Open- and Closed Shell Roles of Sensitiser and Annihilator in Pseudo-Single Component Mixtures for Upconversion

Photochemical upconversion by annihilation of two triplet excitons to a higher-energy singlet state enables energy control of photons in optoelectronics and photonics. Upconversion initiated by a closed-shell sensitiser is limited by energy losses from singlet–triplet intersystem crossing. Here we explore open-shell organic radicals as sensitisers and their closed-shell hydrogenated analogues as annihilators in photon upconversion. The sensitiser combines optical transitions from a triphenylmethyl (TTM-1Cz) radical with energy-degenerate triplet states of an anthracene-based component (DPA) in one molecule (TTM-1Cz-DPA). The difference of one hydrogen atom in its closed-shell counterpart (HTTM-1Cz-DPA) switches the spin-optical properties to an annihilator that can mediate efficient upconversion. Red-to-blue photon upconversion by intermolecular energy transfer, from open-shell sensitiser to closed-shell annihilator, is demonstrated in solution with an apparent anti-Stokes shift higher than 0.9 eV and 7% quantum efficiency. We evaluate this mechanism against true ‘single component’ mixtures for upconversion and find that the presence of hydrogenated precursor with radicals is essential for high performance. Understanding the emergent spin-optical properties from paired open-shell and closed-shell systems enables new opportunities for energy management from the same molecular frame.