organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...

, helicenes are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatics are angularly

annulated In organic chemistry annulation (from the Latin ''anellus'' for "little ring"; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule. : Examples are the Robinson annulation, Danheiser annulation and cert ...

to give helically-shaped chiral molecules. The chemistry of helicenes has attracted continuing attention because of their unique structural,

spectral ''Spectral'' is a 2016 3D military science fiction, supernatural horror fantasy and action-adventure thriller war film directed by Nic Mathieu. Written by himself, Ian Fried, and George Nolfi from a story by Fried and Mathieu. The film stars J ...

, and

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

features.

Structure and properties

The systematic naming for this class of compounds is based on the number of rings: 'n''elicene is the structure consisting of ''n'' rings. According to

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

, only structures where ''n'' is at least 5 are considered helicenes. Some specific compounds also have alternate or

trivial name In chemistry, a trivial name is a non systematic name for a chemical substance. That is, the name is not recognized according to the rules of any formal system of chemical nomenclature such as IUPAC inorganic or IUPAC organic nomenclature. A ...

s. As the number of rings increases, starting at four, the structure becomes non-planar, but instead the planes of consecutive rings tilt to prevent

steric Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

collisions. The resulting helix is chiral. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the

dihedral angle A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common. In solid geometry, it is defined as the un ...

s between the extremities increases going from elicene (26°) to elicene (58°) and then decreases again for example in elicene (30°). Helicenes are notable for having chirality despite lacking both asymmetric carbons and chiral centers. Instead, there is

axial chirality Axial may refer to: * one of the anatomical directions describing relationships in an animal body * In geometry: :* a geometric term of location :* an axis of rotation * In chemistry, referring to an axial bond * a type of modal frame, in music ...

, which results from the handedness of the helicity itself. The clockwise and counterclockwise helices are non-superposable. By convention a left-handed helix is ''minus'' and labeled (''M''), a right-handed helix is ''plus'' and labeled (''P''). Evidence from CD spectroscopy suggests left-handed helices are levorotatory and right-handed helices are dextrorotatory.

Synthesis

The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphthalene. elicene was synthesized in 1918 by Weitzenböck & Klingler. The first elicene (also called ''hexahelicene'') was synthesized by M. S. Newman and D. Lednicer in 1955 via a scheme that closed the two central rings by Friedel–Crafts cyclization of

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...

compounds. Since then, several methods for synthesizing helicenes with different lengths and

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...

s are used. The oxidative photocyclization of a stilbene-type

precursor Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...

is used most often as the key step. The longest helicene prepared by this method is 6elicene in 2015. In one study, elicene was synthesized in an

olefin metathesis Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often create ...

reaction of a divinyl compound (prepared from 1,1′-bi-2-naphthol (BINOL) in several steps), with Grubbs' second generation catalyst: Other approach is also non-photochemical and is based on assembly of biphenylyl-naphthalenes and their platinum-catalyzed double cycloisomerization leading to various elicenes: : Helicene Cycloisomerization

File:Tetrahelicene.jpg, elicene File:Pentahelicene.jpg, elicene File:Hexahelicene.jpg, elicene File:Hexahelicene2.jpg, elicene, other

chirality Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

File:Heptahelicene.jpg, elicene File:Heptahelicene2.jpg, elicene, other chirality File:Octahelicene.jpg, elicene File:Nonahelicene.jpg, elicene File:Decahelicene.jpg, 0elicene File:Undecahelicene.jpg, 1elicene File:Dodecahelicene.jpg, 2elicene File:Tridecahelicene.jpg, 3elicene File:Tetradecahelicene.jpg, 4elicene File:Pentadecahelicene.jpg, 5elicene File:Hexadecahelicene.jpg, 6elicene File:Octadecahelicene.jpg, 8elicene

Applications

Helicenes have been studied with respect to

nonlinear optics Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...

, CPL,

organocatalysis In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic co ...

, conformational analysis, chirality sensing,

chemical sensors A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

and hetero-atom substitution.

References

; General references * {{PAHs Polycyclic aromatic hydrocarbons Geodesic polyarenes

Structure and properties

Synthesis

Applications

See also

References