Size dependence of optical nonlinearity for H-capped carbon chains, H-(CC)-H ( = 3-15): analysis of its nature and prediction for long chains.

Based on a computational approach that can accurately describe their geometric structures and electronic spectra, we have theoretically studied the nonlinear optical (NLO) properties of H-capped carbon chains, H-(CC)-H ( = 3-15), for the first time. Special attention was paid to the size dependence of the molecular (hyper)polarizability of these species through the nonlinear fitting of the data, which formed two power-law formulas of (∞) = -0.206 + 0.264 and (∞) = -0.624 + 0.006 and was thoroughly discussed at the electronic structure level by in-depth wavefunction analyses. The fundamental gap (Δ) between vertical ionization energy (VIE) and vertical electron affinity (VEA) is found to be related to the molecular (hyper)polarizability. The calculated (hyper)polarizability of the carbon chais H-(CC)-H ( = 3-15) is more sensitive to the density functional theory (DFT) applied than to the basis set selected. The results are expected to provide theoretical guidance for the property prediction of arbitrarily long carbon chains not yet synthesized.