pubs.acs.org/joc Article Octapodal Corannulene Porphyrin-Based Assemblies: Allosteric Behavior in Fullerene Hosting ́ Sergio Ferrero, Hectoŕ Barbero, Daniel Miguel, Rauĺ García-Rodríguez,* and Celedonio M. Alvarez* Cite This: J. Org. Chem. 2020, 85, 4918−4926 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: An octapodal corannulene-based supramolecular system has been prepared by introducing eight corannulene moieties in a porphyrin scaffold. Despite the potential of this double picket fence porphyrin for double- tweezer behavior, NMR titrations show exclusive formation of 1:1 adducts. ffi ± × The system exhibits very strong a nity for C60 and C70 (K1 = (2.71 0.08) 104 and (2.13 ± 0.1) × 105 M−1, respectively), presenting selectivity for the latter. Density functional theory (DFT) calculations indicate that, in addition to the four corannulene units, the relatively flexible porphyrin tether actively participates in the recognition process, resulting in a strong synergistic effect. This leads to a very strong interaction with C60, which in turn also induces a large structural change on the other face (second potential binding site), leading to a negative allosteric effect. We also introduced Zn2+ in the porphyrin core in an attempt to modulate its flexibility. The resulting metalloporphyrin ff also displayed single-tweezer behavior, albeit with slightly smaller binding constants for C60 and C70, suggesting that the e ect of the coordination of fullerene to one face of our supramolecular platform was still transmitted to the other face, leading to the deactivation of the second potential binding site. ■ INTRODUCTION buckybowl, has unique properties not seen in planar π- Since its establishment, supramolecular chemistry1 has been conjugated compounds arising from its nonplanarity, which enables it to form supramolecular assemblies with fullerenes used to design and develop new entities with a wide range of 10 applications in various areas of science. Inspired by nature, an through noncovalent interactions. Its net dipolar moment, interesting topic in supramolecular chemistry is the develop- together with its exceptional concave−convex complementary 11 ment of artificial receptors that echo the allosteric control of for π−π stacking, allows it to strongly interact with fullerenes more complex biological systems.2 The term allosterism via the well-known “ball-and-socket” interactions. Not describes structural changes that occur upon the coordination surprisingly, corannulene has proven to be a very useful Downloaded via UNIV DE VALLADOLID on June 19, 2020 at 08:57:53 (UTC). of a substrate to a binding site (allosteric site) of the host.3 The motif for the design of various materials and a very attractive information associated with the first event is transmitted to a building block in the construction of organic electronic different binding site by a change in the conformation, which devices, including solar cells and nonlinear optical materi- See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. can lead to activation (positive cooperation) or deactivation als.8b,12 In particular, it has played a central role in the design (negative cooperation or inhibition) of the binding site. This of efficient receptors for fullerenes. The use of corannulene as a represents a very useful tool to control the coordination of building block, however, has been limited by its tedious different sites, and not surprisingly, it has attracted a great deal synthesis, which requires a large number of steps. Considerable 4 of attention in the supramolecular chemistry community. efforts have been devoted to improving the current synthetic Additionally, carbon nanostructures, particularly fullerenes, methods as well as to its derivatization with suitable functional have found a variety of applications in supramolecular groups to enable its use in producing more complex chemistry due to their unique structures and electronic architectures. Although a single pristine corannulene unit 5 ff properties. Great e orts have been made to construct cannot bind fullerene strongly enough for the corannulene− supramolecular assemblies that host fullerenes through weak intermolecular forces6 to develop high-performance devices, in particular, photoelectrochemical devices and solar cells. Received: January 10, 2020 Size and shape complementarity is crucial to achieving Published: March 10, 2020 efficient supramolecular host−guest binding. In this regard, bowl-shaped aromatic hydrocarbons or buckybowls7 are attracting increasing attention and finding new applications in materials science.8 Corannulene,9 the most-interesting © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.joc.0c00072 4918 J. Org. Chem. 2020, 85, 4918−4926 The Journal of Organic Chemistry pubs.acs.org/joc Article Figure 1. Development of systems bearing different numbers of corannulene units. 13 3 −1 fullerene inclusion complex to be observed in solution, the 10 M (C60) for D and E systems, respectively) seem to stem careful design of molecules incorporating two corannulene from their much greater flexibility, which favors self-interaction units has proven to be a very useful strategy to improve and consequently notably reduces their interaction with corannulene−fullerene interactions. fullerenes. These examples highlight the importance of the Researchers have long focused on improving the strength preorganization and morphology of the supramolecular and selectivity of fullerene receptors. The historical evolution platform in fullerene association, which can be as important of multicorannulene “pincer” systems is shown in Figure 1, as the number of corannulene units attached. Therefore, to which shows selected examples. The first and most obvious increase host−guest surface contact, careful design of fullerene such pincer systems were based on two corannulene units and receptors is required in addition to increasing the number of 18 have had a great impact on supramolecular−fullerene corannulene units. interactions. In particular, Sygula’s buckycatcher I (Figure 1, We have recently explored the use of porphyrin-based ff A)14 represented a breakthrough as the first example of the systems as molecular sca olds for fullerene hosting. Although formation of a stable inclusion complex with fullerene and porphyrins and fullerenes can interact, the presence of only a × 3 −1 single porphyrin core usually results in interactions with presented association constants of 2.78 10 M (C60) and 19 × 3 −1 fullerenes that are too weak to be observed in solution. These 3.03 10 M (C70) in toluene at 298 K. Sygula subsequently engineered the tether between the two corannulene moieties to systems are relatively rigid and allow for the active improve the preorganization of the host and the fullerene participation of the tether in the supramolecular interaction guest, expanding the buckycatcher family (buckycatchers II and are therefore ideally suited to host fullerenes. We and III, not depicted in the figure)15 and improving the C envisaged that a meso-substituted tetraaryl porphyrin core 60 would be an ideal scaffold for supramolecular interactions and binding constant by an order of magnitude. We developed a prepared the meta-substituted porphyrin F with four platinum-based system with two corannulene moieties, which 18b fi corannulene units. However, while this tetrakis-corannulene was the rst system capable of discriminating C60 over C70 (B); porphyrin clip was able to associate with C60, F was present as subsequently, Chen et al. achieved outstanding discrimination adifficult-to-analyze mixture of four atropisomers resulting (K /K = 230:1) using the two-corannulene moiety system C70 C60 from the orientation of the four corannulene units above and C.16 In contrast, systems bearing tris-corannulene clips,17 such below the porphyrin plane. These atropisomers could not be as D and E, have not had the same success as systems bearing separated, resulting in a complex system and marked decrease two corannulene units due to their lower ability to associate in the C60 binding ability. In a recent work, we subsequently fullerenes. The lower binding constants observed for these circumvented the formation of atropisomers by introducing × 3 −1 × tripodal systems (K1 = 1.50 10 M (C60) and K1 = 2.19 eight pyrene units at the meta positions of the tetraaryl 4919 https://dx.doi.org/10.1021/acs.joc.0c00072 J. Org. Chem. 2020, 85, 4918−4926 The Journal of Organic Chemistry pubs.acs.org/joc Article a porphyrin system.20 This resulted in a double tweezer with a Scheme 1. Synthesis of 2H-POctaCor remarkably high association for fullerenes, despite the poor shape complementarity between pyrene and fullerenes, due to the synergistic effect between the porphyrin tether and the pyrene units. As the combination of well-organized corannu- lene fragments and a porphyrin tether resulted in synergistic effects and good fullerene hosting ability, in the present work, we explore the possibility of introducing eight corannulene units on a single porphyrin core (Figure 1, present work) and their impact on the fullerene affinity and selectivity of the new octapodal corannulene supramolecular platform (which contains the most corannulene moieties on a single-molecule receptor to date). The effect of the association on the geometry of the double tweezer (allosteric effect) and its impact on a second association event, which determines whether the receptor will exhibit double- or