Electronic Structure and Absorption Properties of Strongly Coupled Porphyrin–Perylene Arrays

Porphyrin–perylene arrays
are ideal candidates for light-harvesting
systems capable of panchromatic absorption. In this work, we employ
density functional theory (DFT) and time-dependent DFT to investigate
the unique UV–vis absorption properties exhibited by a series
of ethynyl-linked porphyrin–perylene arrays that were previously
synthesized and characterized spectroscopically [Chem. Commun. 2014, 50, 14512−5]. We find that the ethynyl linker is responsible for
strong electronic coupling of porphyrin and perylene subunits in these
systems. Additionally, these arrays exhibit a low barrier to rotation
around the ethynyl linker (<1.4 kcal/mol per one perylene substituent),
which results in a wide range of molecular conformations characterized
by different porphyrin–perylene dihedral angles being accessible
at room temperature. The best match between the calculated and experimental
UV–vis spectra is obtained by averaging the calculated UV–vis
spectra over the range of conformations defined by the porphyrin−perylene
dihedral angles. Finally, our calculations suggest that the transitions
in the lower energy region (550–750 nm) can be assigned to
the excitations originating from the porphyrin subunit; the mid-energy
region transitions (450–550 nm) are assigned to the perylene-centered
excitations, while the high-energy transitions (350–450 nm)
involve contributions from both porphyrin and perylene subunits.