Fusion reactions for improved solar cells

Ribbons of fused benzene rings, the so-called polyacenes, have great potential in flexible organic electronics applications. Indeed, pentacene, the five-ringed ribbon has already been investigated widely. Unfortunately, longer ribbons are notoriously unstable, degrading on exposure to light and oxygen, and having small band gaps. Despite these drawbacks, the longer ribbons may have intriguing optical and electronic properties and so researchers are keenly pursuing them.

Now, Alejandro Briseno of the University of Massachusetts, Amherst, and colleagues there and at Georgia Institute of Technology, Atlanta and the University of Kentucky, Lexington, have now side-stepped those technical problems and designed and constructed a new class of polyacenes comprising eight angular fused rings, which they dubbed bistetracene. [J Am Chem Soc, 2014, 136, 9248-9251].

The team explains that angular geometry of these molecules, not observed in other polyacenes, helps to stabilize the structures in air but does not affect detrimentally the compound's intrinsic charge transport properties. Specifically, it is the presence of more "Clar aromatic sextets" within this structure than are present in pentacene and other conventional polyacenes, the team explains. Not all polyaromatic hydrocarbons, of which polyacenes are an important example, are as stable, or aromatic, as each other. Each fused benzene ring (a sextet of carbon atoms) has a different degree of aromaticity to contribute to the overall stability depending on its precise position in the structure and whether its pi-electrons are fully conjugated or disconnected by the upper limit on carbon's valency of four.

Erich Clar clarified this rule in 1964 and it is now used as an important means to predict aromaticity. The shape of the new class of polyacenes allows full conjugation of more benzene rings within the molecule and so boosts stability, the team reports. Stability aside, this molecule and the bistetracene derivatives can be processed in solution, which promises ease of manufacture of devices. The devices in question being organic photovoltaic solar cells and organic field effect transistors given their low-energy band gaps and high charge-carrier mobility. Theoretical calculations suggest great potential while experimental evidence for one compound revealed charge carrier mobilities as large as 6.1 cm² V⁻¹ s⁻¹ and current on/off ratios of 10⁷, the team reports.

In efforts to explain the nature of stability in these compounds, researchers at University of California Los Angeles led by Ken Houk carried out DFT (density functional theory) calculations. Calculations from the Houk group corroborate experimental results and indicate that bistetracene is about 5 orders of magnitude (about 70 000 times) less reactive than pentacene in Diels-Alder reactions with [60]fullerene [J. Am. Chem. Soc, 2014, online].

The researchers' relatively straightforward synthetic scheme for making these compounds bodes well for the construction of even larger polyacenes in this class and they are now working on building such molecules with relatively high numbers of Clar aromatic sextets for further testing in high performance organic electronic devices.

"Further studies include structure-property relationships of even larger conjugated cores," Briseno told us. "We will also employ these stable building blocks to synthesize polymer semiconductors. These compounds will find use in large-area, roll-to-roll manufacturing of electronic devices."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".