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.

A set of robust molecular cobalt catalysts for the generation of hydrogen from water is reported. The cobalt complex supported by the parent pentadentate polypyridyl ligand PY5Me(2) features high stability and activity and 100% Faradaic efficiency for the electrocatalytic production of hydrogen from neutral water, with a turnover number reaching 5.5 × 10(4) mol of H(2) per mole of catalyst with no loss in activity over 60 h. Control experiments establish that simple Co(II) salts, the free PY5Me(2) ligand, and an isostructural PY5Me(2) complex containing redox-inactive Zn(II) are all ineffective for this reaction. Further experiments demonstrate that the overpotential for H(2) evolution can be tuned by systematic substitutions on the ancillary PY5Me(2) scaffold, presaging opportunities to further optimize this first-generation platform by molecular design.

Organic semiconductors have great potential as the active material in low-cost, large area plastic electronics, whether as light-emitting diodes (LEDs), field-effect transistors (FETs) or solar cells. Organic semiconducting materials retain the processability associated with polymers while maintaining good optoelectronic properties, for example, high absorption coefficients for photons in the visible, and field-effect mobilities comparable with that of amorphous silicon. The elucidation of important structure-property relationships is vital for the design of functional, high-performance organic semiconductors. In this short review, we summarize such relationships stemming from the halogenation of organic semiconductors. While it has been known in the past decade that fluorination lowers the energy levels in carbon based systems, induces stability and electron transport, less is known about the effect of the other halogens. Chlorination has recently been shown to be a viable route to n-type materials. The bandgap of conjugated compounds can also be decreased slightly by the addition of Cl, Br, and I to the aromatic core. The effect of the halogenated moieties on the packing of molecules is discussed.

We show that adding chlorine atoms to conjugated cores is a general, effective route toward the design of n-type air-stable organic semiconductors. We find this to be true for acenes, phthalocyanines, and perylene tetracarboxylic diimide (PDI)-based molecules. This general finding opens new avenues in the design and synthesis of organic semiconductors. We compared a series of fluoro- and chloro-functionalized acenes, phthalocyanines, and PDI-based molecules. The acenes synthesized showed high and balanced ambipolar transport in the top-contact organic field effect transistor (OFET) geometry. The electron-withdrawing halogen groups lowered the LUMO and the charge injection barrier for electrons, such that electron and hole transport occurred simultaneously. If the chlorine added does not distort the planarity of the conjugated core, we found that the chloro-functionalized molecules tend to have a slightly smaller HOMO-LUMO gap and a lower LUMO level than the fluoro-containing molecules, both from calculations and cyclic voltammetry measurements in solution. This is most likely due to the fact that Cl contains empty 3d orbitals that can accept pi-electrons from the conjugated core, while F does not have energetically accessible empty orbitals for such delocalization.

We investigate the relationship between the charge carrier type in organic thin film transistors (OTFTs) and molecular energy levels. We examine a series of functionalized acenes that collectively have their HOMOs range from -4.9 eV to -5.6 eV and LUMOs range from -2.8 eV to -3.7 eV, as measured by cyclic voltammetry. Placed together, these 20 molecules allow us to chart the transition from OTFTs that display only hole transport, to ambipolar, to solely electron transport. Specifically, we note that for octadecyltrimethoxysilane (OTS) treated substrates, with top contact gold electrodes, electron injection and transport occurs when the LUMO < -3.15 eV, while hole injection and transport ceases when the HOMO < -5.6 eV. Ambipolar transport prevails when molecules have HOMO/ LUMO levels within the aforementioned range. This is seen across channel lengths ranging from 50-150 microm and using only gold as electrodes. This empirical plot is the first time such a detailed study has been made on the onset of charge injection and transport for a class of organic semiconductors. It provides guidelines for future molecular design.

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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.