Combined optical spectroscopic study now reveals the photophysical changes of InP QDs upon surface passivation by various methods.

In artificial photocatalysis, sluggish kinetics of hole transfer and the resulting high-charge recombination rate have been the Achilles' heel of photocatalytic conversion efficiency. Here we demonstrate water-soluble molecules as co-catalysts to accelerate hole transfer for improved photocatalytic H2 evolution activity. Trifluoroacetic acid (TFA), by virtue of its reversible redox couple TFA·/TFA−, serves as a homogeneous co-catalyst that not only maximizes the contact areas between co-catalysts and reactants but also greatly promotes hole transfer. Thus K4Nb6O17 nanosheet catalysts achieve drastically increased photocatalytic H2 production rate in the presence of TFA, up to 32 times with respect to the blank experiment. The molecular co-catalyst represents a new, simple and highly effective approach to suppress recombination of photogenerated charges, and has provided fertile new ground for creating high-efficiency photosynthesis systems, avoiding use of noble-metal co-catalysts.

Triplet energy transfer from quantum dots takes advantage of small energy loss during intersystem crossing.

Using two-photon excitation (2PE), molecular nanomachines (MNMs) are able to drill through cell membranes and kill the cells. This avoids the use of the more damaging ultraviolet (UV) light that has been used formerly to induce this nanomechanical cell-killing effect. Since 2PE is inherently confocal, enormous precision can be realized. The MNMs can be targeted to specific cell surfaces through peptide addends. Further, the efficacy was verified through controlled opening of synthetic bilayer vesicles using the 2PE excitation of MNM that had been trapped within the vesicles.

Recently, the implantation of non-noble-metal electrocatalysts into photocatalysts has brought dramatically improved hydrogen evolution activities; yet, the mechanistic details are still under debate, because of the poor understanding of interfacial charge carrier dynamics. Here, for the first time, we unravel that it is the electrocatalytic process that plays the critical role in these heterostructured systems. Spectroscopic characterizations, combined with theoretical calculations, give a clear physical picture that the photoexcited electrons transfer from photocatalysts to phosphides electrocatalysts, then driving H 2 evolution reaction similar to electrocatalysis; and also reveal the Fermi level of electrocatalysts as a feasible descriptor for the photocatalytic activity.

Conspectus The semiconductor-nanocrystal-sensitized, three-component upconversion system has made great strides over the past 5 years. The three components (i.e., triplet photosensitizer, mediator, and emitter) each play critical roles in determining the input and output photon energy and overall quantum efficiency (QE). The nanocrystal photosensitizer converts the absorbed photon into singlet excitons and then triplet excitons via intersystem crossing. The mediator accepts the triplet exciton via either direct Dexter-type triplet energy transfer (TET) or sequential charge transfer (CT) while extending the exciton lifetime. Through a second triplet energy-transfer step from the mediator to the emitter, the latter is populated in its lowest excited triplet state. Triplet–triplet annihilation (TTA) between two triplet emitters generates the emitter in its bright singlet state, which then emits the upconverted photon. Quantum dots (QD) have a tunable band gap, large extinction coefficient, and small singlet–triplet energy losses compared to metal–ligand charge-transfer complexes. This high triplet exciton yield makes QDs good candidates for photosensitizers. In terms of driving triplet energy transfer, the triplet energy of the mediator should be slightly lower than the triplet exciton energy of the QD sensitizer for a downhill energy landscape with minimal energy loss. The same energy cascade is also required for the transfer from the mediator to the emitter. Finally, the triplet energy of the emitter must be slightly larger than one-half of its singlet energy to ensure that TTA is exothermic. Optimization of the sensitizer, mediator, and emitter will lead to an increase in the anti-Stokes shift and the total quantum efficiency. Evaluating each individual step’s efficiency and kinetics is necessary for the understanding of the limiting factors in existing systems. This review summarizes chalcogenide QD-based photon upconversion systems with a focus on the mechanistic aspects of triplet energy transfer conducted by the Tang and Lian groups. Via time-resolved spectroscopy, the rates and major loss pathways associated with the two triplet energy-transfer steps were identified. The studies are focused on the near-infrared (NIR) to visible (VIS) PbS-tetracene-based systems as they allow systematic control of the QD, mediator, and emitter. Our results show that the mediator triplet state is mostly formed by direct TET from the QD and the transfer rate is influenced by the density of bound mediator molecules. Charge transfer, a loss pathway, does not produce triplet excitons and can be minimized by adding an inert shell to the QD. This transfer rate decreases exponentially with the distance between the QD and mediator molecule. The second TET rate was found to be much slower than the diffusion-limited collision rate, which results in the triplet lifetime of the mediator being the main factor limiting its efficiency. Finally, the total quantum efficiency can be calculated using these measured quantities including the TET1 ...

Triplet energy transfer (TET) from quantum dots (QDs) to molecular acceptors has received intense research interest because of its promising application as triplet sensitizers in photon up-conversion. Compared to QD band edge excitons, the role and mechanism of trap state mediated TET in QD-acceptor complexes have not been well understood despite the prevalence of trap states in many QDs. Herein, TET from trap states in CdSe QDs to adsorbed 9-anthracene carboxylic acid (ACA) is studied with steady state photoluminescence, transient absorption spectroscopy, and time-resolved photoluminescence. We show that both band edge and trap excitons undergo direct Dexter energy transfer to form the triplet excited state of ACA. The rate of TET decreases from (0.340 ± 0.002) ns−1 to (0.124 ± 0.004) ns−1 for trap excitons with decreasing energy from 2.25 eV to 1.57 eV, while the TET rate from band edge excitons is 13–37 times faster than trapped excitons. Despite slightly higher TET quantum efficiency from band edge excitons (∼100%) than trapped excitons (∼95%), the overall TET process from CdSe to ACA is dominated by trapped excitons because of their larger relative populations. This result demonstrates the important role of trap state mediated TET in nanocrystal sensitized triplet generation.

Quantum dot (QD) sensitized triplet exciton generation has demonstrated promising applications in various fields such as photon up-conversion through triplet–triplet annihilation. However, how direct triplet energy transfer from the QD to the acceptor through Dexter energy transfer (DET) competes with other processes, including Förster resonance energy transfer (FRET) and charge transfer, remains poorly understood. Herein, the competition of these pathways for QD-sensitized triplet excited state generation in CdSe QD-modified boron dipyrromethene (BODIPY) complexes is studied using transient absorption spectroscopy. After excitation of the CdSe QD with 500 nm pulses, the BODIPY triplet excited state is generated through charge recombination in a charge separated intermediate state (QD−·–BODIPY+·). This intermediate state is populated either through FRET from the excited QD to BODIPY followed by electron transfer from the singlet excited state of BODIPY to the QD or through hole transfer from the excited QD to BODIPY. The triplet excited state generation efficiencies from the FRET and hole transfer pathways are estimated to be (6.18 ± 1.39)% and (13.5 ± 3.1)%, respectively. Compared to these indirect pathways, direct DET from the QD to the BODIPY triplet state is kinetically not competitive. These results demonstrate that sequential charge transfer can be an efficient pathway for triplet excited state generation in QD–acceptor complexes.