Unimolecular Doublet-Triplet Annihilation Upconversion in Iron(III) Coordination Compounds.

Photochemical upconversion usually relies on intermolecular energy transfer between a sensitizer and an annihilator, followed by triplet-triplet annihilation between two annihilator molecules. Here, we depart from both of these fundamental principles and report upconversion in single molecules via a largely unexplored annihilation process involving doublet and triplet excited states. This approach is independent of diffusion, reduces energy losses by eliminating intersystem crossing and bimolecular triplet-triplet energy transfer, and offers more favorable spin statistics than conventional triplet-triplet annihilation. The molecular design integrates three different excited states: a red-light absorbing doublet state on an FeIII carbene complex, a long-lived dark triplet state localized on a perylene unit linked covalently to the FeIII carbene, and a blue fluorescent singlet state on the perylene. By varying the number of attached perylene units from 1 to 4 we gain mechanistic insight and identify the operating regime of unimolecular upconversion that functions not only in fluid solution but also in polymers at room temperature and in frozen glasses at 77 K. Beyond providing a design strategy for diffusion-independent future upconversion architectures, this work establishes unimolecular doublet-triplet annihilation as a fundamentally distinct upconversion concept with strategic advantages over conventional approaches.