For decades, chemists have been racing to build ever-longer carbon chains, molecules that could revolutionize our electronic world.
Imagine an electronic device so thin it could be woven into your clothing, or a screen that rolls up like a piece of paper. At the heart of such futuristic technologies lie extraordinary organic molecules known as acenes—strips of carbon atoms fused together into linear chains of benzene rings. For over eighty years, scientists have pursued an elusive goal: creating ever-longer "higher acenes." This is the story of that scientific quest, a journey that has pushed the boundaries of chemistry and opened new possibilities for the future of electronics.
Years of Research
Rings in Largest Acene
Hole Mobility in Pentacene
Acenes are a family of polycyclic aromatic hydrocarbons (PAHs) consisting of linearly fused benzene rings, with a general formula of C₄N₊₂H₂N₊₄2 . The most well-known member is pentacene (five rings), which has shown exceptional performance as a p-type semiconductor in organic field-effect transistors (OFETs), with hole mobility reaching 1.5 cm² V⁻¹ s⁻¹ in early studies1 .
Extended, rigid, and planar π-conjugated systems allow for enhanced charge delocalization and optimal molecular packing1 .
| Acene Name | Number of Benzene Rings | Status and Key Facts |
|---|---|---|
| Pentacene | 5 | Benchmark semiconductor; good stability |
| Hexacene | 6 | Largest acene synthesized & isolated in 20th century2 4 |
| Heptacene | 7 | First convincing evidence in 20062 4 |
| Nonacene | 9 | Achieved with stabilizing substituents2 |
| Undecacene | 11 | Studied via matrix isolation and on-surface synthesis2 |
| Dodecacene | 12 | Largest acene observed via on-surface synthesis2 |
The central paradox of acene chemistry is that the most desirable electronic properties emerge precisely when the molecules become most difficult to handle. According to Clar's aromatic sextet rule, molecular structures that can host the greatest number of aromatic sextets (resonating benzene rings) tend to be more stable1 .
Longer acenes readily dimerize (form pairs) or polymerize, and react rapidly with oxygen, especially in solution2 .
The strong intermolecular interactions between acene molecules in their herringbone crystal structure make them nearly insoluble in common organic solvents2 .
Good stability, benchmark material
20th century limit for isolation
Requires advanced stabilization techniques
Synthesizing stable "precursor" molecules that can be cleanly converted to the target acene when needed7 .
Using trialkylsilylethynyl groups (R₃SiC₂−) as substituents to enhance both stability and solubility2 .
Creating acenes directly on crystalline metal surfaces under ultrahigh vacuum conditions2 .
| Reagent/Technique | Function in Acene Synthesis |
|---|---|
| Trialkylsilylethynyl Groups | Enhance stability and solubility simultaneously1 2 |
| Precursor Strategy | Provide stable molecules that can be converted to acenes when needed7 |
| On-Surface Synthesis | Enables study of unstable acenes on inert surfaces under vacuum2 |
| Matrix Isolation | Traps reactive acenes in frozen matrices for characterization2 |
| Si/B Exchange Reaction | Constructs boron-doped acene backbones6 |
From hexacene in 1939 to dodecacene in the 2020s, follow the timeline of breakthroughs in acene synthesis.
Recent innovations have expanded beyond pure hydrocarbon acenes to include heteroatom-doped versions, particularly boron-doped acenes6 . Researchers have successfully synthesized quadruply boron-doped pentacene, heptacene, and nonacene, which exhibit unique luminescent properties, enhanced Lewis acidity, and stimuli-responsive emission6 .
The quest to synthesize larger acenes represents more than just a chemical challenge—it's a journey to the fundamental boundaries of aromaticity and electronic behavior in carbon-based materials. From the first isolation of hexacene in 1939 to the recent characterization of dodecacene through on-surface synthesis, this field has consistently pushed the limits of what's possible in organic chemistry.
Each new acene length reveals fresh insights into the quantum behavior of carbon nanomaterials while opening new possibilities for tomorrow's technologies.