Molecular-strain engineering of double-walled tetrahedra

•Double-walled tetrahedra are constructed as a hybrid model from L-shaped dual panels•L-shaped dual panels feature strained bow-like macrocycles called molecular bows•The assembly of building blocks is driven solely by non-covalent bonding interactions•The porphyrin-incorporated double-walled tetrahedra enhance photocatalytic activity Multi-walled nanostructures are architectures that have implications for materials science. Their synthesis, however, remains a formidable challenge on account of their complex (super)structures known as the Russian doll and Parchment models, along with limitations that eliminate the use of all existing synthetic strategies. We report the construction of double-walled tetrahedra, employing a blend of the Russian doll and Parchment models, from L-shaped dual panels composed of strained bow-like macrocycles called molecular bows (MBs). The head-to-tail closed-loop stacking of four L-shaped dual panels, driven solely by non-covalent interactions with conformational self-templation, leads to the formation of a large porphyrin-lined tetrahedron, which encapsulates a small o-xylylene-lined tetrahedron. The development of MBs with tunable conformations from flexible units provides a powerful molecular-strain engineering approach to assemble more complex supramolecular multi-walled Platonic solids. The synthesis of multi-walled nanostructures represents a formidable challenge owing to the limitations imposed by existing synthetic strategies. Herein, we report a strategy involving molecular-strain engineering (MSE) for constructing double-walled polyhedra from strained bow-shaped macrocycles called molecular bows (MBs). As a proof of concept, four double-walled tetrahedra have been assembled from L-shaped dual-panels subtending ∼70.5° and producing MBs, which incorporate porphyrin and o-xylylene units, forming strained shape-persistent conformations. The head-to-tail closed-loop stacking of these units, driven by complementary non-covalent bonding interactions, leads to a large porphyrin-lined tetrahedron encapsulating a small o-xylylene-lined tetrahedron. The synergistic aromatic shielding effects emanating from the porphyrin tetrahedron result in the largest upfield chemical shifts of −7.7 ppm for protons and −8.8 ppm for carbons from those for the free dual panels. This demonstration, which features the launching of MSE for constructing sophisticated supramolecular nanostructures, could herald a fresh approach to the fabrication of multi-porphyrin photocatalytic systems. The synthesis of multi-walled nanostructures represents a formidable challenge owing to the limitations imposed by existing synthetic strategies. Herein, we report a strategy involving molecular-strain engineering (MSE) for constructing double-walled polyhedra from strained bow-shaped macrocycles called molecular bows (MBs). As a proof of concept, four double-walled tetrahedra have been assembled from L-shaped dual-panels subtending ∼70.5° and producing MBs, which incorporate porphyrin and o-xylylene units, forming strained shape-persistent conformations. The head-to-tail closed-loop stacking of these units, driven by complementary non-covalent bonding interactions, leads to a large porphyrin-lined tetrahedron encapsulating a small o-xylylene-lined tetrahedron. The synergistic aromatic shielding effects emanating from the porphyrin tetrahedron result in the largest upfield chemical shifts of −7.7 ppm for protons and −8.8 ppm for carbons from those for the free dual panels. 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These appreciable changes in chemical shifts indicate that the o-xylylene bridge is shielded by the porphyrin plane because of its location above this plane. This observation is consistent with the computationally derived conformation of MB-1. In particular, Hc1 and Hc2 associated with two −CH2− linkers exhibit two doublets, indicating that, upon stretching, both these linkers are constrained as a result of the rigid conformation that exists between the o-xylylene and porphyrin units. Variable-temperature (VT) NMR spectroscopy was carried out in order to investigate the assembly of MB-1 in solution. Upon decreasing the temperature of a CD2Cl2 solution of MB-1 from +30°C to −90°C, we observed (Figure S29) significant upfield chemical shifts for the Ha,b,c and NH resonances, accompanied by significant broadening of the Ha,b,c resonances, as well as changes in the line shapes, from sharp to broad and back to sharp again, of the resonances for the β- and NH protons on the porphyrin unit. These observations suggest that (1) the intermolecular aggregation between the o-xylylene and porphyrin planes through [π∙∙∙π] stacking results in strong mutual shielding between the two planes and that (2) MB-1 undergoes a step-by-step assembly process to form the DWT-1. 1H and 13C NMR spectroscopic investigations of MB-2 confirm (Figure 3) the selective assembly of MB-2 into the corresponding DWT-2 in the solution phase. At 1.2 mM concentration in CDCl3, MB-2 displays (Figure 3B) an 1H NMR spectrum composed of 17 resonances for heterotopic protons—similar to those recorded for MB-1 (Figure 2B) in Figure S5—suggesting (Figure 3A) the presence of a plane (σ) of symmetry in MB-2. The 1H and 13C NMR spectra of MB-2 are, however, both highly concentration dependent. Upon increasing the concentration of MB-2, its 1H NMR spectrum (Figure S32) exhibits a new set of resonances, which arise and increase in intensity compared with those observed for MB-2 at low concentrations. When the concentration of MB-2 reaches 75.9 mM, the new set of resonances become the dominant signals, whereas the original resonances for MB-2 weaken in intensity so as to become unrecognizable. Diffusion ordered spectroscopy (DOSY) of MB-2 at 1.2 and 75.9 mM reveals (Figures 3F and 3G) two distinct single bands with diffusion coefficients (D) of 6.87 × 10−10 and 3.50 × 10−10 m2 s−1, which correspond to diameters of approximately 1.20 and 2.36 nm, respectively, for MB-2 and DWT-2. The diffusion coefficients indicate that, although MB-2 exists as a collection of free monomers at 1.2 mM, it assembles, to all intents and purposes, into the [MB-2]4 tetramer—namely, DWT-2—at an MB-2 concentration of 75.9 mM. 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This number (34) is double the number (17) of resonances present in the 1H NMR spectrum of the completely dissociated MB-2, an observation, which indicates the breaking of the Cs symmetry of the MB-2 building block within DWT-2, leading to the anisochronous nature of all the protons, including the two t-butyl groups. The 1H−1H ROESY spectrum of DWT-2 exhibits (Figure S26) cross peaks between Ha and Hβ2/Hβ3, Hc1 and Hβ4, as well as between Hc1′ and Hβ2′—all of which are spatially close in the expected superstructure (Figure 3A) of DWT-2 but far away from each other within the completely dissociated MB-2, an observation that confirms the formation of DWT-2 in the solution phase. Remarkably, all eight o-xylylene resonances (Figure 3C) for Ha,a′,b,b′,c1,c1′,c2,c2′ exhibit substantial upfield shifts from 4.5 ∼ 6.5 ppm in the completely dissociated MB-2 to −1.5 ∼ 3 ppm in DWT-2, commensurate, to the best of our knowledge, with an unprecedented change (Δδ = −7.7 ppm) in chemical shift for Hb′. In an attempt to rationalize these large changes in chemical shifts, we carried out nucleus independent chemical shift (NICS) calculations (Table S13) based on the structure of DWT-2 on the right in Figure 3A. The differences in the calculated NICS values (Table S4) for the chemical shifts of the pairs of heterotopic resonances for the o-xylylene protons in DWT-2 match very well with the Δδ values (Table S5) in the 1H NMR spectrum (Figure 3C) of DWT-2. These observations indicate that the assembled superstructure of MB-2 at high concentration corresponds with DWT-2, wherein the asymmetric [π∙∙∙π] stacking between o-xylylene and porphyrin units results in substantial upfield shifts of the resonances for their protons,47Shao C. Grüne M. Stolte M. Würthner F. Perylene bisimide dimer aggregates: fundamental insights into self-assembly by NMR and UV/vis spectroscopy.Chem. Eur. J. 2012; 18: 13665-13677Crossref PubMed Scopus (88) Google Scholar,48Mugridge J.S. Bergman R.G. Raymond K.N. 1H NMR Chemical shift calculations as a probe of supramolecular host-guest geometry.J. Am. Chem. Soc. 2011; 133: 11205-11212Crossref PubMed Scopus (31) Google Scholar reflecting the anisochronous nature of all protons in their local anisotropic environments. The 13C NMR spectrum of DWT-2 exhibits (Figure 3E) a total of 56 resonances—i.e., more than twice the 27 resonances observed in the 13C NMR spectrum of the completely dissociated MB-2. 2D HSQC and HMBC spectroscopies allowed (Figures S22 and S24) us to assign the 56 resonances to two heterotopic t-butyl groups (four resonances) and all the other 52 heterotopic carbon atoms, including all 20 carbon atoms on the porphyrin ring, confirming the anisochronous nature of all carbon atoms in the MB-2 building block49The molecular formula of MB-2 is C60H50N4O2Zn in which the 60 carbon atoms correspond to 56 resonances in the 13C NMR spectrum on account that every three homotopic methyl groups in two heterotopic t-butyl groups give only one resonances. within DWT-2. In line with the 1H NMR spectrum of DWT-2, all eight o-xylylene carbon resonances (Ca,a′,b,b′,c,c′,t,t′) exhibit (Figures 3D and 3E), to the best of our knowledge, unprecedented upfield shifts (Δδ = −2.9 ∼ −8.8 ppm) compared with those for the o-xylylene carbon resonances in the 13C NMR spectrum of the completely dissociated MB-2. The aromatic-walled nanospace associated with DWT-2, resulting in strong shielding and anisotropic effects, is reminiscent of a diametrically opposite ex