Preparative-scale synthesis of nonacene

The longer, the better!

January 21, 2022

Long acenes composed of linearly fused benzene rings, have remained for years unreachable compounds despite their high potential interest for applications in molecular electronics, spintronics, and organic electronics. Researchers from the NanoSciences Group in CEMES-CNRS, in collaboration with Hokkaido University (Japan), have found a strategy to prepare nonacene (with 9 rings) and demonstrate its surprising thermal stability in inert conditions.

n-acenes are a class of polyaromatic hydrocarbons consisting of linearly fused benzene rings of formula C4n+ 2H 2n+4, where n is the number of rings. These graphene nanoribbon segments exhibit singular electronic properties such as a rapid decay, as a function of n, of the energy gap between the highest occupied and lowest vacant molecular orbitals. This electronic structure induces a diradical character, small optical gaps, low-energy triplet states, low oxidation potentials and high electronic affinity. This explains why the smallest easily accessible acenes such as tetracene (n=4) and pentacene (n=5) are currently being extensively studied for applications in OFET transistors and organic photovoltaic cells, with hole mobilities exceeding than that of amorphous silicon.

Schematic representation of the precursors used to synthesise nonacene.

Increasing the number of cycles n should improve these properties, but also increases the chemical reactivity of the molecules. This is why, despite this interest and more than 70 years of research, hexacene (n=6) could only be isolated in 2012, then heptacene (n=7) in 2017, and it was accepted until very recently that it was not certain that longer acenes would be stable enough to be synthesized except under very special conditions of high dilution or ultra-high vacuum, and this, at very low temperature.

Researchers from the Nanosciences Group of CEMES-CNRS have just developed a method for preparing pure nonacene (n=9) and demonstrated its surprising thermal stability in the solid phase. Their strategy, which can be extended to even longer acenes, has been preparation of chemically stable, non-planar and soluble precursors with a large gap, which allows synthesis and characterization by classical methods of organic chemistry (in eleven steps of synthesis in total). These precursors have two bridging carbonyl groups that can be removed in the form of carbon monoxide by solid phase heating at 200 ° C or by ultraviolet irradiation. The nonacene thus formed is a stable black solid, which can be kept under an inert atmosphere for months which will allow its use in organic field effect transistors.

Preparative-scale synthesis of nonacene
Andrej Jančařík, Jan Holec, Yuuya Nagata, Michal Šámal, and André Gourdon
Nature Communications 13, 223 (2022)

André Gourdon – andre.gourdon[at]

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