Photon upconversion, the combination of two low energy photons into one high energy photon, has potential applications in solar energy, biological imaging, and data storage. Here, we present a novel molecule-nanocrystal upconversion platform: CdSe and PbSe semiconductor nanocrystals as sensitizers are combined with molecular annihilators to upconvert photons in both the visible and near infrared spectral regions (from 488 to 980 nm). In this process, the absorption of low energy photons by the nanocrystals is followed by energy transfer to molecular triplet states, which then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light. This mechanism was confirmed by the observation of delayed fluorescence of annihilators in time-resolved fluorescence spectroscopy. Advantages of this hybrid upconversion system over conventional molecular and the rare-earth based upconversion systems lie in the large absorption cross-sections of nanocrystals, and the tunability of the nanocrystal light absorber, allowing upconversion to be engineered for arbitrary wavelengths. The system of visible upconversion includes CdSe nanocrystal sensitizers, 9-anthracene carboxylic acid (9-ACA) transmitters, and diphenylanthracene annihilators. For visible upconversion, the efficiency can be enhanced by increasing the surface coverage of 9-ACA transmitter, and using CdSe nanocrystals that have high PLQYs and small sizes. On the other hand, for near infrared upconversion, the combination of PbSe nanocrystals as sensitizers and rubrene as an annihilator is able to upconvert photons as red as 980 nm. This is the longest wavelength that has been upconverted using the triplet sensitization strategy. In this case, conjugated transmitter ligands are not necessary to realize upconversion, but are expected to greatly enhance the upconversion efficiency by up to three orders of magnitude, as observed for the upconversion of visible light with CdSe nanocrystal sensitizers.

Hybrid molecule-nanocrystal photon upconversion across the visible and near-infrared

251st National Meeting (2016)

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