Developing high‐performance visible‐to‐UV photon upconversion systems based on triplet–triplet annihilation photon upconversion (TTA‐UC) is highly desired, as it provides a potential approach for UV light‐induced photosynthesis and photocatalysis. However, the quantum yield and spectral range of visible‐to‐UV TTA‐UC based on nanocrystals (NCs) are still far from satisfactory. Here, three different sized CdS NCs are systematically investigated with triplet energy transfer to four mediators and four annihilators, thus substantially expanding the available materials for visible‐to‐UV TTA‐UC. By improving the quality of CdS NCs, introducing the mediator via a direct mixing fashion, and matching the energy levels, a high TTA‐UC quantum yield of 10.4% (out of a 50% maximum) is achieved in one case, which represents a record performance in TTA‐UC based on NCs without doping. In another case, TTA‐UC photons approaching 4 eV are observed, which is on par with the highest energies observed in optimized organic systems. Importantly, the in‐depth investigation reveals that the direct mixing approach to introduce the mediator is a key factor that leads to close to unity efficiencies of triplet energy transfer, which ultimately governs the performance of NC‐based TTA‐UC systems. These findings provide guidelines for the design of high‐performance TTA‐UC systems toward solar energy harvesting.

Precisely modulated photoluminescence (PL) with external control is highly demanded in material and biological sciences. However, it is challenging to switch the PL on and off in the NIR region with a high modulation contrast. Here, we demonstrate that reversible on and off switching of the PL in the NIR region can be achieved in a bicomponent system comprised of PbS semiconducting nanocrystals (NCs) and diarylethene (DAE) photoswitches. Photoisomerization of DAE to the ring-closed form upon UV light irradiation causes substantial quenching of the NIR PL of PbS NCs due to efficient triplet energy transfer. The NIR PL fully recovers to an on state upon reversing the photoisomerization of DAE to the ring-open form with green light irradiation. Importantly, fully reversible switching occurs without obvious fatigue, and the high PL on/off ratio (>100) outperforms all previously reported assemblies of NCs and photoswitches.

A simple, efficient and general approach for all-visible-light switching of photochromic diarylethenes is developed through triplet sensitization from CdS nanocrystals, which has high potential for all-visible-light activated optoelectronics.

A novel photoswitchable system was developed by combining carboxylated diarylethenes (DAE 1) and CdS quantum dots (QDs), enabling all-visible light reversible interconversion of open and closed forms of DAE 1 in both aerated solution and solid-state.

Triplet‐triplet annihilation photon upconversion (TTA‐UC) has emerged as a promising strategy for enhancing solar energy harvesting efficiency by converting two low‐energy, long‐wavelength photons into a high‐energy, short‐wavelength photon. In recent years, semiconductor nanocrystals have gained significant attention as efficient photosensitizers for TTA‐UC due to their excellent triplet energy transfer efficiency and the ability to tune their bandgap across the solar spectrum. This review focuses on the mechanism of NC‐based TTA‐UC, emphasizing key parameters to evaluate the performance of TTA‐UC systems. The influence of various material‐related factors on the overall NC‐based TTA‐UC performance is thoroughly discussed. Moreover, recent advances in solid‐state approaches for NC‐based TTA‐UC are highlighted, along with an overview of the current status of applications in this field. Lastly, this review identifies the challenges and opportunities that lie ahead in the future development of NC‐based TTA‐UC, providing insights into the potential advancements and directions for further research.

Overcrowded alkene-based molecular motors have high potential in building artificial molecular machines, precise drug delivery, and smart responsive materials due to their unidirectional rotation at the molecular scale under UV light. Powering molecular motors with visible light, especially in the low-energy green, yellow, and red regions, has been a significant challenge, limiting their further application. Herein, we report a general and versatile strategy to overcome this challenge and drive molecular motors using low-energy, low-intensity, noncoherent light across the visible spectrum. Our approach is achieved by simply mixing molecular motors with semiconductor colloidal quantum dots (QDs) and a triplet mediator (9-anthracenecarboxylic acid). The size-tunable absorption of QDs can precisely match the desired color for activating the motor. The mechanism of our design relies on the unique property of QDs, which can efficiently sensitize molecular triplets. Through two-step triplet energy transfers, the rotation of the motor can be efficiently activated using low-energy photons. For the first time, we accomplished driving molecular motors at wavelengths beyond 530 nm under low-intensity and noncoherent light. This breakthrough not only expands the capabilities of visible-light-activated molecular systems to operate in a broad wavelength range but also opens numerous opportunities toward controlling dynamic functions while circumventing competing photochemical processes (i.e., photodegradation).