Synthesis and Nanostructures of Cyclic Triphenylene Trimers Having Long Alkyl and Alkoxy Side-Chains

Large two-dimensional (2D) carbon-rich molecules with defined sizes, shapes, and compositions have attracted considerable attention because of their one-dimensional (1D) stacking in solution, supramolecular 2D crystallinity on surfaces, and optical properties both in solution and in the solid state. In particular, planar cyclic p frames with large inner cavities and long alkyl or alkoxy side chains have been used for the construction of either supramolecular architectures or weakly bound host–guest complexes by the nanoscopic phase separation among the p frame, alkyl moiety, and inner sphere. Thus, large 2D planar condensed aromatics without inner cavities tend to form crystals, liquid crystals, and nanostructures through fairly strong p–p and van der Waals interactions, and the self-association is an enthalpically driven process that is entropically disfavored. In a similar manner, shape-persistent cyclic conjugated molecules with large inner cavities should assemble into supramolecular structures with reduced dimensionality, that is, 1D and 2D nanostructures. However, our preliminary experiments show that their thermodynamic parameters are sometimes different from those of large 2D planar condensed aromatics. We have recently developed a new synthetic method to prepare cyclic oligophenylenes by electrontransfer oxidation of Lipshutz cuprates and reported the synthesis, photophysical properties, and supramolecular architectures of cyclic nonaphenylenes 1a–c (Figure 1). Next, we converted 1a–c into the corresponding triphenylene trimers 2a–c by dehydrogenative bond formation to examine their optical properties and self-assembling behavior. Although 1a–c could be converted into triphenylene trimers 2a–c by using dehydrogenative cyclization, 2a has poor solubility in common organic solvents. Therefore, the optical properties and the formation of nanostructures of 2b,c were investigated and compared with those of 1b,c from a viewpoint of the structure–property relationship. In the case of 1b, fine colorless crystals with high solid-state emission were obtained owing to its nonplanar oligophenylene structure. As for 1c, a fluorescent fibrous material precipitated from a mixture of diisopropyl ether (IPE)/methanol (1:1). The fibers of 1c formed a sheet structure perpendicular to the fiber direction, and the intermolecular distance between the phenyl rings at the apex position was determined to be 4.3 ; this indicated no p–p stacking interactions and a Lego-like interlocking structure. As triphenylene trimers 2b,c have large shape-persistent p frames with long alkyl and alkoxy side chains, 2b,c should show a nanoscopic phase separation and amphiphilicity and, hence, stronger supramolecular self-aggregation. Herein, we report the emission properties and self-aggregation behavior of 2b,c as well as the formation of a dendritic supramolecular architecture comprised of a charge-transfer (CT) complex of 2b and 2,4,7-trinitrofluorenone (TNF). As reported previously, 1b,c were synthesized by using electron-transfer oxidation of Lipshutz cuprates 4b,c prepared from 3b,c in a one-pot procedure (Scheme 1). Although the yields of 1b,c were not high, they were obtained as the major components of the soluble products. Under dehydrogenative cyclization conditions, 1b was treated with CuACHTUNGTRENNUNG(OTf)2 and AlCl3 in CS2 to produce 2b (73%), and 1c was treated with FeCl3 in nitromethane/dichloromethane (3/ [a] Y. Hanai, Dr. M. J. Rahman, Dr. J. Yamakawa, Dr. M. Takase, Prof. T. Nishinaga, Prof. M. Iyoda Department of Chemistry, Graduate School of Science and Engineering Tokyo Metropolitan University Hachioji, Tokyo 192-0397 (Japan) Fax: (+81)42-677-2525 E-mail : iyoda@tmu.ac.jp [b] Dr. M. Hasegawa Department of Chemistry School of Science Kitasato University Sagamihara, Kanagawa 252-0373 (Japan) [c] Dr. K. Kamada Research Institute for Ubiquitous Energy Devices National Institute of Advanced Industrial Science and Technology (AIST) AIST Kansai Center, Ikeda, Osaka 563-8577 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100336.