Excited-State Hund's Rule Violations in Bridged [10]- and [14]Annulene Perimeters

Molecules with Hund's rule violations between low-lying singlet and triplet states may enable a new generation of fluorescent emitters. However, only a few classes of molecules are known with this property at the current time. Here, we use a high-throughput screening algorithm of the FORMED database to uncover a class of compounds where the first excited state violates Hund's rule. We examine this class of bridged [10]- and [14]annulene perimeters with saturated bridges, and relate them to known conjugated polycyclic systems with Hund's rule violations. Despite the structural similarities with the related class of nonalternant polycyclic hydrocarbons, the mechanism is different in these bridged annulene perimeters. Here, two molecular orbital configurations contribute to each excited state. Consequently, a Hund's rule violation can only be unambiguously assigned based on the symmetry of the lowest excited singlet and triplet states. With several examples of synthetically realistic molecules, the class of bridged [10]- and [14]annulenes thus provides a structural link between the known nonalternant and alternant (azaphenalene) classes of molecules violating Hund's rule. These design principles may open avenues for the identification of new types of molecules where the order of the photophysically relevant first excited singlet and triplet states are inverted.