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

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Materials Science

Background:

• Singlet 1,3-diradicals possess unique electronic, photophysical, and redox properties due to their π single-bond character.
• Practical applications of singlet diradicals are hindered by their inherent instability, leading to rapid sigma-bond formation and rearrangements.

Purpose of the Study:

• To investigate the use of spiro-induced strain as a method to control reaction pathways in singlet diradicals.
• To enhance the kinetic persistence and stability of singlet diradicals for potential applications.

Main Methods:

• Employed quantum chemical calculations, specifically Complete Active Space Self-Consistent Field (CASSCF) and broken-symmetry Density Functional Theory (DFT) methods.
• Analyzed the effect of spirocyclic frameworks on the stability of diradicals and their reaction pathways.

Main Results:

• Spiro-induced strain was found to destabilize sigma-bonded products and transition states, effectively suppressing sigma-bond formation.
• The spirocyclic system SP1 (n=1), featuring a methylene bridge, demonstrated the highest activation barrier and longest predicted lifetime.
• Smaller spiro frameworks were shown to suppress intramolecular alkoxy migration, further increasing kinetic persistence.

Conclusions:

• Spiro-induced strain is an effective design principle for controlling reactivity in singlet diradicals.
• This approach enables the realization of long-lived singlet diradicals with π single-bond character, opening avenues for their practical utilization.