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

Abstract

A design strategy towards stable fluorenyl radicals (FRs) 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 FRs, 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 FRs 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.