Luminescent Fe(III)\nComplex Sensitizes Aerobic Photon\nUpconversion and Initiates Photocatalytic Radical Polymerization

Light energy conversion\noften relies on photosensitizers\nwith long-lived\nexcited states, which are mostly made of precious metals such as ruthenium\nor iridium. Photoactive complexes based on highly abundant iron seem\nattractive for sustainable energy conversion, but this remains very\nchallenging due to the short excited state lifetimes of the current\niron complexes. This study shows that a luminescent Fe(III) complex\nsensitizes triplet–triplet annihilation upconversion with anthracene\nderivatives via underexplored doublet-triplet energy transfer, which\nis assisted by preassociation between the photosensitizer and the\nannihilator. In the presence of an organic mediator, the green-to-blue\nupconversion efficiency Φ<sub>UC</sub> with 9,10-diphenylanthracene\n(DPA) as the annihilator achieves a 6-fold enhancement to ∼0.2%\nin aerated solution at room temperature. The singlet excited state\nof DPA, accessed via photon upconversion in the Fe(III)/DPA pair,\nallows efficient photoredox catalytic radical polymerization of acrylate\nmonomers in a spatially controlled manner, whereas this process is\nkinetically hindered with the prompt DPA. Our study provides a new\nstrategy of using low-cost iron and low-energy visible light for efficient\npolymer synthesis, which is a significant step for both fundamental\nresearch and future applications.