started by confirming that the red-emitting DNA-AgNCs, used in this study, can be optically pumped from the D 1 state to the S 1 state (see Fig. SI1, ESI †).…”

Unusually large fluorescence quantum yield for a near-infrared emitting DNA-stabilized silver nanocluster

“…7 It has been previously demonstrated for a large number of DNA-AgNCs that dark state formation from the Frank-Condon state is a common process. [20][21][22][23][24][25][26][27] This dark state can be optically excited by a secondary NIR laser that pumps the dark state to the emissive state in a process termed optically activated delayed fluorescence (OADF). 20,23 We performed OADF measurements, since this allows to estimate the minimum value for the quantum yield of dark state formation (Q D1 ), as was previously demonstrated for a red-emitting DNA-AgNC.…”

“…[20][21][22][23][24][25][26][27] This dark state can be optically excited by a secondary NIR laser that pumps the dark state to the emissive state in a process termed optically activated delayed fluorescence (OADF). 20,23 We performed OADF measurements, since this allows to estimate the minimum value for the quantum yield of dark state formation (Q D1 ), as was previously demonstrated for a red-emitting DNA-AgNC. 20,28,29 After determining the OADF and upconversion fluorescence (UCF) contributions to the secondary fluorescence (SF, see ESI † for details) and normalizing it to the primary fluorescence (PF), a minimum Q D1 value of 4.3% was found.…”

Probing heterogeneity of NIR induced secondary fluorescence from DNA-stabilized silver nanoclusters at the single molecule level

“…1A), which combines some of the advantages of upconversion and time-gating. 11 The imaging modality relies on long-lived (microseconds) dark states in DNA-encapsulated silver nanoclusters (DNA-AgNCs) which are formed upon visible (primary) excitation. 9,[12][13][14][15][16] Nanoclusters in the long-lived dark states can absorb an additional (secondary) NIR photon, which can bring them back to the emissive state.…”

“…The following OADF emission will occur on the anti-Stokes side of the secondary NIR excitation pulse. 11,17 By choosing an appropriate delay time for the secondary NIR pulse, time-gating can easily eliminate all auto-fluorescence coming from a primary excitation pulse. 17 This allows for simple suppression of potential NIR auto-fluorescence by a short-pass filter.…”