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C. S. Kwan, R. Zhao, M. A. Van Hove, Z. Cai and K. C. F. Leung, Nat. Commun., 2018, 9, 1–9 CrossRef CAS PubMed. S. J. Duncan, R. Lewis, M. A. Bernstein and P. Sandor, Magn. Reson. Chem., 2007, 45, 283 CrossRef CAS PubMed . E. Mezzina, R. Manoni, F. Romano and M. Lucarini, Asian J. Org. Chem., 2015, 4, 296–310 CrossRef CAS. A. Herrera, E. Fernandez-Valle, E. M. Gutierrez, R. Martinez-Alvarez, D. Molero, Z. D. Pardo and E. Saez, Org. Lett., 2010, 12, 144 CrossRef CAS PubMed . Cryogenic transmission electron microscopy Techniques Microscopic techniques provide useful complementary tools with which to investigate structures of supramolecular assemblies. In contrast to the previously discussed methods, microscopy enables the direct imaging of macromolecular structures without having the need to interpret complex spectra. A wide range of microscopic techniques were successfully applied to supramolecular molecules with all of them showing different strengths and weaknesses. 12,71 However, cryogenic transmission electron microscopy (cryo-TEM) has arguably emerged as the most promising tool and will therefore be discussed further in this section.

S. J. Elliott, Q. Stern, M. Ceillier, T. El Daraï, S. F. Cousin, O. Cala and S. Jannin, Prog. Nucl. Magn. Reson. Spectrosc., 2021, 126–127, 59 CrossRef CAS PubMed . J. R. N. Haler, D. Morsa, P. Lecomte, C. Jérôme, J. Far and E. De Pauw, Methods, 2018, 144, 125–133 CrossRef CAS PubMed. D. Golowicz, M. Kazmierczak and K. Kazimierczuk, Magn. Reson. Chem., 2021, 59, 213 CrossRef CAS PubMed .M. Sawada, Y. Takai, T. Kaneda, R. Arakawa, M. Okamoto, H. Doe, T. Matsuo, K. Naemura, K. Hirose and Y. Tobe, Chem. Commun., 1996, 1735–1736 RSC. M. E. Fernandez-Valle, R. Martinez-Alvarez, D. Molero-Vilchez, Z. D. Pardo, E. Saez-Barajas and A. Herrera, J. Org. Chem., 2015, 80, 799 CrossRef CAS PubMed . The potential of SABRE for mixture analysis has been explored by Tessari and co-workers, who showed that the method can be applied to complex mixtures of molecules with concentrations in the micromolar range, provided that an appropriate co-substrate is also added to the mixture. 113,114 They also showed that the signal intensity changes linearly with concentration in that regime. With a standard-addition method, they were able to quantify several analytes in an artificial mixture. 115 Also working with a co-substrate, they recorded 2D DOSY data with a flow shuttle. 116 The transfer of polarisation between the hydride ligands, which results from pH 2 addition to a metal centre, to the substrate, can also be performed with pulse sequences. This is exploited in several pulse sequences developed by Tessari and co-workers, who designed 2D experiments that correlate hydride signals with signals from, e.g., the ortho proton of a heterocycle in the target substrate. 117 The resulting 2D spectre provide excellent signal dispersion, and can also be used for quantitative analysis by standard addition.

G. D. Enright, S. Takeya and J. A. Ripmeester, Supramolecular Chemistry: From Molecules to Nanomaterials, John Wiley & Sons, Ltd, 2012 Search PubMed. Considering this, a concomitant need exists to find convincing alternatives for characterisation of the structure(s) and stability of supramolecular compounds. For successful translation of these compounds to applied research and bulk manufacture, structural characterisation should encompass high-throughput techniques. To date principal challenges for structural characterisation include: distinguishing closely related species, including isomers; circumventing or preventing aggregation as well as managing purification and stability issues during the analytical process. Depending on the molecule, different structural characterisation techniques or increasingly a combination of techniques may be pertinent. C. Capici, Y. Cohen, A. D’Urso, G. Gattuso, A. Notti, A. Pappalardo, S. Pappalardo, M. F. Parisi, R. Purrello, S. Slovak and V. Villari, Angew. Chem., Int. Ed., 2011, 50, 11956–11961 CrossRef CAS PubMed. J. Marchand, E. Martineau, Y. Guitton, B. Le Bizec, G. Dervilly-Pinel and P. Giraudeau, Metabolomics, 2018, 14, 60 CrossRef PubMed . A. Fernandez, J. Ferrando-Soria, E. M. Pineda, F. Tuna, I. J. Vitorica-Yrezabal, C. Knappke, J. Ujma, C. A. Muryn, G. A. Timco, P. E. Barran, A. Ardavan and R. E. P. Winpenny, Nat. Commun., 2016, 7, 1–6 Search PubMed.M. Nilsson, M. A. Connell, A. L. Davis and G. A. Morris, Anal. Chem., 2006, 78, 3040 CrossRef CAS PubMed . L. Castanar, P. Moutzouri, T. M. Barbosa, C. F. Tormena, R. Rittner, A. R. Phillips, S. R. Coombes, M. Nilsson and G. A. Morris, Anal. Chem., 2018, 90, 5445 CrossRef CAS PubMed . Additionally to the experimental techniques previously discussed, theoretical calculations have emerged as game changers in supramolecular chemistry. The rapid progress made in the last decades ( e.g. in DFT calculations and empirical methods) is inevitably connected to the massive advance of the field. Because this development will likely continue in the future, it deserves even more attention in the supramolecular regime. The power of NMR spectroscopy also results from the vast space of possible experiments that can be carried out, with a given instrument and a given sample, through the use of different pulse sequences. This makes it possible, through the controlled manipulations of nuclear spins, to access chemical information that would otherwise remain invisible or inaccessible. The process of developing NMR pulse sequences has sometimes been referred to as “spin choreography” or “spin alchemy”. 3

Although the discussed 1D and 2D NMR spectra were in favour of the desired products, they were hardly sufficient for an unambiguous characterisation. As mentioned in Section 2, highly symmetric and big molecules (like the described rotaxane dendrimers and dendrons) are difficult to distinguish with NMR since the chemical environments of the signals are often similar and can overlap. 8 However in this case, ultimate proof was obtained from ESI-MS. Different charge state peaks of the formula [M − z PF 6] z+ ( z = 2, 3, 6, 13, depending on the molecule) were found for all dendrimers and dendrons confirming the correct synthetic products. C. J. Bruns and J. F. Stoddart, The Nature of the Mechanical Bond: From Molecules to Machines, Wiley, 2016 Search PubMed. M. H. Lerche, D. Yigit, A. B. Frahm, J. H. Ardenkjaer-Larsen, R. M. Malinowski and P. R. Jensen, Anal. Chem., 2018, 90, 674 CrossRef CAS PubMed .The most recent method for broadband homonuclear decoupling is the pure shift yielded by chirp excitation (PSYCHE) method. 19 With PSYCHE, the separation between active and passive spins is based on the application of two consecutive pulses with a small tip angle β, as in the anti-z COSY pulse sequence. 20 The active spins are the statistical fraction of the spins that are refocused by the pair of pulses. Their contribution of decoupled spins to the signal (for small angles) scales as β 2, while that of undecoupled spins scales as β 4. F. Zhang, S. L. Robinette, L. Bruschweiler-Li and R. Bruschweiler, Magn. Reson. Chem., 2009, 47(suppl 1), S118 CrossRef CAS PubMed . A. P. Deshmukh, A. D. Bailey, L. S. Forte, X. Shen, N. Geue, E. M. Sletten and J. R. Caram, J. Phys. Chem. Lett., 2020, 11, 8026–8033 CrossRef CAS PubMed.

P. Moutzouri, Y. Chen, M. Foroozandeh, P. Kiraly, A. R. Phillips, S. R. Coombes, M. Nilsson and G. A. Morris, Chem. Commun., 2017, 53, 10188 RSC . M. G. Concilio, C. Jacquemmoz, D. Boyarskaya, G. Masson and J. N. Dumez, Chem. Phys. Chem., 2018, 19, 3310 CrossRef CAS PubMed . Additionally, atomic force microscopy (AFM) was used to probe the size/height of the synthesised dendrimers on a mica surface ( Fig. 14A and B). AFM is a well-established high-resolution microscopic technique, which is mainly used for topographic imaging of surfaces. 92 Here Kwan et al. reported nearly spherical morphologies for all dendrimers, but unsurprisingly different averaged molecule heights/sizes (G1 ≈ 1.15 nm, G2 ≈ 3.94 nm and G3 ≈ 10.96 nm, Fig. 14B right). For the G1 and G2 dendrimers, all three techniques (DLS, DOSY and AFM) showed similar sizes, whereas the size of the G3 dendrimer is significantly higher in AFM images. The authors suggested a decrease in adsorption forces for the higher generation dendrimers as a possible explanation, which could reduce flattening on the mica surface and therefore yield a higher height than measured by DLS and DOSY in solution. In accordance with the hydrodynamic radii obtained from DLS and DOSY, AFM also revealed a significant height increase for the deprotonated species.L. Sellies, I. Reile, R. Aspers, M. C. Feiters, F. Rutjes and M. Tessari, Chem. Commun., 2019, 55, 7235 RSC . T. Gonzalez-Garcia, T. Margola, A. Silvagni, F. Mancin and F. Rastrelli, Angew. Chem., Int. Ed., 2016, 55, 2733 CrossRef CAS PubMed . E. Martineau, J. N. Dumez and P. Giraudeau, Magn. Reson. Chem., 2020, 58, 390 CrossRef CAS PubMed . The magnitudes of a(H) could be predicted for three possible radical conformations in water ( Fig. 3C) and were compared to experimental values. It was demonstrated that the parameters of 5 fit very well to a dihedral angle of 120° between the aryl–C and N–O bonds (referred to as α in the following), which corresponds to structure C, whereas the dimethyl- and dichlorine-substituted species 6 and 7 likely adopt conformation A with α = 0° ( Fig. 3C). A. M. R. Hall, P. Dong, A. Codina, J. P. Lowe and U. Hintermair, ACS Catal., 2019, 9, 2079 CrossRef CAS .