(Aza)Pentacenes Clipped into a Ring: Stabilization of Large (Aza)Acenes

Abstract A doubly alkylene bridged 6,13‐diphenylpentacene and analogously bridged azapentacenes were prepared; they are persistent. The doubly bridged azapentacenes display superior photochemical, oxidative and thermal stabilities compared to azapentacenes protected by bis(TIPS‐ethynyl)‐substituents—clipping an azaacene into a large ring is a viable complement in stabilization.

of the methyl groups with BBr 3 is followed by transformation of the resorcinic intermediate with 1,7-dibromoheptane in DMF (K 2 CO 3 as base) to furnish jacketed pentacene 5. The double cyclization was performed at concentrations of 5 mmol L À1 . For the azaacenes 6 and 7, the bridged diaminonaphthalene 4 was obtained from naphthothiadiazoloquinone. Addition of lithiated 1,3-dimethoxybenzene and reduction with sodium hypophosphite, followed by the opening of the thiadiazole ring by SmI 2 gives 4. Diamino-naphthalene 4 couples under established Pd-catalyzed conditions [9, 10, 11a,b, 12] with 2,3-dihaloarenes to give the azapentacenes 6 and 7. If the doubly bridged diaminoanthracene is employed, 8 results, while 9 is obtained by coupling of ortho-phenylenediamine with the encapsulated 2,3-dibromoanthracene. 6-9 form as the N,N'-dihydro-compounds-these are oxidized by MnO 2 into the azaacenes.
The consanguine TIPS-ethynyl(aza)acenes are literature known and were prepared as reference substances. [2,11,12] Figure 1 displays a photograph of dilute solutions of 5-9 and of 5 TIPS -9 TIPS .The visual colors are similar-their slight variations (cf. 5 and 5 TIPS ) are due to the TIPS-ethynyl groups enlarging the conjugated p-system. The doubly alkylene bridged (aza)acenes display in general broader and blueshifted features in the Uv-vis spectra ( Figure 2). This is not the case for 9-l max of 9 TIPS is blue shifted, possibly due to an increased donor-acceptor character of 9 in comparison to 9 TIPS . The doubly bridged azaacenes appear non-emissive, similar to their TIPS-congeners, only 5 fluoresces notably. Table 1 displays the electronic properties of the targets and their first reduction potentials. As expected, tetraazaderivative 6 is most easily reduced, while the other azaacenes display fairly similar reduction potentials (À1.4 to À1.5 V, vs. Fc/Fc + ) and electron affinities. This trend is echoed in the silylethynylated (aza)acenes, which are more readily reduced due to the electronegative ethynyl substituents with reduction potentials ranging between À1.0 to À1.2 V for the diazaderivatives.
Single crystalline specimen of 5, 6 and 9 were obtained by slow diffusion of methanol into a chloroform solution of the (aza)acene ( Figure 3). Bond lengths and angles of the aromatic cores are in agreement with calculated values. Both 6 and 9 contain chloroform in the crystal lattice. The packing is dominated by van der Waals contacts of the bridging rings with each other. p-p-contacts are not observed for the (aza)pentacenes (packing diagram see SI), as the double bridges dominate the supramolecular structure.
Important is the relative stability 5-9 under irradiation ( Figure 4), performed under air and under argon (10 À5 mol L À1 , DCM, ambient temperature).    Under argon, 5 is of comparable stability to 5 TIPS , but in air, 5 is more easily oxidized than 5 TIPS (Figure 4). We isolated 10, a rearrangement product of the endo-peroxide of 5, identified under mass spectrometric conditions and by its single crystal structure (Figure 5 a). This rearrangement was previously described by Rigaudy et al. in the photolytic decomposition of anthracene. [15] We propose predominant formation of an 5,14-endo-peroxide for 5 due to steric shielding, whereas for 5 TIPS the main product is the 6,13endo-peroxide (98:2; 6,13-vs. 5,14-endo-peroxide). [16] 6-9 are consistently more stable than their TIPS-ethynylcongeners, both under argon but also under air. We note that the position of the pyrazine unit and to a lesser effect the position of the substituents influence the reactivity for the TIPS-ethynyl substituted azaacenes. The bridged azaacenes 6, 8 and 9 were still intact after 18 h irradiation under argon atmosphere. Irradiation in DCM under ambient conditions chlorinates the azaacenes, as verified by mass spectrometry. 7 furnishes 11 as one of the photoproducts (Figure 5 b) we could isolate. Generally, the mixtures formed during the photolysis of the azaacenes are difficult to separate and to characterize.
To expand the clipping-and-jacketing concept, we reacted 12 (Pd-catalyzed) with 2,3-dichloroquinoxaline, treated the coupling-product with PbO 2 and obtained the tetraazahexacene 13 in 53 % yield (Scheme 2, l max abs = 946 nm). [12] An Xray analysis proves the topology; 13 crystallizes without solvent and displays p-p-overlap involving the electron rich and electron poor parts of the hexacene, respectively ( Figure 6). [18] 13 is stable and can be handled without any problem, demonstrating the use of jacketing.
In large azaacenes, Kobayashis double alkylene bridging, termed "clipping-and-jacketing", is superior to TIPS-alkyne substitutens with respect to stabilization. Tetraazahexacene  13 packs in the single crystalline state with p-p-stacking; it has not escaped our attention that molecules like 13 might be useful as n-channel semiconductors in thin-film transistors. Jacketing could emerge as powerful alternative to trialkylsilylalkynylation, particularly as nature and steric demand of the alkylene bridges-the jackets-are easily varied.