Stable 3,6-Linked Fluorenyl Radical Oligomers with Intramolecular Antiferromagnetic Coupling and Polyradical Characters

Organic radicals display unique physical structures and could become next generation functional materials. However, design and synthesis of stable neutral radicals with a significant polyradical character has been an enormous challenge for chemists. In this work, we synthesized a series of stable 3,6-linked, kinetically blocked fluorenyl radical oligomers up to hexamer (FR-n, n = 1-6). Their ground-state geometric and electronic structures were systematically studied by various experimental methods including X-ray crystallographic analysis, variable temperature nuclear magnetic resonance, electron spin resonance, and superconducting quantum interference device measurements, supported by density functional theory and ab initio calculations. Moderate antiferromagnetic coupling between the fluorenyl radicals was observed, and moderate to large diradical and polyradical characters were calculated from dimer onward. Furthermore, their photophysical properties were estimated by steady-state, transient absorption, and two-photon absorption measurements, and their electrochemical properties were investigated by cyclic voltammetry/differential pulse voltammetry and spectro-electrochemical measurements. A clear chain length dependence of their optical, electrochemical, and magnetic properties was found for the oligomers with an odd or even number of spin centers, respectively.