Benzo cd azulenyl: A Structural Isomer of Phenalenyl—Synthesis and Properties of Its Tri‐ tert ‐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 benzocdazulenyl, 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 benzocdazulenyl 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 benzocdazulenyl 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.