This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://dx.doi.org/10.1021/jacs.9b08584. Highly Strained, Radially π-Conjugated Porphyrinylene Nanohoops Youzhi Xu,† Sebastian Gsänger,‡ Martin B. Minameyer,§ Inhar Imaz, Daniel Maspoch, Oleksandr Shyshov,† Fabian Schwer,† Xavi Ribas,# Thomas Drewello,§ Bernd Meyer,‡,* and Max von Delius†,* † Institute of Organic Chemistry, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany. ‡ Interdisciplinary Center for Molecular Materials (ICMM) and Computer-Chemistry-Center (CCC), Friedrich-Alexander Uni- versity Erlangen-Nürnberg, Nägelsbachstraße 25, 91052 Erlangen, Germany. § Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany.  Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and the Barcelona Institute of Science and Technology, Campus UAB, E-08193 Bellaterra, Barcelona (Catalonia, Spain). # Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Campus Montilivi, E- 17003 Girona (Catalonia, Spain). ABSTRACT: Small π-conjugated nanohoops are difficult to prepare, but offer an excellent platform for studying the interplay be- tween strain and optoelectronic properties and increasingly, these shape-persistent macrocycles find uses in host-guest chemistry and self-assembly. We report the synthesis of a new family of radially π-conjugated porphyrinylene/phenylene nanohoops. The strain energy in the smallest nanohoop [2]CPT is approximately 54 kcal mol-1, which results in a narrowed HOMO-LUMO gap and a red shift in the visible part of the absorption spectrum. Due to its high degree of preorganization and a diameter of ca. 13 Å, [2]CPT was found to accommodate C60 with a binding affinity exceeding 108 M-1 despite the fullerene not fully entering the cavity of the host (X- Ray crystallography). Moreover, the π-extended nanohoops [2]CPTN , [3]CPTN and [3]CPTA (N for 1,4-naphthyl; A for 9,10- anthracenyl) have been prepared using the same strategy, and [2]CPTN has been shown to bind C70 five times more strongly than [2]CPT. Our failed synthesis of [2]CPTA highlights a limitation of the experimental approach most commonly used to prepare strained nanohoops, because in this particular case the sum of aromatization energies no longer outweighs the buildup of ring strain in the final reaction step (DFT calculations). These results indicate that forcing ring strain onto organic semiconductors is a viable strategy to fundamentally influence both optoelectronic and supramolecular properties. INTRODUCTION Carbon-rich “nanohoops” exhibiting radial π-conjugation (Figure 1), such as the [n]cycloparaphenylenes, have attracted much attention recently due to their challenging synthesis, in- triguing optoelectronic properties and their vast potential in su- pramolecular chemistry.1 Macrocycles containing porphyrins have been pursued for several decades, 2 leading to important advances in host-guest chemistry3 and catalysis.4 Most reported compounds, however, do not exhibit an uninterrupted conjuga- tion pathway or significant ring strain. An exception regarding conjugation is Anderson’s work on large porphyrin na- norings,1b,5 which has produced new concepts of template syn- thesis6 and spectacular insights into global (anti)aromaticity7 as well as charge delocalization.8 The smallest nanoring synthe- sized by Anderson features five porphyrin moieties linked by butadiyne spacers that appear to bear most of the moderate ring strain.9 Osuka’s porphyrinelene/phenylene hybrids featuring three to six porphyrins within the macrocycle are subject to sig- Figure 1. Radial π-conjugated porphyrinylene/phenylene nano- nificant strain energies (up to 49 kcal mol-1). However, with a hoop. diameter of 16 Å, the smallest nanohoop of this series is still too We wondered whether a smaller variant of Osuka’s macro- large to effectively accommodate fullerenes.10 cycles would be accessible based on recent progress in the syn- thesis of highly strained macrocycles.11 Specifi- Figure 2. Synthesis of the [n]CPT. (a) Reaction conditions: (i) Pd(PPh3)4, Cs2CO3, pyridine, toluene, 125 ⁰ C, 15 h. (ii) TBAF, THF, RT, 1 h. (iii) H2SnCl4, 0 ⁰ C  RT, 12 h, ([2]CPT 10%, [3]CPT 8% 3-step yield) . (b) Partial 1H NMR spectra (CDCl3, 298 K, 400 MHz), [2]CPT-OTES, [2]CPT-OH, [2]CPT, [2]CPTC60 and [3]CPT. (c) UV-vis absorption spectra of 2 (black) [2]CPT-OH (light red) [2]CPT (light green) and [3]CPT (dark red). cally, we anticipated that [2]cyclo-5,15-porphrinylene-4,4,4- that a π-π template effect between electron-deficient pyridine terphenyl ([2]CPT) would have a very similar diameter to and the electron-rich porphyrins may be at work. As shown in [10]cycloparaphenylene ([10]CPP), which has recently been Figure 2b, compound [2]CPT-OTES exhibits a broad peak in shown to be an excellent host for C60,12 enabling studies on non- the 1H NMR spectrum for the signal corresponding to the tert- covalent charge transfer13 and the synthesis of [2]rotaxanes.14 butyl groups. Variable-temperature (VT) NMR spectroscopy Herein we report that [2]CPT as highly strained porphy- and line-shape analysis allowed us to determine the kinetic pa- rinylene/phenylene nanohoop can be synthesized in seven lin- rameters (e.g. G‡298 = 58.9 kJ mol-1, Figure S3-5) for this pro- ear steps. Although the calculated15 ring strain of 54 kcal mol-1 cess, which we attribute to hindered di-(tert-butyl)phenyl rota- in this compound is not as high as in the smallest CPP ([5]CPP: tion due to steric clash between two tert-butyl groups on one 119 kcal mol-1),16 we observed a strong influence of strain on face of the macrocycle (see Figure 3a, right hand side). the absorption and in contrast to the CPPs the bathochromic The isolated compounds [n]CPT-OTES underwent a smooth shift affects the visible part of the spectrum. We also prepared transformation into the corresponding alcohols [n]CPT-OH several π-extended analogues of [n]CPT and found that mem- upon addition of a suitable fluoride reagent. Typically, we con- bers of the [2]CPT nanohoop family are extremely effective re- verted these intermediates immediately into the target com- ceptors for C60 and C70 (e.g. Figure 1). pounds, but in one instance we attempted to purify compound [2]CPT-OH and were able to grow single crystals suitable for RESULTS AND DISCUSSION X-Ray crystallography. The solid state structure (Figure 2a) of Synthesis of Precursors. The key steps in the synthesis of this compound reveals a rectangular (slightly oval) shape with [n]CPT are shown in Figure 2a. Diboronate 1 and meso-por- a Ni−Ni distance of 7.4 Å and two porphyrin macrocycles with phyrin 2 are accessible on a multigram scale in four and three a “ruffle” geometry and an offset angle of 80°, which likely linear steps, respectively. Under standard conditions for Su- helps avoiding an unfavorable interaction between two tert-bu- zuki-Miyaura cross-coupling (125 C, toluene, Cs2CO3), we tyl groups. Because this compound could be of interest as a bi- found that the crucial ring-closing step only gave minuscule metallic catalyst,17 it is worth noting that in this solid state struc- quantities of the desired small nanohoop [2]CPT-OTES. In a ture the cavity is populated by (masked) solvent molecules and parameter optimization study for this reaction step, we discov- that DFT studies point towards negligible ring strain (Table 1), ered that the addition of pyridine (100 equiv.) led to a signifi- as well as the ability to adopt a variety of conformations, in- cantly increased yield of the desired macrocycle and an unex- cluding some with small cavity volumes (Figure S39). pected ratio between [2]CPT-OTES and [3]CPT-OTES of ca. Nanohoop Synthesis and Characterization. We found that 2:1. We initially attributed this effect to the binding of pyridine the final aromatization step (Figure 2a) required rigorously op- to the nickel center, but based on the Ni−Ni distance of 7.4 Å in timized reaction conditions, which is presumably as a result the solid state structure of [2]CPT-OH (vide infra), we believe Table 1. Selected properties of [2]CPT-OH, [3]CPT,[2]CPTN, [3]CPTN and [3]CPTA. Compound Strain Energya max (S, Q)b maxb E0Ox1 ( E0Ox2) c E0red1 ( E0red2) c HOMOd Ege -1 -1 -1 [kcal mol ] [nm] [L·mol ·cm ] [V] [V] [eV] [eV] [2]CPT-OH 0.8 416, 535, － 4.3  10 5 0.54 (－) -1.80 (－) -5.46 2.07 [2]CPT 54.3 446, 575, 617 3.4  105 0.32 (0.68) -1.57 (-1.86) -5.32 1.66 [3]CPT 34.1 432, 552, 590 4.0  105 0.45 (0.84) -1.77 (-1.98) -5.43 1.95 [2]CPTN － 442, 567, 610 2.7  105 0.34 (0.60) -1.69 (-1.92) -5.33 1.78 [3]CPTN － 430, 550, 589 4.0  10 5 0.44 (0.73) -1.79 (-2.04) -5.44 2.04 [3]CPTA － 428, 548, 585 6.2  105 0.49 (0.87) -1.83 (-2.10) -5.45 2.37 a Homodesmotic DFT calculations (B3LYP+D3/def2-TZVP); b Measured in CH2Cl2; c CH2Cl2, TBAPF6 (0.1 M), 295 K, V = 100 mV·s-1, vs. Fc+/Fc; d Set Fc+/Fc EHOMO = -5.1 eV; e Calculated by the difference of the values of Eonsetred1 and Eonsetox1. of the large amount of ring strain generated and the risk of por- avoids excessive ring strain by placing the two face to face por- phyrin.degradation.10 Only when we used the mild reagent phyrins out of horizontal line. The packing diagram reveals ev- H2SnCl411d, were we able to isolate compounds [2]CPT and idence for intermolecular π-π interactions between the ter- [3]CPT in reasonable three-step yields of 10% and 6%, respec- phenyl bridges (3.5 Å, Figure 3c), which leads to “sideway” tively. With pure compounds [2]CPT-OH, [2]CPT and stacking of the molecules. In the third dimension, this packing [3]CPT in hands, we proceeded to compare key properties by leads to uniform pores with “walls” composed nearly exclu- NMR and UV-Vis spectroscopy, cyclic voltammetry and DFT sively from sp2–hybridized carbon atoms, which could be of in- calculations (Table 1). The 1H NMR spectra (Figure 2b) indi- terest for future porous energy storage materials.18 cate that in contrast to the rectangular precursor (vide supra) the Optoelectronic Properties. While the unstrained precursor rotation of di-(tert-butyl)phenyl groups is not hindered at room [2]CPT-OH exhibits an absorption spectrum typical for temperature in both [2]CPT and [3]CPT, which is likely a con- tetraaryl nickel porphyrins, we observed red shifted Soret as sequence of the strain-induced conical arrangement of these well as Q bands and an inversion in the intensity of the Q bands groups. Other notable features in the NMR spectra include sig- for the strained macrocycles [3]CPT and [2]CPT. Of note, nificant differences in the chemical shifts of aromatic protons strain-induced red-shifts are limited to the emission spectra and (red in Figure 2b) and two sets of signals for pyrrole protons the ultraviolet part of the spectrum for the related [n]CPPs.1a (“F” and “E”), pointing towards a “ruffle” rather than “saddle” Compounds [3]CPT and [2]CPT were found to be essentially geometry of the porphyrin moieties, which are bent out-of- non-fluorescent, which is typical for Ni porphyrins however.19 plane by ca. 35° (Cmeso−Ni−Cmeso). Data gathered independently by cyclic voltammetry (Figures S26) and DFT calculations (Figures S46-S48) indicates that the observed bathochromic shifts in the absorption spectrum are to a narrowing of the HOMO/LUMO gap with increasing ring strain. DFT calculations of the frontier molecular orbitals reveal that for both the HOMO and the LUMO the orbital coefficients are delocalized over the entire ring, yet dominant on the por- phyrin moieties (Figure 3d). These results suggest that the in- corporation of organic semiconductor motifs into nanohoops is a viable strategy to systematically tune the band gaps and ab- sorption maxima.20 Fullerene Complexation. We next turned our attention to the inclusion of fullerene guests into the small nanohoop [2]CPT. As shown in Figure 2b, addition of one equivalent C60 led to dramatic changes in all signal sets of the 1H NMR spec- trum as well as a splitting of the pyrrole signals. By means of UV-Vis titrations in toluene (Figure 4a; carried out in triplicate), we were able to determine binding constants of ca. 3108 M-1 for C60 and ca. 2107 M-1 for C70. It is worth noting that the Figure 3. X-ray crystal structure of [2]CPT: (a) ORTEP drawing with strength of fullerene binding is so high that in the MALDI mass 30% probability. (b) Top view with distances and dihedral angles la- spectrum, where non-covalent interactions are typically broken beled. (c) Packing in the unit cell. Hydrogen atoms are omitted for clar- during ionization, the signal for the radical cation of complex ity. (d) Frontier molecular orbitals of [2]CPT calculated at the [2]CPTC60 is of the same intensity as that for the parent com- B3LYP/def2- TZVP level of theory (isovalue, 0.018). pound [2]CPT (Figure 4b). MS/MS experiments revealed that A solid state structure of [2]CPT could be obtained by syn- the nanohoop carries the positive charge in the radical cations chrotron X-ray diffraction (Figure 3). Single crystals of [2]CPT of the complexes [2]CPT⊃C60/70 (Figures S51, S52). Collision- were prepared by slow evaporation of a solution in CH3CN and induced dissociation experiments at variable collision energies CHCl3 (1:1). As shown in Figure 3ab, [2]CPT has an oval shape allowed direct comparison of gas phase relative dissociation en- (approximately C2 symmetry) with an average diameter of ca. ergies of fullerene complexes with a monomeric porphyrin (dis- 13.2 Å. The dihedral angle between neighboring benzene rings sociation onset at Ecom = 0.15 eV), [10]CPP (onset at Ecom = 0.49 is 53 and it seems reasonable to assume that the molecule eV) and [2]CPT, which in the gas phase binds the larger fuller- [2]CPTC60 and [2]CPTC70 complexes, the HOMO orbital ene C70 slightly more strongly than C60 (onset at Ecom = 0.78 and coefficients are localized exclusively on porphyrin moieties, Ecom = 0.76 eV, respectively). while the LUMO orbital coefficients are localized predomi- nately, but not exclusively on the fullerene (Figure 5d, S49). Hence, the DFT calculations suggest that charge transfer plays a role in the non-covalent complexes, which is in agreement with the bathochromic shifts observed during the host-guest ti- trations (Figure 4a). Figure 5. X-ray crystal structure of [2]CPTC60: (a) ORTEP drawing with 30% probability. (b) Top view with distances and dihedral angles labeled. (c) Packing in the unit cell. Hydrogen atoms are omitted for clarity. (d) Frontier molecular orbitals of [2]CPTC60 calculated at the B3LYP/ def2- TZVP level of theory (isovalue, 0.018). π-Extension of Nanohoops. Several π-extended CPPs, Figure 4. (a) UV-vis titration of [2]CPT with C60 (0-2.5 equiv) in tol- which can be considered intermediate structures on the way uene (fit based on 1:1 model). (b) MALDI mass spectrum of from CPPs to armchair carbon nanotube have been prepared in [2]CPTC60, (inset shows the experimental and calculated isotopic recent years.21-23 We wondered whether the inclusion of naph- pattern). (c) Energy-dependent fragmentation experiments for selected thalene or anthracene-moieties would be possible within the fullerene complexes, each fitted with a sigmoidal Boltzmann function [n]CPT architecture. To this end, start our synthesis from com- (see SI section 8 for details). mercially available α-naphthoquinone and anthraquinone to Single crystals of the [2]CPTC60 complex were grown by synthesize the diboronate precursors on a multigram scale. slow diffusion of CH3CN into a mixture of CHCl3 and 1,2-di- Macrocyclic compounds [n]CPTN-OTES and [n]CPTA- chlorobenzene (1:1). The solid state structure clearly shows that OTES (n = 2, 3) were prepared successfully using the same Su- a complex between [2]CPT and C60 with 1:1 stoichiometry is zuki-Miyaura cross-coupling conditions with comparable re- present. As shown in Figure 5a, the encapsulation of C60 is sults to the parent system. The porphyrin macrocycles could clearly facilitate by convex-concave π-π interactions (3.4 Å~3.7 also be transformed smoothly into the corresponding alcohols Å) and induces the nanohoop to adopt a more spherical shape. [n]CPTN-OH and [n]CPTA-OH by treatment with tetrabu- The dihedral angle between neighboring benzene rings de- tylammonium fluoride (TBAF). Strong deviations from the par- creases to 48, lending further support to the presence of π-π ent system were found in the final step of the synthesis. In case interactions between terphenyl bridges and C60. According to of the naphthyl system, we found that for the small-ring pre- the solid state structure, the 1H NMR data and DFT calculations cursor [2]CPTN-OH the final aromatization reaction only pro- (Figure S40), the diameter of [2]CPT is slightly too small for a ceeded at 70 C and gave only 32% yield of [2]CPTN after 12 “perfect” (symmetric) encapsulation of C60, which results in hours, whereas the larger [3]CPTN-OH could be easily trans- “off- center” binding with an offset of 1.9 Å. formed into [3]CPTN in 82% yield at room temperature (6 hours). In case of the anthracenyl system, we failed to convert The complex between [2]CPT with C70 was studied using the small-ring precursor [2]CPTA-OH into [2]CPTA (Figure DFT calculations, because high-quality single crystals could not S24) even at elevated temperature, which indicates that the aro- be obtained in this particular case. The calculations once more matization energy. This interpretation is not only in agreement indicate that the fullerene cannot fully enter into the cavity with with Clar’s “sectet” theory,24 but was corroborated by DFT cal- an offset distance of 2.7 Å, which as expected is larger than the culations, which indeed show that there is a difference of about corresponding offset for C60 (Figure S41). This finding provides 20 kcal/mol (2 moieties per ring) in the aromatization enthalpy an evident opportunity to design related fullerene receptors with gain between neighbouring compounds in this series (Figure even higher binding affinities (vide infra). For both the 6b). Hence, by moving form phenyl (reaction efficient at room (light red) [2]CPTN (light green) [3]CPTN (blue) and [3]CPTA (dark temperature) to naphthyl (reaction inefficient at elevated tem- red). perature) to anthracenyl (reaction impossible), we seem to have Depending on the rotation rate of the phenyl-naphthyl C-C probed the limitations of the aromatization-vs.-strain-genera- bonds, the π-extended nanohoops of type [n]CPTN can in prin- tion approach that is so commonly used in this area. 25 Of note, we can rule out an electronic, and with some confidence also a ciple exist as two different stereoisomers, which is why we con- steric effect, because the larger precursor [3]CPTA-OH could ducted an NMR study to shed light on this issue. The 1H NMR be converted to [3]CPTA in nearly quantitative yield under spectra of [2]CPTN and [3]CPTN showed only one set of sig- mild conditions. nals at room temperature, indicating either fast rotation of the The absorption spectrum of the two naphthyl-bridged nano- C−C bonds or the presence of only one stereoisomer. Conclu- hoops ([2]CPTN, [3]CPTN) are similar to the corresponding sive evidence to this end was obtained by variable temperature [n]CPT nanohoops, but the Soret-band and Q-band absorptions NMR (VT-NMR). The t-Bu group signal of [2]CPT, [2]CPTN are slightly blue-shifted (Table 1, Figure 6c). The [3]CPTA ab- and [3]CPTN splits into two peaks at low temperature (ca. 240 sorption maximum was further blue-shifted (428 nm , Table 1), K, 230 K and 250 K, respectively), which we attribute to the but the most striking observation for this compound is the high slow rotation of the porphyrin-(tBu)2-phenyl C-C bond (see molar absorption coefficient of the Soret band ( = 6.2105 cm- Figures S9, S16，S20, S21 for VT-NMR spectra and Eyring 1 M-1), which significantly exceeds those determined for all analyses). Evidence for a restricted rotation of the naphthalene other nanohoops. Differential-pulse voltammetry and cyclic units within the nanohoops was not observed over the investi- voltammetry experiments of [n]CPTN and [3]CPTA (Figure S26) revealed that these nanohoops exhibit a slightly increased gated temperature range (minimum temperature: 250 K). HOMO/LUMO gap when compared to their CPT analogues.23a To test the limits of fullerene affinity in these new nanohoop architectures, we studied the thermodynamics of [2]CPTNfullerene complexes. UV-Vis titrations revealed an association constant of 3.0108 M-1 (toluene) for [2]CPTNC60 (Figure S32), which is identical within error with the [2]CPTC60 complex and indicates that the π–extension of the nanohoop seemingly does not improve the “contact area” be- tween C60 and the nanohoop. In contrast, when the [2]CPTNC70 complex was studied in the same way, an in- crease of the binding constant (1.0108 M-1, toluene) by a factor of five was observed in comparison to the parent system (Figure 7c). This finding can be rationalized by the larger VdW surface of C70, and indeed this hypothesis was supported by DFT calcu- lations (Figure 7b). Figure 7. (a) Experimental (left) and simulated (right) 1H NMR spectra Figure 6. (a) Synthesis of the [n]CPTN and [n]CPTA: (i) H2SnCl4, of [3]CPTN at various temperatures. (b) Optimized geometries of RT70 C, THF, 12 h. (ii) H2SnCl4, RT, 12 h ([3]CPTN). NaI, [2]CPTNC70 obtained by DFT calculations at the PBE+D3 level of NaH2PO2, AcOH, 100 C, 6 h ([3]CPTA). (b) Relative differences in theory, side chains of [2]CPTN omitted for clarity. (c) UV-vis titration the aromatization enthalpies as calculated at the B3LYP+D3/def2- of [2]CPTN with C70 (0-1.9 equiv) in toluene (fit based on 1:1 model). TZVPP level of theory. (c) UV-vis absorption spectra of [2]CPTN-OH CONCLUSIONS by Dynamic Bond Formation with an Iridium Porphyrin Cyclic Dimer: A “Chemical Friction” for Rotary Guest Motions. J. Am. Chem. Soc. In conclusion, we developed a concise synthesis of a series 2007, 129, 11912–11913. (c) Song, J.; Aratani, N.; Shinokubo, H.; of strained porphyrin macrocycles, which due to their unique Osuka, A. A Porphyrin Nanobarrel That Encapsulates C60. J. Am. molecular design offer opportunities for uses in bimetallic ca- Chem. Soc. 2010, 132, 16356–16357. (d) Kieran, A. L.; Pascu, S. I.; talysis and crystal engineering. The two nanohoops [2]CPT and Jarrosson, T.; Sanders, J. K. M. Inclusion of C60 into an Adjustable [2]CPTN can be considered porphyrinogenic equivalents to Porphyrin Dimer Generated by Dynamic Disulfide Chemistry. Chem. [10]CPP, albeit with ca. 100-fold increased affinity for fuller- Commun. 2005, 1276–1278. (e) Tashiro, K.; Hirabayashi, Y.; Aida, T.; enes, which may prove useful for the regioselective synthesis or Saigo, K.; Fujiwara, K.; Komatsu, K.; Sakamoto, S.; Yamaguchi, K. A Supramolecular Oscillator Composed of Carbon Nanocluster C120 and separation of fullerene bisadducts14,26 and in photoelectroactive a Rhodium(III) Porphyrin Cyclic Dimer. J. Am. Chem. Soc. 2002, 124, devices27. We also observed unusual optoelectronic properties, 12086–12087. (f) Nobukuni, H.; Shimazaki, Y.; Tani, F.; Naruta, Y. A most importantly, a strain-induced red-shift of absorption in the Nanotube of Cyclic Porphyrin Dimers Connected by Nonclassical Hy- visible range of the spectrum, which may inspire further studies drogen Bonds and Its Inclusion of C60 in a Linear Arrangement. Angew. on the bending of organic semiconductors28 or molecular Chem., Int. Ed. 2007, 46, 8975–8978. (g) Xue, S.; Kuzuhara, D.; switches.29 Aratani, N.; Yamada, H. Synthesis of a Porphyrin(2.1.2.1) Nanobelt and Its Ability to Bind Fullerene. Org. Lett. 2019, 21, 2069–2072. (h) Gil-Ramírez, G.; Karlen, S. D.; Shundo, A.; Porfyrakis, K.; Ito, Y.; Briggs, G. A. D.; Morton, J. J. L.; Anderson, H. L. A Cyclic Porphyrin Trimer as a Receptor for Fullerenes. Org. Lett. 2010, 12, 3544–3547. ASSOCIATED CONTENT (4) Walter, C. J.; Anderson, H. L.; Sanders, J. K. M. Q. exo-Selective Supporting Information. Synthesis and characterization data. De- Acceleration of an Intermolecular Diels–Alder Reaction by a Trimeric tails on mass spectrometric, variable-temperature NMR, UV-Vis Porphyrin Host. J. Chem. Soc. Chem. Commun. 1993, 458, 458–460. titrations and theoretical calculations. This material is available free (5) (a) Hoffmann, M.; Wilson, C. J.; Odell, B.; Anderson, H. L. Tem- of charge via the Internet at http://pubs.acs.org. plate‐Directed Synthesis of a π-Conjugated Porphyrin Nanoring. An- Crystallographic data for [2]CPT-OH (CIF) gew. Chem., Int. Ed. 2007, 46, 3122–3125. (b) Rickhaus, M.; Vargas Crystallographic data for [2]CPT (CIF) Jentzsch, A.; Tejerina, L.; Grübner, I.; Jirasek, M.; Claridge, T. D. W.; Crystallographic data for [2]CPTC60 (CIF) Anderson, H. L. Single-Acetylene Linked Porphyrin Nanorings. J. Am. Chem. Soc. 2017, 139, 16502–16505. (6) Anderson, H. L.; Sanders, J. K. M. Amine-Template-Directed AUTHOR INFORMATION Synthesis of Cyclic Porphyrin Oligomers. Angew. Chem., Int. Ed. Corresponding Author 1990, 29, 1400–1403. *max.vondelius@uni-ulm.de (7) Peeks, M. D.; Claridge, T. D. W.; Anderson, H. L. Aromatic and *bernd.meyer@fau.de, Aantiaromatic Ring Currents in a Molecular Nanoring. Nature 2017, 541, 200–203. (8) Peeks, M. D.; Tait, C. E.; Neuhaus, P.; Fischer, G. M.; Hoffmann, M.; Haver, R.; Cnossen, A.; Harmer, J. R.; Timmel, C. R.; Anderson, Notes H. L. Electronic Delocalization in the Radical Cations of Porphyrin The authors declare no competing financial interest Oligomer Molecular Wires. J. Am. Chem. Soc. 2017, 139, 10461– 10471. ACKNOWLEDGMENT (9) Liu, P.; Hisamune, Y.; Peeks, M. D.; Odell, B.; Gong, J. Q.; Herz, L. M.; Anderson, H. L. Synthesis of Five-Porphyrin Nanorings by We are grateful for financial support from the Deutsche For- Using Ferrocene and Corannulene Templates. Angew. Chem., Int. Ed. schungsgemeinschaft (DFG, project number 182849149–SFB953 2016, 55, 8358–8362. “Synthetic Carbon Allotropes”), the University of Ulm, and FAU (10) Jiang, H; Tanaka, T.; Kim, T.; Sung, Y. M.; Mori, H.; Kim, D.; Erlangen-Nürnberg. I.I. thanks the Severo Ochoa Center of Excel- Osuka, A. Synthesis of [n]Cyclo-5,15-porphyrinylene-4,4'- lence Program (ICN2, Grant SEV-2017-0706) and X.R. thank biphenylenes Displaying Size-Dependent Excitation-Energy Hopping ICREA-Acadèmia award. Angew. Chem., Int. Ed. 2015, 54, 15197–15201. (11) (a) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. REFERENCES Synthesis, Characterization, and Theory of [9]-, [12]-, and (1) (a) Darzi, E. R.; Jasti, R. The Dynamic, Size-Dependent Properties [18]Cycloparaphenylene: Carbon Nanohoop Structures. J. Am. Chem. of [5]–[12]Cycloparaphenylenes. Chem. Soc. Rev. 2015, 44, 6401– Soc. 2008, 130, 17646–17647. (b) Golder, M. R.; Jasti, R. Syntheses of 6410. (b) Bols, P. S.; Anderson, H. L. 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