Excited-State Hund’s\nRule Violations in Bridged\n[10]- and [14]Annulene Perimeters

Molecules with Hund’s rule violations between\nlow-lying\nsinglet and triplet states may enable a new generation of fluorescent\nemitters. However, only a few classes of molecules are known with\nthis property at the current time. Here, we use a high-throughput\nscreening algorithm of the FORMED database to uncover a class of compounds\nwhere the first excited state violates Hund’s rule. We examine\nthis class of bridged [10]- and [14]annulene perimeters with saturated\nbridges, and relate them to known conjugated polycyclic systems with\nHund’s rule violations. Despite the structural similarities\nwith the related class of nonalternant polycyclic hydrocarbons, the\nmechanism is different in these bridged annulene perimeters. Here,\ntwo molecular orbital configurations contribute to each excited state.\nConsequently, a Hund’s rule violation can only be unambiguously\nassigned based on the symmetry of the lowest excited singlet and triplet\nstates. With several examples of synthetically realistic molecules,\nthe class of bridged [10]- and [14]annulenes thus provides a structural\nlink between the known nonalternant and alternant (azaphenalene) classes\nof molecules violating Hund’s rule. These design principles\nmay open avenues for the identification of new types of molecules\nwhere the order of the photophysically relevant first excited singlet\nand triplet states are inverted.