Near-Unity Triplet Quantum Yield in a Molecular Cofacial H-Dimer

The quantum yield of a photoreagent’s excited state imposes a ceiling on the overall efficiency of a photoprocess. Organic sensitizers with large or near-unity triplet excited-state quantum yields (Φ T ) are often designed by implementing spin–orbit coupling (SOC) through the incorporation of heavy-atom substituents or heteroatoms with nonbonding electrons, which typically come at the cost of shortened triplet excited-state lifetimes, lowered triplet energies, and/or poor photostability. We show here that the disposition of organic chromophores in an H-dimer offers a path to near-unity Φ T while maintaining a long excited-state lifetime and preserving the excited-state energy. Cofacially poising pyrenes on a xanthene scaffold (Py 2 Xanth) furnishes H-dimer photophysics with a near-unity Φ T of 97% while preserving a 2.1 eV triplet energy and a long-lived, 180 μs, triplet lifetime. Given these properties, we show Py 2 Xanth to be a highly efficient triplet photosensitizer. This work reveals that H-dimer coupling of chromophores is a promising design principle for the development of highly efficient, photostable, organic photosensitizers that avoid the penalties associated with SOC modifications.