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Electronic structure and charge transfer excited states of a multichromophoric antenna

The electronic structure of a multichromophoric molecular complex containing two of each borondipyrromethane dye, Zn-tetraphenyl-porphyrin, bisphenyl anthracene and a fullerene are studied using density functional theory. The snowflake shaped molecule behaves like an antenna capturing photon at different frequencies and transferring the photon energy to the porphyrin where electron transfer occurs from the porphyrin to the fullerene. Molecular structure of this large complex is first optimized using plane wave projector augmented wave methodology. Subsequent electronic structure calculations are performed using the real space methodology using an all electron pseudopotential basis set containing total of 12478 basis functions. The results show that the HOMO and a state below the HOMO are primarily localized on one of the porphyrins while the LUMO resides mainly on the fullerene component of the complex. The energies of the HOMO and LUMO states in the complex, as adjudged by the ionization potential and the electron affinity values, show significant differences with respect to their values in participating subunits in isolation. We have systematically examined the effect of structural strain and the presence of ligands on the ionization energy and the electron affinity. Finally, we have calculated a few lowest charge transfer energies involving electronic transitions from a the porphyrin component to the fullerene subunit of the complex using the perturbative delta-SCF method. Our predicted value of the lowest charge transfer excited state (1.67 eV) is comparable to the experimental estimate of the charge transfer energy of a similar complex.