Open‐Shell Characters, Aromaticities and Third‐Order Nonlinear Optical Properties of Carbon Nanobelts Composed of Five‐ and Six‐Membered Rings.

Geometric features, electronic structures and third‐order nonlinear optical (NLO) properties of belt‐shaped hydrocarbons, namely, carbon nanobelts, are investigated by using the density functional theory (DFT) methods. A carbon nanobelt composed of five‐ and six‐membered rings (IF‐CNB, 42π), which involves repeating indenofluorene (IF) units, is expected to exhibit both open‐shell multiradical character and relatively strong aromatic nature in the singlet state, while that composed only of six‐membered rings (CNB, 48π) is shown to be a closed‐shell weak antiaromatic species. These features of electronic structure of IF‐CNB are shown to be closely related to its p‐quinodimethane (pQM)‐like substructures and several geometric features (bond‐length alternations and symmetry) of the system. Third‐order NLO properties of open‐shell IF‐CNB is expected to be enhanced more than three times as large as those of the closed‐shell CNB. These results indicate the possibility of further multi‐functionalizations of belt‐shaped hydrocarbons by fusing five‐membered rings. A theoretically designed carbon nanobelt composed of five‐ and six‐membered rings (IF‐CNB, 42π) involving repeating indeno[1,2‐b]fluorene units is found to have a potential for exhibiting open‐shell multiradical character, strong aromatic nature and large third‐order nonlinear optical properties in the singlet state based on broken symmetry density functional theory calculations. [ABSTRACT FROM AUTHOR]