Aromaticity-driven singlet-triplet inversions

Molecules with inversion of the singlet and triplet excited state energies are highly promising for the development of a new generation of OLED emitters. Currently, only one class of molecules with these inversions have been identified, in the form of azaphenalenes. Here, we screen a curated database of reported organic crystal structures to identify existing compounds for violations of Hund’s rule in the lowest excited states and identify two new classes of molecules with this behavior. The first is a class of zwitterions where the singlet-triplet inversions occur in the third excited singlet state, which limits their relevance to molecular emitters. The second class consists of two high-symmetry odd-membered polycyclic aromatic hydrocarbons, a fused azulene dimer and a bicalicene, where the lowest excited singlet states violate Hund’s rule. The high D2h symmetry of the two molecules is stabilized by the connectivity of the polycyclic structure, which we rationalize by studying the aromaticity of the ring components. This class of aromaticity-stabilized high-symmetry hydrocarbons shows promise as the next generation of building blocks for organic light-emitting diode emitters.