The Role of Charge-Transfer Interactions and Delocalization in Annelated Nitronyl Nitroxides




Dooley, Brynn Mary

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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.



BNN radicals, spin density