Fluorenyl Radicals Stabilized by Donor‐π‐Radical Conjugation between Nitrogen Atoms and Carbon Radicals

Comprehensive Summary A design strategy towards stable fluorenyl radicals ( FR s) has been developed through introducing donor‐π‐radical (D‐π‐R) conjugation, facilitated by the linkage of amine N atoms and FR centers via phenyl or 9‐anthryl moieties. Four FR s, with or without N atom containing protecting groups, were designed and synthesized for comparative analysis. X‐ray crystallographic studies revealed the planar fluorenyl skeletons of CDP‐FR and MA‐FR , which exhibit significant dihedral angles relative to their protecting groups. Wiberg bond‐order analysis and natural bond orbital analysis conducted on the C‐C and C‐N bond between the radical center and N atoms in all FR s clearly demonstrated the presence of D‐π‐R conjugation. Furthermore, time‐dependent DFT calculations, based on frontier molecular orbital analysis, highlighted the contribution of D‐π‐R structures as donor‐acceptor effect on the lowest energy absorptions of carbazole and diphenylamine substituted CDP‐FR and DPAA‐FR , which would promote charge transfer process and inhibit the photodegradation reaction. Consequently, beneficial from D‐π‐R conjugations, CDP‐FR and DPAA‐FR exhibited superior photostability compared to TP‐FR and MA‐FR . Our study offers a new strategy for the design and synthesis of persistent stable monoradicals.