Large face to face tetraphenylporphyrin/fullerene nanoaggregates. A DFT study Ulises Jiménez Castillo, Patricia Guadarrama, Serguei Fomine ⇑ Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, CU, Coyoacán, Mexico DF 04510, Mexico article info Article history: Received 13 March 2013 Received in revised form 2 May 2013 Accepted 19 May 2013 Available online 11 June 2013 Keywords: Porphyrin Fullerene DFT Dispersion correction Reorganization energy 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/C 60 and TPP-Zn/C 60 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 2 eV. 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 C 60 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. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Donor/acceptor functional nanoaggregates has at- tracted recently much attention as candidates for photo- voltaic cells, optoelectronic devices, and applications in artificial photosynthesis [1–6]. The photoinduced electron transfer (PET) from donor to acceptor is the key step for most of those applications. In fact, the core of natural pho- tosynthesis is a multistep PET along the nanoaggregates of donor and acceptors. The most popular building blocks as electron donors in artificial photosynthetic models by obvious reasons are porphyrins, while fullerenes are excel- lent multielectron acceptors. Porphyrin and fullerene form a very tight complex. Thus, the separation of fullerene carbon and the porphyrin plane is only 2.75 Å, which is notably less than typical separation between molecules experienced p–p interactions which is normally of the or- der 3.3–3.5 Å. Evidently, the interactions between porphy- rin and fullerene are very significant and could be a driving force for the formation of fullerene/porphyrin nanoarrays. In fact, cocrystallization of fullerene with metalloporphy- rin or free base porphyrin frequently leads to the formation of an alternating zigzag motif [7]. Up to date a number of porphyrin/fullerene dyads [8–19] triads [20–24] and oligo- mers involving covalent or noncovalent linkages have been prepared. A recent discovery that among two most com- mon porphyrin/fullerene alignments: face to face and edge to face, the first one allows the PET rates 10 times faster than edge to face orientation [16,25] lead to the synthesis of face to face porphyrin/fullerene nanowires [26]. There- fore, our goal is to study electronic structure of large face 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.05.032 ⇑ Corresponding author. Tel.: +52 56224726. E-mail address: fomine@unam.mx (S. Fomine). Organic Electronics 14 (2013) 2617–2627 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel