Symmetry‐Induced Singlet‐Triplet Inversions in Non‐Alternant Hydrocarbons**

Abstract

Molecules with inversion of the singlet and triplet excited‐state energies are highly promising for the development of organic light‐emitting diodes (OLEDs). To date, azaphenalenes are the only class of molecules where these inversions have been identified. Here, we screen a curated database of organic crystal structures to identify existing compounds for violations of Hund's rule in the lowest excited states. We identify two further classes with this behavior. The first, a class of zwitterions, has limited relevance to molecular emitters as the singlet‐triplet inversions occur in the third excited singlet state. The second class consists of two D2h‐symmetry non‐alternant hydrocarbons, a fused azulene dimer and a bicalicene, whose lowest excited singlet states violate Hund's rule. Due to the connectivity of the polycyclic structure, they achieve this symmetry through aromatic stabilization. These hydrocarbons show promise as the next generation of building blocks for OLED emitters.

∆DFT predicts inverted singlet-triplet gaps with chemical accuracy at a fraction of the cost of wavefunction-based approaches

“…Introduction -Molecules with an inverted singlettriplet energy gap (INVEST) have become a hot research topic as they would enable highly efficient emitters for organic light-emitting diodes (OLEDs). [1][2][3][4][5][6][7][8][9][10][11][12] In OLEDs, excited states (or excitons) are created by recombining electrons and holes. 13 According to Fermi-Dirac spin statistics, the ratio of singlet to triplet excited states generated via recombination is 1:3.…”