N‐Acenoacenes: Synthesis and Solid‐State Properties

Abstract Four N‐acenoacenes were synthesized and analyzed for their optoelectronic properties and solid‐state packings. Two of the regioisomeric acridinoacridines are TIPS‐ethynylated, whereas the other pair are Boc‐ and triflate substituted derivatives. The two TIPS‐ethynyldiazaacenoacenes were processed into organic thin‐film transistors with saturation hole mobilities reaching 2.9×10−2 cm2(Vs)−1.

In this article, we present a synthetic access to TIPSethynylated aza-acenoacenes. They display reduced steric demand compared to 1 -they are substituted with only two morphology-controlling TIPS-ethynyl substituents, similar to their parent azaacenes. [9] X-ray crystallography unveils packing arrangements of 3 and 4 conducive for applications as OFET materials.

Results and Discussion
Pyridinic bisazaacenes 3 and 4 are accessible by i) Buchwald-Hartwig amination of naphthalene 5 or 10, ii) saponification of 6 or 11 and subsequent iii) twofold intramolecular Friedel-Crafts acylation (Scheme 1). Bisacridones 8 and 13 were obtained in high yields (60 %-88 % over 3 steps) starting from dibromo-naphthalenes 5 or 10. We note that intramolecular acylation was only successful starting from a naphthalene core: 1,5naphthyridine yielded intractable and insoluble products in the ring closing reaction (see Supporting Information).
From here on, the synthetic access diverged: Although 8 and 13 were subjected to Boc-protection, only 9 was obtained as the expected N,N-di-Boc-protected bisacridone, similar to our previously synthesized linear acridone systems. 10 The OÀ Bocsubstituted bisacene 14 a was isolated starting from 13 (49 % yield). This is counterintuitive as the valence structure of the respective N,N-diÀ Boc-species suggests increased stability due to an additional Clar sextet. DFT calculations show that 14 a is more stable by 24.5 kcal mol À 1 than the NÀ Boc-derivative (see Table S3).
9 was transformed into acridinoacridine 3 in 24 % yield by addition of lithiated TIPS-acetylene followed by in situ methylation and subsequent deprotection with concomitant elimination of methanol (Scheme 1). [10] As 14 a was unreactive, 14 b was synthesized from 13 with freshly distilled triflic anhydride, also resulting in aromatization. Stille coupling with triisopropylsilylethynyl stannane gave 4 in 51 % yield. All reported bisacenes are stable in air and can be stored at 8°C over long periods of time. They are moderately soluble in common solvents (< 1 mg mL À 1 ). For stability in solution see Figure S5 in the Supporting Information.
The absorption features of the azaacenoacenes are redshifted by approximately 50 nm compared to that of TIPSethynylated acridine 15 (Figure 2), a reference compound (see Supporting Information). The absorption maximum of 3 is most red-shifted (λ max = 468 nm), 14 nm further than that of 4 (λ max = 454 nm). This is backed by quantum-chemical calculations of differences in HOMO LUMO gaps (15 nm, 0.08 eV, see Table S2, Figure S6). In silico, 3 adopts a twisted conformation as a consequence of the two fjord-type regions and to avoid steric repulsion. This twist is also observed in the crystalline state (see below) -we assume that it is responsible for the decrease in the optical gap, an effect previously described by Gidron et al. for substituted anthracenes. [13] This effect on the gap was reproduced in silico for a twisted, unsubstituted and nitrogenfree derivative of 3 compared to its planar form. Its twist (29.8°e nd-to-end twist [16] ) decreased the gap by 0.15 eV compared to the same molecule in planar geometry (Table S2, Figure S6). Unsubstituted planar model regioisomers of 3 and 4 do not significantly differ in their frontier molecular orbital (FMO) energy levels (see Supporting Information). Thus, the regioisomerism does not significantly alter the optoelectronic properties. When comparing 3 and 4 to 1, the difference in substituents/nitrogen content barely influences the strongest absorption maximum (λ = 309 nm for 1) -the effects of additional nitrogen atoms in the backbone and additional ethynyl-substitution nearly cancel each other out. λ max (474 nm) of 1 is red shifted by 6 nm compared to that of 3 and 20 nm compared to that of 4. The normalized absorption of 1 is generally broader and more intense over the whole absorption compared to 3 and 4.
The absorption spectra of 14 a and 14 b are almost identical ( Figure S2). Compared to 4, their absorption maxima at the longest wavelength (14 a: λ max = 421 nm; 14 b: λ max = 427 nm) are blue-shifted by 33 and 27 nm, respectively, attributed to the smaller π-system due to the missing ethynyl substituents. All reported bisacenes show a yellow fluorescence with a small Stokes shift of 6 nm to 9 nm (see Table 1 and Supporting Information Table S1). Although the emission profiles appear similar, 15 exhibits a larger Stokes shift of 27 nm. The low solubility of the bisacenes prevented characterization by cyclic voltammetry.
The solid-state packing of the new bisacenes was determined by X-ray crystallography (Figure 4, Figures S8-S10). 3 is twisted (25.3°end-to-end twist [16] ). It packs in an A 2 B 2 brick wall motif; resulting in π-π-overlap between layers (Figure 4, top) with distances between 3.3-3.6 Å. Planar 4 packs in a brick wall arrangement. π-π-distances of 3.58 Å indicate interaction of the aromatic backbones. The crystal packing of 4 is temperaturedependent, two similar polymorphs of 4 exist at room temperature and À 78°C, respectively ( Figure S10). According to grazing incidence diffraction, thin-films of 3 and 4 display the same packing on surfaces used for device fabriacteion (see below) as observed for single crystals and have their molecular axis oriented orthogonally to the surface ( Figure S13). The flexible substituents in 14 a and 14 b allow for a tighter packing and thus more pronounced π-π interactions. Compound 14 a exhibits a herring-bone packing motif (π-π distance: 3.32 Å).   [14] plot of 3 (left) and 4 (right). The ring current at the edge of the formal acridines is interrupted; isovalue: 0.02; optimization limit: 0.02; maximal arrow length: 1; magnetic field vector is oriented out of plane.  [12] [e] Taken from ref. [5], measured in DCM.

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202201916 14 b packs in a 1D slipped stack (π-π distance: 3.34 Å, see Figure S12). 1, bearing four TIPS-ethynyl substituents, shows no π-π overlap (CCDC: 1946373), [7] underlining the importance number and placement of the substituents on the morphology -see Supporting Information ( Figure S14) for a Hirshfeld analysis [17] comparing short contacts of 1, 3 and 4. Bottom gate/top contact OFEts ( Figure S16) with gold as electrode material and a bilayer dielectric consisting of SiO 2 and Al x O y coated with an alkyl-SAM to prevent trap states were fabricated using 3 and 4. [18] 14 a,b were difficult to process due to their low solubility. The best performing devices resulted from drop-cast thin-films of 4 (toluene/acetone 80 : 20, c = 0.25 mg/mL). Values extracted from transfer measurements of 55 channels measured over 6 different wafers gave average hole mobilities of 5.3 × 10 À 3 cm 2 (Vs) À 1 and maximum saturation hole mobilities of up to 2.9 × 10 À 2 cm 2 (Vs) À 1 and an on/off-ratio of 10 4 ( Figure 5, left).
With SiO 2 as dielectric, devices using 3 with maximum saturation hole mobilities of 2.7 × 10 À 4 cm 2 (Vs) À 1 and an on/offratio of 10 2 could be fabricated ( Figure 5, right). Pictures of the resulting thin-films are provided in the Supporting Information. Measurements of TIPS-pentacene 19 as reference were carried out under the same conditions. For SAM-coated substrates, peak mobilities were 0.58 cm 2 (Vs) À 1 and for neat SiO 2 substrates 2.8 × 10 À 2 cm 2 (Vs) À 1 . 3 and 4 are reasonable p-channel transport materials. Particularly 4 reaches a tenth of the mobility of the

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202201916 reference material TIPS-pentacene extracted in our experiments. Calculated mobilities (see Section 9 of the Supporting Information) suggest that higher mobilities could be achieved with better film morphology.

Conclusion
In conclusion, four novel aza-acenoacenes 3, 4, and 14 a,b were prepared. Their optoelectronic properties do not vastly differ when compared to 1, although solid state packing is dramatically influenced by number and position of the TIPS-ethynyl substituents. As a consequence, favorable π-π interactions result for the newly synthesized azabisacenes. OTFT devices of 4 performed with peak transfer mobilities of 2.9 × 10 À 2 cm 2 (Vs) À 1 . This is one tenth the mobilities reached for single crystal devices of 2 a and of the reference material TIPS-pentacene that was fabricated under similar conditions. Using 3, mobilities up to 2.7 × 10 À 4 cm 2 (Vs) À 1 were obtained. This opens up the field of azaacenoacenes as promising semiconductor materials.