Chiral Dibenzopentalene‐Based Conjugated Nanohoops through Stereoselective Synthesis

Abstract Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π‐systems. They can possess unexpected optoelectronic properties and their radially oriented π‐system makes them attractive for host–guest chemistry. Bending the π‐subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel “corner unit” in the synthesis of the hoops. The [6]DBP[4]Ph‐hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO–LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.


Introduction
Conjugated nanohoops have become am ajor research area in recent years. [1] While hoop-shaped molecules have fascinated chemists since the 1950s, [1][2][3] the interest in the field has rapidly increased since 2008, when the first [n]cycloparaphenylenes ([n]CPPs) were synthesized. [4][5][6] Much progress has been achieved since then. [7] Not only have [n]CPPs of various sizes been synthesized [8][9][10][11] and their properties extensively investigated, but also many derivatives containing aromatic units other than benzene. [12][13][14] Emerging applications for nanohoops were identified, [15] and their use as segments of carbon nanotubes [16,17] as well as in supramolec-ular chemistry [18,19] is ar ising field. Thei ncorporation of psystems other than benzene allows to alter the optoelectronic and structural properties of nanohoops. [12] This has been demonstrated using donor-/acceptor-aromatics or polycyclic aromatic hydrocarbons mostly composed of six-membered rings. [12] We herein present the dibenzo[a,e]pentalene (DBP)based chiral nanohoops (+ +)-1 and (À)-1 consisting of six DBP units and four phenylene rings ( Figure 1A), accessed in as tereoselective synthesis.T he DBP units endow ambipolar electrochemical behavior to the hoop, [20] they possess antiaromatic character, [21,22] and their lack of ad ividing mirror plane when bent makes nanohoop 1 chiral. After rubicene- containing hoops reported by the Isobe group [23] and [2]DBP- [12]CPP nanohoops synthesized by us, [24] this is the third synthetic report incorporating an on-alternant hydrocarbon into ananohoop and the second using an antiaromatic unit. [25] Theh oop incorporation allows to study the effect of radial conjugation and bending on the antiaromaticity of the DBP units.D BP possesses as mall band gap due to an increased HOMO and decreased LUMO energy in comparison to an alternant hydrocarbon of similar size. [1] This causes its ambipolar electrochemical character,m aking it attractive for, e.g.,field-effect transistors. [26][27][28] Chiral conjugated nanohoops have been synthesized on few occasions by unsymmetrically incorporating polycyclic aromatic hydrocarbons,s uch as phenanthrene, [29] anthracene, [30] chrysene, [31,32] anthanthrene, [33] and rubicene, [23] or through topological design. [34][35][36] In most of these cases the hoops were synthesized as racemic or diastereomeric mixtures,and pure stereoisomers were separated by chiral HPLC. This was possible since racemization barriers were high enough to slow down conformational isomerization at room temperature.O ne example has been reported using ac hiral catalyst, which allowed obtaining an enantiomeric excess of ac yclophenylene, [37] and another where an enantioenrichment was obtained for ac yclochrysenylene. [31] We herein present aunique and novel synthetic strategy,which enabled the stereoselective synthesis of (+ +)-1 and (À)-1 as one of the first reports of astereoselective nanohoop synthesis.This was possible by using an ovel bent precursor for nanohoop synthesis,a ss hown in Figure 1B,abent and chiral diketone unit, which can be transformed into DBPs once incorporated into the hoop.Inthe synthesis of dibenzopentalenophanes,we had demonstrated before that this method is suitable to introduce strain and to strongly bend DBP units. [38] Herein we used both enantiomers of this diketone,s ynthesized by racemic resolution, as the only bent precursor in the synthesis of conjugated nanohoops (+ +)-1 and (À)-1.B ending ap referably planar p-system into ahoop shape is one of the highest challenges in nanohoop synthesis. [11] Several strategies leading to abent array of six-membered rings have been reported in the context of CPP syntheses. [4][5][6][39][40][41] Our strategy reported herein adds to this pool and enables the stereoselective synthesis of chiral nanohoops containing antiaromatic and electrochemically ambipolar DBP units.

Racemic resolution of diketone 3
We recently reported on the synthesis of the DBPcontaining nanohoops [2]DBP [12]CPP, [24] accessed using ItamisL -shaped diphenylcyclohexane units [42] as bent precursors to terphenyl units together with tetrafunctionalized DBPs.I no rder to synthesize an anohoop that is chiral and contains ah igher ratio of DBP units,w ew anted to refrain from using bent terphenylene precursors and instead employ diketone 3,which possesses anaturally bent structure (107.28 8 between two six-membered rings) [38] and is achiral precursor to DBPs,m aking it ideally suited to access strained nano-hoops.T he 2,7-bromo substituents in 3 would later allow to perform cross-coupling reactions.Racemic 3 was synthesized in four steps from 2-(4-bromophenyl)acetic acid, as previously reported. [38] In initial synthetic attempts to nanohoop 1 (as mixture of stereoisomers) using racemic 3 we faced two main difficulties:( a) Diastereomeric mixtures of intermediates were formed, containing several of the diketone units,which were difficult to purify and structurally characterize,a nd (b) macrocyclization proceeded with low yield, likely due to the required conformation being hard to achieve in some of the diastereomeric precursors.T oa void these issues we decided to enter the nanohoop synthesis with enantiomerically pure diketone 3 and therefore establish amethod for its racemic resolution. While as eparation of its enantiomers by analytical chiral HPLC was possible (see SI, Figure S88), the low solubility of 3 in common organic solvents restricted its resolution on asemi-preparative HPLC scale to < 50 mg. For the syntheses envisioned herein to obtain chiral DBP-based nanohoops,l arger amounts were required. Thec arbonyl functions in 3 provide au seful handle for its transformation into diastereomers,s eparable on ap reparative scale.W e initially tested the transformation of 3 into abisketal using l-(+ +)-diethyl tartrate,i ts functionalization to ab isimine using (S)-a-methylbenzylamine or to ab ishydrazone using (À)menthydrazide. [43] However, in the first two cases functionalization was unsuccessful, and in the third case the diastereomeric hydrazones formed from 3 were inseparable by column chromatography or crystallization. With success,onthe other hand, proceeded the sulfoximine-mediated racemic resolution developed by Johnson, [44,45] as shown in Scheme 1. Chiral sulfoximine (S)-4 was synthesized from thioanisol in three steps including racemic resolution of the intermediate sulfoximine before N-methylation. [46][47][48] After deprotonation with ethyl magnesium bromide, diastereoselective,c erium trichloride-mediated addition [49] of (S)-4 to 3 afforded diastereomers 5 and 6 in high yields, which were easily separable (DR f = 0.4) on large scale using Scheme 1. Racemic resolution of 2,7-dibromo-diketone 3 and molecular structure of bis-adduct 6 in the solid state (displacement ellipsoids are shown at 50 %p robability;h ydrogen atoms and cocrystallized chloroform molecule are omitted for clarity).
an automated column chromatography system. Thea ddition of the sulfoximine selectively occurred from the convex side of diketone 3.This can be seen in the molecular structure of 6 in the solid state,s hown in Scheme 1, resolved by X-ray diffraction on single crystals,g rown from chloroform/npentane by solvent layering.T hermolysis in toluene allowed removing the chiral auxiliary and furnished (R,R)-3 and (S,S)-3 in excellent yields and ees. Their absolute configuration was determined through the molecular structures of 6 as well as (S,S)-3 (see SI) in the solid state.T his method of racemic resolution allowed accessing both enantiomers of 3 in 1.2 g scale.
Forc omparison of the optoelectronic properties we synthesized 7,r epresenting ap lanar reference compound for nanohoop 1 (Scheme 3). Its synthesis started from 18, which was obtained in analogy to 12 (Scheme 2), but using racemic 3 as starting material (see SI for details). Mesityl groups were attached to the two outer diketone rings in 18 using aS uzuki-Miyaura-coupling reaction to furnish 19. Cerium trichloride-mediated addition of the Grignard-reagent of 15 to the diketone units in 19 followed by acidcatalyzed hexafold water elimination afforded reference compound 7 in good yield of 63 %.

Molecular structure of DBP-based nanohoops (+ +)-1 and (À)-1
Due to their mirror-symmetric structure as enantiomers, the NMR spectra of hoops (+ +)-1 and (À)-1 were identical (see SI). Comparison of the 1 HNMR spectra of nanohoop (+ +)-1 with planar reference compound 7 confirmed the high symmetry and more rigid structure of the hoop (Figure 2, some protons were assigned for comparison, for full assignment see SI). Thedoublets of doublets (highlighted in purple) for the DBP units,located next to the mesityl groups in 7 and next to an eighboring DBP unit in hoop (+ +)-1,w ere shifted downfield by about 0.5 ppm in the hoop compared to planar 7.
TheD BP protons highlighted in green were more differentiated in hoop (+ +)-1 compared to reference compound 7 due to the higher rigidity of the hoop,t he same held for the DBP protons highlighted in yellow.Astronger splitting was also observed for the ethyl protons (highlighted in blue) in (+ +)-1 compared to 7.AVT-NMR experiment did not reveal significant changes in signal width or splitting with increasing temperature (see SI).
To evaluate the structural properties of 1 by DFT calculations all alkyl groups were replaced by methyl groups, furnishing 2-1 (its stereochemistry corresponding to the D 2symmetric-enantiomer drawn in Figure 1o r1a in Figure 4). Theg eometry of 2-1 was initially assessed using molecular dynamics (MD) simulations (with the OPLS3 Force Field as implemented in Schrodinger 2017), followed by geometry optimization at the PBEh-3c [51] -level of theory (see SI for details).

Angewandte Chemie
Research Articles units are bent by V DBP = 32.98 8 on average,w hich is slightly less than in the [2]DBP [12]CPP-hoop mentioned above and significantly less than the bends of up to V DBP = 91.98 8 reported by our group for DBP-phanes. [38] In comparison, the four phenylene units in 2-1 experience abend of only 7.18 8, which is lower than that in [18]CPP of 8.08 8 with similar diameter. [52] While the phenylene units are rotated more strongly out of the hoop shape relative to the DBP units (dihedral angle V DBP-Ph-av. = 53.78 8), the dihedral angles between two DBP units are comparably small (V DBP-DBP-av. = 36. 18 8). Thec alculated strain energy of 2-1 amounts to 44.8 kcal mol À1 (see SI for homodesmotic equation used). This is larger than in the slightly smaller [2]DBP [12]CPPhoop (36.5 kcal mol À1 )r eported by us [24] or in [18]CPP of similar size (31.7 kcal mol À1 ). [52] Ther eason may lie in the larger dihedral angles between the DBP and phenylene units in 2-1 of 548 8 due to the ortho-substituents on the latter compared to large [n]CPPs,where the dihedral angles amount to 368 8 on average.This implies alower conjugation and hence lower conjugative stabilization in 2-1,r esponsible for its relatively high strain energy.

Stereoisomerism in DBP-based nanohoops (+ +)-1 and (À)-1
We next assessed the stereoisomerism in 1.I nt heory,1 4 diastereomers 1a-1n are possible by rotation of one or several DBP units through the hoop (A or Bc onformation, Figure 4), of which ten are pairs of enantiomers and four are meso compounds.I nt he most symmetric (D 2 -symmetric) enantiomer 1a,a ll DBP units are oriented the same way (A conformation). Its enantiomer (all DBPs in Bconformation) would be 1a*. 1a is also the conformation we depicted in Figure 1a nd Scheme 2f or simplicity.
DFT calculations provided insight into the relative energies and interconversion barriers of the 14 diastereomers 1a-1n shown in Figure 4. Thes tructures of 1a-1n were optimized using the semiempirical extended tight-binding model GFN2-xTB [53] with tight convergence criteria and applying the implicit solvation model GBSA [54] with toluene solvent. This method is particularly well-suited to explore the conformational space of molecular systems.B 97-3c [55] DFT single-point energies were computed on all optimized structures,a pplying the implicit solvent model D-COSMO-RS [56] with toluene solvent. With 9.0 kcal mol À1 (maximum energy for 1n)t he energy range is substantial, and 1a as the D 2symmetrical isomer as well as 1c with one DBP unit rotated relative to the others are the most stable diastereomers with relative energies of 1.2 and 0.0 kcal mol À1 ,r espectively. Energetic barriers for the rotation of the two symmetrically inequivalent DBP units were estimated at the B97-3c-(toluene)//GFN2-xTB(gas phase) level of theory using asimplified molecular structure of the hoop,namely 2-2 ( Figure 1). In 2-2 the 4-(n-hexyl)-phenyl substituents Ronthe DBP units were replaced by Ha toms to allow for at ransition state calculation, since the actual hoop was too large for such ac alculation. Thec alculated values for one DBP rotation amounted to 8.9 and 10.8 kcal mol À1 ,a nd they thus mostly reflect the dihedral strain occurring through rotation of the DBP units with respect to the neighboring aryl or DBP rings. Using these estimated values in two possible pathways for complete racemization of hoop 2-2 a to its enantiomer 2-2 a* (see SI, Figure S132), in which all six DBP units have to rotate,p rovided estimated values of 16 or 18 kcal mol À1 for the simplified molecular structure of the hoop 2-2.I nt he actual hoop 1,including all hexylphenyl substituents Ronthe DBP units,b oth energetic as well as entropic contributions would significantly increase the rotational (free energy) barriers for the DBP units.Interestingly,amolecular dynamics simulation of the full system 1aat 400 Katthe GFN2-xTB/ GBSA(toluene) level of theory showed that while the hoop is conformationally highly flexible and its shape changes from circular to oval, no rotation of aD BP unit through the hoop occurred. This can be seen in the animation, provided as additional material online.I nc omparison, the same calculation with highest energy conformer 1n (relative energy 9kcal mol À1 )l ed to almost ac omplete rotation of one DBP unit to furnish al ower energy conformer (see animation as additional material).
We next turned to optical rotation and electronic circular dichroism (ECD) measurements to assess the chirality of the synthesized hoops (+ +)-1 or (À)-1.T he ECD spectra for (+ +)-1 or (À)-1 possess am irror image relationship,c onfirming their enantiomeric relationship ( Figure 5A). TheE CD spectrum of (+ +)-1 shows ap ositive Cotton effect between 500-350 nm followed by an egative Cotton effect from 350-300 ( Figure 5A). In addition, an egative Cotton effect appears between 700-500 nm, corresponding to the HOMO-LUMO transition of the DBP units,which is symmetry-forbidden and therefore weak (see also below,d iscussion of the UV/Vis spectra). Thes ignals correspond well to the UV/Vis absorp-

Angewandte Chemie
Research Articles tion spectrum of (+ +)-1 shown in Figure 6A (an overlay of both spectra can be found in the SI, Figure S111). TheE CD spectrum of (À)-1 shows the opposite signs for the Cotton effects at the same wavelengths as in (+ +)-1 ( Figure 5A), confirming its mirror-symmetric structure.T op otentially assign the ECD-spectrum of (+ +)-1 to one or several of the stereoisomers 1a-1n (see Figure 4), we simulated ECD spectra for all 14 stereoisomers (see SI, Figure S115). Chiral hoops 1a-1j showed Cotton effects of the same signs at similar wavelengths,only with different intensities,depending on the number of DBP units in each orientation (A or Bi n Figure 4). 1a with all DBP units oriented the same way had the highest intensities,f ollowed by 1b and 1c with one unit rotated, and 1d-1i as well as 1j had the lowest calculated intensities of the CD-spectroscopic bands.E xemplarily,t he simulated spectrum for 1a is shown in Figure 5A.I t corresponds well in shape to the experimental ECD-spectrum of (+ +)-1.M easurement of specific optical rotation for the DBP hoops gave [a] 25 D values of opposite sign with + 5.48 8 (c =  These rotation values are low and at the instrumental detection limit (see SI), but their opposite sign confirms the mirror image geometry for both hoops. ECD-spectra were also measured for linear hexaones (R,R) 3 1.03, CHCl 3 )a nd À76.88 8 (c = 0.98, CHCl 3 ), respectively. These are significantly larger than in (+ +)-1 or (À)-1 likely due to the presence of discrete stereocenters in 16.However, aq uantitative evaluation of optical rotation values in different molecules is not possible.
We performed temperature-dependent ECD spectra to gain insight into the conformational behavior of hoop (+ +)-1 ( Figure 5B-D). We chose chlorobenzene as solvent for these experiments,s ince it allowed accessing higher temperatures.The ECD spectrum of (+ +)-1 in chlorobenzene at 20 8 8C ( Figure 5B,b lue line) slightly differs to that in chloroform ( Figure 5A)i nt hat al onger wavelength shoulder around 530 nm is present. To investigate the temperature-dependence of the ECD-signals,w eh eated the sample from 20 to 110 8 8C(at arate of 2 8 8Cper min) and recorded ECD spectra at regular intervals ( Figure 5B). All spectra in Figure 5are UV/ Vis-corrected to rule out concentration effects.W ith increasing temperature,the intensities of all bands slightly decreased. Thel argest change happened at around 60 8 8C, where in particular the shoulder around 530 nm disappeared. Upon further heating,t he signal intensity further decreased for all bands.Wethen held the temperature at 110 8 8Cfor 18 h, during which time no further change occurred ( Figure 5C). Lastly, the solution was again cooled to rt, whereby the intensities of the CD bands slightly increased again ( Figure 5D). However, the shoulder band at 530 nm did not reappear.T hese results show that optical activity of (+ +)-1 is not lost, even after prolonged heating to 110 8 8C, and complete racemization did not occur.T he temperature-dependent changes we observed in the ECD spectra of (+ +)-1 were mostly reversible,with the exception of the shoulder at 530 nm. This result is in line with the MD simulations mentioned above,w hich showed that in 1aat asimulated temperature of 400 Knone of the DBP units rotated. Simulation of the ECD spectrum of 1a at 400 K confirmed aslightly lower intensity of the bands compared to 300 K( see SI, Figure S116). It may be that higher temperatures are required to observe ar acemization of the hoop, however, the CD spectrometer we used only allowed heating to 110 8 8Ca sthe highest temperature.

Electronic structure of DBP-based nanohoop 1
Theoptoelectronic properties of hoop 1 are dominated by the DBP units and reflect its ambipolar character.T he UV/ Visa bsorption spectra of (+ +)-1 and reference compound 7 showed several bands,c haracteristic for substituted DBPs ( Figure 6A). [27,38,49,57] Most constitute transitions involving several orbitals,aswepreviously assigned for small molecule DBP derivatives using TDDFT calculations. [49,57] Electronically,the DBP units in hoop 1 are intermediate between 2,7- [49] and 5,10-arylalkinyl-substituted DBPs, [57] since the bands between 400-500 nm are relatively large in comparison to the absorption maximum at 322 nm due to significant conjugation to the 5,10-aryl substituents.T he slight bathochromic shift of all bands in hoop (+ +)-1 compared to reference compound 7 indicates astronger conjugation in the hoop.Most noteworthy is the lowest energy band, corresponding to the HOMO ! LUMO single excitation and well visible in the inset in Figure 6A.T his transition is forbidden in planar and centrosymmetric DBP derivatives,w here both orbitals are of a u symmetry (Laportesr ule). Since the DBP units in hoop 1 deviate more strongly from planarity than in planar 7,t his shoulder at 500-600 nm had asignificantly higher intensity for the hoop.The optical band gap of (+ +)-1 amounted to 1.66 eV. This value is strongly bathochromically shifted compared to reference compound 7 with 1.87 eV,to[2]DBP [12]CPP hoop recently reported by us with 1.83 eV [24] as well as to small molecule DBP derivatives [57] and indicates strong conjugation around the hoop.N anohoop (+ +)-1 showed no fluorescence, which has been observed for other DBP derivatives. [49,57] Thec yclic voltammogram of (+ +)-1 demonstrates its ambipolar electrochemical character due to the DBP units [20] ( Figure 6B). Ar eversible reduction occurred at ah alf-wave potential of E 1/2 = À1.71 V, and two quasi-reversible oxidations appeared at E 1/2 = 0.63 and 0.80 V( all vs.F c/Fc + ). Compared to the [2]DBP [12]CPP hoop recently reported by our group and a2,5,7,10-tetramesityl-substitutedDBP [24] both reduction and oxidation were facilitated (shifted to higher respective lower absolute potential) in (+ +)-1.F or reference compound 7 the first oxidation occurred at asimilar potential as in the hoop (measured in CHCl 3 for solubility reasons,see SI for CV). Based on this and the UV/Vis data, the HOMO [58,59] and LUMO [60] energies for (+ +)-1 were estimated to À5.35 eV and À3.69 eV,respectively.
Thec alculated frontier molecular orbitals of 2-1 are distributed over several DBP units ( Figure 6D and SI for images of other orbitals). From the LUMO up to LUMO + 5 and the HOMO down to HOMO-11, these orbitals lie very close in energy ( Figure 6C, DE 0.15 eV). Hence the redox events visible in Figure 6B could be processes involving more than one electron, which could explain the higher current for the reduction and second oxidation compared to the first oxidation.
NICS (nucleus independent chemical shift) values [61] provided information on the (anti)aromatic character of the DBP units in 2-1.N ICS(1) iso above/below the five-and sixmembered rings amounted to average values of 4.6 and À6.3, respectively (calculated with the GIAOm ethod on the B3LYP/6-31G* level of theory). Ac omparison with the NICS(1) values for the unsubstituted DBP of 5.9 for the fiveand À6.2 for the six-membered ring [62] shows that the antiaromaticity in the central pentalene unit in hoop 2-1 is slightly reduced while the aromaticity of the six-membered rings remains unaffected.
Complexation of fullerene-C 60 by (+ +)-1 Due to their hoop shape and large p-surface,c onjugated nanohoops are well-suited to bind guest molecules,s uch as fullerenes.Astrong binding of C 60 (diameter 0.71 nm [63] )b y nanohoops has been reported on several instances [18,19] for hoops with diameters of 1.3-1.5 nm, such as [10]CPP. [64] With 2.5 nm, the diameter of hoop 1 (calculated for 2-1)i n spherical shape significantly exceeds these values,h owever, our initial MD simulations indicated the hoop structure to be somewhat flexible.I ndeed, NMR titration experiments [65] performed in triplicate with C 60 in [D 8 ]toluene showed ashift in some of the 1 HNMR-resonances with increasing concentration of C 60 ( Figure 7A). Thes pectra indicated af ast exchange between free and complexed species,asonly one set of signals was visible.T oo btain binding constants,w eu sed hoop (+ +)-1 and a1 :2 ((+ +)-1:C 60 )b inding model, [66][67][68] which considered the formation of 1:1a nd 1:2c omplexes.T his model provided ab etter fit of the experimental data using nonlinear regression and an online tool [69] than a1 :1 model. Ther esulting binding constants were K 11 = (5.4 AE 0.7) 10 3 m À1 and K 12 = (1.1 AE 1.0) 10 2 m À1 for the complex formation between hoop (+ +)-1 and one respective two C 60 molecules.These values lie three to four orders of magnitude below association constants with C 60 reported for smaller nanohoops,such as [10]CPP,due to the large size of (+ +)-1.In the statistical case aratio of 4:1between K 11 and K 12 would be expected; [66] hence our results indicate an anti-cooperative situation for the binding of the second fullerene molecule. This is somewhat surprising,since both C 60 molecules are well accommodated within the hoop,and only asmall distortion of the latter is required (see Figure 7B).
DFT calculations provided insight into the molecular structures of the 1:1-and 1:2-complexes ( Figure 7B). Structures were optimized at the GFN2-xTB level of theory,a s described above.T he nearest distances between C 60 and the DBP and phenylene units amount to 3.24 on average in the 1:1c omplex, which is close to double the van-der-Waals radius of carbon (1.7 ).
Thec alculations also furnished association free energies consisting of the following contributions [Eq. (1)]: where DG RRHO is the thermostatistical contribution in the rigid-rotor-harmonic-oscillator approximation calculated with GFN-FF, [70] DE is the gas phase association energy and dG solv the solvation free energy of each species,the sum of the latter two terms calculated at the GFN2-xTB level of theory. Thea ssociation free energy for the complexation of the first C 60 molecule by (+ +)-1 of À5.3 kcal mol À1 matches very well with the experimental value of À5.1 kcal mol À1 ,while binding of the second C 60 molecule is slightly stronger in the calculation (À6.5 kcal mol À1 )c ompared to the experiment (À3.2 kcal mol À1 ).

Conclusion
We herein reported the stereoselective synthesis of two enantiomers of the chiral conjugated nanohoop 1.T hese hoops contain six dibenzo[a,e]pentalene (DBP) and four arylene units.S tereoselectivity was achieved by using bent and chiral diketone precursors to DBP units in enantiomerically pure form, thereby introducing an ew bent "corner" unit for nanohoop synthesis.E lectronic circular dichroism spectra and MD simulations showed that-inspite of its conformational flexibility regarding the outer shape-hoop (+ +)-1 did not racemize even when heated to 110 8 8C. The antiaromaticity of the DBP units was reflected in the ambipolar electrochemical properties of the hoop allowing for areversible reduction and two quasi-reversible oxidations. Due to its large size of 2.5 nm the [6]DBP [4]Ph-hoop could accommodate up to two fullerene-C 60 molecules,s hown through NMR-based binding studies and DFT calculations.