Cyclopentannulation and Cyclodehydrogenation of Isomerically Pure 5,11-Dibromo-Anthradithiophenes leading to Contorted Aromatics
Isomerically pure 5,11-dibromo-2,8-dihexylanthra[2,3-b:7,6-b']dithiophene, a brominated analog of anthracenedithiophene (ADT), was prepared and utilized for a palladium catalyzed cyclopentannulation reaction with 3,3'-dimethoxyphenylacetylene. The resulting cyclopentannulated-ADT (CP-ADT) was found to be more photo-oxidatively stable than isoelectronic CP-pentacene analogs previously prepared. In addition, the CP-ADT was able to undergo an additional Scholl cyclodehydrogenation reaction to create a contorted aromatic, an incapable feat for previous CP-pentacene analogs. The resulting compound, 4-dihexyl-5,10,17,22tetramethoxytetrabenzo[4,5:6,7:11,12:13,14]rubiceno[2,3-b:10,9-b']dithiophene, was significantly contorted out of planarity owing to four [5]helicene-like arrangements. The density functional theory (DFT) energy minimized structures suggests splay angles of 41.8 0 and 40.9 0 for the cove regions, which are significantly larger than previously published anthracene cyclopentannulated analogs. The contorted aromatic possessed a moderately low optical gap (1.50 eV) and relatively low Lowest Occupied Molecular Orbital energy (-3.70 eV).
<!>Experimental Procedures<!>4,6-i(5-hexylthiophene-2-carbonyl)isophthalic Acid (5a) and 2,5-di(5-hexylthiophene-2carbonyl)terephthalic Acid (5b):<!>SI5<!>2-yl)methyl)terephthalaldehyde (8b):<!>SI8
<p>Contorted aromatics are a class of polycyclic aromatic hydrocarbons (PAH) that contain extended pi networks yet twist out of planarity owing to intramolecular steric congestion. 1 These aromatics are rooted in the decades of research involving twisted aromatics such as helicene-based molecules. 2 Over the past decade, a re-emergence of extended pi-molecules with contorted natures have been described. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The major benefits of the contorted character of these compounds is that they possess a unique shape and that the non-planarity provides significant increases in solubility in comparison to similarly sized flat aromatics, therefore allowing easier characterization and processability. While the unique shapes and contortion help increase solubility, these factors do not always limit pi-pi stacking in the solid state 19 or performance in organic electronics. 20 In fact, the unique three-dimensional shapes may provide value added properties in organic electronics as non-fullerene based electron acceptors in organic photovoltaics. [21][22][23][24] We recently became interested in utilizing a palladium-catalyzed cyclopentannulation to create cyclopentafused polycyclic aromatic hydrocarbons (e.g., 1) that could be cyclodehydrogenated to build up contorted aromatics such as 1,2,6,7tetraarylcyclopenta[hi]aceanthrylenes (2, Figure 1). 25 The contorted compounds contain [4]helicene-like and [5]helicene-like moieties 26 that impart contortion and splay angles of 14.3° and 36.8°, respectively. The compounds were found to be quite soluble, even with methoxy substituents, yet displayed concentration depended aggregation in solution, and lock-and-key pi-pi stacking in the solid state. 25 We had interest in extending this methodology to elongated aromatics, and attempted to create similar contorted aromatics from our recently described 1,2,8,9tetraaryldicyclopenta[fg,qr]pentacenes (3). 27 Unfortunately, we were not successful in applying a variety of well-known cyclodehydrogenation conditions to facilitate the oxidative C-C bond formation of 3 to give the desired contorted aromatics. Although we noted the cyclopenta-fused pentacene cores of 3 was more photo-oxidatively stabilized in comparison to traditional pentacenes, we feared their inherent instability may be limiting the preparation of the expanded contorted aromatics based on this strategy. To address this instability, we turned our focus from pentacene based materials to thienoacene-based materials that have also shown promise as active materials in OFETs. [28][29][30] Anthradithiophene (ADT) is a thienoacene that is isoelectronic to pentacene, with five linearly-fused aromatic rings, yet considerably more stable to oxidative degradation. [31][32][33][34][35][36] For this reason, we sought to access new cyclopentannulated derivatives of anthradithiophene via the utilization of soluble ADT derivatives. ADTs were originally synthesized from regiorandom ADT-quinones and utilized as a mixture of syn and anti-isomers owing to the difficulties in separating the generated isomers. 31 However, recent reports have shown that isomerically pure syn and anti-ADT derivatives can be obtained through new synthetic strategies. [37][38][39][40] Unfortunately, 5,11-dihalogenated ADTs, which would be useful for our desired metal-catalyzed transformations were not known at the undertaking of this project. In this contribution, we have expanded on these recent synthetic advances and have prepared isomerically pure ADT that contain two hexyl solubilizing chains. The additional solubility added to the system by the hexyl chains was then exploited to create the first known 5,11-dibromo-ADT derivatives. We then utilized these new 5,11-di-bromo-ADT derivatives as a substrate for a palladium-catalyzed cyclopentannulation that gives cyclopentafused-ADTs (CP-ADTs). Lastly, these CP-ADTs were found to successfully undergo cyclodehydrogenation to give tetraaryl [4,5:6,7:11,12:13,14]rubiceno[2,3-b:10,9-b']dithiophene (4) that is considerably contorted in structure owing to helicene-like arrangements.</p><p>The synthesis of isomerically pure syn-and anti-2,8-dihexylanthradithophene (DHADT) (9a, 9b) was achieved through modification of a recently described strategy. 40 We used 2hexylthiophene instead of thiophene or methyl-thiophene to give access to more soluble materials via the more flexible hexyl chain. The initial step was a Friedel-Crafts acylation between 2hexylthiophene and 1,2,4,5-benzenetetracarboxylic dianhydride to give a mixture of 4,6-di(  (10) was initially challenging owing to the relative low solubility, even with the added solubilizing chains, and reactive nature of ADTs (e.g., bromination with Br2 was not selective). However, the desired transformation was achieved by bromination with N-bromosuccinimide (NBS) in highly diluted chloroform at room temperature (Scheme 2). A key preliminary step was the prolonged sonication in a large amount of solvent for complete dissolution of 9a before functionalization. To our knowledge, this is the first successful synthesis of a 5,11-dibromo-ADT, which should provide opportunities to access many diverse ADT analogues through metal-catalyzed cross-coupling reactions that are incompatible with nucleophilic addition to ADT quinones.</p><p>Our recently developed palladium-catalyzed cyclopentannulation conditions were applied to the reaction between 10 and 3,3'-dimethoxyphenylacetylene. Our previous small-molecule optimized conditions 25 utilizing the catalyst system of Pd2(dba)3, P(o-Tol)3, and LiCl with KOAc as base in DMF solvent only led to trace amounts of product. However, using a more solubilityfavorable mixture of 1:1 toluene:DMF as a solvent system 41 yielded 4,10-dihexyl-1,2,7,8tetrakis(3-methoxyphenyl)cyclopenta [6,7]aceanthryleno[4,3-b:8,9-b']dithiophene (11) in 39% yield. This compound is considerably more soluble than 9a or 10 owing to the perpendicular arrangement of the attached aryl groups. This compound (11) was subjected to Scholl Although, we were unable to obtain suitable crystals for single crystal analysis, we previously found that the DFT calculated structure of 2 was consistent with its crystal structure. 25 Owing to the similarities of these systems, we employed DFT to analyze the structure of 4.</p><p>The DFT minimized structure of 4 (Figure 2) shows that it is highly contorted owing to the interactions between the central ADT core and the outer phenanthrene units. The arrangement of fused rings gives rise to a total of four [5]helicene-like fragments 26 (Figure 1). These bonding arrangements generate large splay angles of 41.8 ° and 40.9 ° in the coves, respectively, and are considerably larger than the splay angles in the anthracene-based contorted aromatic (2) that possess splay angles of 14.3° and 36.8°, respectively (Figure 1). This higher contortion is attributed to the inclusion of two additional helicene-like arrangements compared to that of 2. (Table 1). 25 The cyclic voltammograms show that these compounds give two reversible reductions and quasi-reversible or irreversibly oxidations (Figure 4). solution under ambient light (Supporting Information). We previously showed that 3 was more stable 27 than TIPS-pentacene and wanted to compare the effects of ring closure to photostability.</p><p>The decomposition half-lives for each compound were obtained by monitoring the disappearance of the prominent absorption band in each compound. We found that CP-ADT 11 (half-life=1572 min) was more stable than our previously synthesized pentacene derivative 3 (half-life=824 min).</p><p>This result agrees with previous stability studies of ADT compared to pentacenes. 31 However, upon ring closure of 11 to 4 the stability of the CP-ADT system was significantly lowered (half-life=367 min), which was similar to TIPS-Pentacene (half-life=286 min). This result suggests a cause for our previous inability to isolate the Scholl cyclodehydrogenated product of 3. Our current hypothesis is that the ring-closed product of the 3 is even more reactive than 4 and was therefore not capable of isolation. These results further demonstrate the stabilizing effect of ADT in comparison to pentacene backbones.</p><p>In conclusion, we have shown that the synthesis of hexyl-containing ADT compounds can be successfully brominated to create isomerically pure 5,11-dibromo-2,8-dihexylanthra[2,3-b:7,6b']dithiophene 10 that can be modified by palladium-catalyzed reactions. These dibrominated a Measurements taken at sample concentration of 0.2 mM and potentials measured relative to a ferrocenium/ferrocene redox couple used as an internal standard (Figure 4). Eox/onset is the onset of oxidation potential, Ered/onset is the onset of reduction potential. HOMO and LUMO values calculated on the basis of the oxidation of the ferrocene reference in vacuum (4.8 eV).</p><p>ADT compounds could provide access to newly functionalized ADTs that are incompatible with traditional nucleophilic addition reactions to ADT quinones (e.g., ester functionalized nucleophiles). In this contribution, we showed that a palladium-catalyzed cyclopentannulation can lead to more stabilized CP-PAHs than the isoelectronic pentacene derivatives and that can be further modified to create significantly contorted PAH structures with reduced optical gaps and low LUMO energies.   SI4</p><!><p>Unless otherwise noted, all reagents were used as received and all reactions were carried out under an argon atmosphere. Column chromatography was performed on a chromatographing system with normal phase silica columns. 1 H NMR and 13 C NMR were recorded on a 400 MHz station at room temperature, unless otherwise noted.</p><!><p>To the mixture of pyromellitic dianhydride (2.53 g, 11.6 mmol) and AlCl3 (6.19 g, 46.4 mmol) in 1,2-dichloroethane (80 ml) was added 2-hexylthiophene (3.9 g, 23.2 mmol) slowly at 0 °C and stirred for 1 h. The reaction mixture was then heated to reflux overnight. After cooling, the reaction was transferred into ice water containing conc. HCl and extracted with CHCl3. The combined organic extracts were evaporated in vacuo to give a beige solid as a mixture of meta-/para-isomers (5.1 g, 79%). The meta-/para-isomers are formed in a ratio of ca. 1:1 determined by NMR spectroscopy. The isomers were separated by taking advantage of the solubility characteristics in 80% acetic acid. The para-isomer is less soluble in 80% acetic acid and precipitated on recrystallization. The meta-isomer could be obtained from the filtrate by removing the solvents or by precipitating by adding water to the filtrate. In some cases, the recrystallization process did not fully purify a given isomer. If this was the case, the compound</p><!><p>was carried forward to 7a or 7b where column chromatography could separate out the minor isomer.</p><p>5a: 1   10% HCl to dissolve the inorganic salts. The organic phase was collected and aqueous phase was extracted with diethyl ether. The combined organic layers were washed with water and dried over sodium sulfate and evaporated. The residue was purified by column chromatography with silica gel using 60% ethyl acetate in hexane to give each product (605 mg (69%) of 7a as a beige solid, 800 mg (80%) of 7b as a gray solid). The Rf values of each isomer on TLC plate were different when eluted with 60% ethyl acetate in hexane; Rf (7a) = 0.6 and Rf (7b) = 0.7. Column SI7 chromatography could be used at this stage if small impurities of a minor isomer was carried through the synthesis.</p><p>7a: 1 4H), 1.29 (s, 12H), 0.87 (t, J = 6.9 Hz, 6H).</p><!><p>A solution of each compound (534 mg, 1.07 mmol of 7a, 600 mg, 1.20 mmol of 7b) in dry pyridine was added slowly to a suspension of chromium(VI) oxide (10 equiv) and dry pyridine. After stirring for 4 h at room temperature, the reaction mixture was filtered and washed with diethyl ether. The filtrate was washed with 10% HCl and 10% sodium carbonate solution successively. The combined organic layers were dried over sodium sulfate and evaporated. The residue was purified by column chromatography on silica gel using 50% dichloromethane in hexane to give each product (202 mg (38%) of 8a as pale yellow oil, 300 mg (50%) of 8b as pale yellow oil).</p><!><p>8a: 1 H NMR (400 MHz, CDCl3) δ 10.31 (s, 2H), 7.81 (s, 2H), 6.55 (d, J = 3.5 Hz, 4H), 4.55 (s, 4H), 2.71 (t, J = 7.6 Hz, 4H), 1.63 (m, 4H), 1.30 (s, 6H), 0.88 (t, J = 6.9 Hz, 6H). 13   mmol of 9a; 50 mg, 0.17 mmol of 9b) that was sonicated in 100 mL chloroform for >40 min was added N-bromosuccinimide (2.1 equiv) and stirred for 1 h at room temperature in the dark. The reaction mixture was transferred into separation funnel with water (50 mL), the organic layer was washed with water (50 mL) and brine solution (50 mL). The combined organic layers were dried over Na2SO4 and concentrated to give each isomer (63 mg, 93% of 10 as red solid; 65 mg, 95% of 10b as red solid).</p>