An sp-hybridized all-carboatomic ring, cyclo[18]carbon: Electronic structure, electronic spectrum, and optical nonlinearity

The electronic spectrum and optical nonlinearity of the sp-hybridized cyclo[18]carbon with novel topology are studied by means of (time-dependent) density functional theory calculations. Molecular orbital analysis shows that the energy levels of σ molecular orbitals of the cyclo[18]carbon are very low, and the energies of two sets of high-lying π molecular orbitals, which are perpendicular with each other, are almost the same. The simulated absorption spectrum shows that the cyclo[18]carbon exhibits two detectable absorption bands corresponding to π→π∗ transition: one strong absorption at 219.4 nm and one weak absorption at 191.4 nm. The considerable oscillator strength of the maximum absorption peak has detailedly investigated via theoretical analysis. Accurate calculation of molecular response properties shows that the cyclo[18]carbon has apparent nonlinear anisotropy due to its two-dimensional looping structure. In particular, the cyclo[18]carbon shows a great second-order hyperpolarizability and displays an obvious optical dispersion in optical nonlinearity. Further analysis of hyperpolarizability density reveals the nature of the great nonlinear optical properties of the studied cyclo[18]carbon.