Alkyne Benzannulations: A Powerful Tool for the Efficient Synthesis of Chiral and Achiral Nanographenes and of Graphene Nanoribbons.

Abstract

ConspectusPolycyclic aromatic hydrocarbons (PAHs) have become ubiquitous in the design of organic electronics due to their extended π-electron systems that give rise to potentially useful optical and electronic properties. These properties─chiefly absorption, emission, and semiconductivity─can be tuned to suit a particular device application through changes to the size and shape of the molecule's backbone, through incorporation of heteroatoms into the core structure, and through functionalization along the periphery. To that end, synthetic design of PAHs necessitates versatile methodology to rapidly expand the π-electron system without sacrificing the solubility that allows the materials to be processed into devices.Alkyne benzannulation is a powerful tool for the synthesis of such PAHs due to the energetically downhill process of making an aromatic ring from a high-energy triple bond. Furthermore, functionalization of the alkynes can increase not only solubility but also steric strain in the backbone, resulting in highly contorted, even chiral, structures. Because the benzannulation process is so energetically favorable, even these very strained PAHs can be synthesized with relative ease under mild conditions.In this Account, we summarize our work and the development of our methods for utilizing alkyne benzannulation to synthesize contorted, and in many cases chiral, PAHs, as well as highly soluble graphene nanoribbons. Trifluoroacetic acid (TFA) is effective for the benzannulation of alkynes to cata-condensed PAH systems, creating phenanthrene-like moieties. However, we found that to generate peri-condensed systems with pyrene-like moieties, a much stronger Brønsted acid, such as triflic acid (TfOH), is required. Our combination of TFA for clean benzannulation of half of the alkynes under mild conditions, followed by TfOH to complete benzannulation of the remaining alkynes, was used to great effect in the synthesis of 5-armchair graphene nanoribbons and their oligomers, the pyrenacenes. This method was, unfortunately, limited to alkynes bearing electron-rich aromatic substituents. To overcome this obstacle and broaden the scope, we screened Lewis acid catalysts and found that InCl3 is effective for alkynes bearing much less electron-rich aromatics, and even alkynes bearing only alkyl chains, under milder reaction conditions than with the Brønsted acids. With these two methods, we synthesized the first chiral peropyrenes and the first chiral teropyrene, as well as other compounds exhibiting twistacene and helicene chirality. We later found that the Lewis acidity of InCl3 can be increased by the addition of a AgNTf2 cocatalyst to effect more difficult benzannulations while keeping the reaction conditions mild. These methods of alkyne benzannulation allow us a great deal of control over the size and shape of the PAH backbone, thereby giving us control over the optical and electronic properties, to give us a suite of compounds that exhibit absorption and fluorescence across the visible spectrum.