metaphenylenes

Metaphenylenes are a class of arylene hydrocarbons in which benzene rings are linked predominantly through meta (1,3) connections, forming a quasi-linear chain of phenylene units. The term encompasses discrete oligomers as well as longer polymers built from 1,3-phenylene bridges. The meta linkage introduces angular connections that interrupt uninterrupted pi-conjugation, distinguishing metaphenylenes from para-linked or orthogonally linked analogs and leading to a mix of localized and partially conjugated electronic structures.

Structure and properties: The repeating motif comprises benzene rings connected at the 1 and 3 positions. The meta bridges can enforce nonplanar or curved backbones, influencing stacking and solid-state packing. Electronic coupling between adjacent rings is generally weaker than in para-phenylene systems, often resulting in larger optical gaps and distinctive electrochemical behavior. The physical properties, including solubility and processability, depend on the length of the chain and the nature of substituents.

Synthesis: Metaphenylenes are typically prepared by modular assembly of meta-linked aryl fragments. Common strategies employ palladium- or nickel-catalyzed cross-coupling reactions, such as Suzuki–Miyaura or Stille couplings, to form sequential 1,3-aryl–aryl bonds. Dehydrogenative couplings and other cross-coupling variants can also be used to close or extend the phenylene backbone, enabling the preparation of well-defined oligomers and short polymers.

Applications and significance: Metaphenylenes provide model systems for studying one-dimensional pi-conjugation with disrupted geometry. They are of interest in fundamental organic chemistry and materials science and are explored for potential optoelectronic applications, including organic semiconductors and light-harvesting components. Challenges include controlling conformation, improving solubility, and achieving scalable synthesis.

See also: para-phenylenes, cyclo-para-phenylenes, arylene polymers, polyphenylene, meta-phenylene.