Synthetic Strategies for Polycyclic Aromatic Compounds

Polycyclic aromatic compounds, characterised by fused aromatic rings, underpin advances in organic electronics, photonics and catalysis. Synthetic approaches broadly divide into bottom-up strategies—where small molecular building blocks are assembled into larger frameworks—and top-down methods that reshape extended carbon networks. Central to bottom-up synthesis are oxidative aromatic coupling reactions, notably the Scholl reaction and related cyclodehydrogenations, which forge new carbon–carbon bonds to extend π-conjugation. Transition-metal-catalysed cross-coupling and annulative π-extension (APEX) enable site-selective growth of defined polyaromatic scaffolds, while mechanochemical activation has recently emerged as a solvent-free alternative. On-surface synthesis under ultrahigh vacuum offers access to novel ring topologies and precise heteroatom doping. Collectively, these strategies afford control over ring size, edge topology and heteroatom incorporation, thereby tuning optical gaps, charge mobility and chemical reactivity for applications ranging from field-effect transistors to molecular sensing.

Research from Nature Portfolio

Mechanochemical Scholl reactions have been shown to convert graphitisation precursors directly into chlorinated curved nanographenes without bulk solvents. Regioselective halogenation at predetermined edge positions permits subsequent Suzuki coupling or nucleophilic aromatic substitution, enabling finely tuned π-extended scaffolds such as corannulene-coronene hybrids and superhelicenes. On-surface fabrication techniques have yielded nitrogen-doped buckybowls featuring inverse Stone–Thrower–Wales ring arrangements. The combination of in-solution and surface-confined steps exploits scanning tunnelling microscopy to confirm bowl-opening conformations and heteroatom placement, paving the way for designer fullerene hosts. Azomethine ylide homocoupling has been employed both in solution and on surfaces to introduce central pyrazine rings into non-planar polycyclic frameworks. This method produces internally nitrogen-doped motifs with localized antiaromaticity, illustrating a versatile route to heteroatom-rich nanographenes.

Synthetic Strategies for Polycyclic Aromatic Compounds publication trend

The graph below shows the total number of articles in synthetic strategies for polycyclic aromatic compounds across all publications each year (not limited to Nature Index journals).

Technical terms

Polycyclic aromatic hydrocarbons (PAHs): Molecules composed of two or more fused benzene or related aromatic rings forming an extended π-conjugated system.

Scholl reaction: An oxidative aromatic coupling that creates new carbon–carbon bonds between adjacent arenes, used to fuse ring systems in PAHs.

Annulative π-extension (APEX): A catalytic method that extends a polycyclic framework by annulating additional aromatic rings at specific edge sites.

Cyclodehydrogenation: A dehydrogenative process that forms new C–C bonds intramolecularly to planarise and extend aromatic networks.

Azomethine ylide: A reactive 1,3-dipolar intermediate employed in cycloaddition sequences to introduce nitrogen into polycyclic architectures.

On-surface synthesis: A technique wherein molecular precursors are assembled and reacted on a solid substrate under ultrahigh-vacuum conditions to access atomically precise nanostructures.

References

1. Laterally π‐Extensed Nitrogen‐Doped Molecular Nanographenes – From Anti‐Kasha Emission to Ping‐Pong Energy Transfer Events. Advanced Energy Materials (2024).
2. Synthesis of precisely functionalizable curved nanographenes via graphitization-induced regioselective chlorination in a mechanochemical Scholl Reaction. Nature Communications (2023).
3. Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds. Chemical Reviews (2021).
4. One-shot K-region-selective annulative π-extension for nanographene synthesis and functionalization. Nature Communications (2015).
5. Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties. Angewandte Chemie International Edition (2020).
6. On-surface synthesis of a nitrogen-embedded buckybowl with inverse Stone–Thrower–Wales topology. Nature Communications (2018).
7. Exploration of pyrazine-embedded antiaromatic polycyclic hydrocarbons generated by solution and on-surface azomethine ylide homocoupling. Nature Communications (2017).