Brominated positions on graphene nanoribbon analyzed by infrared spectroscopy

Determination of the number of bromo groups and brominated positions of edges on carbon materials is essential for selective functionalization because the brominated positions influence their electronic structures. However, the determination is challenging because the analysis of introduced bromo groups on carbon materials is conventionally limited to quantitative analysis by X-ray photoelectron spectroscopy. In this work, infrared spectra (IR) of brominated aromatic compounds were analyzed experimentally and theoretically using reference aromatic compounds to verify that the calculated peak positions can be used to estimate the experimental peak positions of brominated aromatic compounds. IR spectra of brominated graphene nanoribbons (GNRs) with zigzag, armchair, and other edges were simulated as one of the carbon materials with clear edge structures. Their characteristic peak positions and tendencies of shifts were explained from the point of view of Mulliken charge, C–H and C=C length, and orbital hybridization. As the number of Br on GNRs increased, the peak positions basically shifted further, indicating that the density of Br and introduced positions of Br can be estimated from IR spectra. The detailed assignments obtained in this work will lead to selective functionalization of carbon materials.