Singlet–Triplet\nInversions in Through-Bond\nCharge-Transfer States

Molecules where the\nlowest excited singlet state is lower\nin energy\nthan the lowest triplet are highly promising for a number of organic\nmaterials applications as efficiency limitations stemming from spin\nstatistics are overcome. All molecules known to possess such singlet–triplet\ninversions exhibit a pattern of spatially alternating but nonoverlapping\nHOMO and LUMO orbitals, meaning the lowest excited states are of a\nlocal character. Here, we demonstrate that derivatives of the bicyclic\nhydrocarbon calicene exhibit Hund’s rule violations in charge-transfer\n(CT) states between its rings. These CT states can be tuned with substituents,\nso that the first excited singlet and triplet state are energetically\ninverted. This provides a conceptual connection between the emerging\nfields of inverted gap molecules and existing molecular design rules\nfor state-of-the-art thermally activated delayed fluorescence materials.