Vibrational Characterization of Two-Dimensional Graphdiyne Sheets

Graphdiyne is formed by two acetylene bonds conjugatively connecting two phenyl rings, which then extend to a sizable two-dimensional structure. In this work, a molecular size-dependent C≡C stretching vibrational transition intensity is predicted by density functional theory in graphdiyne; in particular, the strongest infrared-active transition is enhanced in intensity by a factor of as high as 2190 from phenyl-acetylenic dimer to graphdiynic 46-mer, showing ca. 300 times enhancement per acetylene bond on average, examined at the level of B3LYP/6-31G*. Such an enhancement of vibrational transition intensity is caused by intramolecular electronic delocalization that gives rise to an enlarged transition dipole moment as well as by intramolecular vibrational delocalization that gives rise to intensity borrowing in such two-dimensionally conjugated graphdiyne molecular sheets. The enhancement is found to be diminished in defected graphdiynes. The results suggest that the periodically appearing C≡C bond may be used as a vibrational spectroscopic marker for assessing the size of perfect graphdiyne oligomers, and the characteristic C≡C stretching absorption can potentially be used to differentiate perfect graphdiyne sheets from defected ones.