Computational approaches to charge transfer excitations in a zinc tetraphenylporphyrin and C70 complex

Electronic charge transfer plays an important role in novel nanostructured photovoltaic materials. Standard density functional theory (DFT) as well as time-dependent DFT severely underestimate the excitation energies related to such transitions. For the paradigmatic case of a donor-acceptor dyad formed by a zinc-porphyrin donor and a C(70) fullerene acceptor these shortcomings are illustrated. A better quantitative estimate of the charge transfer energy is obtained from constrained DFT which is applied to this system in a novel implementation based on a penalty functional.