Toward Tetraradicaloid: The Effect of Fusion Mode on Radical Character and Chemical Reactivity

Open-shell singlet diradicaloids
display unique electronic, nonlinear
optical, and magnetic activity and could become novel molecular materials
for organic electronics, photonics, and spintronics. However, design
and synthesis of diradicaloids with a significant polyradical character
is a challenging task for chemists. In this Article, we report our
efforts toward a tetraradicaloid system. A series of potential tetraradicaloids
by fusion of two p-quinodimethane (p-QDM) units with naphthalene or benzene rings in different modes
were synthesized. Their model compounds containing one p-QDM moiety were also prepared and compared. Their ground-state structures,
physical properties, and chemical reactivity were systematically investigated
by various experimental methods such as steady-state and transient
absorption, two-photon absorption, X-ray crystallographic analysis,
electron spin resonance, superconducting quantum interference device,
and electrochemistry, assisted by density functional theory calculations.
It was found that their diradical and tetraradical characters show
a clear dependence on the fusion mode. Upon the introduction of more
five-membered rings, the diradical characters greatly decrease. This
difference can be explained by the pro-aromaticity/antiaromaticity
of the molecules as well as the intramolecular charge transfer. Our
comprehensive studies provide a guideline for the design and synthesis
of stable open-shell singlet polycyclic hydrocarbons with significant
polyradical characters.