Abstract
A 139-π-electron nanographenoid radical was obtained by expanding the periphery of a naphthalimide–azacoronene hybrid with a methine bridge. The radical was isolated in the form of its σ-dimer, which was shown to possess a conformationally restricted two-layer structure both in the solid state and in solution. The dimer is cleaved into its parent radicals when exposed to ultraviolet or visible radiation in toluene solutions but is resistant to thermally induced dissociation. Under inert conditions, the radicals recombine quantitatively into the σ-dimer with observable kinetics, but they are oxidized into a ketone derivative in the presence of atmospheric oxygen. Combined structural, spectroscopic, and theoretical evidence shows that the σ-dimer contains a weak C(sp3)–C(sp3) bond, but is stabilized against thermal dissociation by a very strong dispersive interaction between the overlapping π surfaces.
Highlights
Dimerization of π-conjugated radicals is a fundamentally important process providing control over magnetism and reversible covalent bond formation in organic materials.[1]
Two π radicals can recombine by formation of a covalent bond to produce a so-called σ-dimer, in which one atom of each π system has been converted into a tetrahedral center
As these bonds often have relatively low dissociation energies, σdimerization provides an important mechanism for assembly of complex architectures by dynamic covalent bonding.[2−11] σ
Summary
Dimerization of π-conjugated radicals is a fundamentally important process providing control over magnetism and reversible covalent bond formation in organic materials.[1]. CD spectra of the enantiomers showed significantly lower intensities than those of the related propeller-shaped HPHACs.[37] Voltammetric analyses of 42 showed two oxidations at 0.60 and 0.85 V and at least eight reduction events in the −1.37 to −1.99 V potential range (vs Fc/Fc+, cf Figures S7−S10) Such an extended electron-accepting ability is a general characteristic of multi-NMI-fused nanocarbon systems.[36,37,39,49−51] For 42, electroreduction was partly irreversible, producing a new species when held at potentials below −1.37 V. Computational data indicate that 4 preferentially adopts a monkey-saddle conformation with alternating helicities of the adjacent bay regions (Figure S20) This conformer is analogous to the structure reported for the parent HPHAC− NMI hybrid.[36] since the subunits of 42 adopt different stereochemistry in the solid-state structure, the radical appears to be sufficiently flexible to adjust its geometry upon dimerization. The energetics of dimerization were probed for 42 (Table 1) and the reference system 82
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