Manuel
Buendía
a,
Anton J.
Stasyuk
bc,
Salvatore
Filippone
a,
Miquel
Solà
*b and
Nazario
Martín
*ad
aDepartamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Av. Complutense, S/N, 28040 Madrid, Spain. E-mail: nazmar@ucm.es
bInstitut de Quimica Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, M. Aurèlia Capmany, 69, 17003 Girona, Spain. E-mail: miquel.sola@udg.edu
cDepartament de Farmàcia i Tecnologia Farmacèutica, i Fisicoquímica, Facultat de Farmàcia i Ciències de l'Alimentació & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona (UB), Av. Joan XXIII 27-31, Barcelona, Spain
dIMDEA-Nanociencia. C/Faraday, 9. Campus de Cantoblanco, 28049 Madrid, Spain
First published on 24th December 2024
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.
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.
![]() | ||
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 (Ha: δ0 = 1.26 ppm; Hb: δ0 = 1.58 ppm), the less shielded methylene proton of the AB system (Hc: δ0 = 2.76 ppm), the doubly benzylic protons of the stereogenic centers (Hd: δ0 = 3.79 ppm), and two notably deshielded protons from the edge of the nanographene layers (He: δ0 = 8.82 ppm; Hf: δ0 = 9.30 ppm). The highest chemical shift difference was found for signal Hc (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).
After plotting the titration data using BindFit fitting tool,36,37 the non-linear fit afforded a moderate binding constant of Ka = 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 Hc 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 Hc signal remained invariable, thus completing a successful titration (Fig. 3).
The non-linear adjust of these data resulted in a binding constant of Ka = 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 BLYP38,39 exchange−correlation functional along with the empirical D3(BJ) dispersion correction40,41 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 (ΔEint) between the CNG-1 tweezers and the C60/C70 units, as well as the deformation energy (ΔEdef) 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, ΔEint was found to be −61.3 and −66.1 kcal mol−1, while ΔEdef was 3.1 and 2.5 kcal mol−1, respectively (Table 1).
Complex | Energy terms | ΔEint | ΔEdef | ΔEcomplex | K a, M−1 | ||||
---|---|---|---|---|---|---|---|---|---|
ΔEPauli | ΔEelstat | ΔEoi | ΔEdisp | Host | C xx | ||||
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 |
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 (ΔEelstat), Pauli repulsion (ΔEPauli), orbital interactions (ΔEoi), and dispersion correction (ΔEdisp).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 (ΔEPauli) 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.
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).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo02071e |
This journal is © the Partner Organisations 2025 |