Large face to face tetraphenylporphyrin/fullerene nanoaggregates. A DFT study

Abstract

Large face to face tetraphenylporphyrin/fullerene nanoaggregates containing up six C60 units and six tetraphenylporphyrin (H2TPP) or tetraphenylporphyrinato-zinc (TPP-Zn) moieties have been studied using dispersion corrected PBE/def2-SVP level of theory. It has been found that most important contribution to the binding energy between fragments comes from dispersion interactions. The binding energies for Zn containing nanoaggregates are slightly higher than those for metal free ones what is not related to the difference in dispersion contributions to the binding energy but comes from DFT term. Center to center distances for large nanoaggregates are shorter than those for the complexes H2TPP/C60 and TPP-Zn/C60 and this effect is more obvious for metal free nanoaggregates. According to the calculations, the band gap of nanoaggregates barely depends on its size being close to 2eV. The nature of electronic excitations in Zn containing nanoaggregates has strong charge transfer (CT) contribution and does not depend on nanoaggregate size, while for metal free nanoaggregate most of low energy excitations are not CT by nature. Ionization potentials and electron affinities of nanoaggregates depend strongly on their size. Polaron cations are uniformly delocalized over donor H2TPP or TPP-Zn units, while polaron anions are delocalized over acceptor C60 units. The reorganization energies calculated for hole and electron transport decreased linearly with 1/n where n is the number of repeating units in nanoaggregate.