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

, helicenes are ortho-condensed polycyclic

aromatic compounds Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

in which benzene rings or other aromatics are angularly annulated to give helically-shaped

chiral molecules In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (). The terms are de ...

. The chemistry of helicenes has attracted continuing attention because of their unique structural,

spectral ''Spectral'' is a 2016 Hungarian-American military science fiction action film co-written and directed by Nic Mathieu. Written with Ian Fried & George Nolfi, the film stars James Badge Dale as DARPA research scientist Mark Clyne, with Max Marti ...

, 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 generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...

collisions. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the dihedral angles 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 center In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...

s. Instead, there is

axial chirality In chemistry, axial chirality is a special case of chirality (chemistry), chirality in which a molecule contains two pairs of chemical groups in a non-planar arrangement about an axis of chirality so that the molecule is not superposable on its mi ...

, 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 Circular dichroism (CD) is dichroism involving circularly polarized light, i.e., the differential absorption of left- and right-handed light. Left-hand circular (LHC) and right-hand circular (RHC) polarized light represent two possible spin angul ...

suggests left-handed helices are

levorotatory Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

and right-handed helices are

dextrorotatory Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

. The stability of the two complementary helical enantiomers with respect to interconversion and the mechanism by which they interconvert depend on ''n''.

Synthesis

The first helicene structure was reported by

Jakob Meisenheimer Jakob Meisenheimer (14 June 1876 – 2 December 1934) was a German chemist. He made numerous contributions to organic chemistry, the most famous being his proposed structure for a group of compounds now named Meisenheimer complex. He also pro ...

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 Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

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

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

s are used. The oxidative photocyclization of a

stilbene Stilbene may refer to one of the two stereoisomers of 1,2-diphenylethene: * (''E'')-Stilbene (''trans'' isomer) * (''Z'')-Stilbene (''cis'' isomer) See also * Stilbenoid Stilbenoids are hydroxylated derivatives of stilbene. They have a C6–C ...

-type

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

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 In organic chemistry, Olefin Metathesis or Alkene Metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the Bond cleavage, scission and regeneration of carbon-carbon double bonds. Because of the ...

reaction of a divinyl compound (prepared from

1,1′-bi-2-naphthol 1,1-Bi-2-naphthol (BINOL) is an organic compound that is often used as a ligand for transition-metal catalysed asymmetric synthesis. BINOL has axial chirality and the two enantiomers can be readily separated and are stable toward racemisation. Th ...

(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 fro ...

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 Nonlinearity, 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 ...

, 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 com ...

, conformational analysis, chirality sensing,

chemical sensors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

and hetero-atom substitution.

References

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