Benzo[ <i>cd</i> ]azulenyl: A Structural Isomer of Phenalenyl—Synthesis and Properties of Its Tri‐ <i>tert</i> ‐butyl‐substituted Derivative and Formation of a Thermal‐ and Photoresponsive σ‐Dimer

The chemistry of nonalternant hydrocarbons has recently experienced a significant resurgence in interest. While extensive research has been conducted on azulenoids, that is, structural isomers of benzenoid polycyclic hydrocarbons, the exploration of doublet open-shell nonalternant systems, that is, isomers of open-shell graphene nanofragments has not yet been undertaken. To clarify the chemical and physical effects of nonhexagonal rings on the unpaired electron in these structures, we focus on benzo[cd]azulenyl, which is a nonalternant isomer of phenalenyl. We synthesized and characterized a tri-tert-butyl substituted derivative, which exists as the stable σ-dimer in the solid state. In solution, the σ-bond of the dimer dissociates in response to external stimuli (light and heat), yielding the monomeric radical. Analyzing the bond-dissociation and recombination processes allowed us to determine the thermodynamic and kinetic parameters. Unlike phenalenyl, the spin density of benzo[cd]azulenyl is unevenly distributed, skewing toward its five- and seven-membered rings. This asymmetrical spin density, combined with kinetic protection, grants the tri-tert-butyl substituted derivative selective and reversible C─C-bond-formation properties. The nonalternant nature of benzo[cd]azulenyl enhances its redox properties and lowers the photoexcitation energy. Our study contributes to the establishment of design strategies for novel open-shell doublet radical materials based on nonalternant hydrocarbon frameworks.