Sensitizing Ketone Triplets Using Low-Toxicity Blue Quantum Dots for Multifunctional Energy-Demanding Applications

Aromatic ketones have been long adopted as photocatalysts for photochemical synthesis owing to their efficient intersystem crossing and high triplet energy. Most of these ketones, however, absorb at ultraviolet wavelengths with low extinction coefficients, limiting the efficiency and scope of accessible photochemical reactions. Herein, we report the use of low-toxicity (Cd, Pb-free) blue ZnSe-based quantum dots (QDs) to sensitize the triplets of their surface-anchored thioxanthone molecules with high triplet energy of 2.75 eV. Ultrafast spectroscopy unambiguously reveals a stepwise, electron-transfer-mediated triplet energy transfer mechanism. This unusual photosensitization, in conjunction with the unique chemical reactivity of the thioxanthone triplets, allows us to use blue light to drive multiple challenging photochemical and photocatalytic applications: i) visible-to-ultraviolet B photon upconversion with large anti-Stokes shift of 0.8 eV through triplet-triplet annihilation; ii) energy transfer photocatalysis such as the disulfide-ene reaction; iii) reductive aryl dechlorination and C-N coupling enabled by hydrogen abstraction by thioxanthone triplets. This environmentally benign QD-ketone hybrid system will inspire the design of advanced photocatalysts for a variety of energy-demanding photochemical applications.