Polarization effects in push-pull buckminsterfullerenes: A semiempirical study

We predict that push-pull Buckminsterfullerenes have a high nonlinear optical response that makes pursuing their synthesis worthwhile. Three different semiempirical quantum chemical models concur to show that some isomers of push-pull C-60 have a static first-order hyperpolarizability, beta, comparable to or larger than that of all-trans 4-(dimethylamino)-4'-nitrodiphenyloctatetraene. Because of the geometrical structure of push-pull Buckminsterfullerenes, standard models that explain the high response in planar conjugated systems cannot be used. Rather, it is found that conjugation and inductive effects contribute at the same time to beta and the separation between the nitrogen-containing groups cannot fully simulate the results of the calculations. Although these complicated materials do not easily lend themselves to simple modeling, some of the correlations of beta with electronic energies, dipole moments, bond orders, and other quantities of physical interest allow us to sift through the different contributions and partially simulate the relative ordering of the hyperpolarizabilities of the various isomers. A simple perturbation approach based on icosahedral C-60 further confirms the results obtained by the correlations and warrants the search of a topological model able to account for the response. It is found that a linear relation exists between beta and the shortest resonant paths between the grafting groups. Two types of resonant structures contribute. They are described in terms of single-double bond alternation. The first resonant structure starts and finishes with hexagon-hexagon alternation (i.e., double bonds) and contributes positively, the second starts and finishes with pentagon-hexagon (i.e., single bonds) and contributes negatively. The ratio of their contributions is roughly two to one.