Fluorenyl Based Macrocyclic Polyradicaloids

Synthesis
of stable open-shell polyradicaloids including control
of intramolecular spin–spin interactions is a challenging topic
in organic chemistry and materials science. Herein, we report the
synthesis and physical characterization of two series of fluorenyl
based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4–6), the fluorenyl radicals are directly
linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3–5), an additional ethynylene moiety
is inserted between the neighboring fluorenyl units. To access stable
macrocyclic polyradicaloids, three synthetic methods were developed.
All of these stable macrocycles can be purified by normal silica gel
column chromatography under ambient conditions. In all cases, moderate
polyradical characters were calculated by restricted active space
spin-flip method due to the moderate intramolecular antiferromagnetic
spin–spin interactions. The excitation energies from the low-spin
ground state to the lowest high-spin excited state were evaluated
by superconducting quantum interference device measurements. Their
physical properties were also compared with the respective linear
fluorenyl radical oligomers (FR-n, n = 3–6). It is found that the geometry, i.e., the distortional
angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl
units, has significant effect on their polyradical character, excitation
energy, one-photon absorption, two-photon absorption and electrochemical
properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due
to cyclic π-conjugation with 36 and 44 π-electrons, respectively.