Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero‐ or Polycyclic Aromatics

Abstract In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size‐dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.


Introduction
Conjugated nanohoops have appealed to chemists for many decades,a st hey can be used to address fundamental questions,p ost synthetic challenges,a nd open up new applications. [1] Their cyclic conjugation, the radial orientation of their p-system, their rigid structure,s ize-dependent physical properties,a nd host-guest chemistry make them attractive for organic chemists,t heoreticians,m aterials engineers, and physicists. [2][3][4][5][6] Bending a p-system out of planarity can significantly alter its optical, electronic,charge-transport, and self-assembly characteristics,g iving nanohoops unique properties as light-emitters,r edox-active molecules,a nd supramolecular structures.D ue to these intriguing properties, conjugated nanohoops were identified as attractive synthetic targets more than 80 years ago. [5,7] However,t heir synthesis remained elusive for decades with only few reported examples of hoops incorporating ring sizes other than six. [4,[8][9][10] More recent synthetic advances led to the extensive synthesis of nanohoops in the past 12 years,i np articular that of [n]cycloparaphenylenes ([n]CPPs,F igure 1a), [11][12][13] for which synthetic attempts had failed as late as 1994. [14] Amongst the most interesting attributes of [n]CPPs are their size-dependent properties,i np articular the invariance of the longest wavelength absorption and the redshift of the emission with decreasing hoop size,w hich stands in contrast to linear oligo(paraphenylenes). [12,15] An umber of potential applications emerged for conjugated nanohoops,such as their use as solution-and solid-state fluorophores,a so rganic electronics components,and as templates for the construction of carbon nanomaterials. [16][17][18][19] Furthermore,c onjugated nanohoops show rich supramolecular chemistry. [20,21] Thes yntheses [11,13] and properties of [n]CPPs,i ncluding some derivatives, [22][23][24] have been reviewed on multiple occasions before, [12,18,[25][26][27][28] as well as their supramolecular chemistry. [20,21] To allow for at uning of the nanohoop properties beyond the sizedependent effect, aromatic rings other than benzene were introduced. This Review aims at providing an overview over such nanohoops beyond [n]CPPs,t hat contain not only benzene but other aromatic p-systems (Figure 1b). These include nanohoops with intrinsic donor-acceptor structure and hoops incorporating donor, acceptor,heteroaromatic, or polycyclic aromatic units,w hich will be discussed in Sections Inthe last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops.An umber of potential applications emerged, including optoelectronic devices,and their use as templates for carbon nanomaterials and in supramolecular chemistry.T otune the structural or optoelectronic properties of carbon nanohoops beyond the sizedependent effect known for [n]CPPs,avariety of aromatic rings other than benzene were introduced. In this Review,weprovide an overview of the syntheses,properties,and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or sucht hat contain acceptor,donor,heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.  2-6. Thec olor code shown in Figure 1b will be used to highlight the respective p-systems within the hoops throughout this Review.F urthermore,c onjugated nanobelts will be covered in Section 7oft his Review. Themain incentives to introduce different p-systems into nanohoops have been to 1) modify their optoelectronic properties,that is,byintroducing donor or acceptor moieties or even donor-acceptor structures,2 )modify their structural properties by breaking the high symmetry of [n]CPPs leading to chiral nanohoops,and 3) attempt to elongate nanohoops in the vertical direction with the aim to develop methods to synthesize single-chirality single-walled carbon nanotubes (SWNTs), [17,29] among others.A part from nanohoop derivatization, synthetic advances have allowed for the synthesis of nanocarbons with unique topologies. [30,31]

Synthetic Strategies Leading to Conjugated Nanohoops
Theh ighest challenge in nanohoop synthesis,a part from the macrocyclization step,istobend the preferably planar psystem into ah oop shape. [32] TheJ asti, [33] Itami, [34] and Yamago [35] groups were the first to develop methods to introduce such abend in oligo(paraphenylene) units,enabling the synthesis of [n]CPPs with n = 5-16, 18, 20, and 21. [11,12,13,36,37] Since then, this synthetic pool has been extended by an umber of other methods.T he synthetic strategies a)-f) used to access the nanohoops discussed herein are summarized in Scheme 1and will be referred to throughout this Review.T he initial bent precursors developed by Jasti, Bertozzi, [33] (Scheme 1a)a nd Itami [34] (Scheme 1b)c an be aromatized by reduction in the first case and elimination + oxidation in the second, while in Yamagosmethod the arylaryl bond is formed by reductive elimination of the square-planar Pt complex (Scheme 1c). [35] All three methods have been extensively used in the syntheses of the compounds reviewed herein. In 2014 Wang introduced acyclohexadienebased corner unit (Scheme 1d), which can be aromatized through oxidation, [38] in the synthesis of naphthalene-containing nanohoops,d iscussed in Section 6.2. Isobe employed an oxanorbornadiene derivative as ap recursor to a9 ,10connected anthracene corner unit in 2017(Scheme 1e), [39] highlighted in the same section. Lastly,o ur group used abent and chiral diketone precursor to incorporate dibenzo-[a,e]pentalenes into nanohoops (Scheme 1f), as will be discussed in Section 6.1. [40,41]

Size-Dependent Properties of [n]Cycloparaphenylenes
[n]CPPs possess intriguing size-dependent properties, which have been reviewed on several occasions. [12,15] For comparison with the nanohoops discussed in this Review,we have listed their optical and electrochemical properties in Table 1. Particularly noteworthy is the invariance of the longest wavelength absorption and the redshift of the emission with decreasing hoop size,d istinguishing [n]CPPs from linear oligoparaphenylenes.A sw ew ill highlight, introducing aromatic units other than benzene provides auseful handle to more strongly influence the optoelectronic or structural properties of conjugated nanohoops,for instance through donor or acceptor units (Sections 2-5) or polycyclic aromatic hydrocarbons (Section 6).

Nanohoops with Donor-Acceptor Structure
Donor-acceptor (D-A)-type systems are relevant in many areas,i ncluding optoelectronic devices,s uch as organic solar cells,l ight-emitting diodes,a nd sensors,i na ddition to biological systems.AD-A structure typically leads to al owering of the optical band gap and aspatial localization of the HOMO and LUMO on the donor respective acceptor moiety, among others.Due to the resulting (partial) charge separation in the excited state,atypical observation for D-A compounds is solvatofluorochromism, where the wavelength of the emitted light depends on solvent polarity, [44] as well as ab athochromically shifted charge-transfer (CT) band in the absorption spectrum. Introducing aD -A structure into an anohoop will significantly alter its optoelectronic properties,asthe examples below demonstrate.Nine reports exist to date on nanohoops with intrinsic D-A structure (see Figure 2 and optoelectronic properties in Table 2). In examples 1-5, 85,a nd 86 the donor part is an oligoparaphenylene moiety, while in compounds 6-8 and 84 the donor character stems from adimethoxynaphthalene or from thiophene units.Since CPP subunits have higher HOMO energies than linear oligoparaphenylenes due to their bent p-system, they can be regarded electronically as donors.
Thefirst D-A nanohoop (1)was reported by Itamisgroup in 2015, who introduced an anthraquinone moiety as an acceptor into a[ 10]CPP donor. [45] Suzuki-Miyaura coupling reactions were used to connect a2,6-disubstituted anthraquinone with an "Itami corner unit" (Scheme 1b), and aN imediated Yamamoto coupling was employed to perform the ring closure.T he acceptor character was further increased by transforming the anthraquinone moiety into at etracyanoanthraquinodimethane group (2). While the absorption maxima of 1 (332 nm) and 2 (335 nm) were almost identical to that of parent [12]CPP (338 nm), the fluorescence differed.
Scheme 1. Synthetic strategies to conjugated nanohoops used for the compounds reviewed herein.

Figure 2.
Donor-acceptor nanohoops 1 and 2, [45] 3 and 4, [46] 5, [47] 6a and b, [48] 7a/b, [49] 8, [50] 84, [51] 85, [52] and 86. [53] 1 exhibited green fluorescence in CCl 4 (l max-em = 496 nm) with ab athochromic shift to orange (591 nm) in the more polar chlorobenzene (Figure 3a). 2 showed red fluorescence in CCl 4 and benzene with abathochromic shift, but was non-emissive in more polar solvents.P arent [12]CPP,f or comparison, exhibits blue fluorescence.Calculations localized the electron density of the HOMOs on the oligoparaphenylene moieties and that of the LUMOs on the acceptor units with similar HOMO energies of À5.39 eV for 1, À5.48 eV for 2,a nd À5.25 eV for [12]CPP,w hile the LUMO energies were strongly affected with À2.71 eV for 1, À3.55 eV for 2,a nd À1.64 eV for [12]CPP in comparison. Also in 2015, the Jasti group published another approach to D-A nanohoops by introducing pyridinium moieties as acceptors (3 and 4)i nto CPPs as donors. [46] Thes yntheses were performed using aS uzuki-Miyaura coupling in the macrocyclization step and employed the Jasti corner unit (Scheme 1a). Several aza [8]CPPs with one,t wo,o rt hree pyridine units replacing phenylene rings were synthesized (27-29 in Figure 8, see also Section 5.1). Thee lectronic properties of these aza [8]CPP were quite similar to [8]CPP in spite of the presence of the pyridine rings,hence they will be discussed in Section 5.1. In the corresponding methylated hoops 3 and 4,o nt he other hand, obtained by reaction with methyl triflate,the acceptor character of the pyridine unit was strengthened, resulting in oxidation potentials shifted by ca. 1Vtowards higher voltages.DFT calculations confirmed the experimental data with al owering of the LUMO energy by 1.15 eV for 4 relative to [8]CPP.T he D-A hoops 3 and 4 showed ab athochromic shift in the absorption maxima compared to [8]CPP with abroad shoulder between 400 and 425 nm. Thef luorescence significantly redshifted to 598 nm (3)a nd 630 nm (4)c ompared to [8]CPP and the nonmethylated aza [8]CPPs 27-29.T he Jasti group further followed up with the smaller derivative aza [6]CPP (30 in Figure 8, see Section 5.1) and N-methylaza [6]CPP (5). [47]   b) Frontier molecular orbitals of 30 and methylated 5,reproduced from ref. [47] with permission from The Royal Society of Chemistry. c) Frontier molecular orbitals of 6c,r eprinted with permission from ref. [48]; copyright 2017 Wiley-VCH.
Their recently developed Pd-catalyzed oxidative homocoupling of boronic esters,which was successful in the synthesis of highly strained [5]CPP, [48] was employed for ring-closing. [54] Similar to aza [8]CPP,t he smaller aza [6]CPP 30 showed little change in optoelectronic properties compared to [6]CPP. After methylation to 5,o nt he other hand, the reduction potential was shifted upwards by 0.71 V( to À1.42 V) compared to aza [6]CPP 30.T he HOMO and LUMO of both compounds further confirmed the electronic nature of aD -A system (Figure 3b). This strengthened the previous findings of aza [8]CPP and its methylated derivative,t hat simple N-substitution has little effect on the energy levels and only methylated aza[n]CPPs possess D-A character.
TheT anaka group employed aR h-catalyzed cross-cyclotrimerization of ad iyne to obtain D-A- [12]CPPs with alternating 1,4-dimethoxynaphtalene units as donor and phthalimide or phthalate esters as acceptor separated by pphenylene units (6a and b). [48] A1 ,4-dimethoxy-5,8-dihydronaphthalene unit acted as the bent aromatic precursor (Scheme 1d)w ith yields up to 13 %o ver two steps (cyclization and aromatization). Single-crystal X-ray diffraction confirmed the conformation shown in Figure 2a sa ll-syn,i n which the acceptor and donor moieties faced each other despite steric bulkiness.T he molecules of 6a adopted ac olumnar packing structure with CH-p interactions between a t-butyl hydrogen atom and the dimethoxynaphthalene moieties.Both 6a and b showed abathochromic shift in the absorption spectra and as trong positive solvatofluorochromism due to their D-A structure.T his was also seen in the HOMO and LUMO electron density distribution calculated for 6c (Figure 3c).
In 2015 Ball et al. published their work on bithiopheneperylene diimide (PDI) donor-acceptor hoop 7,c alled "conjugated corral" due to its shape. [49] By using stannylated precursors and aP t-mediated coupling reaction (Yamago method, Scheme 1c), the authors were able to obtain all three stereoisomers (meso compound (S,R)a nd enantiomers (S,S) and (R,R)) via separation by chiral HPLC.T he enantiomers interconverted at room temperature via the meso isomer, reaching equilibrium after two hours.T he optoelectronic properties reflected the presence of both the bithiophene and the PDI units in addition to an ew bathochromically shifted band in the absorption spectrum.
Asimilar concept-using donors and acceptors frequently employed in co-polymers for organic electronics applications-was reported by Li and co-workers in 2019, who synthesized nanohoop 84 with alternating diketopyrrolopyrrol (DPP) and bithiophene units. [51] YamagosP t-mediated route (Scheme 1c)w as employed, and al inear reference compound with the same number of DPP and thiophene units served for comparison. DFT calculations showed 84 to possess nearly circular shape.B road photoluminescence in the range of 700-1000 nm was observed. Thep otential applicability of D-A-nanohoop 84 was demonstrated in three different device measurements:Asimple organic light-emitting diode measurement showed electroluminescence at 800-1000 nm with an external quantum efficiency of about 0.0001 %; an organic field-effect transistor measurement provided ambipolar charge transport;and when 84 was used as non-fullerene acceptor with P3HT as the donor in ab ulk heterojunction (BHJ) solar cell, an initial power conversion efficiency of 0.49 %w as found.
In 2017 the Wang group incorporated afluorenone unit as an acceptor into athiophene-and phenylene-containing hoop (8). [50] They performed the ring closure using aN i-mediated Yamamoto coupling and employed an oxidative aromatization of ac yclohexadiene precursor (Scheme 1d). 8 can be compared with [10]CPP by replacing four benzene rings with thiophenes and two benzene rings with the fluorenone group. 8 exhibited an intramolecular CT band in the absorption spectrum and solvatofluorochromic behavior with ab athochromic shift for solvents of higher polarity (545 nm in cyclohexane to 597 nm in CHCl 3 ). In contrast to [10]CPP, 8 showed two emission bands in CH 2 Cl 2 ,w ith one at 573 nm significantly redshifted. In the CV ar eduction event was visible,which can be attributed to the fluorenone group,while the oxidation wave was shifted towards higher potential compared to [10]CPP.T wo larger rings with two and three fluorenone groups were also obtained using at hiophenethiophene Yamamoto coupling.
In 2020 the Jasti group published the benzothiadiazolecontaining [10]CPP derivative 85 (BT[10]CPP). [52] Synthesis was afforded by Suzuki-Miyaura coupling of dibromobenzothiadiazole with aC -shaped synthon containing three Jasti corner units (Scheme 1a)f ollowed by reductive aromatization. While with 334 nm the absorption maximum of 85 was almost unchanged compared to [10]CPP (341 nm), the emission maximum was redshifted by 105 nm to 571 nm. Remarkably,t he fluorescence quantum yield (FQY) remained high (F F = 0.59) compared to [10]CPP (F F = 0.65), which stands in contrast to other D-A nanohoops (see Table 2) and made this the first bright orange-emitting nanohoop.A ne ven brighter orange emission was obtained for benzothiadiazole-containing nanohoop 86 (TB [12]CPP), published shortly afterwards in 2020. TheT an group synthesized 86 using YamagosPt-mediated route and confirmed its spherical structure by single-crystal X-ray diffraction. [53] 86 possessed two absorption maxima at 320 and 428 nm, with the latter being redshifted by 89 nm compared to parent [12]CPP. With 569 nm the emission maximum was even more redshifted compared to [12]CPP (450 nm), and 86 showed positive solvatofluorochromism due to its D-A character. TheF QY reached new record values as high as F F = 0.82 in solution for orange-emitting nanohoops.I na ddition, the supramolecular chemistry of 86 was explored using anthracene-C 60 ,among others,asaguest molecule.

Acceptor-Containing Nanohoops
Introducing electron acceptors into nanohoops lowers their orbital energies.T his is particularly interesting for application in optoelectronic devices,i fg ood n-type conduction is required and where aL UMO energy below À3eVi s desired. Three reports exist on such nanohoops,s hown in Figure 4, and with optoelectronic properties listed in Table 3.
TheY amago group expanded the scope of their Ptmediated cyclotetramerization (Scheme 1c)f rom using biphenyl groups to dibenzothiophenes to furnish sulfonesubstituted [8]CPP derivative 9 (see Figure 4, synthesis via 15 in Section 4.1) in 2017. [42] With the help of NMR spectroscopy, DFT calculations,a nd single-crystal X-ray diffraction, they confirmed the anti/anti/anti conformation of 9 to be thermodynamically most favored and 9.8 kcal mol À1 lower in energy than the syn/syn/syn conformation. Theabsorption maximum of 337 nm was nearly identical to that of parent [8]CPP (340 nm). Thef luorescence emission wavelength was also similar to [8]CPP,b ut the FQY much higher with F F = 0.41 compared to 0.081 ([8]CPP). Them ore rigid structure of the sulfur-containing hoop was probably the reason for the higher quantum yield. Cyclic voltammetry of 9 showed no oxidation but aq uasi-reversible reduction wave at À1.57 Vv s. Fc/Fc + , which is shifted to less negative values compared to [8]CPP with À2.33 V.
In 2017 the Wang group used their hexaester-containing [9]CPP [57] in aF riedel-Crafts acylation to obtain nanohoop 10,c onsisting of three indenofluorenedione groups and bearing six carbonyl groups in total. [55] NMR studies and DFT calculations revealed the anti/syn conformation (shown in Figure 4) as the most stable,4.3 kcal mol À1 lower in energy than the all-syn isomer and with arotational barrier of DG°= 23.3 kcal mol À1 for one indenofluorenedione group.T he optoelectronic properties were dominated by the indenofluorenedione moieties rather than the parent [9]CPP structure.T he absorption maximum of 10 at 288 nm was hypsochromically shifted compared to [9]CPP (340 nm), and two fluorescence maxima appeared at 448 and 606 nm upon Ref.  . Acceptor-containing nanohoops 9, [42] 10, [55] and 11. [56] Angewandte Chemie Aufsätze excitation at 395 nm. Thec yclic voltammogram showed ar eversible reduction with ah alf-wave potential of À1.47 V vs.F c/Fc + ,w hereas parent [9]CPP has no observable reduction wave in the respective electrochemical window. Ball et al. expanded their work on "conjugated corrals" with the synthesis of 11,containing four PDI units connected via biphenyl units,which was applied as an n-type material in organic electronics devices. [56] 11 as well as 7a/b ( Figure 2) were compared to their respective linear analogues,m onomeric units,and polymers.Organic photovoltaic devices were fabricated with each compound used as acceptor in aB HJ cell. Thecyclic compounds performed better than the acyclic ones since they absorbed more visible light and showed ab etter energy alignment with the donor material, higher electron transport mobilities,and an optimal phase separation for BHJ solar cells.T he absorption of 11 was bathochromically shifted compared to the acyclicm olecules,indicative of as maller HOMO-LUMO gap. 11 was also later used in an organic photodetector. [58]

Donor-Containing Nanohoops
In contrast to the acceptors discussed above,i ntroducing ad onor moiety can increase orbital energies.I na ddition, introducing S-containing heterocycles,s uch as thiophenes, will impact molecular packing in the solid state due to their strong van der Waals interactions. [67] Thiophenes,furans,and carbazoles as donor moieties were introduced into nanohoops,asdiscussed in this section (see Figure 5and Table 3). This Review excludes cyclo[n]thiophenes, [68] solely consisting of thiophene units,s ince they do not feature radial pconjugation due to the bond angles of the thiophene moiety.

Thiophene and Furan
TheI tami group synthesized as eries of alternating paraphenylene-and thiophenylene-containing nanohoops consisting of 4, 5, and 6u nits each ([n]CPT, [n]12 in Figure 5) in 2015. [59] Thes ynthesis was performed using their "Itami corner" (Scheme 1b)w ith two alkyne units,w hich were coupled using aG laser-Hay cyclo-oligomerization. This approach yielded cyclic diynes of different sizes ranging from dimers to hexamers,s eparable by column chromatography,w hich were transformed into the thiophenes using Na 2 S. After aromatization of the cyclohexane moieties,o nly hoop sizes n = 4-6 could be obtained because of the low thermal stability of the higher strained [3]12.The structure of [4]12 was confirmed by X-ray diffraction and showed the molecule with the thiophene units all pointing in the same direction ( Figure 6a). Them olecules adopted at ubular structure,i nc ontrast to [n]CPPs,w hich usually pack in ah erringbone structure.
[n]12 showed as light bathochromic shift in the absorption spectra compared to the respective [n]CPP with increasing hoop size (333 nm for n = 4to362 nm for n = 6). Like [n]CPP,t he fluorescence emission showed ablueshift with increasing ring size. [69] TheW ang group synthesized as eries of dimethoxynaphthalene-and thiophene-containing nanohoops in 2015. [60] In their synthesis they used ad ihydronaphthalene corner unit (Scheme 1d), and the cyclization was performed using aSuzuki-Miyaura coupling towards [n]13 and aNi-mediated dimerization towards 14.T he most stable conformer of [2]13 was the syn isomer with both naphthalene und sulfur atoms pointing in the same direction. VT-NMR measurements revealed ar otational barrier of the p-phenylene groups of DG°= 11 kcal mol À1 at acoalescence temperature of À41 8 8C. For 14 the anti isomer was the most stable with the naphthalene groups pointing in ad ifferent direction than the thiophene sulfur atoms,while for large hoop [4]13 it was the syn conformation. Thef luorescence maxima were blueshifted for the larger [4]13 compared to [2]13,a sk nown for [n]CPPs,but contrary to the previously mentioned [n]12.   [59] [n]13 and 14, [60] 15, [42] and 16 and [n]17. [61] Angewandte Chemie Aufsätze 15884 www.angewandte.de As mentioned in Section 3, the Yamago group synthesized the cyclic tetramer of dibenzothiophene 15 ([4]CDBT) as the [8]CPP derivative in 2017. [42] The anti/anti/anti structure was the thermodynamically most favored and 1.7 kcal mol À1 lower in energy than the syn/syn/syn conformation. With 337 and 510 nm the respective absorption and fluorescence maxima were close to the values for [8]CPP (340 nm and 533 nm, respectively). Yett he FQY (F F = 0.21) was higher than for [8]CPP (F F = 0.08), likely due to the more rigid structure of 15.Cyclic voltammetry of 15 gave two reversible oxidations at 0.74 and 0.96 Vv s. Fc/Fc + which was higher than that of [8]CPP with 0.59 V. Calculated HOMO/LUMO energy levels revealed the same behavior regarding the HOMO-LUMO energy gap for cyclic vs.l inear structures.C yclic 15 had aH OMO-LUMO energy gap of 3.35 eV,i ts linear analogue [4]DBT 3.78 eV.
TheW ang group investigated furan derivatives 16 and [n]17,analogues of their thiophene-containing nanohoops,in 2016. [61] Thes ynthesis was similar to that of their sulfur analogues using Suzuki and Yamamoto couplings with dihydronaphthalenes as aromatic precursors (Scheme 1d). They obtained the [12]CPP derivative 16 with two furan and two dialkoxynaphthalenes and the [10]-and [15]CPP derivatives [n]17 with four (n = 2) and six (n = 3) furans,a nd two and three naphthalenes,r espectively.T he structure of [2]17 (R = TIPS) was confirmed by X-ray diffraction and showed ac onformation in which all furan oxygen atoms and naphthalene units pointed in the same direction. This stands in contrast to thiophene-based 14,where the thiophene units pointed in the other direction. [60] Interestingly the aromatization reaction of both precursors with different relative configuration of the naphthalene groups (anti and syn) yielded CPPs with lower oxidation potentials with decreasing hoop size,a sh ad been the case for the thiophene-containing compound. Theh alf-wave potentials were lower for the furan than for the thiophene derivatives, that is,0.21 Vlower for [2]17 (0.28 Vvs. Fc/Fc + )compared to 14.T his is even lower than in [10]CPP with avalue of 0.74 V vs.Fc/Fc + .

Carbazole
Carbazole is an electron-rich heterocycle extensively used in molecules for optoelectronics.T his is due to its p-type character and the good hole mobility of its derivatives,and it was lately used as the donor in OLED emitters with D-A structure,s howing thermally activated delayed fluorescence. [70] Carbazole was incorporated into nanohoops in three different types of architectures,s hown in Figure 7. Their optoelectronic properties are listed in Table 3.
Stępień and co-workers synthesized two capped [3]cyclocarbazole derivatives in 2015, 18 and 19. [62] They used am esitylene derivative as the cap and internal template to facilitate ring closure,w hich they performed using aN imediated Yamamoto coupling. 18 and 19 can be regarded as derivatives of [6]CPP, 18 with three 2,7-carbazole groups and 19 with three 2,7-benzo[def]carbazole groups.The ring strain, however, was significantly higher than that of [6]CPP (98 kcal mol À1 )w ith av alue of up to 144 kcal mol À1 ,w hich lies in the region of [4]CPP. [71] Possible reasons are the limited conformational freedom with H-H repulsion, steric congestion because of the capping,and the increased rigidity of the carbazole units.B oth structures were confirmed by X-ray diffraction. Thea bsorption spectra of 18 and 19 were quite different from that of [6]CPP. 18 showed two main absorptions of similar intensity and, unlike non-emissive [6]CPP,was blue-fluorescent with maxima at 440 nm and 465 nm. These bands likely corresponded to transitions from ahigher excited state than S 1 .I nt he CV quasi-reversible oxidations of 18 occurred at 0.06 and 0.18 Vvs. Fc/Fc + ,which lie 0.24 Vlower than for [6]CPP,demonstrating the influence of the electronrich carbazole units.
In 2016, the Suzuki group synthesized aseries of [4]cyclocarbazoles 20 a and b and 21,a sd erivatives of [8]CPP,u sing the Pt-mediated cyclization approach (Scheme 1c)f rom stannylated precursors. [63] 21 has ab ridging alkyl linker, which was initially meant to exert at emplate effect. They confirmed the all-anti conformer to be the most stable by DFT calculations as well as by X-ray diffraction of 20 a.Like for the smaller 18,t wo main absorption maxima were observed, with the lower wavelength absorption in the region of [n]CPPs ( % 340 nm). Thef luorescence maximum was observed at 486 nm for 20 a with an FQY of F F = 0.18, 47 nm blueshifted compared to [8]CPP and with more than double the FQY.Inthe CV the carbazole derivatives showed am ore facile and reversible oxidation than [8]CPP,w ith voltages below 0.31 Vvs. Fc/Fc + .
TheP oriel group further investigated ethyl-substituted [4]cyclocarbazole 20 c in 2019 to gain more insight into its photophysical properties. [64] They optimized the Pt-mediated cyclization (Scheme 1c)u sing borylated precursors.A s before,t he structure of the 20 c was confirmed to be all-anti with respect to the carbazole nitrogen atoms by NMR spectroscopy (the observed high symmetry only allows for  [63] 20 c, [64] and 22. [65] Angewandte Chemie Aufsätze all-syn or all-anti), DFT calculations,a nd X-ray diffraction. With 73 kcal mol À1 the strain energy was the same as that of [8]CPP. [72] Thes hifts in wavelength were only minor in both absorption and fluorescence emission, although several solvents of different polarity (cyclohexane,c hloroform, and THF) were used. Finally, 20 c was used as active material in ap-channel organic field-effect transistor and provided ahole mobility of m = 1.1 10 À5 cm 2 V À1 s À1 with at hreshold voltage of V Th = 24 Vand an on/off ratio of the drain current of 4. 26 10 4 .T his is one of few examples of nanohoops being used in optoelectronic devices. [16] TheS tępień group synthesized the lemniscular-shaped [16]CPP derivative 22 with ac entral bicarbazole in 2019. [65] Starting from af ourfold borylated bicarbazole,J asti corners (Scheme 1a)w ere attached on each functionality,a nd aN imediated Yamamoto macrocyclization was employed. The structure was confirmed by mass spectrometry and NMR spectroscopy,s upported by DFT calculations.A lthough no single crystal of 22 for X-ray diffraction could be obtained, its not fully aromatized precursor was investigated in the solid state and showed the expected connectivity.T he phenylene rings in 22 had different bend angles:12.28 8 for the outermost rings,comparable to [6]CPP,and 3.38 8 for the innermost rings next to the bicarbazole.T he absorption was redshifted by 18 nm compared to [16]CPP,and the emission was even more strongly affected (496 nm vs. 4 15 and 438 nm in [16]CPP, Figure 6c)w ith values similar to [9]CPP,y et alower FQY of F F = 0.36. 22 is ac hiral molecule,a nd its enantiomers (atropisomers) were separated by chiral HPLC.N or acemization at room temperature was observed due to acalculated high racemization barrier of 51.4 kcal mol À1 .T he absolute structure of the enantiomers was verified by comparison of the experimental and calculated CD spectra (Figure 6c). A recent computational study by Si and Yang showed that functionalizing 22 with both ad onor and acceptor unit reduces its HOMO-LUMO gap,m odulates the HOMO/ LUMO distributions,a nd modifies the electronic transition properties and further revealed such derivatives to be excellent candidates for second-order nonlinear optical materials. [73] 5. Nanohoops with Other Heteroaromatics

N-Containing Six-Ring Heterocycles
As discussed in Section 2, exchanging ab enzene for ap yridine ring in [n]CPPs does not significantly alter their electronic properties,i ns pite of the more electron-poor character of pyridine.Y et such hoops are of interest for other reasons,f or instance bipyridines are excellent ligands for metal ions,w hich has been explored by several groups (see below). Furthermore,quaternization of the Natoms provides ah andle for further functionalization. Reported nanohoops incorporating N-containing six-ring heterocycles are shown in Figure 8, and their optoelectronic properties are listed in Table 4. In addition, in 2020 aC PP-based, two bipyridinecontaining lemniscal bis-macrocycle was reported, [74] and in the same year the Jasti group incorporated a meta-linked pyridyl unit into a [ 5]CPP and used the resulting macrocycle in am etal-templated approach to access ad aisy-chain rotaxane. [75] In 2010  [n]24 had the highest strain energies of all four compounds with ah uge variation for even-and odd-numbered n,e ven reaching strain energies higher than those of [n]CPP for n = 13 and 15. This was due to forced unfavorable s-cis conformations of the Na toms,l eading to arepulsion between the nitrogen lone pairs.
32 [ [71] 27-29, [46] 30, [47] 32, [76] 33, [77] [n]31, [78] and 34. [79] = 3-5). [78] While preliminary studies using a2 ,7-diphenylpyrene unit in aP t-mediated coupling reaction (Scheme 1c) of boronic esters did not furnish any macrocycles,t he introduction of nitrogen atoms into the structure allowed for the synthesis of [n]31.T he nanohoops showed simple NMR spectra due to the fast conformational fluctuation of their structures,even at À60 8 8C. An X-ray structure for [4]31 provided ad iameter of 21.3 ,w hich is similar to that of [16]CPP with 22.1 . [80] Thea zapyrene units alternately pointed upward and downward. Thed ihedral angle of the biphenyl units was quite large with 40-508 8 compared to [16]CPP (34.68 8) [71] or other nanohoops such as [4]cyclo-2,8chrysenylene (18.58 8). [81] TheI tami group used their cyclohexane corner unit (Scheme 1b)and Suzuki-Miyaura coupling reactions in 2012 to obtain the 2,2-bipyridine-containing [18]CPP derivative 32. [76] With ad iameter of ca. 25 it has the same size as [18]CPP.T he absorption and fluorescence maxima were slightly redshifted in comparison, as also observed for other nitrogen-substituted CPPs (see below for aza [8]CPPs). The absolute FQY was high with F F = 0.80. Upon protonation of the bipyridine moiety ar edshift of absorption and fluorescence was observed. This is in line with Jastisobservation that methylation of the Na tom creates ad onor-acceptor character of the nanohoop,a sd iscussed above for 3-5.F or protonated 32 the peak shape with as houlder was similar to those of the methylated aza [8]CPPs 3 and 4. [46] After Itami and co-workers had already indicated ap ossible Pd-complexation through bipyridine-containing 32,t he Jasti group followed with their own work on the smaller 33 with one bipyridine unit as a [8]CPP derivative in 2017. [77] 33 was synthesized using the Jasti corner in the bent precursor (Scheme 1a)and aNi-mediated coupling of the 2-chloropyridine units as the ring-closing step on am ultigram scale. Indeed, reaction of 33 with PdCl 2 led to aP dCl 2 -complexed species,w hich could be transformed into the corresponding Pd-complexed dicationic dimer by abstracting the chloride ions with AgBF 4 .S ingle-crystal X-ray diffraction proved a trans geometry of the dimeric structure at the Pd center (Figure 9b). AR uc omplex was also synthesized. While the optical properties of 33 did not significantly differ from [8]CPP (main absorption at 345 nm compared to 340 nm), the main absorptions of the Ru complex were redshifted with abroad band from 425-575 nm for the metal-to-ligand charge transfer.
Lastly,Z hu, Cong,a nd co-workers synthesized catenane 34 in 2018, where each nanohoop of the catenane consists of eight para-phenylene and one phenanthroline unit. [79] Tw o monomeric precursors with incorporated Jasti corners (Scheme 1a)w ere transformed into ac atenane using Sauvagesc opper(I)-templated method. [84] Ring closure was achieved using aP d-catalyzed oxidative homocoupling of boronic esters,f ollowed by removal of the copper ion and aromatization. Forc omparison the single hoop was also synthesized. Single-crystal X-ray diffraction unambiguously confirmed the formation of the catenane and the Mçbius topology of 34 (Figure 9a). With adiameter of 1.1-1.4 nm, its size is comparable to [8]- [10]CPP.T he highly symmetric 1 HNMR spectrum suggested fast conformational changes. Theabsorption and fluorescence maxima of 34 and its single hoop were similar and showed the same fluorescence wavelength as [10]CPP of 466 nm. ACID (anisotropy of the current induced density) calculations indicated alocal aromaticity in each benzene ring and the phenanthroline unit. Calculations showed that nonbonding interactions contributed À84 kcal mol À1 of stabilization to the catenane structure.

Porphyrins
Cyclic porphyrin arrays have been of interest as models for artificial light-harvesting antenna complexes,h osts for molecular recognition, and scaffolds for efficient hole delocalization. [85,86] Figure 10 provides an overview of reported nanohoops containing porphyrin units,a nd their optoelectronic properties are summarized in Table 5. Thef irst examples of porphyrin nanohoops solely consisting of paraconnected aromatic units and porphyrins were reported in the past six years.In2015, Kim, Osuka, and co-workers employed aPt-mediated coupling reaction (Scheme 1c)toconnect 1,12diborylated Ni-porphyrins to form cycloporphyrins [n]35 ([n]CP, n = 3-5). [82] Thes tructures were highly symmetric as T hese sizes were comparable to those of [7]CPP, [9]CPP,a nd [12]CPP. [80] Thec alculated ring strain energies were up to 8kcal mol À1 lower than for the parent [n]CPPs. Themain absorption (Soret band) of the porphyrin units was redshifted with increasing size.E lectrochemical investigations revealed several oxidation and reduction events.T he half-wave oxidation and reduction potentials both increased in absolute value with hoop size.

Nanohoops Based on Polycyclic Aromatic Hydrocarbons
In contrast to electronically modulating nanohoops,many groups set their goal on synthesizing hoops containing polycyclic aromatic hydrocarbons (PAHs) to investigate their dynamic properties,t ouse them as model segments for carbon nanotubes,ortoinduce chirality in the hoops.Several examples were published for smaller PA Hs,such as naphthalene and anthracene,a sw ell as larger structures,s uch as hexabenzocoronene.I nm any cases VT-NMR measurements or CD spectroscopy was used to investigate rotational barriers of the bent PA Hs through the hoop.I nt his section we will discuss PA H-containing nanohoops sorted by the type of the PA Hi ncorporated. Theo ptoelectronic properties of these Angewandte Chemie Aufsätze nanohoops can be found in Table S1 in the Supporting Information.

Nanohoops Incorporating Five-Ring-Containing PAHs
Fluorene is one of the most abundant co-monomers in conjugated co-polymers for OLED applications.The bridging of two phenyl rings by as aturated carbon increases rigidity and conjugation, leading to ar edshift in absorption and emission and as mall Stokes shift compared to biphenyl. There are several reports of nanohoops with incorporated fluorene groups (Figure 11). In the first example from 2015, Yamago and co-workers synthesized [3]-and [4]40 a. [87] In contrast to most nanohoops synthesized via the Pt-mediated route (Scheme 1c)t hey obtained at rinuclear in addition to the tetranuclear Pt intermediate by changing the solvent and Pt precursor in the cyclization step.T his was possible due to the geometry of the fluorene moiety.
[3]40 a was received as am ixture of two rotamers,w hich were separable by chiral HPLC,while [4]40 a was only observed as the all-anti rotamer shown in Figure 11. This was due to ah igh calculated rotational barrier of the fluorene units in [3]40 a of 58 kcal mol À1 ,n ot allowing interconversion even at 180 8 8C, as shown by VT-NMR. In comparison, [4]40 a has al ow barrier of 18 kcal mol À1 ,t heoretically allowing for dynamic conformational changes.T he higher rigidity of [n]40 a compared to [n]CPPs led to an enhanced conjugation and bathochromic shift of the absorption maxima as well as ac athodic shift of the first oxidation potentials compared to [6]-and [8]CPP. Most noteworthy is the significantly enhanced FQY of F F = 0.32 for [4]40 a compared to [8]CPP (F F = 0.081) together with asmaller Stokes shift due to this rigidity.Similar to these hoops is [4]40 c with propyl side chains,s ynthesized by Loh, Huang,a nd co-workers and published in 2016. [88] Here,a lso Yamagosp rocedure using aP t-mediated route (Scheme 1c) was used. NMR spectroscopy and single-crystal X-ray diffraction revealed the all-anti rotamer with aslightly oval hoop shape (d:1 .19 1.00 nm). Both in solution and in thin film [4]40 c showed an emission at 512 nm with an even higher FQY than [4]40 a of F F = 0.45. To prove its practical applicability,t hey used [4]40 c as the emitter in an OLED showing strong green emission with ab rightness of 878 cd cm À2 at 10 Vand am aximum luminescence efficiency of 0.83 cd A À1 .T his was one of the first examples of an anohoop being used in an optoelectronic device. [16] In 2018 Quinton, Poriel, and co-workers reported [4]40 b with ethyl side groups on the fluorene units,s ynthesized in aP t-mediated route (Scheme 1c). [89] X-ray crystallography revealed the hoop to possess an all-anti conformation, as had been reported for the other [4]cyclofluorenes discussed above.I nterestingly,c ompared to the emission maxima of  [74] Thes yntheses started from at etrabromospirobifluorene,w hich was coupled in aS uzuki-Miyaura reaction with two C-shaped units of different sizes containing two Jasti corners each (Scheme 1a). In addition, aderivative of [6,6]41 was synthesized containing four pyridine units in the outer loops.T hree of the lemniscal bis-macrocycles were characterized by single-crystal X-ray diffraction. Compared to [n]CPPs (l max = 335-340 nm), the absorption maxima were redshifted to l max 353-358 nm. Thef luorescence blueshifted with increasing size (from 493 nm for n = 8to454 nm for n = 12), which is acommon feature for CPP-derived macrocycles. Due to their porous solid-state structures,t he gas and vapor analyte uptake of [n,n]41 was investigated and showed an increased guest uptake compared to [n]CPPs.
In contrast to fluorene with ap artially saturated fivemembered ring, PA Hs with conjugated five-membered rings can significantly alter the electronic properties of the hoops due to their non-alternant p-system. Four examples exist for such PA Hs incorporated into nanohoops,n amely rubicene ([n]42), fluoranthene (88), and dibenzo[a,e]pentalene (DBP, 43 and 44) (Figure 11). Thecylinder-shaped nanohoops [n]42 ([n]CR) consisting of rubicene units were synthesized by Isobe (2017) using the Pt-mediated cyclization (Scheme 1c). [92] Both the three-and four-membered congeners were isolated (n = 3, 4). All four possible diastereomers of [4]42 could be separated by chiral HPLC,w hile [3]42 was formed as one of two possible diastereomers (Figure 12 a) with all-syn orientation of the rubicene groups,a sc onfirmed by NMR spectroscopy and X-ray diffraction. UV/Vis spectra indicated conjugation around the hoop and an extended psystem compared to monomeric rubicene.T his was in line with ab ond length evaluation, which indicated reduced aromaticity and an enhanced quinoidal character for [3]42 compared to rubicene.
In 2020 the Du group reported [10]CPP derivative 88 with an incorporated fluoranthene group. [93] Synthesis was afforded using aC -shaped synthon for a [ 7]paraphenylene linker based on three Jasti corner units (Scheme 1a)i n aS uzuki-Miyaura reaction with ab is-borylated diphenylfluoranthene unit. While the absorption spectrum strongly resembled that of [10]CPP,t he emission maximum was redshifted by 22 nm to 488 nm with an FQY of F F = 0.49. 88 can host one C 60 molecule with ab inding constant of 1.4 10 6 m À1 ,which is similar to that of [10]CPP (3 10 6 m À1 in the same solvent toluene).
In 2020 our group published the first synthesis of ananohoop containing an antiaromatic system, namely 43,with two dibenzo[a,e]pentalene units. [94,95] Thesynthesis included using the Itami corner unit (Scheme 1b)a nd aY amamoto cyclization followed by oxidative aromatization to yield two electronically different derivatives 43 a and 43 b.I na ccordance with previous results from our group,bending the DBP slightly reduced its antiaromaticity. [96] Ther edox properties reflected the ambipolar character of the DBP units with reversible oxidations and reductions and the expected electronic impact of the substituents R. Thef rontier molecular orbitals were separated between the DBP units and the para-phenylene linkers with electron densities in HOMOÀ1, HOMO,L UMO,a nd LUMO+ +1m ainly localized on the DBPs (Figure 12 b). Thep resence of these two p-subsystems was also reflected in an additive way in the UV/Vis absorption spectra. 43 a was emissive due to the para-phenylene linkers, however, with aF QY of < 1% due to energy or charge transfer to the non-emissive DBP units.
In 2021 we followed up with chiral DBP-based nanohoops 44 accessed in astereoselective synthesis. [40] Both enantiomers (+ +)-and (À)-44 were synthesized separately by using enantiomerically pure and bent diketone precursors (Scheme 1f). Thes mall HOMO-LUMO gap and ambipolar electrochemical character of the DBP units were reflected in the optoelectronic properties of the hoops.Electronic circular dichroism spectra measured at elevated temperatures and molecular dynamics simulations showed that (+ +)-44 did not racemize even when heated to 110 8 8Cinspite of its conformational flexibility regarding the outer shape.D ue to its large diameter of ca. 2.5 , 44 was able to accommodate two C 60 molecules,a sN MR-based binding studies showed (Figure 12 c).

Naphthalene
Naphthalene is the PA Hm ost often incorporated into nanohoops.T he reported examples are shown in Figure 13. Shortly after their first synthesis of [n]CPPs,I tami and coworkers published the synthesis of a( 2,6)-naphthalenecontaining [13]CPP,n amely [13]45. [97] They used the Itami corner unit (Scheme 1b)a nd Suzuki-Miyaura couplings for ring closure.A lthough 45 possessed helical chirality,t he calculated racemization barrier was low with 8.4 kcal mol À1 due to the large size of the hoop.In2019 Dusgroup used aCshaped synthon for a [7]paraphenylene linker based on three Jasti corner units (Scheme 1a)i naS uzuki-Miyaura reaction with a2 ,6-bifunctionalized naphthalene to synthesize the more strained [7]45. [98] Its main absorption at 335 nm was comparable to the absorption maxima of [n]CPPs.T he vibrational structure of the emission spectrum of [7]45 resembled that of the larger [10]- [12]CPPs while being redshifted due to the lower optical band gap.T he FQY (F F = 0.30) lay between those of [8]-and [9]CPP.A nother hoop consisting of alternating aryl-and 2,6-connected naphthyl units (six each) was published by Chi, Miao,a nd coworkers in 2019. [99] It served as ap recursor to ac onjugated nanobelt 83 (see Figure 23 in Section 7), and no optoelectronic data were published.
In 2015 Isobesgroup published the first synthesis of a2,6connected, solely naphthalene-containing hoop ([8]46)u sing Yamagosr oute (Scheme 1c)w ith diborylated binaphthyl units. [100] Then aphthyl units in [8]46 rapidly rotated at room temperature due to alow calculated barrier of 10.5 kcal mol À1 . Three out of 18 possible diastereomers of [8]46 were crystallized by changing the crystallization solvent and characterized by X-ray diffraction. They also synthesized analogue 47 with bridged binaphthyl units,i ntroducing more rigidity into the hoop and influencing its dynamic behavior. Theb inaphthyl units did not rotate in solution on the NMR timescale.Indeed, 47 had ar edshifted absorption, indicating ahigher degree of conjugation. In alater work the Isobe group synthesized five further derivatives of [n]46 with n = 6-11. [101] They combined precursors of different sizes in aY amago nanohoop synthesis (Scheme 1c). Am athematical model was evaluated to describe the number of possible stereoisomers.W ith an increasing number of repeating units,the number of possible diastereomers increased up to 63 for n = 11. For n = 7, out of nine possible diastereomers,t wo (in pairs of enantiomers) were characterized in the solid state,which crystallized in two sets of disordered structures.T he calculated rotational barriers ranged from 38.5 kcal mol À1 for n = 4t o5 .8 kcal mol À1 for n = 11.
Examples of 1,4-connected naphthyl groups are more abundant. In 2012 the Itami group published the synthesis of the 1,4-connected, nine-naphthyl-unit-containing hoop [9]48 ([9]CN) by applying an adapted cyclohexadiene-fused corner unit (Scheme 1a)inaNi-mediated "shotgun" cyclization. [102] They observed ad ynamic conformational change that was slow on the NMR time scale,w hich corresponded well with the calculated rotational barrier of 21 kcal mol À1 .The absorption maximum was redshifted (378 nm) compared to [9]CPP (339 nm), but emission occurred at almost the same wavelength with al ower FQY of F F = 0.35. Further research on 1,4-connected [n]48 was done by the Itami (2017) [103] and Du groups (2018). [104] While Itami proved the applicability of his synthesis for even-numbered [n]48 with n = 8, 10, 12, and 16, Du showed that it was also possible to use YamagosP tmediated route (Scheme 1c)for n = 8, 9, and 12 with aslightly higher yield in the last strain-inducing step.There are several trends observable from decreasing n. Theo ptical band gap decreased for smaller n with increasing strain, and the emission maxima were shifted to higher wavelengths,w hich is in agreement with theoretical predictions and the observations made for [n]CPPs (Figure 14 a). Akinetic study (via VT-NMR) on the thermal conversion of the C S -symmetric [10]48 into the more stable D 5d conformer revealed an isomerization barrier of 27.1 kcal mol À1 ,c onsistent with the theoretically predicted isomerization barrier. Dusresults supported these findings with only minor variations of the spectroscopic values.
In 2014 Wang and co-workers introduced their Diels-Alder synthesis of cyclohexa-1,4-dienes (Scheme 1d), enabling the synthesis of many different nanohoops. [38] In this publication they presented adihydronaphthalene corner unit and as ubsequent Ni-mediated "shotgun" cyclization leading to hexamethoxy-substituted 50 a as a [9]CPP analogue.NMR spectra revealed arapid rotation of the naphthalene moieties at room temperature.T he absorption and fluorescence maxima of 50 a were both redshifted by 18 nm to 362 nm and 512 nm, respectively,incomparison to parent [9]CPP.The Wang group also introduced these dimethoxynaphthalene units into hoops together with thiophenes [60] and furans, [61] as was discussed in Section 4.1 ( Figure 5).
Nanohoop 50 b,a nother [9]CPP analogue with electronwithdrawing substituents,was published by the Wang group in 2016 and was synthesized following asimilar strategy. [57] X-ray diffraction showed that the ester groups in 50 b all canted towards the inner plane of the [9]CPP and the naphthyl groups were oriented in a syn,anti fashion. With 346 nm the absorption maximum of 50 b was similar to [9]CPP (340 nm), while the fluorescence maximum at 445 nm was blueshifted by 49 nm compared to the latter. In 2017 the Wang group extended their synthetic route to naphthalene-containing hoops 51 and 52. [106] 51 b is the tetrameric congener of 50 a.An NMR analysis showed at ime-averaged D 4h -symmetric structure for 51 b,w hile DFT calculations predicted the all-anti conformer to be the most stable.B oth 51 a and b had an absorption maximum at 358 nm, redshifted by 20 nm compared to parent [12]CPP,w hile for smaller 52 the absorption maximum at 336 nm was similar to that of parent [10]CPP.A similar redshift of 34 nm compared to [12]CPP was observed in the fluorescence of 51 a and b at 484 nm, while 52 experienced ab lueshift to 453 nm (vs.4 70 nm for [10]CPP).

Phenanthrene
Further research into the stereoisomerism of hoop-shaped molecules was conducted by Isobesg roup,w ho investigated the dynamic behavior of the phenanthrene-based nanohoop 53 in 2016 ( Figure 13). [107] It can be regarded as aderivative of the previously mentioned cyclonaphthylenes with annelated benzene rings in the periphery of the hoop. 53 was synthesized using Yamagosr oute (Scheme 1c)w ith biphenanthrene monomers.X -ray diffraction showed ar acemic mixture of the (E,R,E,R,E,R,E,R)(shown in Figure 13) and (E,S,E,S,E,-S,E,S)isomers (Figure 14 b) with atime-averaged D 4 symmetry,a ss hown by NMR spectroscopy.I nV T-NMR measurements no splitting of the resonances was observed (see also [n]46 above). Thet wo D 4 -symmetric enantiomers were separated by chiral HPLC.AVT-CD spectroscopic measurement revealed rapid racemization between 30 and 60 8 8C. The racemization barrier (DH°= 25 kcal mol À1 )w as in the range of that observed for 1,1'-binaphthyl (DH°= 22 kcal mol À1 ).

Chrysene
In 2011 the Isobe group synthesized nanohoops 54 ([4]CC) with four 2,8-connected chrysene panels using Yamagosm ethod (Scheme 1c,F igure 15). [108] Due to their chirality they can serve as models for SWNTs.A ll six stereoisomers (two pairs of enantiomers and two mesocompounds) were separated and characterized by chiral HPLC.T he authors explored the possibility of asymmetric induction and were able to obtain an enantiomeric excess of 11 %a nd 17 %f or one set of the diastereomeric pairs of enantiomers by carrying out the reductive elimination in the presence of cholesteryl stearate.I nalater publication the authors investigated the fullerene complexation of one single enantiomer ((M)- (12,8)) of 54 by synchrotron X-ray diffraction, among others, [81] and furthermore elucidated the solidstate structure of the racemic mixture of the (12,8)-isomers of 54.
In 2017 the Isobe group investigated the separate stereoisomers of 54 in more detail;a mong others they structurally characterized the (P)-and (M)- (12,8) isomer and performed CD spectroscopic as well as circularly polarized luminescence measurements (Figure 16 a). [112] A high luminescence dissymmetry factor of g lum = À0.152 was measured in conjunction with ahigh FQY of F F = 0.80, which suggests that organic molecules could rival lanthanide complexes in terms of circularly polarized luminescence performance.

Anthanthrene
TheI sobe group incorporated anthanthrene panels into two different nanohoop architectures ( Figure 15). Thef irst example from 2013 was 56,acyclic 2,8-connected tetramer, which was synthesized from borylated precursors using Yamagosm ethod (Scheme 1c). [110] 56 a with TIPS substituents was formed as the D 4 -symmetric isomer shown in Figure 15 out of four possible diastereomers,a ss hown by X-ray diffraction. For 56 b with hexyl substituents,a ll four diastereomers formed and were separated by chiral HPLC. Ther otational barrier for the anthanthrene units in 56 b was experimentally determined to be 21 kcal mol À1 .A na ssociation study from 2017 using the (12,8) diastereomer of 56 b with the fullerene dimer C 120 provided association constants of K a1 = 6.9 10 8 m À1 and K a2 = 3.2 10 3 m À1 for the first and second binding event (in 1,2-dichlorobenzene). [113] An X-ray crystallographic structure of both the single and the double complex was obtained (Figure 16 c). In 2015 Isobesg roup synthesized the larger nanohoop 57 with incorporated biphenyls using Yamagosm ethod (Scheme 1c). [111] Due to its larger size,the structure of 57 rapidly fluctuated among the four possible diastereomers on the NMR timescale.D FT calculations provided as mall rotational barrier of 8kcal mol À1 .

Anthracene
Due to its optical applications and Diels-Alder-reactivity, several groups introduced anthracene into carbon nanohoops. In 1996, long before the first synthesis of [n]CPPs,the Herges group reported "picotube" 58,w hich is ac yclic anthracene tetramer and [4]CPP derivative ( Figure 17). [114] It was synthesized by ap hotochemically induced ring-expanding metathesis reaction from tetradehydrodianthracene.N MR spectroscopy and X-ray diffraction confirmed the high D 4h symmetry with ad iameter of 5.4 and al ength of 8.2 .I n spite of its strained structure, 58 was extraordinarily stable and unreactive.
Jastisapproach in 2016 was to introduce anthracene into a [ 12]CPP. [115] 9,10-Dihydroanthracene was coupled with aJ asti corner unit (Scheme 1a). Subsequent ring closure by Suzuki-Miyaura reaction followed by aromatization of the ring as well as the anthracene unit gave nanohoop 59.I ts structure was confirmed by X-ray diffraction. Thea uthors compared 59 to anonconjugated cyclic as well as to an acyclic anthracene derivative.T he optical band gap increased in this order from 2.72 to 2.78 and 2.98 eV with al ower degree of electron delocalization, and 59 showed al ower oxidation potential of 0.65 Vv s. Fc/Fc + compared to the reference compounds and to [12]CPP.T he electron density in the HOMO and LUMO was mainly localized on the diphenylanthracene unit. 59 did not undergo photodimerization of the anthracene unit, but Diels-Alder reactions with strong dienophiles were possible.
Three further anthracene-containing nanohoops [n]61 were published in 2017 by Tokuyama, Isobe,a nd co-workers. [39] Fort his purpose,t he new corner unit 9,10-epoxyanthracene was used with ar elatively large directing angle of 1268 8 (Scheme 1e)a nd aN i-mediated coupling for macrocyclization. Thea uthors were able to crystallize two of the nonconjugated cyclic precursors (n = 5, 7) as well as [5]61, which at the time was one of the largest nanohoops to be structurally determined by X-ray diffraction. Thed ihedral angles between anthracenes and neighboring phenylene rings were found to be large,w ith 618 8-898 8.T his was,a ccording to theoretical calculations,the thermodynamic minimum. NMR spectroscopy revealed ahighly symmetric structure with timeaveraged D 5h symmetry.T he rotational barrier for the 9,10connected anthracene groups was 10 kcal mol À1 ,w hich is slightly higher than that for the 2,6-substituted anthracene nanohoop 60 (see above). Thesimilarly connected anthracene nanohoop 62 was published by the Gaeta and Peluso groups shortly thereafter. [117] Thes ynthesis was based on the Jasti approach (Scheme 1a)w ith aS uzuki-Miyaura coupling for cyclization. Thesmaller hoop diameter hindered the rotation of the anthracene unit and led to asignificant distortion of its central six-membered ring, shown by calculations. 62 showed absorptions attributable to the diphenylanthracene unit as well the [8]CPP frame.T he emission, however,s ignificantly differed from that of [8]CPP with ab road maximum at 485 nm (F F = 0.47). In the presence of ap orphyrin Pd II complex as sensitizer, 62 showed visible light up-conversion.
Apart from the Herges groupspicotube 58,the first solely anthracene-containing nanohoop was reported in 2020 by the Du group. [118] Theauthors proved the versatility of Yamagos Pt-mediated approach (Scheme 1c)and synthesized [4]cyclo-2,6-anthracene (63). Thec ompound was obtained in ag ood yield of 59 %over the last two steps.T he 1 HNMR spectrum featured four signals,indicative of D 4 symmtery,and no signal splitting occurred between À80 8 8Cand 125 8 8C. Rotation of the anthracene units was hindered due to the small hoop size,and the two enantiomers of the most symmetric D 4 conformer (shown in Figure 17) were separated by chiral HPLC.T heir configurational stability was further confirmed by spectroscopic methods and theoretical calculations (Figure 18 a). In comparison to anthracene,aredshift of the longest wavelength absorption was observed owing to the larger cyclic pconjugated system. Thepronounced emission bands were also shifted to lower energy with aF QY of F F = 0.18. Theh oopshaped structure of 63 was confirmed by scanning tunneling microscopy on Au (111). Circularly polarized luminescence provided sizeable dissymmetry factors g lum of 0.103 and 0.090 for the two enantiomers (Figure 18 c).

Pyrene
Incorporating pyrene units into CPPs leads to their vertical extension as subunits of armchair carbon nanotubes. Tw oe xamples have been reported ( Figure 19). Thet wopyrene-unit-containing hoop 64 was synthesized by the Itami group in 2014 as a [16]CPP analogue. [121] Pyrene was coupled with the Itami corner unit (Scheme 1b), cyclized in aY amamoto fashion, and the hoop aromatized. NMR spectroscopy showed 64 to be highly symmetric.Poor conjugation between the para-phenylene and the pyrenylene moieties was observed in the absorption spectra as well as in theoretical calculations.T he UV/Vis absorption spectrum was ac ombination of the pyrene and [16]CPP bands,a nd DFT calculations showed orbital separation between the two.N anohoop 64 showed ar edshifted emission at 430 nm (F F = 0.21) compared to pyrene and [16]CPP.
6.2.7. Hexabenzocoronene, Tetraphene, and Pentaphene As ad isc-shaped subunit of graphene,h exabenzocoronene (HBC) has been used as astarting point to create larger graphenic structures. [122] Incorporating HBC as well as substructures thereof into CPPs provides templates for the synthesis of structurally defined SWNTs ( Figure 20). Thefirst HBC-containing nanohoop 66 b was reported by Nishiuchi, Müllen, and co-workers in 2015. [123] Synthesis was conducted using ap ost-construction method, where the HBC was formed in aS choll reaction after the hoopss ynthesis.T his had been attempted before by the same group,b ut the cyclodehydrogenation had been hampered by,among others, 1,2-phenyl shifts. [124] To circumvent this,m ethyl groups were introduced at the critical positions.T he [15]-and [21]CPP precursors to 66 a and 66 b were synthesized using Jasti corner units (Scheme 1a)a nd aN i-mediated coupling, followed by successful cyclodehydrogenation for the larger 66 b.N MR analysis proved as ymmetric structure for 66 b,a nd the absorption and emission spectra showed patterns distinctive for substituted HBC units.
In 2016 the Du group reported HBC-containing [18]CPP 67,w hich was synthesized using Itamisc orner unit (Scheme 1b)together with bis-borylated HBC units and aNi-mediated ring-closing reaction. [125,126] Its cyclic structure was confirmed by STM on Au(111) (Figure 21 a). With 375, 431, and 458 nm, the optical absorption bands were redshifted compared to both [18]CPP and HBC,t he same was observed for the emissions.DFT calculations showed that in both HOMO and LUMO the electron density was localized on the HBC units.
In 2016, the Jasti group incorporated benzotetraphene and dibenzopentaphene into CPP scaffolds (71-73)t od emonstrate anew concept for the synthesis of conjugated nanobelt fragments. [129] Thea uthors used ac ombination of CPP synthetic strategies,i ncluding the Jasti corner (Scheme 1a) and aS uzuki-Miyaura coupling for hoop closure,a nd the olefin metathesis of vinyl side groups to close the second set of six-membered rings.W ith 106, 79, and 71 kcal mol À1 the strain energies for 71-73 were 5-9 kcal mol À1 higher than those of the corresponding [6]-, [8]-, and [9]CPPs.T he major absorptions of 72 at 310 nm and 73 at 325 nm were blueshifted compared to [n]CPPs (340 nm), and cyclic voltammetry indicated slightly higher HOMO energies than for [n]CPPs.

Conjugated Nanobelts
We will close this Review with conjugated nanobelts-the synthetically most challenging hoop derivatives.C yclacenes, conjugated nanobelts consisting of annelated small rings,have been discussed since 1954 in connection with the conjugation in hoop-shaped p-systems. [2,6] In particular, [6] n cyclacenes were long-standing synthetic targets, [4,5,9,10] but they could never be synthesized as isolable species due to their predicted open-shell character. [10,131] Recently, [ 6] 8 cyclacene was detected in the gas phase, [132] providing evidence of its successful synthesis.A fter acceptance of this manuscript, the first two reports of benzannulated [6] n cyclacenes,n amed zigzag carbon nanobelts,appeared, as will be discussed below. Changing to an angular annelation pattern of the sixmembered rings or incorporating ring sizes other than six, on the other hand, successfully led to the synthesis of several conjugated nanobelts to date,asdetailed below. [133,134] Heterocyclacenes [135,136] and nanobelts incorporating pentalene units [137] have also been identified as attractive synthetic targets.

Nanobelts with Annelated Four-, Six-, and Eight-Membered Rings
Thev ery first example of ac onjugated nanobelt was reported by the Wudl group in 2002 with [6.8] 2 cyclacene 74 ( Figure 22). [138] 74 was synthesized by successive [4+ +2] cycloadditions of dibenzocyclooctadiyne using ar eactive pyrimidinium betaine as the diene,a nd its structure was confirmed by X-ray diffraction. Thea bsorption spectrum featured two bands at 225 and 285 nm and ab road band at 330 nm, significantly redshifted compared to other cyclophanes.I n 2004 the Gleiter group introduced Co-capped cyclobutadiene rings into nanobelts in the form of [4.8] 3 cyclacenes 75. [139] The synthesis used the Co-mediated alkyne dimerization reaction to link three dibenzocyclooctadiynes into am acrocycle, furnishing nanobelt 75 as the main product. Its structure was confirmed by X-ray diffraction. Thes ynthetic strategy was extended to derivatives 76 including substituents Ratthe Cp rings or Rh instead of Co in 2009. [140] In 2008 the Gleiter group reported the bottom-up synthesis of the first unsubstituted conjugated nanobelt, namely [6.8] 3 cyclacene 77. [141] 77 consists of alternating six-and eightmembered rings and was synthesized by Wittig reactions to make ah examethyl [2 3 ]-metacyclophane.O xidation of the methyl to aldehyde groups and af inal threefold McMurry coupling reaction for the second set of double bonds led to 77. [142] X-ray diffraction proved its D 3h -symmetric structure (Figure 24 a). Its absorption maximum lay at 220 nm with shoulders at 278 and 290 nm, and fluorescence occurred with am aximum at 370 nm. After acceptance of this manuscript, N-doped [(6.) m 8] n cyclacenes of different sizes and containing dibenzodiazocines were reported by the Wu group. [143]

Nanobelts Solely Consisting of Annelated Six-Membered Rings
In 2003 Nakamura et al. reported on the carbon-capped [10]cyclophenacene derivatives 78 a and b (Figure 23). [144,145] Theg roup approached the challenging synthetic target of ac yclophenacene in at op-down strategy starting from fullerene-C 60 and saturating the top and bottom part using organocopper chemistry.T he structure of 78 a was confirmed by X-ray diffraction. 78 a and b showed broad absorption bands with maxima at 260 nm and further broad bands that extended up to 500 nm, and they emitted with maxima at 560 and 620 nm (F F = 0.10, Figure 24 b).
In 2017, the group of Segawa and Itami was the first to succeed in the bottom-up synthesis of ac arbon nanobelt solely consisting of six-membered rings,n amely 79,a s a [ 12]cyclophenacene isomer. [146] Thes ynthetic strategy consisted of sequential Wittig reactions to construct adecabrominated [2 5 ]-paracyclophane as ak ey precursor followed by subsequent Ni-mediated aryl-aryl coupling.The synthesis was later optimized and 79 made accessible in 0.8 %overall yield from p-xylene. [147] X-ray diffraction confirmed its C 2h -symmetric structure (Figure 24 c). Ther ing strain was calculated Figure 22. Conjugated nanobelts 74, [138] 75 and 76, [139,140] and 77. [141,142] Angewandte Chemie Aufsätze to be 120 kcal mol À1 .N ICS calculations showed the main resonance form to be the one with most Clar sextets, [148] one in each of the six aryl rings along the equator. 79 displayed several absorption bands reaching up to 550 nm with two maxima at 284 and 313 nm, while the emission was found in the red region with amaximum at 630 nm (Figure 24 c). This seemingly large Stokes shift was due to the fact that the S 0 !S 1 transition is symmetry-forbidden, and only the S 0 !S 2 absorption band was visible.T he group recently showed that 79 can undergo sixfold Diels-Alder reaction of its six central six-membered rings with arynes and alkynes,resulting in cycloiptycenes. [149] In 2018 the Segawa and Itami group published larger congeners 80 and 81 of this nanobelt as [16]-and [24]cyclophenacene isomers. [147] Synthesis was performed with the same strategy as for 79 using Wittig reactions and Nimediated couplings.X -ray diffraction proved the highly symmetric structures of these nanobelts with diameters of 11.2 and 17.5 for 80 and 81,respectively (Figure 24 c). Due to the larger p-systems,t he absorptions were redshifted compared to 79 with maxima at 333 nm (80)a nd 351 and 356 nm (81). Thee missions,o nt he other hand, showed interesting features.A tr oom temperature,b oth the S 1 !S 0 and S 2 !S 0 emissions were visible for 80 at 524 and 478 nm (F F = 0.13), respectively,w hile for 81 only the S 1 !S 0 transition appeared at 466 nm (F F = 0.10), as assigned by TDDFT calculations (Figure 24 c).
As mentioned above,t he very first two reports of zigzag carbon nanobelts appeared in 2021 by Chi [150] (87)a sw ell as Segawa and Itami and coworkers (88) [151] after acceptance of this manuscript. [152] 87 is a[6] 12 cyclacene derivative,while 88 is ad erivative of a [ 6] 18 cyclacene.I no rder to overcome the small predicted singlet-triplet gaps of [6] n cyclacenes,b oth groups annulated six-membered rings along the belt rim, resulting in atotal of 12 and 18 aromatic sextets,respectively. Thes yntheses both employed Diels-Alder cycloadditions of furan or furan derivatives with arynes.Noteworthy is the use of hexafluorobenzene as at emplate in the macrocyclization step during the synthesis of 88.X-ray crystallography revealed diameters of 0.92 nm for 87 and 1.4 nm for 88.Inspite of these different sizes,t he absorption maxima of 87 at 332 nm with as houlder peak at 405 nm and of 88 at 336 nm with as mall peak at 405 nm were found at similar wavelengths.T he emission bands,o nt he other hand, were shifted to longer wavelength in smaller 87 (peak maxima at 422, 429, and 442 nm) compared to 88 (maxima at 407 and 432 nm). [152]

Conclusions and Outlook
Synthetic advances in the last 13 years in nanohoop synthesis,inparticular for the parent [n]cycloparaphenylenes, have enabled synthetic chemists to apply these concepts to incorporate al arge variety of aromatic groups other than benzene into hoops.A sw eh ave shown here,t his made it possible to modify the optoelectronic properties of the hoops, that is,b yi ntroducing donor or acceptor moieties or even donor-acceptor structures,t oc hange their structural properties and induce e.g. chirality,leading to intriguing chiroptical properties,and to provide vertically extended hoops as model systems and templates for single-walled carbon nanotubes. These examples provide ab ase to-in the future-study applications of conjugated nanohoops [16] with designed properties in materials science or biology,t om ake use of the chiroptical properties of nanohoops in optoelectronic devices, and to further develop methods for the synthesis of singlechirality carbon nanotubes starting from suitable templates.