Efficient Red-to-Blue Triplet-Triplet Annihilation Upconversion Using the C70-Bodipy-Triphenylamine Triad as a Heavy-Atom-Free Triplet Photosensitizer

Triplet-tripletannihilation upconversion (TTA-UC) withheavy-atom-free organic triplet photosensitizers has attracted extensiveattention recently, however, the successful examples with absorptionin red and first near-infrared (NIR-I, 650-900 nm) region arestill insufficient. Herein, we conducted a new TTA-UC system of peryleneusing C-70-bodipy-triphenylamine triad (C ( 70 ) -BDP-T) as the heavy-atom-freephotosensitizer. Efficient red-to-blue (663 to 450 nm) TTA-UC wasachieved in this system with an anti-Stokes shift of 0.88 eV and aquantum yield up to 5.2% (out of a 50% maximum) in deaerated toluene.Notably, this is the highest quantum yield to date in similar TTA-UCsystems with heavy-atom-free organic photosensitizers. Using steady-stateand transient absorption spectroscopy, together with cyclic voltammogramand quantum chemical calculations, photophysical and photochemicalmechanisms were elucidated. Specifically, two triplet triads, C ( 70 ) - ( 3 ) BDP*-T and ( 3 ) C ( 70 ) *-BDP-T, were produced efficientlyupon photoexcitation, with lifetimes of 2.0 & PLUSMN; 0.1 and 32.2 & PLUSMN;0.3 & mu;s, respectively. Electron transfer and recombination mechanismswere confirmed to play crucial roles in the formation of these triplets,instead of intersystem crossing. Our results shed light on the superiorityof fullerenes in the development of heavy-atom-free photosensitizers.