Published on Web 07/07/2005
Synthesis and Structure of Fused r-Oligothiophenes with up to Seven Rings Xinnan Zhang, Adrien P. Coˆ te´ , and Adam J. Matzger* Department of Chemistry and the Macromolecular Science and Engineering Program, UniVersity of Michigan, 930 North UniVersity, Ann Arbor, Michigan 48109-1055 Received May 21, 2005; E-mail:
[email protected] R-Oligothiophenes are one of the most prominent organic semiconducting materials and have been widely studied both as compounds of interest in electronic devices and as models for understanding polythiophenes.1 In particular, R-sexithiophene (Figure 1) and its derivatives have been employed as the active layer in organic electronic devices.2 However, deviation from planarity may decrease conjugation in R-oligothiophenes through torsion about single bonds or S-syn defects. Such effects do not arise in fully fused conjugated oligomers, such as pentacene, because of its rigid structure.3 Although pentacene exhibits outstanding charge carrier mobility,4 its environmental stability5 raises concerns about device lifetime. Furthermore, theoretical studies predict that the molecular packing is not optimized for maximum charge carrier mobility.6 This raises the possibility of creating R-oligothiophenes that are fully fused because stability will be vastly improved relative to acenes and molecular packing may change from a herringbone to π-stacked structure. This latter effect is expected to arise from the relatively high C/H ratio, which reduces the role of C-H‚‚‚π interactions in the crystal.7 Although the synthesis of an impressive array of fully fused β-oligothiophenes has been accomplished, annelation in this manner gives rise to relatively large optical band gaps (λmax ) 260 nm for n ) 7).8 In contrast, fully fused R-oligothiophenes, oligothienoacenes (Figure 1), have planar structures with extended conjugation, which suggests potential applications in electronic devices should a suitable synthetic route be achieved. They are also of considerable interest as models for the hypothetical thienoacene (C2S) polymer. The synthesis of fully fused R-oligothienoacenes with up to five rings has previously been reported.9,10 However, the synthetic route is inefficient and not applicable to longer thienoacenes because of the extremely poor solubility of the longer oligomers. Here, we introduce new synthetic methodology designed to efficiently prepare oligothienoacenes (Figure 1). This route allowed production of pentathienoacene (1) in large scale and with improved yield. In addition, heptathienoacene (2), which contains seven fused rings, was also successfully prepared. Significantly, diffraction data indicate that compounds 1 and 2 adopt face-to-face (π-stacked) packing motifs in contrast to the packing arrangement of R-oligothiophenes (face-to-edge or herringbone packing).1 The solubility of oligothienoacenes is generally low, causing isolation difficulties and contributing to low yields. In our synthetic method, TIPS (triisopropylsilyl)11 groups were introduced by Li-Br exchange of dibromide 312 followed by quenching with TIPS-Cl (Scheme 1). The β-bromo substitution was obtained by the halogen dance reaction13 of 4 with LDA to afford 5. The synthesis of sulfide 6 was achieved by the Pd-catalyzed coupling of Bu3SnSSnBu3 with 2 equiv of 5 in excellent yield (95%).14 This constitutes a significant improvement over the traditional method of introducing sulfur linkages via Li-Br exchange followed by quenching with bis(phenylsulfonyl)sulfide, which leads to substantial amounts of the R,β-linked isomer as a byproduct. By blocking 10502
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J. AM. CHEM. SOC. 2005, 127, 10502-10503
Figure 1. Chemical structures of R-oligothiophenes (n ) 4, R-quaterthiophene; n ) 6, R-sexithiophene) and oligothienoacenes (n ) 3, dithieno[3,2-b:2′,3′-d]thiophene; n ) 5, pentathienoacene, 1; n ) 7, heptathienoacene, 2). Scheme 1. Synthesis of Fully Fused Oligothienoacene 1
Scheme 2. Synthesis of Fully Fused Oligothienoacene 2
the terminal R-positions with TIPS groups, competing deprotonation at the outside R-positions is eliminated and the yield of oxidative ring closure is 52% for 7 and 68% for 12 (Scheme 2), excellent results considering that unsubstituted 6 yields 1 in 0-20% yield. Pentathienoacene (1) was isolated in high yield after protodesilylation with TFA and is a fully fused analogue of R-terthiophene containing only six double bonds and with a relatively large HOMO-LUMO gap.9,15 The solubilization strategy allowed us to prepare a longer oligothienoacene, heptathienoacene (2), with extended conjugation and a lower band gap (Scheme 2). TIPSsubstituted heptathienoacene, 12, was obtained in a similar manner as 7 from dibromide 8.16 Protodesilylation of 12 with TFA led to an inseparable mixture of 2 and mono-TIPS-substituted 2 as evidenced by EI-MS. However, both TIPS groups were smoothly removed upon treatment with tetrabutylammonium fluoride (TBAF). The solubility of 2 is exceedingly low in common organic solvents, and both it and 1 could be further purified by sublimation. TIPS-substituted oligomers 7 and 12 produced single crystals suitable for X-ray structural analysis. This confirmed the backbone 10.1021/ja053326m CCC: $30.25 © 2005 American Chemical Society
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Figure 2. (left) ORTEP diagram with ellipsoids at 50% probability and (right) packing motif of 1.
structure but yielded distorted molecular geometries, a likely consequence of accommodating the bulky TIPS groups (see Supporting Information). To elucidate the molecular packing of oligothienoacenes, the single-crystal X-ray structure of 1 was solved, which confirmed that the design of a π-stacked packing motif was successful (Figure 2). Negligible atomic deviation (
