Toward an Air-Stable Triradical with Strong Spin Coupling: Synthesis of Substituted Truxene-5,10,15-triyl

With the aim to achieve
air-stable polyradical species manifesting
strong spin coupling, synthetic endeavors are made toward triradical
molecules featuring a truxene-triyl skeleton. Commonly used steric-hindering
side groups such as 2,4,6-trichlorophenyl and 9-anthracenyl are both
found to be incompetent at stabilizing the targeted truxene triradical,
which appears to be elusive from isolation and characterization. Nonetheless,
single-crystal structures of adducts formed by relevant radicals are
obtained, which strongly suggests the transient existence of the designed
triradicals. Finally, a truxene triradical comprising 1-anthracenyl
along with two 9-anthracenyl substituents is successfully isolated
and found to exhibit decent stability in air. We propose that because
of the smaller dihedral angle assumed by 1-anthracenyl with respect
to the plane of truxene-triyl, more effective π-conjugation
allows the spin density to be more widely delocalized and distributed
to the anthracenyl side groups. Thus, higher stability is gained by
the triradical molecule.