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Rigid Core Anthracene and Anthraquinone Linked Nitronyl and Iminoyl Nitroxide Biradicals
journal contribution
posted on 2016-05-25, 17:18 authored by Handan Akpinar, John A. Schlueter, Rafael
A. Allão Cassaro, Jonathan R. Friedman, Paul M. LahtiThe first syntheses of bis(nitronyl
nitroxide) and bis(iminoyl
nitroxide) (diNN, diIN) biradicals linked through rigid acene core
conjugating anthracene (A) and anthraquinone (AQ) units are reported.
Computational modeling predicts weak intramolecular exchange in AQ-linked
systems, but A-linked biradicals to have ground state multiplicities
consistent with the Borden-Davidson disjointness model. Solution electron
spin resonance spectra showed inter-radical exchange-coupled triplet
states, except for 2,6-AQ biradicals showing isolated spin spectra.
Crystallography of the A-linked biradicals shows a key role for inter-radical
contacts for molecular packing. DiINs showed lower-dimensional dyad
packing with disorder at the radical units: the conformationally more
symmetrical diNNs gave staircase one-dimensional or brickwork two-dimensional
lattices. Core anthracene unit stacking was only seen in two systems
with bromine on the central anthracene ring: the (large) bromine occupies
alternate side placement in dyad stacks for the diIN, chain stacks
for the diNN. Magnetism of 2,7-A-linked systems showed predominant
ferromagnetic intramolecular triplet-singlet splitting of 24–28
K for diNNs and 8 K for diINs, plus weak antiferromagnetic (AFM) interactions
from intermolecular contacts. The 2,6-A-linked biradicals showed AFM
exchange between spins. Both A and AQ cores offer possibilities for
electronic material development, with a combination of multiple radical
spins and π-electron-rich acene cores.
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Keywords
diINacene coreAFM exchangedyad stacksRigid Core Anthraceneresonance spectranitroxidecore anthracene unitintramolecular exchangebromine8 Kground state multiplicitiesside placementchain stacksanthracene ringbiradicalComputational modelingAQ cores offer possibilitiesdiNNmaterial developmentsolution electroncontactIminoyl Nitroxide Biradicals
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