Optically Activated Delayed Fluorescence through Control of Cyanine Dye Photophysics

Merocyanine 540 fluorescence can be enhanced by optically depopulating dark photoisomer states to regenerate the fluorescence-generating manifold of the all-trans isomer. Here, we utilize a competing modulation route, long-wavelength coexcitation of the trans triplet population to not only modulate fluorescence through enhanced ground-state recovery but also generate optically activated delayed fluorescence (OADF) with longer-wavelength co-illumination. Such OADF (∼580 nm) is directly observed with pulsed fluorescence excitation at 532 nm, followed by long-wavelength (637 nm) continuous wave depopulation of the photogenerated triplet by repopulating the emissive S₁ state. Such reverse intersystem crossing (RISC) results in ns-lived fluorescence delayed by several microseconds after the initial primary excitation pulse and the prompt 1 ns-lived fluorescence that it induces. The dark state from which OADF is generated decays more rapidly with increased secondary laser intensity, as the optically induced RISC rate increases. This first OADF from organic dyes is observed, as the red secondary laser excites ∼580 nm, <1 ns-lived fluorescence from the previously optically prepared ∼1 μs-lived triplet state. This sequential two-photon, repumped fluorescence yields background-free collection with potential for new high-sensitivity fluorescence imaging schemes.