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Fused Polycyclic Aromatic Compounds: [njAcenes, [njHelicenes, and Their Heterocyclic Analogues
Published in Atsushi Nagai, Koji Takagi, Conjugated Objects, 2017
Helicenes have been known for more than 100 years since the first synthesis in 1903.55 Helicenes composed of six or more benzene rings exhibit stable helical chirality because of an intramolecular steric repulsion. The P (plus) isomer possesses a right-handed helix, and the M (minus) isomer does a left-handed helix. The stable helical structure of helicenes provides unique chiroptical properties such as optical rotatory power, circular dichroism, and circularly polarized luminescence, compared to usual organic chiral molecules. After 50 years later from the first synthesis of helicene, Newman et al. reported the preparation of enantiomerically pure [6]helicene for the first time.56 They synthesized racemic [6]helicene by using Friedel-Crafts acylation and separated the enantiomers through charge transfer complexation with enantiomerically pure 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionic acid (TAPA). It is not too much to say that this result opened up the greater interest in the chiroptical properties of helicenes as well as efficient synthetic methods including asymmetric versions.
Aromatic Helicenes
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
The numerical prefix (or a number in square brackets) before the helicene name expresses the number of fused cycles as exemplified by hexahelicene or simply [6]helicene 1. Provided all these rings are benzenes, such compounds are called carbohelicenes (1, Figure 4.3). If one (or more) benzene unit is formally displaced with a heterocycle, such a skeletal modification leads to heterohelicenes. Accordingly, sub-families of heterohelicenes are mentioned in the literature, such as azahelicenes or pyridohelicenes (4), azoniahelicenes (5) or thiahelicenes (6). Helicene-like compounds represent another type of abundant helicenes that preserve a typical helical shape of the molecule, but some (hetero)cycles forming the helical backbone are contracted, expanded or non-aromatic (partially hydrogenated). The class of helicene-like compounds features a considerable structural diversity, and even specific terms were coined to some entities. It encompasses, e.g., heliphenes (7), tetrahydrohelicenes (8), oxahelicenes (9), helquats (10), cationic heterohelicenes (11), cyclometallated helicenes (12), borahelicenes (13), phosphahelicenes (14) and silahelicenes (15). A meta-annulated cycle might also be embedded into the otherwise all-ortho-annulated cycles (16, Figure 4.3). Finally, several helicene units might be combined within a single molecule to speak about multipole helicenes (17), or the helicene backbone can be laterally extended to form a twisted nanographene (18). Obviously, the family of helicenes has considerably expanded and diversified in the structure patterns as helicene chemistry has significantly developed over time. Nowadays, it is generally accepted that (i) helicene is a molecule composed of four and more carbo/heterocycles that are mostly aromatic and annulated normally in an ortho fashion, (ii) its curved and twisted backbone forms a helix owing to the steric repulsion between its termini and (iii) it resembles a molecular shape of the original Newman’s hexahelicene 1 (at least in its part). Accordingly, in the following text, the generalized term “helicene” will be used in the broadest sense of its definition regardless of the specific structural variations.
Relative energies of increasingly large [n]helicenes by means of high-level quantum chemical methods
Published in Molecular Physics, 2023
Helicenes are ortho-fused polycyclic aromatic hydrocarbons (PAHs) that adopt a chiral 3D topology due to their spatial helical arrangement. Helicenes have attracted significant interest due to their unique catalytic, nanochemical, optical and electronic properties [1–7]. The smallest helicene, and the first one to be synthesised, is [4]helicene (a.k.a. benzo[c]phenanthrene). Small helicenes (e.g. [4] and [5]helicene) exhibit a flip-flop chirality inversion that can be catalysed by nanographene flakes and cyclophanes [8–11]. The activation energy for the enantiomerization of larger helicenes have been measured to be 100.8 ([5]helicene), 148.1 ([6]helicene), 172.4 ([7]helicene), 175.3 ([8]helicene) and 180.3 ([9]helicene) kJ mol–1 [12]. Thus, larger helicenes exist as highly stable axial enantiomers and have found applications in asymmetric synthesis and catalysis.
Cation–π interaction of thallium (I) with [7]helicene: an experimental and theoretical study
Published in Molecular Physics, 2022
Petr Vaňura, David Sýkora, Stanislav Böhm, Tereza Uhlíková
Ortho-fused aromatic molecules can be rendered helical or spirally coiled due to the inability of conjoined rings to occupy the same plane. This is a case for molecules, where the number of benzene rings exceeds 4. These molecules termed the helicenes display a non-planar topology with C2-symmetric axis perpendicular to the axis of helicity because of the steric repulsive interaction between terminal aromatic rings [3–6]. Helicenes are good π-donors and can form charge-transfer complexes with many π-acceptors. In addition, the π–π interactions play an important role in determining both the properties and the self-assembly behaviour of helicenes in either the solution or the solid state. Helicenes are notable for having chirality despite lacking both asymmetric carbons and chiral centres. Instead, there is axial chirality, which results from the handedness of the helicity itself. The helical topology of carbohelicenes causes a high optical rotation, high circular dichroism values and several enhanced physical-organic properties [7].