Spin-optical design of organic radicals for photochemical 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 closed-shell sensitisers is limited by energy losses from singlet–triplet intersystem crossing. Here we explore open-shell organic radicals as sensitisers that can maximise the energy shift of photon upconversion. We design a new class of sensitiser that combines optical transitions from a triphenylmethyl radical (TTM-1Cz) component with energy-degenerate triplet states of an anthracene-based annihilator (DPA) in one molecule (TTM-1Cz-DPA). Following photoexcitation, doublet and quartet excited states form by coupling between the anthracene-triplet state and the radical. This leads to extended excited-state lifetimes of the sensitiser and efficient intermolecular triplet energy transfer to annihilator molecules. Red-to-blue photon upconversion is demonstrated in solution with anthracene derivatives showing an apparent anti-Stokes energy shift by up to 1.15 eV. Understanding the relation between exciton nature and spin-optical properties of radicals can improve designs for upconversion and energy management in molecules beyond singlet and triplet excitons.