Photocatalysts based on g-C 3 N 4 by loading cocatalysts or constructing heterojunctions have shown great potential in solar-driven water oxidation. However, the intrinsic drawbacks of g-C 3 N 4 , such as poor mass diffusion and charge separation efficiency, remain as the bottleneck to achieve highly efficient water oxidation. Here we report a simple protonation method to improve the activity of g-C 3 N 4 . Studies using valence band X-ray photoelectron spectra and steady-state and time-resolved spectroscopy reveal that the promotion of catalytic ability originates from the higher thermodynamical driving force and longer-lived charge separation state, which may provide guidance in designing efficient polymeric semiconductor photocatalysts with desirable kinetics for water oxidation.

Sensitized triplet-triplet annihilation (sTTA) is the most promising mechanism for pooling the energy of two visible photons, but its applications in solution were so far limited to organic solvents, with a current maximum of the excited-singlet state energy of 3.6 eV. By combining tailor-made iridium complexes with naphthalenes, we demonstrate blue-light driven upconversion in water with unprecedented singlet-state energies approaching 4 eV. The annihilators have outstanding excited-state reactivities enabling challenging photoreductions driven by sTTA. Specifically, we found that an arylbromide bond activation can be achieved with blue photons, and we obtained full conversion for the very energy-demanding decomposition of a persistent ammonium compound as typical water pollutant, not only with a cw laser but also with an LED light source. These results provide the first proof-of-concept for the usage of low-power light sources for challenging reactions employing blue-to-UV upconversion in water, and pave the way for the further development of sustainable light-harvesting applications.

Selective oxidation of alcohols to aldehydes/ketones has been achieved with the help of 3-mercaptopropionic acid (MPA)-capped CdSe quantum dot (MPA-CdSe QD) and visible light. Visible-light-prompted electron-transfer reaction initiates the oxidation. The thiyl radical generated from the thiolate anion adsorbed on a CdSe QD plays a key role by abstracting the hydrogen atom from the C-H bond of the alcohol (R CH(OH)R ). The reaction shows high efficiency, good functional group tolerance, and high site-selectivity in polyhydroxy compounds. The generality and selectivity reported here offer a new opportunity for further applications of QDs in organic transformations.