Azaarenes: 13 Rings in a Row by Cyclopentannulation

Abstract Cyclopentannulation was explored as a strategy to access large, stable azaarenes. Buchwald–Hartwig coupling of previously reported di‐ and tetrabrominated cyclopentannulated N,N′‐dihydrotetraazapentacenes furnished stable azaarenes with up to 13 six‐membered rings in a row and a length of 3.1 nm. Their optoelectronic and semi‐conducting properties as well as their aromaticity were investigated.

strategies (Alonso et al., [21] Baumgarten et al. [22] ) or by zipping up a long precursor (Mastalerz et al., [14c, 23] Chalifoux et al. [24] ). The current record holder in length is compound B with a length of 14.9 nm. [25] Most of these examples are based on the annulation of six-membered rings, whereas cyclopentannulation is underexplored for the synthesis of molecular nanocarbons, although pentacene was previously stabilized in such a fashion. [26] In this contribution, we use our recently reported di-and tetrabrominated pentannulated dihydrotetraazapentacene building blocks (1 a,b) [27] for cross-coupling reactions with privileged ortho-diamines to obtain soluble molecular Ndoped nanocarbons reaching up to 3.1 nm (calculated by DFT for 3 c) in length.
Compounds 1 a,b were postfunctionalized via Buchwald-Hartwig [29] couplings with TIPS-ethynylated diamines [30] (see Supporting Information) furnishing mono-(2 a-c) and bis-πextended (3 a-c) cyclopentannulated azaarenes along with their corresponding NH-species (Scheme 1). After column chromatography, the product mixtures were oxidized with manganese(IV) oxide in DCM to yield the oxidized species. The yields ranged between 21 % and 81 %. In contrast to the sparingly soluble starting bromides, the products are well soluble in common organic solvents, testimony to the presence of four or six silyl groups, respectively. Nevertheless, we noticed decreasing solubility with increasing number of rings. Figure 2 displays the absorption spectra of the doubly post-functionalized but non-emissive compounds 3 a-c (see Supporting Information, Figure S13, for the spectra of compounds 2 a-c). As expected, the extension of the πsystem results in a red shift of the absorption maximum, increasing number of annulated rings (102 nm for 3 a, 215 nm for 3 b and 355 nm for 3 c) compared to 1 b. The corresponding mono-postfunctionalized compounds 2 a-c are blue shifted by 25 nm for the smallest system and up to 32 nm for the largest system with respect to their doubly functionalized counterparts. This indicates only a small electronic interaction between both terminal acene moieties. NICS(1.7) zz -xy-scans [31] (Figure 3a) as well as anisotropy of the induced current density (AICD-π-only) [32] calculations support this interpretation. The central six-membered ring (ring 7) of 3 c exhibits NICS(1.7) zz values up to À 1 ppm indicating (close to) no aromaticity. In contrast, to the five membered rings are attributed NICS values up to À 14 ppm, which is in the same region as the eastern/western rings of the molecule (see Figure 3, rings 1 and 13), a sign of aromaticity. The current density vectors on the AICD-πonly isosurface show (diatropic) ring currents over the complete molecule except the two bonds between the central ring and the neighboring six membered rings. In these bonds, scalar field/current density vectors are not observable, indicating the lack of conjugation/aromaticity, in agreement with the NICS calculations ( Figure 3c). We calculated the diradical character for 3 a and 3 c. Both molecules show a diradical character of 0.
The calculated (DFT, B3LYP/def2-TZVP) and experimental optoelectronic properties are summarized in Table 1. For the arenes, two reversible reduction events (three for 3 a) and up to two non-reversible oxidation events (one for 2 a, 3 a, 3 b, 3 c and two for 2 b, 2 c) were observed in their cyclic voltammograms. Some of the measured values for the electron affinity differ significantly from the calculated ones for the LUMO, probably due to degradation of the compounds on the electrode. TD-DFT calculations indicate that the absorption maxima correspond to the HOMO-LUMO-transitions.  We grew crystals for x-ray diffraction analyses by diffusion of methanol into a saturated solution of the arenes in chloroform or dichlormethane. The crystal structure of 2 b and 3 b (Figure 4) unveil that 2 b forms a 1D staircase with overlapping acene units. The distance between the layers is estimated to 3.64 Å. Besides the π-π overlap there are also TIPS-π-interactions to the next 1D stack. In the crystalline state, 2 b is 1.9 nm long (measured without hydrogen atoms).
Compound 3 b shows a similar packing motif as 2 b with additional (two sided) π-π interactions. The π-π distance amounts to 3.29 Å. The length of the molecules is determined to 2.6 nm for 3 b (measured without hydrogen atoms using the crystal structure) and 3.1 nm for 3 c (measured without hydrogen atoms using the optimized DFT structure). [35] Due to their electrochemical properties and the possibility to obtain solvent-free crystal structures, 2 b and 3 b were promising candidates for thin film transistors (TFTs). TFTs were fabricated in bottom gate/top contact architectures ( Figure 5 and Supporting Information) with silver as contact electrodes, using 12-cyclohexyldodecylphosphonic acid as self-assembling monolayer (SAM) to modify the dielectric (SiO 2 /AlO x ). [36] Thin films of 2 b and 3 b were obtained by drop-casting (2 b: toluene, 0.5 mg mL À 1 ; 3 b: DCM, 0.5 mg mL À 1 ). The average electron mobility of 3 b (μ ave = 1.4 × 10 À 3 cm 2 V À 1 s À 1 ) is one order of magnitude higher than that of the mono-postfunctionalized compound 2 b (μ ave = 2.1 × 10 À 4 cm 2 V À 1 s À 1 ) probably due to π-π-interactions at the western and eastern benzene rings of 3 b compared to 2 b. As 3 c did not furnish satisfactory thin-films due to its decreased solubility compared to 3 b, OFETs could not be obtained.
To verify the measured electron mobilities, we calculated transfer integrals of all different types of neighboring molecular pairs out of the crystal structure using the ADF software package. [37] Reorganization energies were calculated by the four-point method [38] using Gaussian16. These values were employed for the calculation of a theoretical electron transport mobility μ ( Table 2). [39] Only the highest transfer integral of each compound is shown-all other transfer integrals, including images of the corresponding dimer pairs, are listed in the Supporting Information, Figure S19, S20. The calculated electron mobilities (2 b: μ theo = 0.69 cm 2 V À 1 s À 1 ; 3 b: μ theo = 2.6 cm 2 V À 1 s À 1 ) also predict that the value for 3 b is one order of magnitude higher compared to 2 b. The absolute calculated mobilities differ from the measured ones, possibly due to differences in the solid state structure in the film compared to that of the single crystal.  [a] First reduction potentials determined via cyclic voltammetry (CV) in DCM at room temperature with Bu 4 NPF 6 as the electrolyte against Fc/Fc + as an internal standard (À 5.10 eV) [33] at 0.1 V s À 1 , 0.2 V s À 1 or 0.5 V s À 1 ; [b] ionization potential = electron affinityÀ λ onset,abs ; [c] obbtained from DFT calculations (Gaussian16 [34] B3LYP/def2-TZVP) [d] electron affinity = À e(5.1 V + E (0/À ) ); [e] measurement not possible due to insufficient solubility.
To assess the stability of the compounds in solution, time dependent absorption spectroscopy was performed on 2 b,c and 3 b,c in dichloromethane. We chose TIPS-TAP [40] as reference ( Figure 6). The solutions were irradiated with UVand white light under ambient conditions (for more details, see the method section in the Supporting Information). In Figure 6 time dependent decay of the absorption intensity is depicted. All synthesized molecules are more stable than TIPS-TAP (t 1/2 = 15 min). The mono-functionalized derivatives (2 b: t 1/2 = 25 min, 2 c: t 1/2 = 17 min) are less stable in comparison with their difunctionalized congeners (3 b: t 1/2 = 45 min, 3 c: t 1/2 = 35 min). The most stable material is 3 b, whose half-life is three times higher compared to that of TIPS-TAP.
To characterize the electron transporting species in the n-type OFETs, we attempted to synthesize the corresponding radical anion and dianion of 3 b by adding 1 or 2 equivalents of potassium anthracenide to its THF solution. After addition of 1 equivalent, the deep blueish solution turned purple under N 2 . EPR measurements (Supporting Information, Figure S14) support the presence of a radical anion. Unfortunately, the compound decomposes quickly under ambient conditions to the starting material -we were neither able to obtain suitable single crystals nor to measure    a suitable absorption spectra. Adding another equivalent of the reducing reagent results in a color shift to orange. Again, the instability of this species under air prevented further characterization.
In conclusion we synthesized a new class of azaarenes via modular π-extension of a cyclopentannulated azaarene core via Buchwald-Hartwig couplings. All aza-nanocarbons are stable under ambient conditions -the largest compound comprises 13 annulated rings in a row. 2 b and 3 b were used as semiconductors in organic field effect transistors with a bottom gate/top contact architecture. Di-postfunctionalized 3 b shows an electron mobility which is one order of magnitude higher compared to the mono-postfunctionalized compound 2 b. This is most likely the consequence of π-πinteractions in two directions.
Deposition Numbers 2208801 (for 2b), 2208802 (for 2c), 2208803 (for 3b) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.