%0 Generic %A Kirk, Martin L. %A Shultz, David A. %A Stasiw, Daniel E. %A Habel-Rodriguez, Diana %A Stein, Benjamin %A Boyle, Paul D. %D 2016 %T Electronic and Exchange Coupling in a Cross-Conjugated D–B–A Biradical: Mechanistic Implications for Quantum Interference Effects %U https://acs.figshare.com/articles/dataset/Electronic_and_Exchange_Coupling_in_a_Cross_Conjugated_D_B_A_Biradical_Mechanistic_Implications_for_Quantum_Interference_Effects/2371750 %R 10.1021/ja405354x.s001 %2 https://acs.figshare.com/ndownloader/files/4011244 %K quantum interference effects %K EPR %K wire component %K phenylene bridge character %K exchange mechanism %K type configuration %K singlet ground state %K Quantum Interference EffectsA combination %K absorption spectroscopy %K LUMO %K meta %K electron approximation %K exchange interaction %K HOMO %K NN %K transfer %K biradical %K superexchange mechanisms %K bridge moiety %K SQ %K polarization mechanism %K ground state %K susceptibility measurements %K molecule %K contribution %K Mechanistic Implications %X A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on TpCum,MeZn­(SQ-m-Ph-NN) (1-meta) a donor–bridge–acceptor (D–B–A) biradical that possesses a cross-conjugated meta-phenylene (m-Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of 1-meta. The results reveal important excited-state contributions to the ground-state singlet–triplet splitting in this cross-conjugated D–B–A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D–B–A biradical possessing a para-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D–B–A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the m-Ph bridged D–A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D–B–A molecules where, in the case of 1-meta, the magnetic exchange in the active electron approximation is expected to be J ∼ 0 cm–1. We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D–B–A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated. %I ACS Publications