Linear and nonlinear optical properties of [60]fullerene derivatives.

Using a wide variety of quantum-chemical methods we have analyzed in detail the linear and non-linear optical properties of [60]fullerene-chromophore dyads of different electron-donor character. The dyads are composed of [60]fullerene covalently linked with 2,1,3-benzothiadiazole and carbazole derivatives. Linear scaling calculations of molecular (hyper)polarizabilities were performed using wave function theory as well as density functional theory (DFT). Within the former approach, we used both semiempirical (PM3) and ab initio (Hartree-Fock and second-order Møller-Plesset perturbation theory) methods. Within the latter approach only the recently proposed long-range (LRC) schemes successfully avoid a large overshoot in the value obtained for the first hyperpolarizability (β). Calculations on model fullerene derivatives establish a connection between this overshoot and the electron-donating capability of the substituent. Substitution of 2,1,3-benzothiadiazole by the triphenylamine group significantly increases the electronic first and second hyperpolarizabilities as well as the two-photon absorption cross section. For [60]fullerene-chromophore dyads we have, additionally, observed that the double harmonic vibrational contribution to the static beta is much larger than its electronic counterpart. The same is true for the dc-Pockels β as compared to the static electronic value, although the vibrational term is reduced in magnitude; for the intensity-dependent refractive index the vibrational and electronic terms are comparable. A nuclear relaxation treatment of vibrational anharmonicity for a model fulleropyrrolidine molecule yields a first-order contribution that is substantially more important than the double harmonic term for the static β.