The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. In this Letter, CdSe and PbSe semiconductor nanocrystals are combined with molecular emitters (diphenylanthracene and rubrene) to upconvert photons in both the visible and the near-infrared spectral regions. 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. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process could be enhanced by up to 3 orders of magnitude. The use of different combinations of nanocrystals and emitters shows that this platform has great flexibility in the choice of both excitation and emission wavelengths.

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Third generation photovoltaics are inexpensive modules that promise power conversion efficiencies exceeding the thermodynamic Shockley-Queisser limit, perhaps by using up- or down-converters, intermediate band solar cells, tandem cells, hot carrier devices, or multiexciton generation. Here, we report the efficient upconversion of infrared to visible light at excitation densities below the solar flux. Colloidally synthesized core-shell lead sulfide-cadmium sulfide nanocrystals in combination with tetracene derivatives absorb near-infrared light and emit visible light at 560 nm with an upconversion quantum yield (QY) of 8.4 ± 1.0%, which is a factor of 4 lower than the maximum upconversion QY possible. This is achieved with 808 nm cw excitation at 3.2 mW/cm, approximately three times lower than the available solar flux. The molecular and nanocrystal engineering here paves the way toward utilizing this hybrid upconversion platform in photovoltaics, photodetectors and photocatalysis.

Molecular control of energy transfer is an attractive proposition because it allows chemists to synthetically tweak various kinetic and thermodynamic factors. In this Perspective, we examine energy transfer between semiconductor nanocrystals (NCs) and π-conjugated molecules, focusing on the transmitter ligand at the organic−inorganic interface. Efficient transfer of triplet excitons across this interface allows photons to be directed for effective use of the entire solar spectrum. For example, a photon upconversion system composed of semiconductor NCs as sensitizers, bound organic ligands as transmitters, and molecular annihilators has the advantage of large, tunable absorption cross sections across the visible and near-infrared wavelengths. This may allow the near-infrared photons to be harnessed for photovoltaics and photocatalysis. Here we summarize the progress in this recently reported hybrid upconversion platform and point out the challenges. Since triplet energy transfer (TET) from NC donors to molecular transmitters is one of the bottlenecks, emphasis is on the design of transmitters in terms of molecular energetics, photophysics, binding affinity, stability, and energy offsets with respect to the NC donor. Increasing the efficiency of TET in this hybrid platform will increase both the up-and down-conversion quantum yields, potentially exceeding the Shockley−Queisser limit for photovoltaics and photocatalysis.

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