  We have calculated several low-lying Charge Transfer (CT) excited-state
energies for four non-covalently bound dyads composed of a sulfonated-ZnPc
coupled to C60. Our results show that the di- and tri-sulfonated systems yield
a CT state as the lowest-energy excited state in the system. In contrast, an
energy re-ordering for the tetra-sulfonated ZnPc system leads to local
excitations lying lower in energy than the CT state, displaying a possible
deactivation pathway obstructing charge separation. Since several different
donor-acceptor relative orientations may co-exist at an organic heterojunction,
we compare the energetics of a few low-lying CT states for the end-on geometry
of a di-sulfonated system to its co-facial orientation counterpart. The
calculated CT excitation energies are larger for the end-on orientation in
comparison to the co-facial structure by ~1.5 eV, which results principally
from a substantial decrease in exciton binding energy in going from the
co-facial to the end-on orientation. Furthermore, changes in relative
donor-acceptor orientation have a larger impact on the CT energies than changes
in donor-acceptor distance. TDDFT calculations on the various sulfonated ZnPc
donor molecules show a significant splitting of the Q-band for only one of the
four donor systems. Our present calculations, in line with previous
experimental studies, show that the systematic variation of chemical functional
groups is a promising avenue for the substituent-level tuning of various
physical properties of organic semiconductors.
