Successful synthesis of polyacene with "numerous connected benzene rings" — World's first synthesis by the University of Tokyo

Acenes, which have a structure of linearly connected benzene rings, have attracted considerable attention in the fields of organic electronics and spintronics because they exhibit excellent electrical conductivity, luminescence, and magnetic properties because electrons can easily move over a wide range as the number of benzene rings increases.

Since the synthesis of the pentacene molecule with five fused benzene rings in 1912, various methods of synthesizing longer acenes have been developed. However, longer acene molecules have significantly lower solubility and stability, making them extremely difficult to synthesize. The longest acene to date, reported in 2020, is dodecacene, which consists of twelve benzene rings.

Professor Takashi Uemura, Assistant Professor Takashi Kitao, and graduate student Takumi Miura of the Graduate School of Engineering at the University of Tokyo have achieved the world's first synthesis of polyacenes consisting of numerous linearly linked benzene rings through the precise synthesis of a polymer precursor and subsequent thermal conversion using a porous metal complex (MOF) with molecular‐scale pores as a reaction field.

Polyacene is a substance that many scientists have been trying to synthesize for over 100 years but have been completely unsuccessful. In the future, there are expectations for its use in nanodevices, and further clarification of its physical properties.

The research group has previously succeeded in the controlled synthesis of polymers and nanocarbon materials using the nanopores of MOFs as a reaction field. In the current study, polymers that serve as precursors for polyacenes were synthesized by introducing monomers, which are the raw materials for polyacenes, into MOFs with one‐dimensional spaces, followed by linking reactions. Simply heating the monomer would lead to the formation of a branched structure because the reaction position cannot be controlled. Within the pores of the MOF, in contrast, the monomers are arranged one‐dimensionally. Thus, they can only be linked at the desired reaction position. The resulting complex was treated with a base to selectively remove only the MOF backbone, isolating the high molecular weight precursor. It was then converted into polyacene by heat treatment.

In collaboration with a research group led by Professor Hiroko Yamada of the Nara Institute of Science and Technology (now at Kyoto University), the structure of polyacene was analyzed in detail using various spectroscopic techniques. It was found that the longest polyacene has more than several dozen connected benzene rings (the average number is 19), which significantly surpasses the previous record for the longest polyacene.

The team also collaborated with a group of researchers led by Professor Taro Hitosugi at the University of Tokyo and Professor Shu Seki at Kyoto University to investigate several aspects of the structural stability and electronic and magnetic properties of the resulting polyacene.

In this research, they succeeded in synthesizing polyacene, which had previously existed only in theory. Because the reaction can be scaled up for mass synthesis, the researchers will work to clarify the optical, electronic, and magnetic properties of polyacene, which have not yet been explored, and aim to develop a wide range of applications, including solar cells and nanodevices that exploit the unique functions of the thinnest graphene.