The design and synthesis of stable organic radicals with delocalized spin density
distribution and low energy optical and redox processes is central to the development of
magneto-conducting materials. Towards this end, a generalized synthetic methodology
has been developed allowing for the synthesis of a series of annelated benzonitronyl
nitroxide (BNN) radicals. The BNN radicals exhibited remarkably low reduction
potentials (~0.0 V vs SCE) and a near-infrared absorption attributed to a HOMO–SOMO
charge-transfer excitation.
The annelated BNN radicals were found to be less stable than the closely related
tetramethyl nitronyl nitroxide radicals, particularly in solution. A series of π-delocalized
and heteroaromatic radicals were synthesized to determine if the instability was due to
the delocalization of electron density onto the carbon skeleton or the low reduction
potential. DFT calculations with the EPR-II basis gave rise to calculated electronic
structures that were consistent with EPR spectroscopy and suggested changes in spin
density distribution are occurring with perturbation of the annelated ring. Cyclic
voltammetry revealed the heteroaromatic and π-delocalized radicals had reduction
potentials lower than BNN, with some systems reducing at potentials of 0.2 V vs SCE,
comparable to that of 7,7,8,8-tetracyanoquinodimethane. The distribution of spin
throughout the molecular framework and the low reduction potential of the annelated
nitronyl nitroxide radicals were both found to contribute to the lowered stability of the
annelated nitronyl nitroxides relative to the far less redox active tetramethyl nitronyl
nitroxides.
The low reduction potential of the BNN radicals suggested that incorporation into
acceptor–donor (A–D) systems would allow for investigation of the role of charge
transfer interactions on the electronic structure and properties of neutral open-shell A–D
radicals. Two D–A–D radicals were prepared using metal catalyzed coupling and furoxan
condensation methodologies which resulted in incorporation of a second donor in the C5
position of the BNN moiety. The radical D1–A–D2 triads, where D1 = thiophene and D2 =
thiophene or phenyl, exhibited three intramolecular charge-transfer excitations (λmax =
550, 580 and 1000 nm) that were investigated by variable temperature absorption
spectroscopy. Structural characterization of the triads in the solid state by single crystal
and powder X-ray diffraction revealed slipped π stacks that arise from intermolecular π–
π and D–A interactions, providing pathways for antiferromagnetic (AFM) and
ferromagnetic (FM) exchange. While the phenyl substituted triad (Th–BNN–Ph)
exhibited antiferromagnetic interactions and a room temperature conductivity of σRT =
10−7 S cm−1, the thienyl substituted derivative (Th–BNN–Th) exhibited short-range FM
interactions and increased conductivity (σRT = 10−5 S cm−1), giving rise to an organic
semiconductor exhibiting FM exchange. The differences in conductivity and magnetic
behavior were rationalized by the degree of slippage dictated by an interplay between π–
π and intermolecular D−A interactions.
Finally, a series of BNN–D radicals were investigated where the donor ability of D was
systematically varied from Eox = 2.30 V vs SCE (benzene) to 0.32 V vs SCE
(tetrathiafulvalene). Calculations of the near-infrared charge transfer excitation suggested
that the HOMO–SOMO gap could be significantly decreased with increasing donor
ability, consistent with charge transfer theory. A subset of the series of BNN–D radicals
with D = anisole, benzo[b]thiophene, N-methylindole, N-ethylcarbazole, and N,Ndiphenylaniline
were synthesized. Solution state spectroscopic studies of the series by
EPR and electronic absorption spectroscopy revealed spin-delocalized structures with
extremely low reduction potentials (~0 V vs SCE). The solid state properties of the
BNN–D radicals were investigated by magnetometry and room temperature conductivity
measurements. Due to primarily steric interactions, weak D–A coupling was observed,
with weak intermolecular interactions in the solid state leading to paramagnetic and
insulating behaviour. The BNN-(N,N-diphenylaniline) radical structure was characterized
by single crystal XRD and found to exist as well isolated radical moieties with extremely
weak intermolecular interactions, consistent with magnetometry and conductivity
measurements. / Graduate / 0490 / 0794
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/5000 |
| Date | 28 October 2013 |
| Creators | Dooley, Brynn Mary |
| Contributors | Frank, Natia L. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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