ja405354x_si_001.cif (39.42 kB)
Electronic and Exchange Coupling in a Cross-Conjugated D–B–A Biradical: Mechanistic Implications for Quantum Interference Effects
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posted on 2016-02-18, 18:54 authored by Martin L. Kirk, David A. Shultz, Daniel E. Stasiw, Diana Habel-Rodriguez, Benjamin Stein, Paul D. BoyleA 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.
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Keywords
quantum interference effectsEPRwire componentphenylene bridge characterexchange mechanismtype configurationsinglet ground stateQuantum Interference EffectsA combinationabsorption spectroscopyLUMOmetaelectron approximationexchange interactionHOMONNtransferbiradicalsuperexchange mechanismsbridge moietySQpolarization mechanismground statesusceptibility measurementsmoleculecontributionMechanistic Implications
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