A DFT analysis of the ground and charge-transfer excited states of Sc3N@Ih-C80 fullerene coupled with metal-free and zinc-phthalocyanine

Endohedral metallofullerenes and phthalocyanine derivatives are recognized as excellent active materials in organic photovoltaics (OPVs). The tri-metallic nitride endohedral C-80 fullerenes have greater absorption coefficients in the visible region and electron-accepting abilities similar to C-60, which can allow for higher efficiencies in OPV devices. In this work, we have investigated the ground and charge transfer excited states of two co-facial donor-acceptor (D-A) molecular conjugates formed by the non-covalent coupling of trimetallic nitride endohedral fullerene (Sc3N@I-h-C-80) with metal-free (H2Pc) and zinc-phthalocyanine (ZnPc) chromophores using DFT calculations. The charge transfer (CT) excitation energies are calculated using the perturbative delta-SCF method that enforces orthogonality between the ground and excited states. The binding energies calculated using the PBE and DFT-D3 methods indicate that the dispersion effects play an important role in the stabilization of these complexes. The ground state dipole moment of the Sc3N@C-80-H2Pc dyad is much larger than that of Sc3N@C-80-ZnPc, but this is reversed in the excited state where the dipole moment of Sc3N@C-80-ZnPc increases significantly. The lowest few excitation energies in the gas phase for the two complexes are very close, in the range of 1.51-2.66 eV for Sc3N@C-80-ZnPc and 1.51-2.71 eV for the Sc3N@C-80-H2Pc complex. However, the lower ionization potential and lower exciton binding energy make the Sc3N@C-80-ZnPc dyad a better candidate for OPVs as compared to the Sc3N@C-80-H2Pc dyad.