Singlet–Triplet Inversions in Through-Bond Charge-Transfer States

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