Synthesis of Graphene Nanoribbons by Diels-Alder Cycloadditions of Dimesitylbisanthene and p-Benzoquinone Followed by Aromatization

The study of graphene ribbons is currently a topic of great interest in many branches of the scientific community. Most procedures for making graphene or graphene ribbons have focused on chemical vapor deposition of carbon-containing gasses on metal surfaces in order to form single layers. Solution-based approaches are presently lacking. With this in mind, we planned to synthesize graphene nanoribbons by using simple chemical processes.

         Our group previously reported that dimesitylbisanthene undergoes Diels-Alder reactions at both of its bay regions to afford ovalene derivatives. By related chemistry, we are preparing alternating oligomers of bisanthene with p-benzoquinone and aromatizing the quinones. This strategy provides an effective procedure for synthesizing fully aromatic linear graphene nanoribbons through Diels-Alder chemistry. The 2:1 bisanthene:p-benzoquinone oligomer (BQB) was synthesized in 69% yield by using Diels-Alder cycloaddition of dimesitylbisanthene and p-benzoquinone. To aromatize BQB, we first tried using 5 equivalents of lithium aluminium hydride. This experiment produced the corresponding diol in 73% yield. The aromatization was also attempted using tin (II) chloride and concentrated hydrochloric acid. Using these conditions, we obtained comparatively pure compound by proton NMR and mass spec. The proton NMR shows a distinctive peak around 11 ppm, arising from the protons of the central benzene ring. A peak at m/z 1244 was observed by MALDI-MS, as expected for the desired fully aromatic compound (calculated exact mass for C₉₈H₆₇ [M+H⁺]:1244). This product exhibits poor solubility in common organic solvents.

         We also attempted the synthesis of substituted diol compounds for the improvement of solubility. When we used 20 equivalents of lithium trimethylsilyl acetylide for the addition to BQB, we succeeded in synthesizing of the corresponding diol in 85% yield. In contrast, when we used 20 equivalents of phenyllithium, we obtained an inseparable mixture of diol and ketoalcohol.

         We established an effective procedure for the preparation of fully aromatic, linear graphene ribbons through Diels-Alder cycloadditions of dimesitylbisanthene. We also succeeded in the synthesis of substituted precursors of fully aromatic linear type graphene nanoribbons. Efforts to extend π conjugation system from the obtained graphene nanoribbon using similar procedures are currently in progress.