Shedding Light on the Nature of HostâGuest Interactions in PAHs

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Article

Shedding Light on the Nature of Host-Guest Interactions in PAHs-ExBox Complexes 4+

Glaucio R. Nagurniak, Giovanni Finoto Caramori, Renato Luis Tame Parreira, Pedro Augusto de Souza Bergamo, Gernot Frenking, and Alvaro Muñoz-Castro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04844 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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The Journal of Physical Chemistry

Shedding Light on the Nature of Host-Guest Interactions in PAHs-ExBox4+ Complexes Glaucio R. Nagurniak,† Giovanni F. Caramori,∗,† Renato L. T. Parreira,‡ Pedro A. S. Bergamo,‡ Gernot Frenking,¶ and Alvaro Muñoz-Castro§ Departamento de Qu´ımica, Universidade Federal de Santa Catarina, Campus Universit´ ario Trindade, 88040-900 Florian´ opolis, SC, Brazil., N´ ucleo de Pesquisas em Ciências Exatas e Tecnol´ ogicas, Universidade de Franca, Franca, SP, Brazil., Fachbereich Chemie Philipps-Universit¨ at Marburg, Hans-Meerwein-Strasse, D-35032, Marburg, Germany., and Lab. de Qu´ımica Inorg´ anica y Materiales Moleculares, Universidad Autonoma de Chile, Llano Subercaceaux 2801, San Miguel, Santiago, Chile. E-mail: [email protected]

∗

To whom correspondence should be addressed Departamento de Qu´ımica, Universidade Federal de Santa Catarina, Campus Universitário Trindade, 88040-900 Florianópolis, SC, Brazil. ‡ N´ ucleo de Pesquisas em Ciências Exatas e Tecnológicas, Universidade de Franca, Franca, SP, Brazil. ¶ Fachbereich Chemie Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032, Marburg, Germany. § Lab. de Qu´ımica Inorg´ anica y Materiales Moleculares, Universidad Autonoma de Chile, Llano Subercaceaux 2801, San Miguel, Santiago, Chile. †

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Abstract Host-Guest systems formed by polycyclic aromatic hydrocarbons and ExBox4+ are suitable models to gain a deeper understanding in π-π interactions, which are fundamental in supramolecular chemistry. The physical nature of Host-Guest (HG) interactions between ExBox4+ (1) and polycyclic aromatic hydrocarbons (PAHs) (2-12) is investigated at the light of the energy decomposition (EDA-NOCV), non-covalent interactions (NCI), and magnetic response analyses. The EDA-NOCV results show that the dispersion forces play a crucial role on the HG interactions in PAHs⊂ExBox4+ complexes. The HG interaction energies are dependent on both, the size of the PAH employed and the number of π-electrons in the guest molecules. The parallel face-toface arrangement between HG aromatic moieties is also fundamental to maximize the dispersion interaction and consequently for the attractive energy which leads to the inclusion complex formation.

Introduction Polycyclic aromatic hydrocarbons (PAHs) are constituted by fused aromatic rings without heteroatoms or substituents in the chain. 1–3 The risks offered to the environmental and human health has placed PAHs in the limelight, 4–7 since carcinogenic, teratogenic, and mutagenic effects are related to them. 8 For that reason, different sort of compounds, able to recognize PAHs such as cyclodextrins, 9–14 calix[n]arenes, 15–17 colic acid, 18 and diazapyreniumbased metallocycles, 19,20 have been extensively synthesized. Recently, Stoddart and coworkers 21–29 reported the preparation, solid-state characterization, and HG binding affinities of a semi-rigid family of tetracationic cyclophanes, named ExBox4+ (1), which are constituted by two π-electron-poor 4,4’-bipyridinium units tethered by two p-xylylene linkers (Figure 1a). Stoddart 21 have described the ability of 1 to scavenge eleven different electron-rich PAHs (azulene (2); anthracene (3); phenanthrene (4); pyrene (5); tetracene (6); tetraphene (7); chrysene (8); helicene (9); triphenylene (10); perylene (11); and coronene (12)) (Fig2


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also explored the constitutional modifications of ExBox4+ via efficient synthetic protocols, by increasing the size of the cavities, making them able to interact with large PAHs. An example is Ex2 Box4+ , which contains two bipyridinium moieties extended by two p-phenylene rings and tethered by two p-xylylene linkers, which is able to host two guest molecules simultaneously. 22 Spectroscopic and computational studies 23 concerning structure and energetics of different oxidation states of ExBox4+ , such as ExBox3+ and ExBox2+ , has shown that, after the photoreduction, the p-phenylene spacers become more coplanar in regard to the pyridinium units, suggesting that 1 is an adjustable system, that is able to reorganize its structure to stabilize multiple charges. Particularly, the presence of through-bond electron transfer from p-xylylene towards bipyridinium units and the ability of 1 to accept and stabilize two electrons was also reported by Stoddart and Wasielewski. 24 ExBox4+ , 1, has also been employed to prepare molecular switches like [2 ]catenane, 25 which comprises the cyclophane 1 interlocked by a porphyrin-containing polyether. The versatile structure of 1 was also identified as a potential biomimetic catalyst, in which the bowl-to-bowl inversion of corannulene and ethylcorannulene is achieved. 26 In this particular case, 1 stabilizes the planar transition state of the guest (corannulene and ethylcorannulene) through the stereoelectronic reorganization of the host 1. 26 The extended version of 1, the Ex2 Box4+ was also identified to be able to bind with π-electron-rich and -poor guests. This amplified molecular recognition is possible due to the electronic structure of Ex2 Box4+ , which comprises two pyridinium rings located at the edge of the cyclophane, which form donoracceptor interactions with π-electron-rich guests, while the spacers (biphenylene), located between the pyridinium rings, are more electron-rich and prefer to interact with π-electronpoor guests. 22,28 DFT calculations 21–28 have been extensively employed to evaluate the geometries, electronic structure, relative energies, vibrational modes, excitation energies, and other properties of ExBox4+ , Exn Box4+ , and their inclusion complexes as guest⊂ExBox4+ . However, theoretical studies devoted to explain the physical nature of HG interactions involving 1 are

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still limited. Recently, Bachrach 30,31 studied the binding free energy of some linear acenes in 1 using different DFT functionals, and on attempt to understand ring strain and electrostatic influence at complex structures, Bachrach also design a neutral analogue of ExBox4+ , giving the idea that electrostatics contribution account for 10-20% of the total binding energy. Another structural change was proposed, giving the ”Flatter-ExBox4+ ”, another host that was predicted to bind more tightly with PAHs than 1. 32 Chattaraj and Das 28 reported a theoretical study, in which they describe the HG interactions between azines and 1, (azine⊂1). They explored the different conformations of the guest inside the cavity of 1 and confirmed that electron-rich guests interact preferentially with pyridinium moieties of 1. Particularly, benzene and pyridine present parallel displaced conformations as the most stable one, while guests like pyrimidine binds in a Tshape manner. Chattaraj and Das claim that the stability and aromaticity of these guest molecules are important factors that may influence the extension of the interaction. In the present contribution, we critically examined the nature of HG interactions in a set of PAHs⊂ExBox4+ inclusion complexes (2⊂1−12⊂1) (Figure S1), originally synthesized and characterized by Stoddart, 21 by considering the 11 different π-electron-rich PAHs (2-12) (Figure 1b). The nature of HG interactions in PAHs⊂ExBox4+ complexes were evaluated with EDA-NOCV scheme, 33 adopting the crystallographic geometries 21 in conjunction with the dispersion corrected functional BP86-D3 and triple-ζ quality basis set augmented by two sets of polarization functions, TZ2P.

Computational Methods The geometries of complexes (2⊂1−12⊂1) were optimized without symmetry constraints by using the Becke Perdew exchange correlation functional 34,35 (BP86) with inclusion of the D3 dispersion correction of Grimme (BP86-D3). 36 A triple-ζ STO basis set, TZ2P, as implemented in the ADF2013 software was employed. 37,38 All images were created with

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Chemcraft 39 and VMD. 40 The HG interactions in complexes (2⊂1−12⊂1) were analysed by means of the EDA-NOCV analysis, 33 where the host (1) and the guests (2-12) were taken as interacting fragments. In this case, the same level of theory was employed BP86-D3/TZ2P. According to EDA-NOCV approach, the total interaction energy, ∆Eint , is decomposed into physical meaningful terms according to (Equation 1), where ∆Eelst corresponds to the classical electrostatic interaction between the fragments, considering the frozen charge distribution at the geometry of the complex. The repulsive Pauli interaction between occupied orbitals of the fragments is taken into account by the ∆EPauli term. The orbital interactions between occupied and unoccupied orbitals of interacting fragments (charge transfer) and interactions between occupied and unoccupied orbitals on the same fragment (polarization) are given by the ∆Eorb term. The dispersion correction term ∆Edisp is computed, since the dispersion corrected functional (BP86-D3) is employed. The EDA-NOCV scheme decomposes the differential density, ∆ρ(r), into deformation densities, ∆ρi (r), describing the directions of the flow of charge. The orbital component, ∆Eorb , is decomposed into contributions, ∆Eorb i , corTS responding to the charge transfer channels, ∆ρi (r) (Equation 2), where Fi,i are the diagonal

transition-state Kohn-Sham matrix elements defined over NOCV eigenvalues. Details about the theoretical fundamentals of EDA-NOCV methodology can be found in the original paper of Mitoraj and coworkers. 33

∆Eint = ∆Eelst + ∆EPauli + ∆Eorb + ∆Edisp

∆E

orb

=

X i

∆Eorb i

=

N/2 X

TS TS + Fi,i νi −F−i,−i

(1)

(2)

i=1

The molecular response under a uniform external magnetic field (Bext ) is obtained by mapping the induced magnetic field (Bind ) quantity. Such terms are related to the secondrank shielding tensor (σij ) expressed in ppm units, according to, Bind = σij Bext which j i are described in terms of the more familiar chemical shift tensor (δij ), according to δ =

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(σ ref /(1-σ)) ≈ σ ref − σ. For a nucleus-independent probe, σ ref is equal to zero, resulting in a relationship for each component of such tensor, given by δij = −σij . The i and j indexes are conveniently related to the x-, y- and z-axes of a molecule-fixed Cartesian system, where the isotropic, δiso = (δxx + δyy + δzz )/3, is related to the averaged magnetic response under different orientations of the applied field, resembling the tumbling of the molecule in solution as occurs in normal in-solution NMR experiments, whereas certain components of are related to specific orientations of the applied field. 41–43

Results and Discussion Geometries and Bonding Analysis The PAHs (2-12) adopt parallel or quasi-parallel displacement inside the cage of 1, when inclusion complexes are formed, as revelead by the obtained relaxed geometries. The orientation of PAHs is parallel in relation to the pyridinium rings of 1 and perpendicular to the p-xylylene rings of 1 (T-shape orientation) (Figure 2), which is in line with the results reported by Chattaraj and Das 28 concerning the HG interactions between azines and 1.

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 2: Optimized geometries of ExBox4+ (1) (front view) and selected geometrical parameters, including distances between centroids of p-xylylenes (a) (˚ A), distances between the A), internal angles of bridges (c1-c4) centroids of rings in bipyridinium moieties (b1-b3) (˚ (◦ ) and dihedral angles between bipyridium and p-phenylene moieties (d1-d4) (◦ ) (a). Top and side views of selected inclusion complexes: 2⊂1 (b) and (e); 9⊂1 (c) and (f); 11⊂1 (d) and (g).

In general, the optimized structures of inclusion complexes (2⊂1−12⊂1) correspond to minima on the potential energy surface, excepting (4⊂1) and (7⊂1), which features very small imaginary eigenvalues in the Hessian matrix -7.06 and -22.3 cm−1 , respectively (even after exhaustive re-optimizations employing very tight energy and scf orbital gradient

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Table 1: EDA-NOCV of inclusion complexes (2⊂1 - 12⊂1) formed between ExBox4+ (q1 ) and the PAHs 2-12 (q2 ) (kcal·mol−1 ), by using BP86-D3/TZ2P level of theory. ∆Eint ∆EPauli ∆Eelst ∆Eorb tot ∆Eorb 1 ∆Eorb 2 ∆Eorb 3 ∆Eorb 4 ∆Eorb res ∆Edisp ∆Estr h ∆Estr g ∆Eprep b q1 q2 a

a

2⊂1 3⊂1 4⊂1 5⊂1 6⊂1 7⊂1 8⊂1 9⊂1 10⊂1 11⊂1 -32.8 -43.3 -44.1 -48.4 -54.6 -55.5 -52.9 -55.1 -52.1 -62.1 26.6 33.8 37.6 40.8 52.8 54.7 42.7 51.3 46.4 56.0 -14.8 -19.6 -21.1 -23.0 -28.4 -29.9 -24.5 -28.4 -25.8 -31.5 (24.9%) (25.4%) (25.8%) (25.8%) (26.5%) (27.1%) (25.6%) (26.7%) (26.2%) (26.7%) -7.2 -9.0 -9.0 -10.3 -17.8 -16.2 -12.8 -14.5 -14.0 -16.0 (12.1%) (11.7%) (11.0%) (11.5%) (16.6%) (14.7%) (13.4%) (13.6%) (14.2%) (13.6%) -1.9 -0.7 -1.3 -1.1 -5.6 -3.1 -1.6 -1.6 -1.7 -1.6 -1.0 -0.9 -0.9 -1.2 -0.7 -0.9 -1.0 -0.9 -1.2 -1.4 -0.6 -0.4 -0.4 -0.6 -0.9 -0.8 -0.5 -0.9 -0.8 -0.9 -0.4 -0.5 -0.4 -0.5 -0.8 -0.7 -0.5 -0.5 -0.6 -0.9 -3.0 -5.8 -5.3 -6.3 -9.0 -9,7 -8.3 -9.2 -8.6 10.0 -37.4 -48.5 -51.7 -55.9 -61.1 -64.1 -58.3 -63.4 -58.8 -70.5 (63.0%) (62.9%) (63.2%) (62.7%) (56.9%) (58.2%) (61.0%) (59.6%) (59.6%) (59.7%) 3.54 2.14 2.23 2.87 3.85 3.73 2.86 4.12 2.61 3.26 0.02 0.03 0.03 0.05 0.08 0.07 0.05 0.40 0.11 0.11 3.56 2.17 2.26 2.92 3.94 3.80 2.91 4.52 2.71 3.37 3.94 3.96 3.96 3.95 3.82 3.90 3.93 3.94 3.93 3.92 0.06 0.04 0.04 0.05 0.18 0.10 0.07 0.06 0.07 0.08

12⊂1 -62.8 46.7 -27.1 (24.7%) -15.0 (13.7%) -1.2 -0.8 -0.7 -0.5 -10.8 -67.4 (61.6%) 3.43 0.13 3.56 3.94 0.06

str ∆Estr h and ∆Eg denote the strain energy of the host and guest, respectively. q1 and q2 denote the Hirshfeld charges of host (1) and guests (2-12), respectively.

a b

A considerable decrease in the bridge angles (c1-c4) is observed with the inclusion complexes formation, specially on going from 1 to (2⊂1 - 8⊂1 and 10⊂1) (Figure 3c). The bridge angles (c1-c4) of 1 (Figures 2a and 3c) reveal that the host structure in some inclusion complexes, as (2⊂1), (9⊂1), and (11⊂1) (Figures 2b-2g and 3c) is more tilted than in others, such distortion stems from the parallel displacement adopted by 1 to include guests from different sizes inside its cage, in order to maximize the HG interaction energy, as also evidenced by EDA-NOCV. For instance, in (2⊂1) a preferential face-to-face orientation between the π-electron-rich phenylene spacer of 1 and the azulene 2 is observed, while the pyridinium rings (π-electron poor) become more tilt. In complexes (9⊂1-12⊂1) the distortion of the host structure that leads to a parallel displacement of the pyridinium rings of 1 is more accentuate, generating the symmetric differences between (c1-c4) values (Figure 3c). The most significant changes in the geometrical 10


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parameters of 1 are directly related with the strain energy imposed to 1 (increase of ExBox str tension), ∆Estr h , during the complex formation (Table 1). ∆Eh is calculated as the electronic

energy difference between optimized geometry of 1 and the geometry that it acquires in the complex formation. On the other hand, no significant tension on the guests structures was observed, ∆Estr g . The EDA-NOCV analysis of inclusion complexes 2⊂1 - 12⊂1 was performed by considering the host 1 and each PAHs 2-12 as interacting fragments, both in singlet electronic states. The interaction energy, ∆Eint depends on both, the size of the PAH employed and the number of π-electrons in the guest molecules (Table 1). ∆Eint becomes more stabilizing (more negative) on going from 2 to 12. The smallest ∆Eint value (-32.8 kcal·mol−1 ) is observed in 2⊂1, where azulene 2 is employed as guest. On the other hand, the most significant ∆Eint values (-62.1 kcal·mol−1 and -62.8 kcal·mol−1 ) are observed in 11⊂1 and 12⊂1, where the guests perylene 11 and coronene 12 are employed, respectively. According to the Hirshfeld charges (q1 and q2 ), no significant charge transfer is observed in the HG interactions, since 1 maintains its charge (≈ +4.0), while the PAHs charges remain close to zero. Among the stabilizing contributions to the interaction energy, the dispersion contribution, ∆Edisp , is the most significant one (58.2% - 63.0%), followed by the electrostatic, ∆Eelst , (24.7% - 27.1%), and orbital, ∆Eorb tot , (11.0% - 14.7% ) contributions. As shown in Figure 4, the dispersion contribution, ∆Edisp strongly depends on the number of π-electrons in the guest molecule.

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∆ρ1 , ∆Eorb = −5.6, ∆q1 = 0.551 ∆ρ2 , ∆Eorb = −0.9, ∆q2 = 0.119 ∆ρ3 , ∆Eorb = −0.7, ∆q3 = 0.115 1 2 3

(a)

∆ρ1 , ∆Eorb = −3.1, ∆q1 = 0.363 ∆ρ2 , ∆Eorb = −0.9, ∆q2 = 0.124 ∆ρ3 , ∆Eorb = −0.8, ∆q3 = 0.110 1 2 3

(b) Figure 5: Contours of deformation densities, ∆ρi (r), describing the HG interaction in 6⊂1 (a) and 7⊂1 (b) inclusion complexes. The corresponding energy ∆Eorb (kcal·mol−1 ) and i charge transfer estimation ∆qi (in au) values are included. Red surfaces indicate density outflow and blue surfaces density inflow (contour value 0.003).

The amount of charge transferred is relatively small ∆q1 = 0.551e and ∆q1 = 0.363e, respectively. Such charge transfer corresponds to the highest energetic stabilization values observed for these complexes: ∆E1orb = −5.6 and ∆E1orb = −3.1 kcal·mol−1 , respectively. The other less significant density deformations with contributions ranging from −0.7 to −0.9 kcal·mol−1 comprise density polarization in both host and guest structures. Therefore, in cases where orbital contributions are more significant, EDA-NOCV results show clearly that even in these situations the density deformation channels, ∆ρi (r), involve density outflows and inflows that tend to push the density from the guests (PAHs) to the host ExBox4+ .

Non-Covalent Interaction and Induced Magnetic Field Analysis The formation of the PAH⊂ExBox4+ pair, involves the presence of weak non covalent interactions related to the π-π interaction between the aromatic rings. Among other stabilizing 13



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interactions (vide infra), π-π interactions have been recognized as important intermolecular forces that influence the formation of extended architectures from building blocks with aromatic moieties, ranging from small molecules like benzene to large biological systems, contributing significantly towards self-assembly or molecular recognition processes. 44–47 The impact of noncovalent interaction can be obtained from the NCI analysis which reveals the nature and spatial distribution of intermolecular interaction in the HG pair. The NCI plot allows us to identify the interacting regions as well as assessment of the type of interaction, based on the topological analysis of the electron density by means of the reduced density gradient (s(ρ)) at low-density regions (ρ(r) < 0.06). s(ρ) exhibits small values in regions where both covalent bonding and noncovalent interactions are located. To distinguish the nature of the interaction, each point in this region is correlated with the second eigenvalue of the electron density Hessian (λ2 ) which accounts for the accumulation (attractive) or depletion (repulsive) of density in the plane perpendicular to the interaction. The product between ρ(r) and the sign of λ2 has been proposed as a useful descriptor to reveal stabilizing or attractive interactions (λ2 < 0), van der Waals type interaction (λ2 ≈ 0), or repulsive interactions (λ2 > 0); thus, the ρ*sign(λ2 ) ranges from negative to positive values according to the nature of the noncovalent interactions. In Figure 6a the NCI analysis for 3⊂1 and 2⊂1 is presented, which denotes a large region displaying van der Waals interaction between the ExBox4+ and the PAH moiety, with stabilizing π-π interaction in zones were the aromatic carbon atoms are overlapped.

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[2.2]paracyclophane, a transannular interaction leads to an enhanced magnetic response due to the additive interaction of the induced magnetic fields generated by the ring current effect of each aromatic ring. Here we set to increase the understanding of the interplay between the aromatic and pyridyl rings of the ExBox4+ and the fused-ring based guest, from where the spatial distribution of noncovalent interactions has been depicted above. Magnetic response properties provide a sensitive and powerful tool to account for the chemical environment. In addition to the chemical shift tensor centered at each atom, accounting for the NMR experiments, the overall magnetic response can be conveniently generalized through the space. This leads to a graphical representation of the short- and long-range magnetic behavior driven by the presence of induced currents. The through-space magnetic response or induced magnetic field (IMF) for representative systems 8⊂1 and 4⊂1, is given in Figure 6b, where positive and negative values denote deshielding and shielding regions as result of paratropic and diatropic induced currents, respectively. The term δiso suggests the mid range behavior of the induced currents contained in the rings of both ExBox4+ and PAH structures, depicting shielding IMF in the phenyl rings and deshielding IMF in the pyridyl fragments, with shielding zones in both C-C and C-N bonds. For the side aromatic rings of ExBox4+ , the magnetic response is largely affected from the deshielding character of the IMF from the pyridyl , denoting an outer shielding cone and a almost zero IMF in the inner section. The shielding region from the guest PAH exhibits an additive interaction similar to [2.2]paracyclophane with the above and below phenyl rings of the ExBox4+ structure, which contrast to the pyridyl-PAH induced magnetic field overlap of subtractive character. Thus, the observed stabilizing van der Waals character of the π-π interaction involves both aromatic-aromatic and aromatic-antiaromatic pairs. The resulting additive interaction between the shielding region from both ExBox4+ and PAH observed in Figure 6b, accounts for the experimental 1 H-NMR shifts due to the shielding of the face-toface aromatic rings. 21 To evaluate the aromatic character of each ring, magnetic criteria of aromaticity has

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been widely employed to describe the ring current effect, where the application of a perpendicular external magnetic field (Bext ) leads to an opposed induced field (Bind ), resulting in a shielding response at the center of the ring. In contrast, for antiaromatic molecules, such induced magnetic field enhances Bext leading as consequence to a deshielding region. Hence, a perpendicular external field, which is accounted by δzz (Figure 6b), denotes that the pyridyl moieties exhibit an antiaromatic character, in contrast to the aromatic benzene rings of both host and guest structures. The resulting inner face of the long-range shielding cone from the perpendicular aromatic rings exhibits an enhanced response because of the aromatic-aromatic interaction due to the overlap of respective shielding regions. In addition, the deshielding region from the antiaromatic pyridyl rings are summed with the outer deshielding region of the PAH guest leading to a long range deshielding region surrounding such guest. The aromatic character of the side rings from ExBox4+ was evaluated through an external field oriented from the y-axis, which exhibits the respective external long-range shielding cone depicting the aromatic character of such rings. In contrast, for the internal shielding region the presence of the PAH moiety depicting the interaction between the shielding and deshielding regions from both ExBox4+ and PAH.

Concluding Remarks The physical nature of Host-Guest interactions between ExBox4+ (1) and polycyclic aromatic hydrocarbons (PAHs) (2-12) was investigated at the light of the energy decomposition (EDA-NOCV), non-covalent interactions (NCI), and magnetic response analyses. The results reveals that the PAHs (2-12) adopt parallel or quasi-parallel displacement inside the cage of 1, when inclusion complexes are formed. The enlargement of distances between centroids of p-xylylenes rings in (1) followed by the by the simultaneous decrease of the distances between the centroids of bipyridinium moieties suggest the presence of a significant

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π-π interaction between the host and guests, as also confirmed the dispersion contributions in EDA-NOCV analysis, which reveal to be the most significant to total HG interaction. The EDA-NOCV results also show that the interaction energy is dependent on both the size of the PAH employed and the number of π-electrons in the guest molecules. The parallel face-to-face arrangement between HG aromatic moieties is fundamental to maximize the dispersion interaction and consequently the interaction energy leading to the inclusion complex formation. The NCI analysis confirms the presence of π-π interaction between the HG by denoting a large region displaying van der Waals interaction between the ExBox4+ and the PAH moiety, with stabilizing π-π interaction in zones were the aromatic carbon atoms are overlapped. In contrast, when the carbon-carbon distance is larger, no van der Waals type interactions can be noted. The magnetic response analysis gives support to EDA-NOCV and NCI results indicating that the observed stabilizing van der Waals character of the π-π interaction involves both aromatic-aromatic and aromatic-antiaromatic pairs. It also denotes that the pyridyl moieties exhibits an antiaromatic character, in contrast to the aromatic benzene rings of both host and guest structures. The inner face of the long-range shielding cone from the perpendicular aromatic rings exhibits an enhanced response because of the aromatic-aromatic interaction due to the overlap of respective shielding regions.

Acknowledgement The authors thank FAPESC, CNPq and Capes for the financial support. GRN thanks CNPQ (166254/2013-4) for the financial support. GFC thanks CNPq (302408/2014-2) for the research fellowship and both the Hochschulrechenzentrum of the Philipps-Universität Marburg and the Centro Nacional de Supercomputa¸caõ CESUP-UFRGS for the excellent computational service provided. RLTP thanks FAPESP (2011/07623-8) for the financial support. AMC thanks the financial support from FONDECYT and PROJECT MILLENIUM (1140359 and RC120001).

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Supporting Information Available Optimized structures, geometrical parameters, contour plots deformation densities, NCI contour plots, complete references and the cartesian coordinates of inclusion complexes are provided as supporting information material. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry

Host-Guest system formed between helicene and ExBox4+ .

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Shedding Light on the Nature of HostâGuest Interactions in PAHs

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