The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. Triplet-triplet annihilation using all-organic systems is a highly efficient approach to upconversion, but organic molecules have limited photostability and poor spectral coverage in the infrared. One way to sensitize molecular triplet states is to use semiconductor nanocrystals (like CdSe and PbSe) to absorb the long-wavelength photons (visible and near-infrared) and then sensitize the triplet states of molecular emitters (diphenylanthracene and rubrene). Absorption of low energy photons by the nanocrystals is followed by energy transfer to the molecular triplet states, which then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light in the visible and ultraviolet regions. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process can be enhanced by up to three orders of magnitude. The mechanism of the nanocrystal-to-triplet energy transfer is investigated using time-resolved spectroscopy. An alternate approach for solid-state upconversion is to take advantage of low-energy intermolecular states in organic crystals to sensitize triplet states. Upconverted photons are generated by triplet-triplet fusion in the solid-state. We show that this process can be surprisingly efficient in certain molecular crystals, even in the absence of nanocrystal sensitizers.

New ways to activate organic triplet states for photon upconversion in the visible and near-infrared

251st National Meeting (2016)

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