Tetrabenzononacene: “Butterfly Wings” Stabilize the Core

Abstract In combination with bulky substituents at the core, fourfold benzannulation at the cata‐positions stabilizes a nonacene sufficiently to allow its isolation and characterization by 1H NMR and X‐ray analysis. The four benzo units blueshift the absorption spectrum in comparison to a solely linear nonacene, but significantly increase the stability in the solid state.

The higher acenes have been am agnificent challenge, accepted since Clarsf irst synthesis of pentacene. [1] Unsubstituted, they are both insoluble and vulnerable towards ambient conditions.A nthony et al. [2] have introduced bulky silylethynyl substituents to the larger acenes,and, depending on size and steric demand, even heptacenes can be stabilized ( Figure 1). [3] However,t he stabilization of octacenes and nonacenes remains challenging.A part from surface- [4] or matrix-based [5] approaches,tothe best of our knowledge only two approaches yielded nonacene-type structures.M iller et al. employed thioether substituents, [6] while Anthony et al. combined steric repulsion with fluorination [7] to achieve the stabilization of nonacene derivative Non.Other methods of stabilization allow as ignificant number of linearly annulated benzene rings but with starkly diminished acene character. [8] We recently prepared stable tetrabenzoheptacenes (and azaheptacenes) such as B 4 Hep,e xhibiting hexacene-like absorption maxima, the blueshift being ac onsequence of the diminished conjugation of the quadruply annulated acene core. [9] Here we extend this approach to ar easonably stable nonacene derivative, B 4 Non,b ye mploying amodification of Anthonysroute.
Bis(bromomethyl)phenanthrene 1 and an excess of pbenzoquinone furnished 2 (Scheme 1);asecond Cava reaction with 3 results in 4 (crude yield 87 %), fourfold ethynylation of which gave intermediate 5 (40 %) using al arge excess of lithium acetylide ( % 100 equiv). Reductive aromatization with SnCl 2 furnishes tetrabenzononacene B 4 Non.The concentration was adjusted to precipitate B 4 Non during synthesis. B 4 Non,unlike other higher acenes, [7] is surprisingly stable in the solid state.I ts proton NMR spectrum shows sharp resonances (see the Supporting Information), in contrast to the broad signals observed for other nonacenes [7] -the well-resolved signals being due to the stabilization of its closed-shell ground state.
Single crystals were grown by subsequently layering nhexane and MeOH on aT HF solution of B 4 Non under nitrogen;s olvent molecules are included in the crystal packing but are heavily disordered and cannot be resolved. B 4 Non crystallizes with two independent molecules per unit cell with am inor p-p interaction of two phenanthrenylenes. These molecules form 1D-stacks and are oriented perpendic-  Table 1), 576 cm À1 blue-shifted in com-parison to that of Anthonysnonacene Non and red-shifted by 2859 cm À1 compared to the p-band of B 4 Hep.T he first reduction potential of B 4 Non occurs at À1.19 V( cyclovoltammetry,reversible,T able 1, SI), more negative than that of Non (À0.51 eV) due to the absence of electron-withdrawing fluorine substituents. [7,9] Thef irst oxidation potential is at 0.98 V( irreversible).
Similar to most higher acenes, [3a,7,10] there is ad ramatic difference between solid state and solution persistability: B 4 Non is stable for more than 6weeks under nitrogen in the solid state,a lthough its half-life in n-hexane solution is only 30 min under ambient conditions (see SI) and 7h under nitrogen atmosphere (Figure 4). In air probably the endoperoxide forms (see SI), [7] while the mode of decomposition under nitrogen is less clear.
Analysis of the decomposition products via recycling gel permeation chromatography (see SI) suggests the formation of dimeric and oligomeric species,both common degradation products for higher acenes. [3a, 10] Compared to Non, B 4 Non is less stable under air and nitrogen in solution (Figure 4), as Nons electron-withdrawing halogen substituents retard endo-peroxide formation. In contrast, Non is persistent for   [a] Lowest energy absorption maxima.
2d at 10 8 8Ci nt he solid state, [7] while B 4 Non is stable for weeks. Figure 5d isplays the calculated NICS values for B 4 Non' '. Thes ystem shows NICS(1) values in accord with expectations,with the formal inner triphenylene ring being the least aromatic one-"empty" in the Clar formalism-and the other rings displaying high aromaticity.T his is further illustrated in FMO calculations (see SI), in which the central triphenylene rings show small coefficients in the outer rings compared to their nonacene congeners.
In conclusion, we have prepared an ovel, reasonably stable tetrabenzononacene, B 4 Non,inwhich the stabilization is due to the attachment of four benzo units at the cataposition of the acene unit. Thematerial displays sharp NMR resonances and as lightly blue-shifted absorption in comparison to Anthonysnonacene Non,yet it is only one of the very few isolated and structurally characterized nonacenes. B 4 Non is highly stable in the solid state.B enzo-wings therefore should allow stabilization of other, hitherto only moderately persistent, reactive aromatics.