Spiro-Induced Strain as a Design Principle for Long-Lived Singlet Diradicals with π-Single Bond Character

Singlet 1,3-diradicals with a π single-bond character exhibit unique electronic structures and attractive photophysical and redox properties; however, their practical utilization is severely limited by rapid σ-bond formation and competing rearrangement pathways. Here, we demonstrate that spiro-induced strain provides a design principle for controlling competing reaction pathways in singlet diradicals. Quantum chemical calculations using CASSCF and broken-symmetry DFT methods reveal that this strain destabilizes both σ-bonded products and the corresponding transition states, thereby suppressing σ-bond formation. This effect is most pronounced for a spirocyclic system SP1 (n = 1) bridged by a methylene (-CH₂-) unit, which achieves an optimal balance between strain and structural flexibility, resulting in the highest activation barrier and the longest predicted lifetime among the systems examined. In addition, intramolecular alkoxy migration is suppressed in smaller spiro frameworks, further contributing to kinetic persistence. These findings establish spiro-induced strain as an effective strategy for realizing long-lived singlet diradicals with a π single-bond character.