Dehydrative π-extension to nanographenes with zig-zag edges

Zig-zag nanographenes are promising candidates for the applications in organic electronics due to the electronic properties induced by their periphery. However, the synthetic access to these compounds remains virtually unexplored. There is a lack in efficient and mild strategies origins in the reduced stability, increased reactivity, and low solubility of these compounds. Herein we report a facile access to pristine zig-zag nanographenes, utilizing an acid-promoted intramolecular reductive cyclization of arylaldehydes, and demonstrate a three-step route to nanographenes constituted of angularly fused tetracenes or pentacenes. The mild conditions are scalable to gram quantities and give insoluble nanostructures in close to quantitative yields. The strategy allows the synthesis of elusive low bandgap nanographenes, with values as low as 1.62 eV. Compared to their linear homologues, the structures have an increased stability in the solid-state, even though computational analyses show distinct diradical character. The structures were confirmed by X–ray diffraction or scanning tunneling microscopy.

A 250 mL round bottom flask, was charged with hexamethylenetetramine (8.50 g, 60.0 mmol) and dissolved in CHCl3 (100 mL). A solution of Br2 (20.0 g, 125 mmol) in CHCl3 (100 mL) was added dropwise at rt. A yellow solid precipitated. The mixture was stirred for an additional 30 min, and the yellow solid was collected by vacuum filtration, yielding the desired complex in 92 % (25.5 g, 55.2 mmol). The complex was used as obtained without further characterization.
A 5 L two-neck round bottom flask, equipped with a magnetic stir bar and a dropping funnel was charged with pyrene (30.0 g, 148 mmol) and dissolved in CHCl3 (1.5 L). A solution of Br2 (15.3 mL, 297 mmol) in CHCl3 (500 mL) was added dropwise at rt over 8 h. The formed precipitate was collected after 16 h, washed with MeOH and purified by recrystallization from hot xylene (mixture of isomers), yielding the product as beige solid in 36 % (19.0 g, 52.8 mmol).
NMR data of pure product were in accordance with literature.
A dry 250 mL Schlenk round bottom flask, equipped with a magnetic stir bar and a dropping funnel was charged with 1,2,4,5-tetrabromobenzene (3.74 g, 10.0 mmol) and dissolved in toluene (100 mL) and furan (5 mL) under a N2-atmosphere. The mixture was cooled and a temperature between -20 °C to -30 °C was maintained while adding a mixture of 2.5 M n-BuLi/hexanes (4.0 mL; 11.0 mmol) dissolved in 100 mL hexanes over 3 h. The mixture was allowed to warm to rt over a period of 12 h. The white suspension was quenched with MeOH (1.0 mL). The clear solution was washed with H2O (2x 20 mL) and the aqueous layer was extracted with Et2O (1x 10 mL). The combined organics were dried over MgSO4, concentrated to give an oil, which upon precipitation from CH2Cl2 and hexanes yielded dibromoepoxynaphthalene as white solid. The intermediate product was used without further characterization in the next step.
A dry 500 mL Schlenk round bottom flask equipped with a magnetic stir bar and a condenser was charged with Zn-powder (6.00 g, 92.0 mmol) and suspended in THF (150 mL) at 0 °C. TiCl4 (6.0 mL, 55.0 mmol) was added slowly via syringe at 0 °C, and the mixture was brought to reflux for 5 min until all yellow solids dissolved. The mixture was cooled down to 0 °C again. A solution of the prepared epoxydibromonaphthalene in THF (60 mL) was added slowly over 10 min to the Zn/Ti mixture.

General procedure
A 250 mL round bottom flask equipped with a magnetic stir bar is charged with the precursor (3, 4, 6, S3, 8, or 9) (20 mg) and dissolved in CH2Cl2 (100 mL). While stirring, a solution of SnCl2 . 2H2O (500 mg, 2.22 mmol) in i-PrOH (2.0 mL) is added at rt. At this point a slight color change to yellowish is typically observed. Then, conc. H2SO4 (1.0 mL) is added and the mixture is allowed to stir at rt with protection from daylight for 18 h. A dark purple coloration and the formation of a white precipitate is typically observed. The dark purple mixture is quenched with 1 M HCl (2.0 mL). Vigorous mixing is necessary at this point. The mixture is diluted with CH2Cl2 (20 mL) and washed with H2O (1x 50 mL). The aqueous layer is extracted with CH2Cl2 (3x 20 mL) including the dark insoluble solids (product). The combined CH2Cl2 layers are diluted with MeOH (100 mL) and the CH2Cl2 is removed on the rotavap at atmospheric pressure at 50 °C. The formed precipitate in the MeOH layer is centrifuged, the MeOH layer is decanted and the solid is washed again with MeOH. The product is dried in vacuo and obtained in quantitative yield.

Bulk synthesis of 2.3,8.9-Dibenzanthanthrene (DBATT)
A 4 L round bottom flask equipped with a magnetic stir bar was charged with 1,6-bis(2-formylphenyl)pyrene 3 (0.50 g, 1.22 mmol) and dissolved in 2.5 L CH2Cl2. A solution of SnCl2 . 2H2O (12.5 g, 55.4 mmol) dissolved in i-PrOH (50.0 mL) was added, followed by the addition of conc. H2SO4 (25.0 mL). The mixture was stirred for 18 h. 1 M HCl (50.0 mL) was added and the solution was mixed thoroughly. H2O (1.0 L) was added and the mixture was extracted with CH2Cl2 (5x 200 mL). The organic layer was diluted with MeOH (500 mL) and the CH2Cl2 layer was evaporated on the rotavap. The remaining MeOH layer was centrifuged, the supernatant was decanted and the solid was washed again with MeOH. The dark blue solid was dried in vacuo, giving DBATT in 96 % (0.44 g, 1.17 mmol).

STM Measurements
All low-temperature STM experiments were performed in a two-chamber ultra-high vacuum (UHV) system from Scienta-Omicron GmbH that is operated at a base pressure below 1 × 10 -10 mbar. The STM measurements were conducted in the constant current mode at a sample temperature of around 4.7 K or 77 K, respectively. The bias voltages mentioned here refer to the sample, which was grounded in the experiment. The STM topography images were analyzed with the WSxM software. 2

Sample Preparation
The Ag(111) and Au(111) (from MaTecK) single crystals were cleaned by subsequent cycles of Ar + ion sputtering (1 keV) and annealing at 750 K, respectively. The TTc, BPc, and TPc were thermally evaporated from a commercial Knudsen cell (Kentax GmbH) with the quartz crucible held at 350°C, 300°C, and 395°C, respectively. The molecules were thoroughly degassed before the deposition on the clean metal surface. The evaporation rates were determined by a quartz crystal microbalance. The metal substrates were kept at room temperature during the evaporation of molecules. Figure S7a shows that the deposition of nearly a monolayer of TTc on Au(111) held at room temperature results in the formation of a close-packed self-assembly. The TTc monomers within the islands can be identified by the x-shape of the carbon backbone. Along the direction of the short axis of the x-shaped carbon backbone, the molecules form perfectly ordered columns. Weak intermolecular van der Waals forces are presumably responsible for the formation of the well-ordered columns. We observe two rotational domains of the assembly, which are distinguished by a rotation of the end-to-end connection between the branches of two neighboring molecules (highlighted by the green lines). The corresponding unit cells are shown in figure S7b, where the molecules are connected horizontally in the unit cell highlighted in red and the other domain (blue unit cell) has a vertical connection line. The rhombic unit cell measures a = 1.44 ± 0.11 nm, b = 1.64 ± 0.13 nm and ϴ = 77°± 3° for both domains. The domain boundary line is indicated by a white line in figure S7b. The Au(111) herringbone reconstruction, 3 is clearly visible through the self-assembled molecular overlayer and seems not affected by adsorption of the molecules. The intact herringbone reconstruction of the Au(111) surface underneath the adsorbed molecules is generally considered as a sign for a weak molecule-substrate interaction suggesting a physisorption-type interaction. 4 Figure 7. Low-temperature STM images (77 K) of self-assembled TTc on Au(111). (a) Overview STM image of the TTc self-assembly. (b) Detailed STM image showing the two rotational domains (red and blue unit cell) of the TTc self-assembly. The two domains are distinguished by a rotation of α = 120° ± 4.2° of the endto-end connection between the branches of two neighboring molecules (highlighted by the green lines). Scale bars: 2 nm. Tunneling conditions: a) 100 pA/1 V; b) 100 pA/1 V.

Adsorption of TPc on Ag(111)
The overview STM image in figure S8a shows TPc molecules adsorbed on Ag(111) after deposition at room temperature. Two kinds of molecules are observed. The majority is xshaped, while there are also a few y-shaped molecules co-adsorbed. The y-shaped species is considered as a TPc with one naphthyl fragment missing (three-fold coupled precursor instead of four-fold), most probably being a minor byproduct during the synthesis process. The yshaped molecule is expected to sublime at lower temperature than the x-shaped molecule and therefore observable in room temperature depositions. The size of the x-shaped molecules from the STM image is consistent with the size of the TPc. Besides the two kinds of molecules, bright protrusions are observed on Ag(111). Figure S8b shows a detailed STM image of the bright protrusions, which might arise from halogen atoms that could be residues from the synthesis. The protrusions form chains as well as networks (highlighted by the white circles in figure S8b). The networks have a √ × √ structure with 0.46 ± 0.2 nm spacing of neighboring atoms, which is consistent with the previously observed halogen-halogen distances on Ag(111). 5,6 The amount of halogen residue was significantly reduced by degassing the compound under UHV conditions. To avoid lager amounts of dissociated halogens on the surface, we propose experiments with surfaces preheated above the desorption temperature of the halogens (>250 °C) a strategy that was successfully used for growing graphene from hexabromobenzene on Cu(111). 7 Under these conditions, perturbing halogens are supposed to desorb, while the TPc molecules are expected to stick to the surface owing to their larger mass. However, chemical reactions within and among the molecules cannot be excluded by the deposition at elevated sample temperatures and trace amounts of byproducts from the synthesis are less likely to be seen. Therefore, we preferred here to use room temperature evaporations for the structural identification of the molecules and the analysis of possible byproducts. We note that the overall trace concentration of halogen is small as confirmed by mass spectrometry but is pronounced in this preparation technique.

Computations
Geometries were optimized at the DFT R(U)B3LYP 6-311+G(d,p) level of theory (RB3LYP for closed shell structure, UB3LYP for open shell structure) using the Gaussian '09 work package. 1

Closed shell
Open shell Tetracene, pentacene, DBATT, TTc and TPc revealed a closed shell singlet ground, whereas hexacene and BPc favored an open shell singlet ground state (bold energies). The occupation number of the HONO and LUNO were obtained from the DFT-optimized minimum structures and evaluated at the UHF 6-31+G(d,p) level of theory. 8 Diradical character y0 was obtained from equation S1 and the Eigenvalues of the respective HONOs and LUNOs. 9 Occupation number