Molecular Design Using a Cage Structure to Extend the Lifetime of Singlet 1,3‐Diradicals with π‐Single‐Bonding Character

ABSTRACT The carbon–carbon covalent bond is a cornerstone of organic molecular structures. Among these, π bonds exhibit redox activity and govern fundamental photophysical functions such as light absorption and emission. In efforts toward solar energy conversion and bioimaging, significant attention has been devoted to developing molecules with visible–to–near‐infrared (NIR) absorption and emission properties. Conventional strategies to narrow the HOMO–LUMO gap have relied on π‐conjugation extension or the inclusion of heteroatoms bearing soft lone‐pairs. In contrast, we have discovered singlet 1,3‐diradicals exhibiting π single‐bond character and strong visible–NIR absorption and emission. Nevertheless, its inherently short lifetime underscores the need for novel molecular architectures that can persist stably at room temperature. This computational study reveals that incorporating a rigid cage‐like framework stabilizes this singlet diradical relative to its σ‐bonded and rearranged isomeric forms. Furthermore, substituent and elemental effects indicate that such π single‐bonded species could be thermally stable at ambient conditions. This work provides new molecular design principles for open‐shell π‐electron systems, offering insights into their unique reactivity, stability, and electronic properties, and paving the way for next‐generation functional π‐materials.