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.