Can MP ( P ) 4 Compounds Form Complexes with C 60 ?

Numerous complexes between versatile derivatives of metalloporphyrins MP (with M being Mn, Co, Ni, Cu, Zn and Fe) and C60 have been synthesized and characterized recently. Favorable van der Waals attractions between the curved π-surface of the fullerene and the planar π-surface of MP assist in the supramolecular recognition, overcoming the necessity of matching a concave-shaped host with a convex-shaped guest structure. Recently, we reported the computational studies of the structures and electronic properties of the series of metalloporphyrins where all the four pyrrole nitrogen atoms are replaced with P-atoms, MP(P)4, M = Sc-Zn. Motivated by the numerous examples of the complex formation between regular planar or quasi-planar MP and C60, we computationally investigated possibility of the complex formation between two MP(P)4 species, ZnP(P)4 and NiP(P)4, and C60 without any linkers, using the CAMB3LYP/6-31G* approach, both in the gas phase and with implicit effects from C6H6. We found that the binding energies in the MP(P)4-C60 complexes for these two MP(P)4 compounds are relatively low, ca. 1-1.6 kcal/mol and ca. 5 kcal/mol for M = Zn and Ni, respectively. The ZnP(P)4 species was found to be noticeably distorted in the ZnP(P)4-C60 complex whereas NiP(P)4 inside the NiP(P)4-C60 complex essentially retained its bowl-like shape. Thus, we showed the possibility of the formation of complexes between MP(P)4 species and C60 without any linkers and showed dependence of the complex stability on the transition metal M. Further investigations are in progress.


INTRODUCTION
Porphyrins are among the most studied macrocyclic systems because of their prominent characteristics including photochemical and photophysical properties [1].The porphyrin molecule has an 18π-electrons conjugated system within a square-planar or quasi planar framework giving rise to the aromatic character responsible for the porphyrin species stabilization [1].Also, porphyrins are important chromophores that absorb visible light and are good candidates for photoelectronic materials which are used, for instance, in photosensitized solar cells [2,3].The rich and extensive absorptions (i.e., π-π* transitions) of porphyrins hold particular promise for an efficient use of the solar spectrum.During recent years porphyrins have attracted significant attention as light-harvesting building blocks for the construction of molecular architectures [1].
Fullerene C 60 , on the other hand, is an excellent novel three-dimensional multielectron acceptor with a favorable reduction potential (−0.169 V).It has played an important role in the development of molecular building blocks for photovoltaic applications [4,5].High electron affinity combined with low reorganization energy in charge-transfer processes make the fullerenes ideal electron acceptors [6].The delocalization of charges within the spherical carbon framework (diameter > 7.5 Å) together with the rigid, confined structure of the aromatic π-sphere offers unique opportunities for stabilizing charged entities.
A variety of supramolecular assemblies with porphyrin moieties was proposed as functional models for light-harvesting and photoinduced electron transfer systems [7][8][9].Porphyrins and fullerenes are complementary compounds due to their strongly aromatic large structures, and thus they can form molecular architectures with strong π−π interactions [10,11].Also, formation of such supramolecular assemblies has been shown to occur due to metal−ligand bonds [12,13], hydrogen bonds [14][15][16], electrostatic interactions [17], mechanical bonds [18,19], or a combination of several of these interactions [20][21][22].The assemblies of porphyrins and fullerenes are considered as one of the most extensively organic donor−acceptor pairs studied [23][24][25][26][27][28][29][30][31][32][33][34].So far, numerous complexes between versatile derivatives of metalloporphyrins MP (with M = Mn, Co, Ni, Cu, Zn, and Fe) or a free porphyrin and C 60 have been synthesized and characterized .Virtually any functional group can be covalently linked to the highly reactive C 60 carbon framework.As mentioned above, the formation of MP-C 60 complexes involves the utilization of strong π−π interactions between metalloporphyrins and fullerenes.Favorable van der Waals attractions between the curved π-surface of the fullerene and the planar π-surface of MP assist in the supramolecular recognition, overcoming the necessity of matching a concave-shaped host with a convexshaped guest structure.This leads to complexes with unusually short contacts (2.7-3.0Å), shorter than ordinary van der Waals contacts (3.0-3.5 Å), and a variety of crystal structures [see, for instance, Ref. 10].
Recently, we reported the computational studies of the structures and electronic properties of the series of metalloporphyrins where all the four pyrrole nitrogens are replaced with P-atoms, MP(P) 4 , M = Sc-Zn [36][37][38].We showed that the prominent structural feature of all the MP(P) 4 compounds is the significant bowl-like distortion from planarity.Furthermore, motivated by the phenomenon of stack formation by regular metalloporphyrins, we performed the computational check of the stack formation between the MP(P) 4 species without any linkers or substituents, choosing the ZnP(P) 4 species as the simplest MP(P) 4 compound [39].For the current research, we were motivated by the numerous examples of the complex formation between regular planar or quasi-planar MP and C 60 , along with the recent report by Anand and co-workers about the complex formation between the dimer of antiaromatic tetraoxaisophlorin and C 60 [40].Thus, we decided to computationally investigate possibility of the complex formation between MP(P) 4 species, taking as examples ZnP(P) 4 and NiP(P) 4 compounds, and C 60 without any linkers or substituents.
The paper is organized as follows.The next section describes computational details of the study.Then, we consider the results obtained and make discussion.Finally, conclusions and perspectives are provided.

COMPUTATIONAL DETAILS
The study described here was performed using the Gaussian 09 package [41].The MP(P) 4 (M = Zn, Ni), C 60 , and MP(P) 4 -C 60 species were optimized without any symmetry constraints, and the resulting structures were assessed using vibrational frequency analysis to probe whether they represent true minimum-energy geometries.We performed the geometry optimizations and frequencies calculations using the Handy and coworkers' long range corrected version of B3LYP, CAM-B3LYP [42] with the split-valence 6-31G* basis set [43,44], further referred to as CAM-B3LYP/6-31G*.So far, different theoretical approaches have been used to study the complexes between porphyrins and their derivatives and fullerenes, for example: pure generalized gradient approximation (GGA) functional PBE, global hybrid meta-GGA functional M06, hybrid functional B3LYP, and wB97XD hybrid density functional including empirical atomic-pairwise dispersion corrections following the Grimme's D2 dispersion scheme (D) by Baruah and co-workers in the studies of systems based on fullerene/graphene oxide and porphyrin/smaragdyrin [33]; hybrid functional B3LYP in the recent study of noncovalent interactions between C 60 -dipyridyl and zinc porphyrin dimer by Stangel et al. [45]; B97-D3 functional including dispersion corrections in the study of supramolecular ensembles of conjugated porphyrin dimers with C 60 by Moreira et al. [32]; PBE with included semi-empirical 'DFT+D3' term in the studies of fullerene-porphyrin supramolecular nanocables by Buldum and Reneker [31]; CAM-B3LYP along with wB97XD, TPSSh, M06, and M06L functionals in the studies of Rhoda et al. on noncovalent complexes of subphthalocyanine derivatives with C 60 and C 70 fullerenes [46].We decided to choose the CAM-B3LYP approach because of its superiority over the standard "pure" and "hybrid" exchange-correlation functionals in treating noncovalent weak long-distance interactions which should take place in the studied complexes.
Computational studies were performed using the CAM-B3LYP/6-31G* approach, both in the gas phase and with implicit effects from C 6 H 6 (dielectric constant 2.2706).With implicit solvent effects the geometries and vibrational frequencies were calculated using the self-consistent reaction field IEF-PCM method [47] (the UFF default model used in the Gaussian 09 package, with the electrostatic scaling factor α set to 1.0).
The binding energies (E bind ) of the complexes studied were computed using the following formula: E bind = E(complex) -E(C 60 ) -E(MP(P) 4 ), where E(complex) is the energy of the MP(P) 4 -C 60 species and E(C 60 ) and E(MP(P) 4 ) are the energy of C 60 and MP(P) 4 , respectively.Below we consider the gas phase and implicit solvent results without the zeropoint correction ZPE (ΔE 0 ).The charge analysis was performed using the Natural Bond Orbital (NBO) scheme with the 'pop=nbo' command as implemented in the Gaussian 09 package [48].Molecular structures were visualized using OpenGL version of Molden 5.0 visualization software [49].The work was partially done using the Extreme Science and Engineering Discovery Environment (XSEDE) [50].

RESULTS AND DISCUSSION
Figure 1 shows the calculated structures of the complexes ZnP(P) 4 -C 60 and NiP(P) 4 -C 60 and Table 1 provides selected computed data for these complexes along with MP(P) 4 and C 60 .Interestingly, in the NiP(P) 4 - C 60 species the bowl-like shape of NiP(P) 4 is generally preserved ("an apple at a saucer" structure) whereas in the ZnP(P) 4 -C 60 complex the ZnP(P) 4 molecule is quite noticeably distorted ("an apple on a crumpled paper plate" structure).It should be noticed that for the ZnP(P) 4 -C 60 complex similar distortions were observed when we did test calculations at the lower levels of theory, too (B3LYP/3-21G* and B3LYP/6-31G*).A closer look at the two complexes and comparison with the bare MP(P) 4 species shows the following.(i) In ZnP(P) 4 -C 60 , the two Zn-P bond distances, Zn-P 3,4 (Figure 1), shorten by 0.09 Å compared to the Zn-P bond distances in the free ZnP(P) 4 species (cf.Table S1 in Supporting Information), and the two other Zn-P bond distances, Zn-P 1 and Zn-P 2 , are shortened by 0.04 and 0.03 Å, respectively.The phosphole unit containing P 2 becomes tilted oppositely relative to the other three phospholes in the porphyrin ring, and the C 60 -P 2 distance, 3.67 Å, is the shortest distance between ZnP(P) 4 and fullerene in this complex (compare with 4.08 Å between Zn and C 60 , see Figure 1).The P-Zn-P bond angles in the complex are smaller by ca. 14 and 9 o compared to free ZnP(P) 4 (Table S1), and dihedral angles (P-P-P-Zn) increase by ca. 10 o .
While in the free ZnP(P) 4 Zn-center is slightly "in" the plane formed by the four P-atoms, in the complex it is located essentially inside this plane.(ii) In NiP(P) 4 -C 60 , all the Zn-P bond distances have the same length as in the free NiP(P) 4 species (Table S1).The P-Ni-P bond angles in the complex are essentially the same as in free NiP(P) 4 (Table S1), and dihedral angles (P-P-P-Ni) are just diminished by merely 0.7 o (see Table S1).Both in the free NiP(P) 4 and in the complex Ni retains slightly "out" of the plane formed by the four P-atoms.The P-C 60 distances in this complex vary from 3.91 to 4.21 Å (Figure 1), being longer than the Ni-C 1,2 distances, 3.85 and 3.90 Å, respectively (Figure 1).(iii) Comparison of the two complexes shows that ZnP(P) 4 -C 60 should be considered as essentially having C s symmetry, and NiP(P) 4 -C 60 as having C 2v symmetry.We agree that the calculated M-C 60 distances in our complexes, 3.85-4.08Å, and the P-C 60 distances, 3.67-4.21Å, should be considered as relatively long, compared to the previously reported values for other complexes, for instance, experimentally determined distances of 2.7-3.0Å [10], or computed distances of 2.88-3.14Å (metal•••C 60 distances) [32], or experimentally determined very close contacts (2.58 and 2.61 Å) between the isophlorin π-surface and the curved surface of C 60 [40].However, the differences in considered complexes and in the computational approaches employed should be taken into account here.
Considerations of the C 6 H 6 calculated energies of the HOMO/LUMO and HOMO-LUMO gaps for the MP(P) 4 -C 60 complexes and free C 60 and MP(P) 4 show the following upon the complex formation: (i) the HOMO-LUMO gaps become noticeably smaller, decreasing by 1.23(Zn)/0.42(Ni)eV when compared to free C 60 and by 0.26(Zn)/0.22(Ni)eV when compared to free MP(P) 4 (we understand that calculating the HOMO-LUMO gaps as energy differences of HOMO and LUMO should be taken with caution, but our task here is to show general trends in change of the compounds properties); (ii) the MP(P) 4 -C 60 HOMO is somewhat destabilized for M = Zn and slightly stabilized for M = Ni compared to free MP(P) 4 ; (iii) the MP(P) 4 -C 60 LUMO is stabilized for both M compared to free MP(P) 4 ; (iv) the MP(P) 4 -C 60 HOMO is noticeably destabilized for both M compared to free C 60 ; (v) the MP(P) 4 -C 60 LUMO is noticeably stabilized for M = Zn and slightly destabilized for M = Ni compared to free C 60 .So, the LUMO energies of the two complexes are quite close to the LUMO energy of C 60 , whereas the HOMO energies of the two complexes are quite close to the HOMO energies of the MP(P) 4 (see Table 1 and Table S1).Thus, the HOMO of the MP(P) 4 -C 60 complexes would have dominating contributions from the MP(P) 4 moieties, and the LUMO of the MP(P) 4 -C 60 complexes would have dominating contributions from the fullerene part.Therefore, charge-transfer interactions within such complexes could have nonzero possibility, which, in turn, might result in appearing the charge-transfer absorption band in the spectra of these complexes.To further analyze the possibilities for the charge transfer within the complexes, we performed the analysis of the NBO charges of the MP(P) 4 -C 60 complexes and free MP(P) 4 .This analysis shows the following: (i) in ZnP(P) 4 -C 60 , the charge on the Zn-center becomes slightly smaller, by 0.07 e, whereas the charge on the P 2 -center, the closest one to the C 60 , increases noticeably, by 0.16 e, the charges on the P 3,4 -centers increase by 0.04 e, and the charge on the P 1 -center decreases by 0.04 e.Thus, there apparently exists certain charge transfer between the Zn-center and C 60 , between the P 2 -center and C 60 , and between the total P4-unit and the rest of the porphyrin moiety.The NBO calculations show some small charges appearing on the C 60 atoms in the ZnP(P) 4 -C 60 complex but they are generally in the order of 0.001 e. (ii) In NiP(P) 4 -C 60 , the charge on the Ni-center becomes slightly smaller, by 0.04 e, whereas the charges on the P-centers retain the same as in the free NiP(P) 4 .Thus, the NBO analysis did not give the conclusive response to the question about the chargetransfer interactions within the complexes studied.Further, more profound, investigation will be necessary to clarify this issue.
Finally, consideration of the binding energies of the two complexes shows that they are indeed bound by the relatively weak van der Waals interactions: ZnP(P) 4 -C 60 has the binding energy calculated in implicit benzene mere 1.1 kcal/mol but a little higher E bind in the gas phase, 1.6 kcal/mol (see Table S1) whereas NiP(P) 4 -C 60 is slightly stronger bound complex, with the E bind 4.7 kcal/mol in the implicit C 6 H 6 and 4.8 kcal/mol in the gas phase (Table S1).This could be explained by shorter distances between the Ni-center and C 60 unit (see Figure 1) compared to the ZnP(P) 4 -C 60 complex, apparently due to the smaller size of the Ni-center compared to Zn-center, and maybe also by negative charge on the Ni-center which might be -probably -donated to extended high-lying unoccupied orbitals of C 60 .It would require further studies to figure out the exact picture of interactions between MP(P) 4 species and C 60 leading to the formation of ZnP(P) 4 -C 60 and NiP(P) 4 -C 60 .Currently, we have demonstrated the possibility of the complex formation between the MP(P) 4 species and C 60 leading to the formation of MP(P) 4 -C 60 complexes.

CONCLUSIONS AND PERSPECTIVES
Motivated by the numerous examples of the complex formation between regular planar or quasiplanar metalloporphyrins and their derivatives and C 60 , along with the recent report by Anand and co-workers about the complex formation between the dimer of antiaromatic tetraoxaisophlorin and C 60 [40], we decided to computationally investigate possibility of the complex formation between MP(P) 4 species, taking as examples ZnP(P) 4 and NiP(P) 4 compounds, and C 60 without any linkers or substituents.We have demonstrated the possibility of the complex formation between the MP(P) 4 species and C 60 leading to the formation of MP(P) 4 -C 60 complexes for M = Zn and Ni.In our computational studies, the CAM-B3LYP/6-31G* approach, both in the gas phase and with implicit effects from C 6 H 6 , was employed.
We found that the two MP(P) 4 -C 60 complexes for M = Zn, Ni, are indeed bound by the relatively weak van der Waals interactions: ZnP(P) 4 -C 60 has the binding energy calculated with implicit effects from C 6 H 6 mere 1.1 kcal/mol (1.6 kcal/mol in the gas phase) whereas NiP(P) 4 -C 60 was calculated to be slightly stronger bound complex, with the E bind 4.7 (4.8) kcal/mol.This could be explained by shorter distances between the Ni-center and C 60 unit compared to the ZnP(P) 4 -C 60 complex, apparently due to the smaller size of the Nicenter vs. Zn-center, and maybe also by charge on the Ni-center which might be -probablydonated to extended high-lying unoccupied orbitals of C 60 .The clarification of the picture of interactions between MP(P) 4 species and C 60 requires further studies.
The ZnP(P) 4 unit was found to be noticeably distorted in the ZnP(P) 4 -C 60 complex giving rise to the "apple on the crumpled paper plate" structure whereas NiP(P) 4 inside the NiP(P) 4 -C 60 complex essentially retained its bowl-like shape giving rise to the "apple at the saucer" structure.Distances between M and C 60 were computed to be 3.85-3.90Å (for M = Ni) and 4.08 Å (for M = Zn).The shortest distances between MP(P) 4 and C 60 were calculated to be ca.3.67 (for M = Zn) and 3.91 (for M = Ni) Å. Upon the complex formation, the HOMO-LUMO gaps become noticeably smaller, decreasing by 1.23(Zn)/0.42(Ni)eV when compared to free C 60 and by 0.26(Zn)/0.22(Ni)eV when compared to free MP(P) 4 .In the ZnP(P) 4 -C 60 complex, there apparently exists certain charge transfer between the Zn-center and C 60 , between the P 2 -center and C 60 , and between the total P4-unit and the rest of the porphyrin moiety.In NiP(P) 4 -C 60 , the charge on the Ni-center becomes slightly smaller than in the free NiP(P) 4 whereas the charges on the P-centers retain the same as in the free NiP(P) 4 .
Based on the results obtained, the following questions to be answered can be formulated: (iii) Which approaches could be used to facilitate the complex formation between MP(P) 4 and C 60 ?Could further porphyrin core-modification (replacing P with other elements or using porphyrins with smaller number of coremodifying atoms) or adding various electrondonating/withdrawing substituents be helpful?

Figure 1 :
Figure 1: Structures of the complexes ZnP(P)4-C60 (left) and NiP(P)4-C60 (right) calculated using the CAM-B3LYP/6-31G* approach with implicit effects from C6H6 included.The color coding is as follows: dark brown for C, light grey for H, dark blue for P, light blue for Zn, and dark grey for Ni.Bond distances are given in Å, bond angles and dihedral angles are given in degrees.
(i)How will the binding and structures in MP(P) 4 -C 60 complexes depend on the M nature (main group element or transition metal)?How will charge and size of M and number of unpaired delectrons of M (spin state) affect the MP(P) 4 -C 60 complexes formation?(ii) What is the exact nature of bonding between MP(P) 4 species and C 60 ?

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iv) What are charge accumulation/charge transfer and photophysical properties of MP(P) 4 -C 60 complexes?Some research projects aimed to answer these questions are ongoing currently.ACKNOWLEDGEMENTSThis work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) grant "Estudo Teórico Computacional de Sistemas Nanoestruturados com Potencial Aplicação Tecnológica", number 402313/2013-5, approved in the call N° 70/2013 Bolsa de Atração de Jovens Talentos -BJT -MEC/MCTI/CAPES/CNPq/FAPs / Linha 2 -Bolsa de Atração de Jovens Talentos -BJT.The computational resources of the centers Centro Nacional de Processamento de Alto Desempenho em São Paulo, Núcleo de Tecnologia da Informação, and Centro Nacional de Processamento de Alto Desempenho -UFC are highly apprecaited.This work also used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575.