Chemists Build a Giant Molecular Screw That Glows in Circularly Polarized Light

A team of chemists in Japan has pulled off one of the most demanding feats in modern molecular construction: the enantioselective synthesis of a π-extended carbo[17]helicene, a gigantic helical molecule made exclusively of carbon and hydrogen atoms. The achievement, reported in Nature Synthesis by Futo Morita, Takeshi Ichimura and colleagues at the Institute of Science Tokyo under the direction of Ken Tanaka, demonstrates how careful molecular programming can overcome the steric chaos that has long blocked access to very long, purely hydrocarbon helicenes. The resulting nanocarbon is not merely a structural trophy. It emits circularly polarized light with dissymmetry factors reaching 0.043, among the highest values ever recorded for helical hydrocarbons, placing it firmly in the spotlight of chiral optoelectronics research.

Helicenes are ortho-fused polycyclic aromatic molecules whose benzene rings twist into a rigid screw-like backbone, giving rise to two mirror-image forms, labeled P and M for right- and left-handed helices. The longer the helix, the more pronounced its chiroptical behavior, particularly circularly polarized luminescence, in which emitted light preferentially rotates in one direction. Theory predicts that the degree of circular polarization in carbo[n]helicenes increases with the number of fused rings, making long helicenes highly desirable targets. Yet there is a catch: fluorescence quantum yields drop sharply as helicenes grow longer, because the floppy excited-state structures of extended helices relax non-radiatively. Lateral π-extension, adding extra fused benzene rings around the helical periphery, stiffens the framework and suppresses this relaxation, restoring brightness. Combining longitudinal growth with lateral expansion, however, multiplies molecular strain and steric congestion to levels that conventional synthesis simply cannot handle.

The classical workhorse for making carbohelicenes has been oxidative photocyclization of stilbene derivatives. Using this approach, Murase and Fujita reported the racemic synthesis of carbo[16]helicene in 2015 through a remarkable sextuple photocyclization, albeit in low yield. More recently, Jin and coworkers achieved an efficient racemic route to carbo[17]helicene using stilbene precursors containing heteroatom functionalities. But racemic products are only half the story. For applications in circularly polarized light detection, spin filtering and chiral photonics, enantioenriched material is essential, and no one had previously made a long carbohelicene composed solely of carbon and hydrogen in enantioenriched form. Heterohelicenes, which incorporate nitrogen, sulfur or other heteroatoms, are easier to make enantioselectively because the heteroatoms assist reactivity and selectivity; all-carbon helicenes offer no such handles.

The Tokyo group’s alternative strategy centers on the nickel-catalyzed [2+2+2] cycloaddition, a reaction in which three alkyne units cyclotrimerize to form a benzene ring. Chiral nickel catalysts can impose handedness during this ring formation, directly producing helicenes with high enantiomeric purity without the need for chiral chromatography. In 2024, the same laboratory used triple [2+2+2] cycloadditions followed by Scholl reactions, oxidative dehydrogenative cyclizations that fuse peripheral aryl groups, to build π-extended carbo[11] and [13]helicenes. But that earlier approach suffered from partial disordering of the cyclization sequence, which eroded yields and selectivities. Scaling up to a quadruple cyclization, with four separate cyclotrimerizations on a single dodecayne substrate, demanded something new: a way to force the four cyclizations to happen in the right order.

The solution is a sterically differentiated, wavy dodecayne precursor built around a central triphenylene core. The molecule contains two inner triyne units, each destined to form a relatively unhindered [5]helicene, and two outer triyne units that must generate far more sterically demanding [6]helicenes. The triphenylene backbone serves two roles: it elongates the eventual helicene and it increases the distance between the inner triyne units, reducing congestion during their cyclization. Crucially, the inner triynes react faster than the outer ones, so the quadruple cyclization proceeds from the molecular center outward, assembling the two [5]helicenes first and only then tackling the crowded [6]helicene formations. This programmed sequence suppresses dead-end pathways in which an outer triyne cyclizes prematurely and blocks the rest of the cascade.

The experimental route began with iodination of 2,7-dimethoxytriphenylene and demethylation to give a diiodotriphenylenediol, which was coupled by Sonogashira chemistry with pentayne building blocks to form decaynes, then elaborated by triflation and Suzuki–Miyaura coupling into the final dodecaynes. Subjecting dodecayne 1a to the chiral Ni(cod)₂/(R)-Quinap catalyst system under a two-temperature protocol, 72 hours at 0 °C followed by 72 hours at 60 °C, delivered the desired expanded [21]helicene (P,P,P,P)-8a in 31% yield with an outstanding 97:3 enantiomeric ratio, alongside a diastereomeric product. Subsequent Scholl reactions, first with DDQ and triflic acid to fully fuse the backbone and then under harsher conditions to close the peripheral aryl–aryl bonds, converted 8a into the π-extended carbo[17]helicene 2a while preserving the enantiomeric ratio at 98:2. No racemization occurred, mirroring the configurational stability of the smaller [5] and [6]helicene substructures under the same strongly oxidative and acidic conditions.

The importance of sequence control was proven by a series of elegant control experiments. When the team extracted the individual triyne fragments and raced them against each other under the chiral nickel catalyst, the triphenylene-bridged inner triyne produced its [5]helicene in up to 97% yield, roughly four times faster than the naphthalene-bridged triyne that models the outer units. In contrast, a benzene-bridged triyne corresponding to the less hindered analogue dodecayne 1b reacted at nearly the same rate as the inner fragment, meaning no sequence preference existed. Consistently, dodecayne 1b failed to deliver its expanded [19]helicene under the chiral conditions and required a high-temperature achiral catalyst, producing complex mixtures. Density functional theory calculations reinforced the picture: the (P,P) intermediate formed after two inner cyclizations is more stable than its (P,M) diastereomer by 10.9 kcal per mole, and the alternative (M,P,P,M) product is destabilized by 21.9 kcal per mole relative to the desired (P,P,P,P) helicene, explaining why dead-end isomers never appear.

Single-crystal X-ray diffraction confirmed the extraordinary geometry of the products. The π-extended carbo[17]helicene 2a contains a continuous helical backbone of 17 fused benzene rings surrounded by 35 additional benzene rings along its periphery, with 32 overlapping rings compressing the helix to a length of 9.26 Å, among the longest helical nanographenes reported. The precursor 9a displayed interplanar angles as large as 80.6°, among the highest ever measured for helicene derivatives. Thermal studies showed that the kinetically formed (M,P,P,P) isomer is configurationally stable at 60 °C but irreversibly converts to the thermodynamic (P,P,P,P) isomer at 120 °C, with calculated activation barriers of 25.1 and 33.3 kcal per mole for the forward and reverse processes.

Photophysical measurements revealed why the effort was worthwhile. The long helicenes absorb at 465 to 512 nm, red-shifted by roughly 100 nm relative to the precursor, and fluoresce with quantum yields of 14 to 29%, remarkable for helicenes longer than nine rings, which are normally weak or non-emissive. Absorption dissymmetry factors reach 0.017 to 0.040, driven by large magnetic transition dipoles nearly parallel to their electric counterparts. Most strikingly, the luminescence dissymmetry factors of the fully fused long helicenes, 0.038 to 0.043, match or exceed their absorption values, an unusual outcome attributed to the rigid fused framework suppressing excited-state structural relaxation. The resulting circularly polarized luminescence brightness values of 186 to 471 M⁻¹ cm⁻¹ rank among the highest for helical hydrocarbons, and far surpass recently reported donor–acceptor [17]helicenes, underscoring that lateral π-expansion matters as much as helix length. The authors note that future chiral nickel catalysts with even higher stereodiscrimination should suppress the minor diastereomer and push yields and enantioselectivity further, opening a programmable route to ever-larger all-carbon chiral nanocarbons for photonic and spintronic technologies.

Subject of Research: Enantioselective synthesis of a π-extended carbo[17]helicene via sequence-controlled Ni-catalysed quadruple [2+2+2] cycloadditions and Scholl reactions

Article Title: Enantioselective synthesis of π-extended carbo[17]helicene via sequence-controlled quadruple [2+2+2] cycloadditions

Article References: Enantioselective synthesis of π-extended carbo[17]helicene via sequence-controlled quadruple [2+2+2] cycloadditions. (n.d.). https://doi.org/10.1038/s44160-026-01193-3

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01193-3

Keywords: helicene, chiral nanocarbon, [2+2+2] cycloaddition, nickel catalysis, circularly polarized luminescence, enantioselective synthesis, Scholl reaction, polycyclic aromatic hydrocarbon, chiroptical properties, stereochemistry, nanographene, organic synthesis