A large enhancement of (hyper)polarizabilities of polyyne capped by Cu

We calculated the dipole polarizability (α) and second hyperpolarizability (γ) of the linear CuC2nCu and CuC2nH (n = 1–7) molecules. The determination of geometrical parameters of these molecules was based on the experimental geometry of CuC2H and the ab initio calculations. We find that the cap of Cu largely enhance the nonlinear optical responses for the short chain molecule. However, this enhancement rapidly decreases and trends to vanish at the effective conjugation length. These behaviors were understood by the redistribution of charge driven by an external field and the natural bond orbital delocalization analysis.