The synthesis of polycyclic aromatic hydrocarbons (PAHs) has been the focus of extensive research due to their applications in organic electronics.¹ In the context of tuning their electronic properties, the isosteric [CH→N] substitution of polycyclic aromatics provides a method to vary the electronic structure of the material while maintaining the molecular shape and rigidity of the π-system.² Whereas perylene-based dyes, such as perylene-3,4 : 9,10-bis(dicarboximides) (PDIs) have been among the most intensively studied chromophores in organic dye chemistry,³ a similar development of the structurally similar coronene was hampered by limited synthetic accessibility for a long time.⁴ More recently, the exploration of coronene dyes has experienced a renaissance based, inter alia, on the combination of the coronene core with the PDI-derived imido units.⁵ Examples for N-atom doping of coronene derivatives remain extremely rare,^{6, 7, 8} and only a single fully characterized tetraazacoronene has been reported.^8b

Recently, we have begun to apply isosteric [CH→N] substitution to perylene chemistry giving synthetic access to various types of peri-functionalized tetraazaperylene derivatives.^9-11 Herein we report two different strategies to laterally expand the azaperylene core in its bay-positions to give access to the corresponding tetraazacoronenes. On the basis of density-functional theory (DFT) simulations at the B3LYP/Def2-TZVPP-GD3(BJ)^12-17 level of theory, the aza substitution should lower both the HOMO and LUMO energies about 1 eV (see Figure 1).

In the first approach two adjacent sp hybridized carbon atoms are coupled via cyclization giving rise to a cyclobutene-annulated azacoronene. An alternative access to the tetraazacoronene core is achieved by fourfold Suzuki–Miyaura coupling of a >C=C< synthon which not only gives rise to the azacoronene parent compound but also to its two-, three- and four-fold condensed congeners.

The starting compound for the tetraazacoronene synthesis was the previously reported octaazaperopyrenedioxide (OAPPDO) derivative 1 (Scheme 1).¹¹ Pd-catalyzed coupling of OAPPDO 1 with trimethyl((tributylstannyl)ethynyl)silane led to the fourfold alkynylated OAPPDO 2 (see SI). Compound 2 was then subjected to a zirconium mediated cyclization as established by D. Tilley and co-workers.¹⁸ The reactive Zr^II species labelled as “Cp₂Zr” were prepared in situ through reduction of Cp₂ZrCl₂ with n-butyllithium (“Negishi's reagent”),¹⁹ followed by addition of 2 as well as subsequent workup with acetic acid gave the trimethylsilyl (TMS) substituted doubly cyclobutene-annulated azacoronene 4 as the main reaction product. In 4 the cyclobutene moiety represents a strained four membered ring (bond angles of 85.4(2)–94.9(2)°) with an sp²−sp² carbon bond length of 1.371(4) Å similar to coronene²⁰ and a significantly elongated sp³−sp³ bond length of 1.614(4) Å (Figure 2).²¹ Compared to the parent coronene, the bond lengths of the aromatic core are similar except for the C−N bonds which are, as expected, slightly shorter.²⁰

Analysis of the electrochemical properties revealed that azaperylene 2 and azacoronene 4 possess two quasi reversible one electron oxidation steps (0.954/1.191 V for 2 and 1.039/1.349 V for 4 against a saturated calomel electrode, see SI).

The TMS groups of 4 could be removed with TBAF as a F⁻ source forming the cyclobutene-annulated azacoronene 5. Its strained four-membered ring renders it a possible precursor for an electrocyclic ring-opening to afford the diene for consecutive Diels–Alder reactions. We have not observed reactions with substituted alkynes, however using N-ethylmaleimide as dienophile yielded the corresponding products. Separation of the stereoisomers remains subject of a future study.

The diastereoselective formation of compound 4 is already pre-determined by the relative orientation of the Zr-centers on both sides in the cyclization process. This was established by the isolation and structural characterization of the organometallic intermediate 3 by X-ray structure analysis. Its molecular structure, and particularly the coordination of the metal complex fragments, is depicted in Figure 3.²¹ The Zr fragment in 3 adopts a bent sandwich structure with two chlorido ligands, one of them bridging to a bis(thf-solvated) lithium cation. The latter connects one molecular unit with the urea oxygen atom of another azacoronene complex on each side which leads to the formation of a coordination polymer (Figure 3, top). The two π-coordinated ligands at Zr are the remaining Cp ligand and as well as a η⁴-bound cyclobutadiene moiety derived from the cyclization of the diynes in each bay area of the OAPPDO precursor.

The four-membered π-ligand in 3 has distorted square geometry, with the C−C−C bond angles lying between 87.7(2)° and 92.1(2)°, while the C−C bond lengths range from 1.430(4) Å to 1.535(4) Å. The variation of the C−C distances is complementary to that of the Zr−C bonds (2.296(3)–2.520(3) Å) in that longer C−C bonds are associated with shorter Zr−C bonds.

We note that only Cp₂-zirconacyclopentadienes have been isolated and characterized as intermediates of this type of cyclization reaction so far.^{18, 19b} In general, η⁴-cyclobutadiene complexes of early transition metals are rare,²² the sole example for another zirconium cyclobutadiene species having been reported by Milsmann's group.²³ Based on DFT modeling (B3LYP-GD3(BJ)/Def2-TZVPP)^12-17 and localized orbital bonding analysis^{24, 25} the zirconium in intermediate 3 is best described as Zr^IV (d⁰). A doubly reduced cyclobutadienyl moiety at both ends leads to an overall fourfold negative charge of the azacoronene-bis(cyclobutadienyl) unit (see SI). The long-ranging electrostatic repulsion between the zirconate units might be the main reason for the diastereoselectivity whereas short range steric effects (Pauli repulsion) are probably irrelevant. The HOMO and HOMO−1 π-orbitals resemble the two non-bonding orbitals of cyclobutadiene with only minor contributions of Zr d-orbitals, which is similar to the mononuclear complex previously studied by Milsmann.²³

A second synthetic strategy for azacoronene derivatives of the OAPPDO precursor 1 without the annulated cyclobutene rings has been based on Suzuki–Miyaura cross coupling of (E)-(1,2-bis(pinacolatoboryl)vinyl)trimethylsilane as a vinylene building block for the bay-area of 1 and subsequent desilylation (Scheme 2). 1,2-Bis(pinacolatoboryl)alkenes are readily accessible by reaction of the corresponding alkynes with B₂Pin₂ in the presence of [Pt(PPh₃)₄]²⁶ and are used as building blocks in the chemistry of functional PAHs,²⁷ while the (E)-(1,2-bis(pinacolatoboryl)vinyl)trimethylsilane employed in this work has only been reported as a product of borylation catalysis.²⁸ The azacoronene target 6 was obtained by this reaction sequence along the laterally condensed “dimeric” azacoronene 7 (after desilylation with TBAF) as major product when using 2.1 equivalents of the boronic ester.

To obtain some mechanistic insights, model reactions of the C₂ synthon under the same reaction conditions were performed (see SI) indicating that no Pd mediated isomerization to the corresponding Z diastereomer takes place. Analysis of the ¹H- and ¹¹B-NMR spectra revealed a desilylation of the synthon and a formation of other boron species. Reaction with bromobenzene showed no signs of a Heck type coupling competing against the Suzuki coupling. On this basis a tentative mechanistic proposal for the bay-annulation and competing oligomerization is displayed in Scheme 3. The first can be rationalized by a double Suzuki cross coupling reaction of the synthon similar to the mechanism described by Shimizu and co-workers.^27a Concerning the oligomerization, after the first Suzuki coupling of the synthon a further azaaromatic system needs to be coupled to the vinyl moiety. This can proceed via desilylation or CH activation. Similar Pd catalyzed couplings with simpler systems are described in the literature.^{29, 30} A detailed investigation of the mechanism involving appropriate model compounds will be addressed in future studies.

In order to assess whether higher oligomers of 6 were also formed during the reaction, we optimized the reaction conditions to promote further oligomerization (see SI). Using 1.5 equivalents of the boronic ester, not only 6 and 7 but also the corresponding three- and fourfold condensed expanded congeners 8 and 9 could be isolated and characterized. To the best of our knowledge, the azacoronene oligomers 7, 8 and 9 represent the first examples of heteroatom doped PAHs consisting of directly fused coronenes. In previous examples of lateral π-extension in PDI chemistry described by Nuckolls and co-workers, the PDI subunits are fused with >C=C< bridges.³¹ These nanoribbons were applied as materials for lithium batteries³² or in its twofold fused form as a polymer in solar cells.³³ Further connection motifs are pyrene³⁴ and naphthalene diimide units,³⁵ or a direct fusion of two PDI molecules³⁶ X-ray diffraction studies of 7 and 8 revealed that the individual planar coronene moieties are twisted with respect to each other by 32.998(15)° and 27.145(18)°, primarily due to steric pressure of the hexyl substituents (Figure 4 and Supporting Information for 7).²¹ In particular, the threefold extended compound 8 crystallizes in its meso and not a helically chiral conformation. Compared to doubly cyclobutene-annulated azacoronene 4, the bond lengths of the aromatic cores of 7 and 8 are similar except for the bond of the connecting carbon atoms being longer (1.439(3) vs. 1.371(4) Å).

The UV/Vis absorption spectra of the azacoronene 6 and its fused extended congeners 7, 8, and 9 in solution are characterized by absorption bands in the visible spectral region with a distinct vibrational progression (Figure 5, see Supporting Information for a plot on a scale of molar absorption coefficient and for the spectra of azacoronenes 4 and 5). The effect of the π-extension on the bathochromic shift of the absorption maxima is most pronounced upon going from 6 to 7 (103 nm) and decays towards the higher oligomers (53 nm and 29 nm), while the molar extinction coefficients increase by one order of magnitude (Table 1). All compounds are highly fluorescent with the emission band being Stokes shifted by 150–200 cm⁻¹ and the luminescence quantum yields of the oligomers higher than for the parent compound 6 (67–80 % versus 45 %).

Based on TD-DFT modeling,^12-17 the absorption maxima in the visible can be assigned to the first singlet transition (S₀→S₁) for all compounds and show a similar pattern of vibrational progression towards shorter wavelengths. Notably, there is another sharp absorption band in the long wave UV region observable in the spectrum of 6 which corresponds to the S₀→S₂ transition.

To analyze and quantify the local aromaticity of the azacoronenes and the effect of oligomerization on the aromatic ring current, the anisotropy of the induced current density (ACID) was analyzed (Figure 6).³⁷ As for coronene itself,^37b the density current vectors of 6 indicate a strong diatropic ring current running clockwise around the circulene periphery and a weak paratropic ring current in the central six membered ring. Fusion of further azacoronene moieties causes an expansion of the ring current leading to a delocalization around the whole molecule, shown here for 7 and similar to previously reported laterally benzo-fused coronenes³⁸ (see Supporting Information for the ACID plots of the higher oligocoronenes and OAPPDO). It is therefore apparent that the lateral π-extension of the OAPPDO starting material, which itself displays the electronic characteristics of a tetraazaperylene (containing two fused diazanaphthalene units), leads to a full delocalization of the π-electrons. This is in stark contrast to a longitudinal π-extension in the form of the “rylene” series in which independent naphthalene units are fused with only partially coupled π-systems.³⁹

The synthetic approach to a new class of azacoronene dyes has additionally provided a viable synthetic method to extended nanoscale azaaromatic systems with a high level of “N-doping”, the first examples of this series having been reported in this work. The relative ease of their isolation along with their pronounced fluorescence properties render this class of compounds an attractive target for further expansion towards (atomically precise) nanoribbons with adjustable optoelectronic properties and may find application as materials in lithium batteries as similar edge fused perylene diimide coronenes.