There are various types of helical-shaped molecules reported in the literature, start- ing from the single-stranded to multi-stranded helical systems.3,28,29 Below we discuss typical helical systems reported in the literature.
1.1.1 Single-stranded helical systems
The single-stranded helical systems consist of single helicenic units of either P (right- handed) or M (left-handed) configurations. Pristine carbo[n]helicenes and their deriva- tives are the most common example of this class of helical systems.
1.1.1.1 Ortho-substituted ring systems
Small-sized oligomers of ortho-substituted heterocyclic rings are linear in structure and achiral in nature. However, chain elongation of such oligomers results in non-planar structures and induces chiral properties in the systems. In this regard, many five- membered ring-based conducting polymers such as polythiophene30–33 and polypyr- role,31,34–38 have been studied. In recent times, helical polyfurans have also been syn- thesized.39–42 Among the polymers of phenylene, ortho-phenylenes have been shown to form helices.43–46
1.1.1.2 Fused-ring systems
In 1956, Newman and Lednicer first synthesized the fully helical carbo[6]helicene.47 Thereafter, many medium to large-sized multi-layered carbohelicenes up to carbo[16]helicene48 have been synthesized till date.3,29 In addition, several modified carbohelicenes based on either substitution49–57 or lateral ring-fusion7,58–63 have also been synthesized.3,29 For example, perylene diimide60 and hexabenzocoronene62 have been commonly used skeletons for lateralπ extension. In addition, fluorene-64 and azaine-fused65 are also in use. In addition, many heterohelicenes based on oxygen66,67, nitrogen68, sulphur68,69 and others are also reported.29
1.1.2 Multi-stranded helical systems
In addition to conventional single-stranded helicenes, double and multiple helicenes, entailing more than one helicene subunit in one molecule, have emerged as a new class of chiral polycyclic aromatic hydrocarbons (PAHs).
1.1.2.1 Double helicenes
Among the various multiple helicenes, double helicenes are the simplest and most stud- ied family of helicenes. These include double carbo[5-8]helicenes. In 1959, Clar and co- workers very first reported the [X]-shaped double [5]helicene or Hexabenzoperylene.70
Further, Miao et al. synthesized its alkoxy derivatives in 2012.71 In these [X]-shaped helicenes, each component shares the same benzene ring in the middle70,71 or the C- C bond of that ring.72 In addition to these [X]-type double carbo[5]helicenes, [S]-type double carbo[5]helicene73sharing one terminal ring of each unit was also reported. Fur- ther, in some cases, the terminals of two moieties are connected via one or two C-C bonds74,75producing either [S]-type or figure-eight shaped molecules, respectively. Sim- ilar to double carbo[5]helicenes, fusion of two pristine carbo[6]helicenes produced dou- ble carbo[6]helicenes of various shapes.28 For example, two fused carbo[6]helicene units produce [S]- and [M]-shaped double carbo[6]helicenes, sharing the central76,77 and ter- minal78 naphthalene rings, respectively. In the former case, the two terminals are anti to each other, while those are cis in the latter case. Further, there are some double carbo[6]helicenes, sharing either one ring78,79 or a bond.80 Recently, Mori et al.81 re- ported an isometric [X]-type double carbo[6]helicene, sharing the naphthalene unit in the middle. In addition, [S]-shaped double aza[6]helicene has also been reported by Tanaka et al.82Unlike the carbo[5,6]helicenes, no [S]-type double carbo[7]helicene in the anti-form has been reported to date. However, a classic example of M-shaped double carbo[7]helicene that shares a terminal naphthalene unit was reported as early as 1974 by Martin et al..78In 2003, Cobas et al. reported an asymmetric double helicene made up of lateral fusion of carbo[7]helicene and carbo[5]helicene.83 In recent times, there has been an increase in studies ofπ-extended multiple helicenes.28,60,84–88These are based on some commonπ-conjugated units such as hexabenzo-fused double carbo[6]helicene85, pyrene- fused [X]-shaped double carbo[7]helicene88, hexapole carbo[7]helicene fused with hexa- peri-hexabenzocoronene core86, perylene diimide-embedded double carbo[8]helicenes60 etc.. Further, in addition to benzo-annulated double carbohelicenes, multiple helicenes containing one or more hetero atoms in the core have also been explored extensively.28,89
1.1.2.2 Triple helicenes
groups. In 1999, Barnett et al. reported two stereoisomers of hexabenzotriphenylene of D3 and C2 symmetries, sharing triphenylene unit in the core.91 Further, Watanabe et al.94synthesized the very first more twisted and three-bladed propeller-shaped triple carbo[5]helicenes of C3 symmetry in 2017. A trimer of bowl-shaped corannulene, a π- extended version of triple [5]helicene, i.e., hexabenzotriphenylene, was also prepared.84 Further triperylene hexaimides based triple carbo[5]helicenes along with its nitrogen doped and selenium derivatives are also reported.95,96
1.1.2.3 Quadruple and larger helicenes
Unlike double and triple helicenes, quadruple helicenes are limited. In 2013, Dichtel et al.97 synthesized extended and partially fused quadruple carbo[5]helicene as a π- elongated derivative of double [5]helicene70 (i.e., hexabenzoperylene). It contains four carbo[5]helicene units and none of these four units share any ring among themselves. Fur- ther, quadruple hetero[5]helicene was synthesized by Itami and co-workers from dithia[6]
and carbo[5] helicenes.98Recently, Xiao et al. reported quadruple carbo[5]helicene, com- posed of four indene units, in which no fully shared benzene ring is present.99
Fewer numbers of quintuple helicenes are reported in literature. Those quintuple helicenes are based on corannulene units. For example, Itami and co-workers100,101, syn- thesized propeller-shaped quintuple carbo[6]helicene and quintuple decathia[6]helicene.
Both of these are composed of a corannulene unit and six helicenic moieties. In 2022, Tanaka et. al reported azahelicene-fused corannulenes.102In 2017, the groups of Kamikawa103 and Gingras104 synthesized chiral propeller-shaped D3 symmetric sextuple helicene.
It consisted of three external and three internal carbo[5]helicene units. In addition, carbo[5]helicene-based unique circular and cage shaped molecules are also reported, where two helicene units get connected via several linkers, instead of ring fusion.105–107