Photoredox catalysis powered by triplet fusion upconversion: arylation of heteroarenes

Abstract

In this work, the feasibility of triplet fusion upconversion (TFU, also named triplet–triplet annihilation upconversion) technology for the functionalization (arylation) of furans and thiophenes has been successfully proven. Activation of aryl halides by TFU leads to generation of aryl radical intermediates; trapping of the latter by the corresponding heteroarenes, which act as nucleophiles, affords the final coupling products. Advantages of this photoredox catalytic method include the use of very mild conditions (visible light, standard conditions), employment of commercially available reactants and low-loading metal-free photocatalysts, absence of any sacrificial agent (additive) in the medium and short irradiation times. The involvement of the high energetic delayed fluorescence in the reaction mechanism has been evidenced by quenching studies, whereas the two-photon nature of this photoredox arylation of furans and thiophenes has been manifested by the dependence on the energy source power. Finally, the scaling-up conditions have been gratifyingly afforded by a continuous-flow device. Graphical abstract

Metal–Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

“…Aside from reacting via photoinduced electron transfer, 1* PPO can furthermore be expected to serve as an energy donor, which could open new perspectives for energy transfer catalysis. Photoredox catalysis via upconversion is an emerging field, − ,,,− whereas energy transfer catalysis via upconversion is yet very uncommon. ,, Two Paternò–Büchi model reactions (Figure c) were chosen to explore the concept of converting a lower energy triplet-excited state into a higher energy triplet-excited state. The [2 + 2] cycloaddition reaction of benzaldehydes with furan usually requires UV irradiation at 365 nm (3.40 eV) because the energy of the reactive triplet-excited state of benzaldehydes is ca.…”

Sensitizer-controlled photochemical reactivityviaupconversion of red light

“…20,21,22,39 Sensitized triplet-triplet annihilation upconversion (sTTA-UC) is an attractive approach to combining the energy input of multiple photons. 1,[23][24][25] In this case, the co-catalyst (the socalled annihilator) is not directly excited, but instead its lowest triplet excited state is formed via triplet-triplet energy transfer (TTET) from the primary photosensitizer and then the annihilator's uorescent singlet excited state is populated by triplet-triplet annihilation. That singlet excited state can either activate the substrate directly, or it can be quenched by sacri-cial electron donors to yield a radical anion, which subsequently engages with the substrate.…”

Photochemically deoxygenating gels for triplet–triplet annihilation photon-upconversion performed under air

“…UC emissions in fluid solutions have been successfully used as the light source in photocatalysis. 3 However, in photovoltaic devices, the use of TTA-UC in fluid solutions is limited.…”