  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.
