# Helicene
**Helicene**, under the IUPAC definition, is an ortho-fused polycyclic aromatic or heteroaromatic compound in which all rings, at least five, are angularly arranged so that the molecule takes a helical shape and is therefore chiral, the prototype being hexahelicene[1](https://goldbook.iupac.org/terms/view/H02762.html). Although helicenes contain no stereogenic centers, they are inherently chiral because of the helical shape they adopt[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of). The name was introduced by Newman in 1955 for the benzologues of phenanthrene in which extra ortho-condensed rings give rise to a regular cylindrical helix[3](https://onlinelibrary.wiley.com/doi/10.1002/anie.197406491), and the bracketed notation [n]helicene, where n is the number of aromatic rings in the helical skeleton, was accepted alongside the systematic name proposed by Newman and Lednicer in 1956[4](https://download.e-bookshelf.de/download/0007/9058/68/L-G-0007905868-0016773358.pdf).
| Key fact | Detail |
|---|---|
| Definition | IUPAC definition: ortho-fused polycyclic aromatic or heteroaromatic compounds with a minimum of five rings, angularly arranged into a helically shaped chiral molecule[1](https://goldbook.iupac.org/terms/view/H02762.html) |
| Chirality | No stereogenic centers; chirality arises from the helical shape, with P (plus, right-handed) and M (minus, left-handed) enantiomers[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of) • [5](https://iupac.qmul.ac.uk/BlueBook/PDF/P9.pdf) |
| Inversion barriers | 4.1, 23.9, 37.3, and 42.0 kcal·mol⁻¹ for [4], [5], [6], and [7]helicene; [5] racemizes slowly at room temperature, [6] is stable[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287) |
| Chiroptics | Very high optical rotation and circular dichroism values; CPL brightness up to 513 M⁻¹ cm⁻¹ and |g_lum| up to 0.028 in optimized derivatives[7](https://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35111k) • [8](https://www.nature.com/articles/s44160-024-00527-3) • [9](https://waseda.elsevierpure.com/en/publications/tetraaza715helicenes-synthesized-by-two-step-strategy-length-cont/) |
| General syntheses | Photocyclodehydrogenation of diaryl olefins and transition-metal-catalyzed [2+2+2] cycloisomerization; the latter has reached oxa[19]helicene by closing 12 rings in one operation[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of) |
| Applications | Enantioselective catalysis, chirality sensing, circularly polarized light emission, and spintronics based on chirality-induced spin selectivity[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of) |
## Structure and chirality
Carbohelicenes consist of ortho-fused benzenoid rings forming a helically distorted conjugated polyaromatic system, and they have been described as "molecules in distress": the chiral helicity arises from severe intramolecular steric strain between the overlapping terminal rings[7](https://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35111k).
The two enantiomers are assigned by the IUPAC helicity rule, which designates a left-handed helix as M (minus) and a right-handed helix as P (plus), following Prelog and Helmchen; (P)-hexahelicene and (M)-hexahelicene are the preferred IUPAC names[5](https://iupac.qmul.ac.uk/BlueBook/PDF/P9.pdf). Experimental work on (+)- and (−)-hexahelicene and its 2-bromo and 2-methyl derivatives, combining vibrational and electronic circular dichroism with [X-ray diffraction](https://www.edgepedia.org/x-ray-diffraction), found the (−) optical isomer consistently associated with M helicity[10](https://re.public.polimi.it/retrieve/e0c31c0d-ac36-4599-e053-1705fe0aef77/Helical%20sense-responsive%20and%20substituent-sensitive%20feature%20in%20vibrational%20and%20electronic%20circular%20dichroism_11311-764547_Tommasini.pdf).
Helical geometry is characterized by three parameters: pitch (the height of one helical turn), diameter, and length[11](https://www.researchgate.net/publication/395195542_Expanding_Chemistry_of_Expanded_Helicenes). In the higher homologues a second coil forms, and the helical pitch becomes a meaningful measure of the overlap between successive turns[7](https://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35111k). For parent [7]- and [9]helicene, pitches determined from centroid–centroid distances of overlapping rings are 3.87 and 3.52 Å; π-extended analogs give 3.95 and 3.54 Å[12](http://pubs.acs.org/doi/suppl/10.1021/jacs.0c13197/suppl_file/ja0c13197_si_001.pdf). In the oxa[n]helicene series, the torsion angle between terminal-ring centroids grows with helical extension, from 16.6° in oxa[5]helicene to 36.7° in oxa[7]helicene, and 72.8° in oxa[9]helicene[13](https://ir.library.osaka-u.ac.jp/repo/ouka/all/97140/Chirality_36_5_e23673.pdf).
## Nomenclature and the [n]helicene series
Under IUPAC rule FR-2.1.7, a hydrocarbon parent component of six or more rings consisting of a benzene ring fused to the 3,4-position of phenanthrene, with further benzene rings fused similarly, is named from the numerical prefix denoting the total number of rings followed by the ending "-helicene"; hexahelicene's CAS and Beilstein name is phenanthro[3,4-c]phenanthrene[14](https://iupac.qmul.ac.uk/fusedring/FR21.html). The simpler [n]helicene notation counts only the rings of the helical skeleton, not additional rings fused to the backbone[4](https://download.e-bookshelf.de/download/0007/9058/68/L-G-0007905868-0016773358.pdf).
Properties change markedly as n grows. A computational study of carbo[n]helicenes for n = 4–10 found that the anisotropy (g) factor of the ¹B(b) band and the specific rotation are inversely proportional to n and correlate with helical pitch, but are discontinuous at n = 6, where the aromatic rings begin to overlap; excitation energies of the ¹B(b) and ¹B(a) bands are inversely proportional to n across the whole range[15](https://scispace.com/pdf/racemization-barriers-of-helicenes-a-computational-study-1xfogyr9um.pdf).
## Racemization and configurational stability
The enantiomerization transition-state barriers for [4]–[7]helicene are 4.1, 23.9, 37.3, and 42.0 kcal·mol⁻¹ respectively; for [4]helicene the transition state adopts a planar C2v symmetry with a barrier small enough for inversion at ambient temperature[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287). A 2023 Nature Chemistry study reports closely comparable values, 4.1 kcal mol⁻¹ for carbo[4]helicenes and 24.1 kcal mol⁻¹ for carbo[5]helicenes, attributing their room-temperature racemization to these low barriers[16](https://www.nature.com/articles/s41557-023-01174-5). Owing to these inversion barriers, [5]helicene racemizes slowly at room temperature whereas [6]helicene is stable at this temperature, making [6]helicene the minimum member for thermally stable chiral materials[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287). Hexahelicene and higher homologues are conformationally stable within a reasonable range of temperatures and can therefore be obtained enantiopure by racemate resolution or asymmetric synthesis[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of).
Two qualifications apply. First, the stability threshold is not exact: one review states that barriers of 100 kJ mol⁻¹ (about 24 kcal mol⁻¹) or higher are generally required for configurational stability of expanded helicenes[11](https://www.researchgate.net/publication/395195542_Expanding_Chemistry_of_Expanded_Helicenes), yet [5]helicene, at roughly that barrier, still racemizes slowly at room temperature[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287). Second, structure matters as much as ring count: [4]helicenes are extremely unstable under ambient conditions, and terminal fjord-region substituents stabilize the lower helicenes[17](https://www.cell.com/chem-catalysis/fulltext/S2667-1093(22)00552-8). π-Extension barely affects the backbone of large helicenes: calculated P/M isomerization barriers of π-extended [7]- and [9]helicenes are 42.4 and 41.6 kcal mol⁻¹, close to the parent values[12](http://pubs.acs.org/doi/suppl/10.1021/jacs.0c13197/suppl_file/ja0c13197_si_001.pdf).
## Synthesis and enantiopurity
Only a few synthetic methods have proved general, robust, and practical: traditional photocyclodehydrogenation of diaryl olefins and transition-metal-catalyzed [2+2+2] cycloisomerization[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of). The cycloisomerization of triynes, catalyzed mainly by Ni(0), Co(I), or Rh(I) complexes, has prepared [5]- through [19]helicenes, including oxa[19]helicene by closing 12 rings in a single synthetic operation[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of). Other practical methods include Diels–Alder addition; some reactions deliver gram-scale product and some afford helicenes in greater than 99% enantiomeric excess[4](https://download.e-bookshelf.de/download/0007/9058/68/L-G-0007905868-0016773358.pdf).
**Catalytic asymmetric routes** form a growing family: transition-metal-catalyzed [2+2+2] cycloaddition, hydroarylation, olefin metathesis, and oxidative coupling, plus organocatalysis with chiral phosphoric acids and amides[17](https://www.cell.com/chem-catalysis/fulltext/S2667-1093(22)00552-8). In 2020 Alcarazo and coworkers made 1,12-disubstituted [4]carbohelicenes by Au-catalyzed asymmetric hydroarylation with TADDOL-derived α-cationic phosphinite ligands[17](https://www.cell.com/chem-catalysis/fulltext/S2667-1093(22)00552-8). A 2026 review groups catalytic asymmetric helical synthesis into three strategies: extending conjugation via cycloaddition, locking configuration through single-site functionalization, and kinetic resolution[18](https://www.cell.com/trends/chemistry/abstract/S2589-5974(26)00178-4).
**Resolution** complements asymmetric synthesis. Chiral HPLC on commercial stationary phases resolves the stable members; oxa[7]- and oxa[9]helicenes were fully resolved on a Daicel chiral stationary phase, while oxa[5]helicene racemizes too rapidly at ambient temperature for resolution[13](https://ir.library.osaka-u.ac.jp/repo/ouka/all/97140/Chirality_36_5_e23673.pdf). Some helicene racemates crystallize as conglomerates, and a few were historically resolved by hand-picking homochiral single enantiomeric crystals for optical rotation measurement[7](https://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35111k). [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography) of 3D π-extended carbo[11] and [13]helicenes confirmed that their rigid backbones form conglomerates favoring preferential crystallization[8](https://www.nature.com/articles/s44160-024-00527-3). The absolute stereochemistry of helicene enantiomers was first established using X-ray diffraction data[11](https://www.researchgate.net/publication/395195542_Expanding_Chemistry_of_Expanded_Helicenes).
## Heterohelicenes and structural variants
Replacing backbone carbons with heteroatoms gives heterohelicenes named oxa-, aza-, thia-, phospha-, and sila[n]helicenes for O, N, S, P, and Si respectively; two helicene moieties joined by a single bond are bihelicenyls, fused pairs are double helicenes, and bridged ones are helicenophanes[4](https://download.e-bookshelf.de/download/0007/9058/68/L-G-0007905868-0016773358.pdf). Derivatives are built by three design moves: helical elongation, lateral π-extension, and heteroatom substitution with B, N, O, Si, P, or S[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287).
Heteroatoms change both stability and electronics. In 2015 Tanaka and coworkers achieved a rhodium-catalyzed enantioselective synthesis of sila[7]helicene with high ee; X-ray analysis showed the angle of the two newly formed C=C double bonds in the annulated silole to be 41.8°, larger than the 37.2° of the carbonyl-linked analog, which explains its high racemization tolerance[17](https://www.cell.com/chem-catalysis/fulltext/S2667-1093(22)00552-8). Gold-catalyzed hydroarylation gave dithia[5]helicenes in up to 98% yield and 98% ee, extended to dithia[9]helicenes with enantiopurity retained[17](https://www.cell.com/chem-catalysis/fulltext/S2667-1093(22)00552-8). Nitrogen embedding also works: a pyridine-embedded S-shaped double [6]helicene was obtained in 71% yield with 89% ee[19](https://pubs.rsc.org/en/content/articlehtml/2025/ob/d5ob00966a).
## Chiroptical and electronic properties
The molecular distortion produces very high chiroptical and circular dichroism values, the most distinctive properties of the class[7](https://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35111k). Chiroptical response grows steeply with helical length: going from [7] to [9] helical length causes an approximately tenfold increase in the dissymmetry factors, and a π-extended [9]helicene reaches g_lum = 7.4 × 10⁻³ with a circularly polarized luminescence brightness of 12.6 M⁻¹ cm⁻¹, among the highest for carbohelicenes, with a photoluminescence quantum yield of 0.41[12](http://pubs.acs.org/doi/suppl/10.1021/jacs.0c13197/suppl_file/ja0c13197_si_001.pdf).
Recent designs push these numbers further. 3D π-extended carbo[11] and [13]helicenes reached a maximum CPL brightness of 513 M⁻¹ cm⁻¹, reported as the highest value among helicene derivatives[8](https://www.nature.com/articles/s44160-024-00527-3). A homologous series of tetraaza[7]–[15]helicenes showed optical spectra converging beyond [11]helicene, defining a conjugation ceiling, while chiroptical responses amplified sharply to |g_lum| up to 0.028; [7]H and [15]H maintained high fluorescence quantum yields of 0.39 and 0.36 with CPL brightness values of approximately 490[9](https://waseda.elsevierpure.com/en/publications/tetraaza715helicenes-synthesized-by-two-step-strategy-length-cont/).
## Comparison with other chiral contorted π-systems
Helicenes sit within a broader family of helical nanographenes and helical one-dimensional nanostructures, collectively called nanohelicenes, which include helical polymers and nanorods with intrinsic chirality; in carbo-[n]helicenes, n denotes the number of ortho-fused benzene rings[20](https://pmc.ncbi.nlm.nih.gov/articles/PMC10458037/). Within the helicene class itself, the literature distinguishes carbohelicenes from expanded, π-extended, multiple, and heterohelicene variants, each tuning geometry and electronics differently[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287) • [11](https://www.researchgate.net/publication/395195542_Expanding_Chemistry_of_Expanded_Helicenes).
## Applications and recent developments
Documented applications span enantioselective catalysis, chirality sensing, circularly polarized light emission, organic and molecular electronics, and spintronics based on chirality-induced spin selectivity[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of). Under ultrahigh vacuum, helicenes on surfaces serve as model systems for studying chiral crystallization, chiral amplification, stereospecific C–C coupling, and chirality-induced electron spin selectivity, making them candidates for spintronic applications[21](https://www.dora.lib4ri.ch/empa/dload/empa%3A38859/PDF/Ernst-2024-Helicenes_on_surfaces-(published_version).pdf).
**Recent advances (2023–2026)** include a [C–H activation](https://www.edgepedia.org/c-h-activation)-based enantioselective synthesis of lower carbo[n]helicenes reported in 2023[16](https://www.nature.com/articles/s41557-023-01174-5); the 2024 enantioselective triple [2+2+2] cycloaddition followed by [Scholl reaction](https://www.edgepedia.org/scholl-reaction) that delivered 3D π-extended carbo[11] and [13]helicenes with up to 87:13 enantiomeric ratio[8](https://www.nature.com/articles/s44160-024-00527-3); and the tetraaza[7]–[15]helicene series, in which the [15]helicene constitutes the longest helicene ever resolved into its enantiomers[9](https://waseda.elsevierpure.com/en/publications/tetraaza715helicenes-synthesized-by-two-step-strategy-length-cont/). On length records the sources differ by series: one review states the longest carbohelicene is [16]helicene[6](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287), while the cycloisomerization literature reports oxa[19]helicene in the heterohelicene series[2](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of). Multiple helicenes, containing two or more helicene units, are now classified into linear, cyclic, and cross-linked types; Tanaka's group progressed from double [6]helicenes with 73% ee that racemized at room temperature to a double [2+2+2] cycloaddition product with stable helical chirality at greater than 99% ee, where two additional fused benzene rings significantly enhanced configurational stability[19](https://pubs.rsc.org/en/content/articlehtml/2025/ob/d5ob00966a).
## References
1. [IUPAC Gold Book – helicenes (H02762)](https://goldbook.iupac.org/terms/view/H02762.html)
2. [Helically Chiral Aromatics: The Synthesis of Helicenes by Transition-Metal-Catalyzed [2+2+2] Cycloisomerization, Accounts of Chemical Research (2020)](https://pubs.acs.org/achre4/article/53/1/144/1265772/Helically-Chiral-Aromatics-The-Synthesis-of)
3. [The Helicenes, Angewandte Chemie (1974)](https://onlinelibrary.wiley.com/doi/10.1002/anie.197406491)
4. [Helicene Chemistry: From Synthesis to Applications, Chapter 1](https://download.e-bookshelf.de/download/0007/9058/68/L-G-0007905868-0016773358.pdf)
5. [Nomenclature of Organic Chemistry, IUPAC Recommendations and Preferred Names 2013, Section P-92](https://iupac.qmul.ac.uk/BlueBook/PDF/P9.pdf)
6. [Symmetric Multiple Carbohelicenes, Synlett](https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0037-1610283.pdf?issue=10.1055%2Fs-009-42287)
7. [One hundred years of helicene chemistry. Part 2, Chemical Society Reviews (2013)](https://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35111k)
8. [Design and enantioselective synthesis of 3D π-extended carbohelicenes for circularly polarized luminescence, Nature Synthesis (2024)](https://www.nature.com/articles/s44160-024-00527-3)
9. [Tetraaza[7]–[15]helicenes Synthesized by Two-Step Strategy](https://waseda.elsevierpure.com/en/publications/tetraaza715helicenes-synthesized-by-two-step-strategy-length-cont/)
10. [Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism of Some Simple Optically Active Hexahelicenes](https://re.public.polimi.it/retrieve/e0c31c0d-ac36-4599-e053-1705fe0aef77/Helical%20sense-responsive%20and%20substituent-sensitive%20feature%20in%20vibrational%20and%20electronic%20circular%20dichroism_11311-764547_Tommasini.pdf)
11. [Expanding Chemistry of Expanded Helicenes (2025)](https://www.researchgate.net/publication/395195542_Expanding_Chemistry_of_Expanded_Helicenes)
12. [Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes, JACS supporting information](http://pubs.acs.org/doi/suppl/10.1021/jacs.0c13197/suppl_file/ja0c13197_si_001.pdf)
13. [Impact of helical elongation of symmetric oxa[n]helicenes, Chirality](https://ir.library.osaka-u.ac.jp/repo/ouka/all/97140/Chirality_36_5_e23673.pdf)
14. [IUPAC FR-2.1.7 Polyhelicene components](https://iupac.qmul.ac.uk/fusedring/FR21.html)
15. [Theoretical and Experimental Studies on Circular Dichroism of Carbo[n]helicenes](https://scispace.com/pdf/racemization-barriers-of-helicenes-a-computational-study-1xfogyr9um.pdf)
16. [A C–H activation-based enantioselective synthesis of lower carbo[n]helicenes, Nature Chemistry (2023)](https://www.nature.com/articles/s41557-023-01174-5)
17. [Advances in catalytic enantioselective synthesis of chiral helicenes and helicenoids, Chem Catalysis (2022)](https://www.cell.com/chem-catalysis/fulltext/S2667-1093(22)00552-8)
18. [Catalytic asymmetric synthesis strategies of helically chiral molecules, Trends in Chemistry (2026)](https://www.cell.com/trends/chemistry/abstract/S2589-5974(26)00178-4)
19. [Recent advances in the synthesis of multiple helicenes, Organic & Biomolecular Chemistry (2025)](https://pubs.rsc.org/en/content/articlehtml/2025/ob/d5ob00966a)
20. [Current State of Computational Modeling of Nanohelicenes](https://pmc.ncbi.nlm.nih.gov/articles/PMC10458037/)
21. [Helicenes on Surfaces: Stereospecific On-Surface Chemistry, Single Enantiomorphism, and Electron Spin Selectivity (2024)](https://www.dora.lib4ri.ch/empa/dload/empa%3A38859/PDF/Ernst-2024-Helicenes_on_surfaces-(published_version).pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Organic reactions, structure and reference › Hydrocarbon and arene structure and reactivity › Polycyclic and non-benzenoid aromatics › Contorted aromatics and helicenes*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license