Helicenes

Helicenes are a class of polycyclic aromatic hydrocarbons in which several benzene rings are fused in an ortho-configuration to generate a rigid, nonplanar helical framework. The term typically denotes the [n]helicenes, where n is the number of ortho-fused rings. The helical shape makes each molecule chiral, producing stable enantiomers that cannot interconvert without breaking aromaticity; racemization barriers rise with the number of rings, so enantiomers of [6]‑ and larger helicenes can be isolated and studied.

In terms of properties, helicenes absorb in the UV region and often emit in the visible, with strong chiroptical responses evidenced by circular dichroism and, in some derivatives, circularly polarized luminescence. Substitution and incorporation of heteroatoms lead to heterohelicenes with tailored electronic properties.

Synthesis methods include Mallory-type oxidative photocyclization of stilbene precursors to form the helical backbone, as well as other cyclization strategies and enantioselective syntheses using chiral catalysts or auxiliaries. The choice of rings and substituents allows tuning of the optical and electrochemical behavior.

Applications of helicenes span catalysis and materials science. They are used as chiral ligands and organocatalysts in asymmetric synthesis, as components in chiroptical materials for OLEDs and circularly polarized light emitters, and as probes for studying chirality in solid-state environments. Heterohelicenes and substituted helicenes broaden the utility across materials chemistry and molecular recognition.