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Aromatic Helicenes
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
There is another way of expanding the helicene backbone by fusing several helicene units into a single molecule. This concept of molecular design leads to multipole helicenes whose number in the literature is rapidly increasing. Hexabenzotriphenylene 89 synthesized from a corresponding o-trimethylsilyl (o-TMS) triflate as a benzyne precursor 88 by Pérez, Guitián et al. (Peña et al., 2000) represents a multipole helicene prototype in which a central aromatic unit (here benzene) is a joint component of the three surrounding helicene units (here [5]helicenes) (Figure 4.23). Similarly, Sygula et al. cyclotrimerized in situ generated corannulyne under palladium catalysis to provide a highly nonplanar hydrocarbon 90 with three both corannulene and [5]helicene subunits that prefers a twisted conformation of C 1 symmetry and exhibits dramatically different bowl-to-bowl inversion barriers (Yanney et al., 2011). Employing Yamamoto-type coupling, Coquerel, Gingras et al. succeeded in cyclotrimerizing [5]helicene dibromide 91 into hexapole [5]helicene 92 of D3 symmetry that represents chiral nanographene propeller with six conformationally stable [5]helicene units (Berezhnaia et al., 2017). Independently, the same compound was prepared by Tsurusaki, Kamikawa et al. using the aforementioned Pd-catalyzed helicenyl aryne cyclotrimerization (Hosokawa et al., 2017). Not only benzene but also other polycyclic aromatic hydrocarbons can form a central core. To this end, Segawa, Itami et al. employed pentaborylated corannulene to synthesize, via multiple Suzuki–Miyaura coupling, pentaarylated corannulene 93 that underwent Pd-catalyzed fivefold intramolecular direct arylation to receive the quintuple [6]helicene 94 as a C 5-symmetric propeller-shaped structure (Kato et al., 2018). The most complex multiple helicene, here the hexapole [7]helicene 96, was prepared by Wang et al. in two steps from arylated tolane 95 by employing Co-mediated alkyne [2 + 2 + 2] cycloisomerization followed by Scholl reaction to transform polyphenylene precursor into a circularly twisted chiral nanographene with a propeller structure (Zhu et al., 2018). Optically pure 96 displays high thermostability and a wealth of remarkable chiroptical and electronic properties.
Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics
Published in Combustion Science and Technology, 2019
Kimberly Bowal, Jacob W. Martin, Alston J. Misquitta, Markus Kraft
Recently, we have proposed that interactions between curved PAHs (cPAHs) and ions may play a significant role in soot formation (Martin et al., 2018a, 2018b). Curved aromatics, which differ from their planar counterparts through the integration of non-hexagonal rings into their predominately hexagonal arrangement, are found to comprise a significant portion of young soot particles (Botero et al., 2016; Martin et al., 2018a; Wang et al., 2017). This nonplanar structure has been shown to produce significant polarity in otherwise nonpolar molecules. For example, coronene and corannulene are both PAHs consisting of a single concentric arrangement of aromatic rings, although in contrast to the planar coronene molecule corannulene is curved due to a central pentagonal ring. Coronene is nonpolar while corannulene has a dipole moment of 2.07 debye (Lovas et al., 2005), which is similar to that of water at 1.85 debye (Shostak et al., 1991) (although a better comparison is made with the local dipole moment at the pentagonal site of corannulene which is 1.63 debye given our previous analysis (Martin et al., 2017)). This charge polarization is primarily due to the strain-induced shift of the electron density from the concave to convex side of the curved molecule, known as the flexoelectric effect (Martin et al., 2017). Experimental and computational work has determined that a representative nucleating soot molecule contains 15 rings, 2 of which are pentagons, and has a dipole moment of 5.32 debye (Martin et al., 2018a).