Third order NLO properties of corannulene and its Li-doped dimers: effect of concave-convex and convex-convex structures

Buckybowls involving pi-pi interactions offer exciting future opportunities in terms of designing novel smart nonlinear optical (NLO) materials. Dimeric species of corannulene (C20H10), the smallest buckybowl, was considered as a model for the host-guest assemblies, which exhibited the convex-concave stacking of curved conjugated carbon surfaces. Different stacking motifs (concave-convex and convex-convex dimers) of C20H10 dimers that affect the NLO properties are the focus of our study. We performed density functional theory calculations on the structure, binding interactions, electronic absorption spectra and second hyperpolarizabilities of pi-stacking dimers of C20H10 and its Li-doped derivative. It was found that the concave-convex dimers exhibit stronger binding interactions because of larger electrostatic interactions and thus are more stable with respect to the convex-convex dimers. The doping of Li ion significantly enhances the orbital interaction between the monomers but slightly affects the spectra and the second hyperpolarizabilities of the dimers. The convex-convex dimers exhibit larger polarizabilities and second hyperpolarizabilities (gamma(zzzz)) as a result of enhanced interlayer charge transfer properties. There is an increasing linear relationship between the electronic coupling and gamma(zzzz) values. The results presented in this article provide important evidence for the convex-convex stacking motif that enhances the NLO properties of the pi-stacking dimers. Thus, controlling molecular stacking is an important way in terms of designing novel smart NLO materials.