Probing Electronic Communication in Stable Benzene-Bridged Verdazyl Diradicals

Two benzene-bridged N,N‘-bis(isopropyl)6-oxoverdazyl diradicals 7a (1,4-benzene-bridged) and 7b (1,3-benzene-bridged) were prepared and studied by an array of physicochemical techniques aimed at elucidating
the intramolecular electronic and magnetic coupling between verdazyl chromophores. The very high
stability of these diradicals permits comprehensive investigations of their properties for the first time.
The UV−vis spectra suggest negligible direct conjugative overlap involving the radical SOMOs, although
significant differences in higher energy absorptions suggest that radical orbitals other than the SOMO
are likely communicating via the central p-phenylene bridge in 7a. The electrochemical features of the
two diradicals are nearly identical; in each derivative, both radicals are oxidized essentially independently
while the reductions occur in a stepwise manner with differences of ∼100mV between the two reductions.
EPR and magnetic susceptibility collectively indicate that the para-bridged diradical is weakly
antiferromagnetically coupled while the meta analogue is weakly ferromagnetically coupled, in accord
with the topology of the substitution pattern on the central benzene ring.