Triple phosphorescence in compact donor-acceptor dyads

Charge transfer triplets and their interplay with other electronic states are critically important for prolonged charge separation and thermally activated delayed fluorescence in organic molecules, primarily as they act as long-lived energy storage due to the spin-forbidden nature of their decay to the ground state. They are, however, generally hardly detectable and little is known about the mechanisms of their formation and decay, in part because of a poor understanding of how intersystem crossing processes are related to molecular structure. A group of phthalide-based dyads with orthogonally aligned donor and acceptor displaying unusual phosphorescence composed of three bands emitted from locally excited, charge transfer and biradical triplets allows for getting an insight into various singlet-triplet interconversion mechanisms and a better understanding of the structure-intersystem crossing relationships.