Octazethrene and Its Isomer with Different Diradical Characters and Chemical Reactivity: The Role of the Bridge Structure

The fundamental relationship between
structure and diradical character
is important for the development of open-shell diradicaloid-based
materials. In this work, we synthesized two structural isomers bearing
a 2,6-naphthoquinodimethane or a 1,5-naphthoquinodimethane bridge
and demonstrated that their diradical characters and chemical reactivity
are quite different. The mesityl-or pentafluorophenyl-substituted
octazethrene derivatives OZ-M/OZ-F and their isomer OZI-M (with mesityl substituents) were synthesized via an
intramolecular Friedel–Crafts alkylation followed by oxidative
dehydrogenation strategy from the key building blocks 4 and 11. Our detailed experimental and theoretical studies
showed that both isomers have an open-shell singlet ground state with
a remarkable diradical character (y0 =
0.35 and 0.34 for OZ-M and OZ-F, and y0 = 0.58 for OZI-M). Compounds OZ-M and OZ-F have good stability in an ambient
environment, while OZI-M has high reactivity and can
be easily oxidized to a dioxo product 15, which can be
correlated to their different diradical characters. Additionally,
we investigated the physical properties of OZ-M, OZ-F, and 15.