Acenes

Acenes are molecules in which benzene rings, shown below, are fused together.

 

Researchers are very keen on making acenes because of their interesting properties.

Naphthalene, produced from the distillation of coal tar in the nineteenth century and traditionally used in mothballs, is a molecule in which 2 benzene rings are fused:

 

 Anthracene, a component of coal tar that is used to make dyes, has 3 fused benzene rings:

Tetracene (or naphthacene), is a semiconductor used to make organic light-emitting diodes (OLEDs) and organic field-transmitters (OFETs), has 4 fused benzene rings:

Pentacene is a semiconductor relevant to the building of photoelectronic devices. It oxidises when exposed to UV and visible light. It has 5 fused benzene rings:

The longer the acene molecule is, the less stable it is so the longer acenes are so unstable they do not occur in nature. Indeed, nonacene, in which 9 benzene rings are fused together was only detected recently, in 2010. Yet researchers are still interested in developing longer acenes because of their unique electronic properties.

In 2017, researchers from the Institute for Materials Science and Center for Advancing Electronics at Technische Universität of Dresden (TUD) and the research center CiQUS (University of Santiago de Compostela) were able to make the elusive decacene in which 10 benzene rings are fused together.
Individual decacene molecules were visualised by high-resolution Scanning Tunneling Microscopy (STM):

The decacene molecule is so reactive that it had to be prepared on a gold surface under a high vacuum in order to stabilise it.

Reference

Technische Universität Dresden. "Researchers obtain decacene, the largest acene synthesized ever." ScienceDaily. ScienceDaily, 14 August 2017. .

Further Reading

Benzene: Structure and Properties

Introduction to IUPAC Nomenclature of Organic Compounds

Empirical Formula

Molecular Formula

2-Dimensional Structural Formula

Condensed Structural Formula

Skeletal Structural Formula

Suggested Study Questions:

1. Draw the 2-dimensional structural formula for each of the following molecules:
• benzene
• naphthalene
• anthracene
• tetracene
• pentacene
2. Draw the condensed structural formula for each of the following molecules:
• benzene
• naphthalene
• anthracene
• tetracene
• pentacene
3. Give the molecular formula for each of the following molecules:
• benzene
• naphthalene
• anthracene
• tetracene
• pentacene
4. Give the empirical formula for each of the following molecules:
• benzene
• naphthalene
• anthracene
• tetracene
• pentacene
5. Name the following molecules found in the acene series:
• 6 benzene rings fused together
• 7 benzene rings fused together
• 8 benzene rings fused together
• 9 benzene rings fused together
6. Consider the structure of the molecules in the acene series. Explain why Chemists use the term "fused" rather than "bonded" or "linked" to refer to how the benzene rings are joined together to make the acene molecules.
7. Place the molecules in the acene series from benzene to decacene in order of increasing reactivity.
8. What feature of the molecules in the acene series makes them useful semiconductors?
9. Explain why the reactivity of the molecules in the acene series increases as the number of benzene rings fused together increases.
10. Explain why decacene had to be prepared under a high vacuum.