Spectroscopic Signatures of Resonance Inhibition Reveal Differences in Donor–Bridge and Bridge–Acceptor Couplings

The
torsional dependence of the ground state magnetic exchange
coupling (J) and the corresponding electronic coupling
matrix element (HDA) for eight transition
metal complexes possessing donor–acceptor (D-A) biradical ligands
is presented. These biradical ligands are composed of an S = 1/2 metal
semiquinone (SQ) donor and an S = 1/2 nitronylnitroxide (NN) acceptor,
which are coupled to each other via para-phenylene,
methyl-substituted para-phenylenes, or a bicyclo[2.2.2]­octane
ring. The observed trends in electronic absorption and resonance Raman
spectral features are in accord with a reduction in electronic and
magnetic coupling between D and A units within the framework of our
valence bond configuration interaction model. Moreover, our spectroscopic
results highlight different orbital mechanisms that modulate coupling
in these complexes, which is not manifest in the ferromagnetic JSQ‑B‑NN values. The work provides
new detailed insight into the effects of torsional rotations which
contribute to inhomogeneities in experimentally determined exchange
couplings, electron transfer rates, and electron transport conductance
measurements.