Excited States of Light-Harvesting Systems Based on Fullerene/Graphene Oxide and Porphyrin/Smaragdyrin

In this work, we present a theoretical study at the density functional theory (DFT) level and time-dependent DFT of the ground and singlet excited states of electron donor–acceptor complexes formed by porphyrin (TPP)/smaragdyrin (TPOS) (expanded porphyrin), as light-harvesting systems, and fullerene (C60)/graphene oxide (GO), as acceptor nanocarbon structure. We investigate the effect of the nanocarbon on UV–vis electronic absorption of porphyrin/smaragdyrin, using the functionals B3LYP, PBE, M06, and wB97XD. The results showed the lowest deviation of the Q-band for the functional M06 (0.01–0.02 eV). Electronic absorption spectra calculated for the complexes with M06 predict that (a) graphene oxide increases the intensity of the Soret band, (b) fullerene produces a red-shift of the Q bands (4 nm) with respect to graphene oxide, and (c) smaragdyrin causes a red-shift of Q (27–48 nm) and Soret (37 nm) absorption bands as compared to porphyrin. We also investigate the effect of the nanocarbon structure on the charge-transfer (CT) excited states. Using the perturbative delta-SCF method with the PBE functional, we found that the charge-transfer excitation energy increases as TPOS-C60 (2.53 eV) < TPP-GO (2.89 eV) < TPP-C60 (3.01 eV) < TPOS-GO (3.28 eV). The presence of a nanocarbon structure affects more strongly smaragdyrin (∼0.8 eV) than porphyrin (∼0.1 eV). We show that the binding between smaragdyrin and fullerene C60 favors the charge separation states with a lower energy cost, which means that these systems present an advantage for its application in photovoltaic cells.