Molecular vibration assisted triplet-triplet annihilation nir-upconversion luminescence of fluorescein

Abstract Here we report the near-infrared-upconversion (NIR-UC) luminescence of the organic dye fluorescein, without the need of metal-based NIR-UC materials or the use of high-energy and high-cost two-photon absorption excitation (TPA) systems. Fluorescein upconverts NIR light (λ = 920 nm) to visible light (λ = 545 nm) in aqueous solutions, with a pH-dependent light-intensity, by using a low-energy and low-cost nanosecond pulsed laser. The NIR-UC of fluorescein is explained with a triplet-triplet annihilation (TTA) mechanism, assisted by molecular vibrations. This work opens the door for deep-penetration bioimaging and photodynamic therapy, as well for NIR-excited TTA, photocatalysis and photovoltaics, by using solely high quantum yield organic molecules.