Size effects of Möbius carbon nanobelts (MCNBs) from the (7-7)MCNB to the (21-21)MCNB on their aromatic properties and third order nonlinear optical properties†

Möbius carbon nanobelts (MCNBs) have attracted the attention of more and more researchers because of their torsional π conjugated structures and Möbius aromatic characteristics. The Möbius rule showed that 4n π-electron MCNBs were aromatic. Several MCNBs have been successfully synthesized experimentally recently. However, the theoretical studies on the third-order NLO properties and excited state electron transition characteristics of MCNBs have not been reported. We used density functional theory to study the structure, molecular orbitals, aromaticity, absorption spectra and third-order nonlinear optical properties of the MCNB rings. The results show that the aromatic properties of the ring come from the twisted benzene ring on the ring. With the increase of the number of benzene rings and the volume of the rings, the charge transfer region of the MCNBs gradually concentrates on the twisted benzene ring, and the maximum absorption spectrum shows a small red shift. The polarizability α_tot values of the MCNBs increased as the number of benzene rings increased from 751.4 a.u. (1) to 3168.0 a.u. (8). And the second-order hyperpolarizability γ_tot values of the MCNBs increased from 7.4 × 10⁵ a.u. (1) to 2.2 × 10⁷ a.u. (8). The results showed that the larger NLO response depends on the size of MCNBs. This work provided the novel interplay between the size of MCNBs and the third order NLO responses and provided a theoretical basis for the MCNBs as an excellent nonlinear optical material.