All-carbon supramolecular complexation of a bilayer molecular nanographene with [60] and [70]fullerenes

Abstract

Supramolecular chemistry of carbon-based materials provides a variety of chemical structures with potential applications in materials science and biomedicine. Here, we explore the supramolecular complexation of fullerenes C₆₀ and C₇₀, highlighting the ability of molecular nanographene tweezers to capture these structures. The binding constant for the CNG-1⊃C₇₀ complex was significantly higher than for CNG-1⊃C₆₀, showing a clear selectivity for the more π-extended C₇₀. DFT calculations confirmed these experimental results by showing that the interaction energy of C₇₀ with CNG-1 is more than 5 kcal mol⁻¹ higher than that of C₆₀. Theoretical calculations predict that the dispersion interaction provides about 58–59% of the total interaction energy, followed by electrostatic attraction with 26% and orbital interactions, which contribute 15–16%. The racemic nanographene tweezers effectively recognize fullerene molecules and hold promise for future applications in chiral molecule recognition.

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Introduction

The supramolecular chemistry of carbon-based materials has been a recurring topic in organic chemistry with the uncountable possibilities and combinations stemming from the size and shape of graphene,^1,2 carbon nanotubes (CNTs),^3,4 and fullerenes.⁵ As expected for these carbon nanostructures, non-covalent interactions such as C–H⋯π and, mostly, π⋯π interactions are those mainly controlling the formation of the resulting supramolecular ensembles.^6–9 Interestingly, the non-covalent functionalization of these carbon allotropes has provided a great variety of chemical architectures, many of them showing unexpected properties with potential biomedical¹⁰ and materials science applications.¹¹

In particular, the supramolecular complexation of fullerenes has received a lot of attention due to their singular spherical shape as well as their electron-acceptor nature, affording a variety of supramolecular complexes exhibiting new structural and optoelectronic properties.^6,12 Thus, a plethora of molecules of diverse structural complexity have been synthesized acting as hosts to capture guest fullerenes, especially C₆₀ and C₇₀. In this regard, chemical structures based on macrocycles such as calixarenes,^13,14 cucurbituryls,¹⁵ cyclodextrins,^16,17 molecular nanobelts,^18,19 cycloparaphenylenes (CPPs),^20–22 molecular cages,^23,24 as well as molecular tweezers endowed with curved π-extended moieties with positive Gaussian curvature such as corannulenes,²⁵ or with electronically complementary electron-donor molecules such as porphyrins or π-extended tetrathiafulvalenes (ex-TTFs)^26,27 have extensively been reported.

More recently, molecular nanographenes have attracted the attention of the scientific community due to their remarkable chiroptical and optoelectronic properties, their quick access and tunability through benchtop synthesis and their amazing structural diversity.^28–30 These nanometric fragments of graphene typically present a higher number of π–π interactions with fullerenes than other Polycyclic Aromatic Hydrocarbons (PAHs). Accordingly, molecular nanographenes have been used as hosts in host–guest chemistry with C₆₀ and C₇₀ species, being embedded into the aforementioned macrocycles as CPPs and nanobelts,^18,31,32 and also as simple molecules with either positive or negative Gaussian curvatures.³³ Interestingly, these supramolecular complexes not only have enhanced binding constants with fullerenes than their previous counterparts, but they can also showcase photoinduced electron transfer processes between the fullerene and the nanographene units.³⁴

In this work, we report on the molecular complexation of C₆₀ and C₇₀ with a molecular nanographene tweezer (CNG-1) as a suitable host previously obtained by a six-step synthetic process. Furthermore, theoretical calculations using the DFT BLYP exchange–correlation functional, along with the empirical D3(BJ) dispersion correction and def2-SVP basis set, nicely underpin the experimental values, matching the difference in interaction energy with binding constant values.

Results and discussion

Racemic and chiral triindane nanographene CNG-1 was synthesized according to a procedure previously reported by our group.³⁵ Interestingly, this bilayer molecule exhibits a suitable concave geometry for further supramolecular complexation experiments. In particular, we envisaged to seize the inner cavity of this molecular tweezer with different fullerenes, namely C₆₀ and C₇₀, endowed with complementary convex surfaces (Fig. 1).

Fig. 1 All-carbon nanostructures involved in the supramolecular complexation of CNG-1 with fullerenes C₆₀ and C₇₀.

The X-ray crystal structure of CNG-1 revealed that both nanographene layers of the tweezer connected to the central triindane unit form an all-carbon host with an interlayer distance of around 9 Å, being a suitable host to accommodate either C₆₀ or C₇₀ fullerenes and, thus, affording their respective supramolecular complexes (Fig. 1). For this purpose, ¹H NMR titration experiments were performed to carry out the supramolecular complexation by following the evolution of the proton signals mostly affected in the complexation process between racemic CNG-1 with fullerenes C₆₀ and C₇₀.

These titration experiments were performed in deuterated chlorobenzene (PhCl-d⁵) to ensure that both fullerene species were properly dissolved. Additionally, a complete NMR study was performed to assign all proton signals of CNG-1 and gain a deeper insight into the complexation processes, which can be found in Fig. S1–S6, ESI.† The first titration involved CNG-1 as host and fullerene C₆₀ as guest. ¹H NMR spectra of this titration showcased chemical shifts of single time-averaged signals of CNG-1 with additions up to 19 equivalents of guest C₆₀, evidencing a fast exchange rate in solution between complex and free host and guest species as well as the supramolecular complexation process (Fig. S7†). Notable differences in chemical shifts were observed for six proton signals: two tert-butyl protons (H_a: δ₀ = 1.26 ppm; H_b: δ₀ = 1.58 ppm), the less shielded methylene proton of the AB system (H_c: δ₀ = 2.76 ppm), the doubly benzylic protons of the stereogenic centers (H_d: δ₀ = 3.79 ppm), and two notably deshielded protons from the edge of the nanographene layers (H_e: δ₀ = 8.82 ppm; H_f: δ₀ = 9.30 ppm). The highest chemical shift difference was found for signal H_c (Δδ = 0.087 ppm) which, according to the X-ray data, belongs to the proton pointing towards the inner cavity of the nanographene tweezer, nicely confirming the complexation process (Fig. 2).

Fig. 2 Model for the supramolecular complexation of the CNG-1⊃C₆₀ complex with a 1 : 1 stoichiometry (left), and H_c signal chemical shift variation upon addition of C₆₀ equivalents throughout the titration in PhCl-d⁵ (right).

After plotting the titration data using BindFit fitting tool,^36,37 the non-linear fit afforded a moderate binding constant of K_a = 61 ± 1 M⁻¹ for the CNG-1⊃C₆₀ supramolecular complex, matching a 1 : 1 stoichiometry model (Nelder Mead method, Fig. S8;† for molar fraction evolution of both host and complex see Fig. S9†), thus confirming the affinity of CNG-1 for fullerene C₆₀.

The second titration involving fullerene C₇₀ as guest revealed a similar behavior for CNG-1 host proton signals, where the same six signals from the first titration with C₆₀ also shifted largely (Fig. S10†), with the greatest difference in chemical shift for the methylene signal H_c as well (δ₀ = 2.76 ppm, Δδ = 0.049 ppm). Moreover, complete saturation of complex in solution was reached (8 equivalents of C₇₀) as the H_c signal remained invariable, thus completing a successful titration (Fig. 3).

Fig. 3 Model for the supramolecular complexation of the CNG-1⊃C₇₀ complex with a 1 : 1 stoichiometry (left), and H_c signal chemical shift variation upon addition of C₇₀ equivalents throughout the titration in PhCl-d⁵ (right).

The non-linear adjust of these data resulted in a binding constant of K_a = 400 ± 17 M⁻¹ for the CNG-1⊃C₇₀ supramolecular complex, also matching with a 1 : 1 stoichiometry model (Nelder Mead method, Fig. S11;† for molar fraction evolution of both host and complex see Fig. S12†). This result demonstrates the higher affinity of CNG-1 C₇₀ over C₆₀ with a binding constant value around one order of magnitude higher. Such enhancement can be accounted for by the larger π-extended surface of the C₇₀ guest compared to the former C₆₀ and, hence, increasing the π–π interactions between host CNG-1 and guest C₇₀ molecules.

To gain insight into the nature of these supramolecular complexation processes, a series of quantum chemical calculations were performed. Systems of interest were optimized utilizing the DFT BLYP^38,39 exchange−correlation functional along with the empirical D3(BJ) dispersion correction^40,41 and def2-SVP basis set.^42,43 The calculations were performed within the ORCA 5.0.3 program⁴⁴ (see ESI for more details on the computational method used†).

To assess the stability of the complexes, we calculated the interaction energy (ΔE_int) between the CNG-1 tweezers and the C₆₀/C₇₀ units, as well as the deformation energy (ΔE_def) associated with the deformation of the units from their equilibrium geometries to the geometries they adopt in the complexes. DFT optimized structures of both complexes are provided in Fig. 4. For CNG-1⊃C₆₀ and CNG-1⊃C₇₀, ΔE_int was found to be −61.3 and −66.1 kcal mol⁻¹, while ΔE_def was 3.1 and 2.5 kcal mol⁻¹, respectively (Table 1).

Fig. 4 Optimized DFT structures for supramolecular complexes CNG-1⊃C₆₀ and CNG-1⊃C₇₀.

Table 1 EDA results (in kcal mol⁻¹) and experimentally determined binding constant values (K_a) for CNG-1⊃C₆₀ and CNG-1⊃C₇₀ complexes^a

Complex	Energy terms				ΔEint	ΔEdef		ΔEcomplex	Ka, M^−1
	ΔEPauli	ΔEelstat	ΔEoi	ΔEdisp		Host	Cxx
a Relative values (in parentheses) are given as a percentage and express the contribution to the sum of all attractive energy terms: ΔEelstat + ΔEoi + ΔEdisp. Complexation energy: ΔEcomplex = ΔEint + ΔEdef.
CNG-1⊃C60	104.56	−43.24 (26%)	−25.80 (16%)	−96.78 (58%)	−61.25	2.74	0.37	−58.14	61 ± 1
CNG-1⊃C70	116.86	−47.36 (26%)	−28.01 (15%)	−107.57 (59%)	−66.08	2.18	0.27	−63.63	400 ± 17

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Most of the deformation energy is attributed to the CNG-1 deformation, with deformation of the fullerenes contributing only 11 to 12%. It is interesting to note that despite the significant difference in the spatial sizes of C₆₀ and C₇₀ fullerenes, the deformation energy of the host unit in both complexes is nearly the same. This can apparently be explained by the mutual arrangement of the fullerenes and CNG-1 units in the complexes. In particular, C₇₀ in the CNG-1⊃C₇₀, complex is positioned in such a way that its main axis runs along the cavity of the tweezers, and thus, the effective size of both fullerenes appears to be very similar.

To analyze the nature of the host–guest interactions, we employed the Morokuma-like interaction energy decomposition analysis (EDA)⁴⁵ implemented in the ADF program.⁴⁶ The EDA decomposes the interaction energy into four components: electrostatic (ΔE_elstat), Pauli repulsion (ΔE_Pauli), orbital interactions (ΔE_oi), and dispersion correction (ΔE_disp).⁴⁷ This decomposition enables us to assess the role of the specific interactions in the systems. Table 1 shows the EDA analysis results for CNG-1⊃C₆₀ and CNG-1⊃C₇₀ complexes.

As expected for both complexes, the nature of the host–guest interactions is nearly the same. In particular, dispersion interaction provides about 58–59% of the total interaction energy, followed by electrostatic attraction with 26%, and orbital interactions which contribute 15–16%. The destabilizing term (ΔE_Pauli) is slightly larger for CNG-1⊃C₇₀ than for CNG-1⊃C₆₀, 116.9 and 104.6 kcal mol⁻¹, respectively. However, this difference is compensated to a significant extent by a larger dispersion correction term for CNG-1⊃C₇₀ (Table 1). Overall, based on the complexation energy values, the CNG-1⊃C₇₀ complex is more stable than CNG-1⊃C₆₀ by 5.5 kcal mol⁻¹, supporting the higher binding constant for the complex with C₇₀ fullerene observed in the titration experiment.

The analysis of the host–guest interaction topology for the complexes of interest, performed using the non-covalent interaction (NCI) index analysis,⁴⁸ revealed two distinct types of areas between the fullerenes and the CNG-1 host unit. One type of these areas is located between the fullerene and the nanographene units of the tweezer.

Considering the π-conjugated nature of the fullerene and nanographene units, the interactions can be assigned as π⋯π interactions. The other type of area can be classified as C–H⋯π interactions due to its location between the fullerene and the hydrogen atoms of the central triindane fragment or the hydrogen atoms of the tert-butyl groups. The NCI isosurfaces for both complexes are presented in Fig. S13, ESI.†

Although these binding constants are lower with respect to other previously reported in literature for molecular nanographenes,⁴⁹ it is important to remark that CNG-1 as a molecular receptor for fullerenes (especially C₇₀) is endowed with an inherent chiral triindane which, eventually, could open a door for the selective supramolecular recognition of sp² carbon-based chiral molecules.

Conclusions

The supramolecular complexation of guest fullerenes C₆₀ and C₇₀ with racemic molecular nanographene CNG-1 as a host is presented in this work. Relatively moderate binding constant values were afforded for CNG-1⊃C₆₀ and larger for CNG-1⊃C₇₀ complexes, both accommodating the respective guest fullerene inside its cavity with a 1 : 1 stoichiometry. Moreover, the binding constant for CNG-1⊃C₇₀ was one order of magnitude higher than that of CNG-1⊃C₆₀, thus revealing a clear selectivity towards the more π-extended fullerene. DFT calculations supported these experimental values, matching the difference in interaction energy with binding constant values. In this manner, we have demonstrated that racemic molecular nanographene CNG-1, can be used for the supramolecular recognition of carbon-based π-extended molecules such as fullerenes. Besides, its inherent chirality could potentially play a major role in the near future for the recognition of chiral molecules, including fullerenes.

Experimental section

Materials and methods

CNG-1 was prepared according to the procedure reported in the literature.^{35 1}H NMR spectra of the titration were recorded at 300 (Bruker AVIII) MHz at 298 K using deuterated chlorobenzene (PhCl-d⁵) as solvent. All titrations were performed in an NMR sample tube charged with 0.5 mL of a starting CNG-1 host solution (5 × 10⁻⁴ M). Sequential aliquots of a guest stock solution were added to the initial solution of CNG-1 with a micropipette and mixing properly, recording their ¹H NMR spectra afterwards. These guest fullerene stock solutions were prepared dissolving the respective fullerene with a CNG-1 solution of the same molarity as the initial to prevent dilution factors and keep a constant host concentration throughout the titration. Therefore, C₆₀ + CNG-1 (9.5 × 10⁻³ M and 5 × 10⁻⁴ M, respectively) and C₇₀ + CNG-1 (4 × 10⁻³ M and 5 × 10⁻⁴ M, respectively) stock solutions were afforded. The resulting titration data was fitted using BindFit fitting tool to determine the association constants.³⁷

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

A. J. S. and M. S. thanks the Ministerio de Ciencia, Innovación y Universidades (MCIN/AEI/10.13039/50110001103) (Projects PID2023-147424NB-I00, PID2023-146849NB-I00, RED2022-134939-T, and FPI predoctoral grant to C. C.) and the Generalitat de Catalunya (Project 2021-SGR-623). A. J. S. gratefully acknowledge Polish high-performance computing infrastructure PLGrid (HPC Center: ACK Cyfronet AGH) for providing computer facilities and support within computational grant no. PLG/2023/016841.

M. B., S. F., and N. M. acknowledge financial support from the Spanish MICIN (project PID2020-114653RB-I00), they also acknowledge financial support from the ERC (SyG TOMATTO ERC-2020-951224) and from the ‘(MAD2D-CM)-UCM’ project funded by Comunidad de Madrid, by the Recovery, Transformation and Resilience Plan and by NextGenerationEU from the European Union. M. B. also acknowledges financial support from the Spanish MICIN (project CTQ2017-83531-R).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo02071e

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