%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