Synthetic Tailoring of Graphene Nanostructures with Zigzag‐Edged Topologies: Progress and Perspectives

Abstract Experimental and theoretical investigations have revealed that the chemical and physical properties of graphene are crucially determined by their topological structures. Therefore, the atomically precise synthesis of graphene nanostructures is essential. A particular example is graphene nanostructures with zigzag‐edged structures, which exhibit unique (opto)electronic and magnetic properties owing to their spin‐polarized edge state. Recent progress in the development of synthetic methods and strategies as well as characterization methods has given access to this class of unprecedented graphene nanostructures, which used to be purely molecular objectives in theoretical chemistry. Thus, clear insight into the structure–property relationships has become possible as well as new applications in organic carbon‐based electronic and spintronic devices. In this Minireview, we discuss the recent progress in the controlled synthesis of zigzag‐edged graphene nanostructures with different topologies through a bottom‐up synthetic strategy.


Introduction
In 2004, Novoselov,G eim et al. isolated the first stable graphene as single-to few-layer hexagonal carbon nanosheets by using amicromechanical cleavage method. [1] This groundbreaking experiment stimulated research studies in graphene and carbon nanostructures.T he outstanding electronic and optical properties of graphene not only constitute an important issue in fundamental physics but also hold promise for use in future nanoelectronics. [2][3][4][5] Nonetheless,the conduction and valence bands of graphene cross at the Dirac points, thereby leading to az ero-band gap semiconductor,w hich dramatically limits the integration of graphene into digital electronic devices. [6] Therefore,f inding aw ay to open the band gap in graphene is of great importance.Many top-down approaches involving band-gap opening, such as substrateinduced band-gap tuning, [7,8] bilayer graphene, [9] hydrogen passivation, [10] or nanoscale holes to create graphene nanomeshes [11] have been reported. Themost prominent method is to realize quantum confinement of charge carriers with tailorable band gaps in graphene nanostructures by cutting graphene into finite graphene fragments and strips,s o-called nanographenes (NGs) and graphene nanoribbons (GNRs). [12][13][14][15][16][17] According to theoretical calculations,t he magnetic and electronic properties of NGs and GNRs,a sw ell as their chemical reactivities,a re crucially determined by their edge types. [18,19] In general, there are five types of edge structures for graphene:z igzag, armchair, cove,g ulf,a nd fjord (Figure 1a). In contrast to armchair-, cove-, gulf-, and fjord-edged graphene nanostructures,w hich generally present semiconducting properties,m ost zigzag-edged NGs exhibit distinct magnetic features because of the spin polarization associated with their edge states. [20,21] Fore xample,E noki and coworkers experimentally confirmed the edge state of zigzagedged hydrogen-terminated NGs by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). [22,23] Tapasztó and co-workers demonstrated spin ordering along the edges in narrow zigzag-edged graphene nanoribbons (ZGNRs;F igure 1b). Thef erromagnetic and antiferromagnetic coupling with the zigzag and armchair edges as well as their switching behavior were elucidated. [24] Although substantial efforts have been dedicated in the last decade to preparing high-quality NGs,t he above topdown approaches suffer from difficulties in controlling the sizes and edge structures. [25] Fore xample,t he topdown-fabricated narrow GNRs show ruffled edges,w hich make the band gap poorly defined, thereby resulting in dramatically degraded charge-carrier transport properties. [26] This constitutes the main reason why achieving NGs with atomically smooth edges has been am ajor obstacle for applying graphene in nanoelectronic devices.
In contrast with the top-down method, the bottom-up synthetic strategy has the incomparable advantage of controlling the widths and edge topologies of NGs at the atomic level. [3,14,27] Theclassical bottom-up synthesis of NGs or large polycyclic aromatic hydrocarbons (PAHs) involves at ypical oxidative intramolecular cyclodehydrogenation of dendritic oligophenylene precursors.Byusing such asynthetic strategy, ab road class of PA Hs with different topological structures have been synthesized in recent decades. [3,28] In contrast to the armchair-edged NGs (A-NGs) with af ully benzenoid structure,z igzag-edged NGs (Z-NGs) have another ring fused at the bay position (pink;F igure 2), and the two additional pelectrons cannot be drawn as aC lar sextet. [29] Fore xample, the fully benzenoid hexa-peri-hexabenzocoronene (HBC, 1) can be annulated with three,f our, or six additional benzene rings at the bay regions to generate at ri-zigzag HBC (2), [30] tetra-zigzag HBC (3), [31] and full-zigzag HBC (4;a lso called supercoronene), respectively ( Figure 2). Such az igzag K-Experimental and theoretical investigations have revealed that the chemical and physical properties of graphene are crucially determined by their topological structures.Therefore,the atomically precise synthesis of graphene nanostructures is essential. Aparticular example is graphene nanostructures with zigzag-edged structures,whichexhibit unique (opto)electronic and magnetic properties owing to their spinpolarized edge state.R ecent progress in the development of synthetic methods and strategies as well as characterization methods has given access to this class of unprecedented graphene nanostructures,w hich used to be purely molecular objectives in theoretical chemistry.Thus, clear insight into the structure-property relationships has become possible as well as new applications in organic carbon-based electronic and spintronic devices.I nt his Minireview,w ediscuss the recent progress in the controlled synthesis of zigzag-edged graphene nanostructures with different topologies through abottom-up synthetic strategy.
region has double-bond character,t hus enabling its electrophilic substitution reaction or oxidation to the diketone structure. [32] Despite the failure in the synthesis of full zigzag HBC (4)t hus far, another family of PA Hs with af ull zigzag periphery,namely,circumacenes,has received much synthetic success ( Figure 3). Among these,c oronene (5)a nd ovalene (6)d ate back to efforts from Clar in the last century. [33][34][35][36] In 1991, Broene and Diederich described as ynthetic route for the next generation circumarenes,namely,circumanthracene (7;F igure 3). [37] However,t hey could not characterize this compound because of its poor solubility.Even three decades after its discovery,t here have been no significant efforts towards the synthesis and application of circumanthracene,its derivatives,ort he next generation of circumacenes.
Thei ntegration of zigzag edges or reactive double bonds into NGs exerts al arge influence on the electronic and magnetic properties,aswell as their chemical reactivity,such as in oxidation reactions or electrophilic substitutions.F or example,o ur group demonstrated an edge chlorination method for the functionalization of coronene,w hereby the chlorine atoms at the edges provide the opportunity for further chemical derivatization to produce athiol-substituted coronene. [32,38] On the other hand, zigzag-edged NGs generally have non-KekulØ structures or open-shell structures because of the unpaired electrons present in the molecules, [39,40] which render them ideal candidates for application in nanocarbon-based spintronics.Aprominent example is phenalenyl radical [41,42] (8;F igure 4) with ad elocalized spin/ radical structure.Inthe first study of 8 in the 1950s, [43,44] it was found to be very reactive and only survived under an inert gas atmosphere in solution. Later, 8 was stabilized by protection with three tert-butyl groups,w hich allowed its isolation and characterization. [45,46] Since the 1990s,p henalenyl radicals have been widely explored as building blocks for constructing open-shell polycyclich ydrocarbons (PHs), and Nakasuji and Kubo have intensively investigated as eries of biphenalenyl derivatives (9-11;F igure 4). [47,48] Another outstanding molecule rich in zigzag edges is zethrene (12;F igure 4), au nique PA Hw ith formally fixed C À Cd ouble bonds.C lar first synthesized zethrene (12)i n1 955. [49] Later,i nt he 1960s,    Staab and Sondheimer developed more straightforward synthetic methods through the cross-coupling of copper acetylides and iodoarenes. [50][51][52] Recently,t he groups of Tobe and Wu have paid particular attention to zethrene and its higher homologues (such as heptazethrene and octazethrene) with open-shell characteristics. [53][54][55][56][57] Very recently,w ei n collaboration with the Fasel group demonstrated the onsurface synthesis of super-heptazethrene on Au (111). [58] In contrast to its open-shell singlet ground state in solution, super-heptazethrene presents ac losed-shell character on Au (111). Since there have already been excellent reviews summarizing phenalenyl-based [47,48] and zethrene-type radicals, [59][60][61][62][63] we will not discuss them further in this Minireview.
In the last decade,aremarkable breakthrough was achieved in the bottom-up synthesis of zigzag-edged NGs and GNRs,w hich benefited from the advancement of synthetic methods and analytical tools.Inparticular,powerful and complementary on-surface and in-solution synthetic strategies have been established;i nt he former case,t he characterization of NGs at the atomic/molecular level is currently possible. [17,64] Fore xample,o pen-shell peri-tetracene [65,66] and peri-pentacene, [67] the next highest analogues of bisanthene,which have been pursued for several decades but hampered by their poor chemical stabilities,a re now accessible by solution-based or surface-assisted syntheses. p-Extended triangulene [68] with three unpaired electrons and chemically unstable full zigzag-edged GNRs [69] have also been recently realized by on-surface synthesis.A ll of these graphene nanostructures show ac ritical dependence of their electronic properties on the topological structures,especially with the dominant zigzag edges.
In this Minireview,wewill highlight recent advances in the synthetic strategies and physiochemical properties of graphene nanostructures rich in zigzag-edges,i ncluding periacenes,t riangular-shaped NGs,r hombus NGs,h eteroatomdoped NGs,a nd zigzag-edged GNRs.W ew ill provide our views on the advantages and challenges of the respective synthetic methods and routes.M oreover,t he unique chemical, electronic,p hotophysical, and magnetic properties of these graphene nanostructures will be discussed in this context.

peri-Acenes
Acenes are linearly cata-condensed PA Hs,w hile periacenes consist of two or more rows of peri-fused acenes ( Figure 5). [70] Generally,t he solution synthesis of longer acenes (larger than hexacenes) remains elusive because of their poor stability under ambient conditions, [71] whereas higher acenes up to dodecacene have been achieved through on-surface synthesis. [72][73][74][75] Bisanthene (14)can be regarded as alaterally extended perylene (13;F igure 5), and was synthesized for the first time in 1948. [76,77] Recently,h omologous PA Hs with zigzag-edge peripheries,s uch as teranthene (15) and quateranthene (16), [40] were synthesized by expanding the bisanthene core in the longitudinal direction ( Figure 5). From the resonance structures,t here are three and four additional Clar sextets for teranthene (15-1)a nd quateranthene (16-1), respectively.According to the Clar sextet rule,more sextets in the open-shell biradical form results in higher aromatic stabilization energies and thus amore dominant contribution of the biradical form to the ground state ( Figure 5). [40] From calculations,t he biradical indexes (y 0 )o f15 and 16 are estimated to be 0.42 (CASSCF) and 0.84 (UBHandHLYP), respectively,i nc ontrast to only 0.07 (CASSCF) for 14.T he open-shell characteristics of 15 and 16 also lead to their unique optical and electronic properties. [78] Fore xample, 15 and 16 display weak low-energy absorption at l = 1054 nm (15)and l = 1147 nm (16)inthe NIR region, which indicates that the energy gap between their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is small.
According to theoretical calculations,t he energy gap of peri-acenes drastically decreases as their length increases. [79,80] In 2015, we reported the synthesis of bistetracene 17 (tetrabenzo[a,f,j,o]perylene), [81] in which two tetracenes are fused together by two bonds ( Figure 6). First, 20 was synthesized in eight steps.T hen, bistetracene 17 was synthesized through aGrignard reaction, ring fusion, and oxidative dehydrogenation (Scheme 1a). Theoptical energy gap (E g opt ) Figure 4. Phenalenyl radical (8)and its resonanceforms, biphenalenyls 9-11 and their resonanceforms, and zethrene (12). of 17 was estimated to be 1.56 eV from its UV/Vis absorption spectrum. Interestingly,b ased on the calculation, 17 has abiradical nature (y 0 = 0.61) in the ground state.T he driving force to form biradical 17-1 is due to the three additional Clar sextets in the open-shell form compared to its closed-shell form ( Figure 6). However, 17 is unstable under ambient conditions,w ith ah alf-life (t 1/2 )o fo nly about 30 min (Scheme 1c). Ar ed-colored solution was obtained during the oxidation, which corresponds to the formation of tetrabenzo[a,f,j,o]perylene-9,19-dione (23;Scheme 1b).
Researchers have not abandoned the pursuit of higher peri-acenes,e ven though their synthesis has been met with tremendous challenges.T hus far, two synthetic approaches have been proposed for the synthesis of peri-pentacene (19)in solution;unfortunately,both were unsuccessful. [84,85] In 2015, peri-pentacene (19)w as reported through on-surface synthesis from the precursor 6,6'-bipentacene (29)u nder ultrahigh vacuum (UHV) conditions ( Figure 7). [67] After depositing 29 on the Au(111) surface,itarranged into highly ordered linear chains,a sv isualized by STM imaging (Figure 7b). Annealing at 200 8 8C, led to 29 being fully cyclized to form  Interestingly,the highly reactive 19 could be stabilized on the surface through interaction with the free valences of the Au substrate. (30)was first studied by Clar in 1941, [86] and with two unpaired electrons is one of the most fundamental non-KekulØ PA Hs (Figure 8). [87] Moreover,t he singlet-triplet energy gap (DE S-T )o ft riangulene (30)w as calculated to be 20 kcal mol À1 ,thus suggesting that 30 displays at riplet biradical feature in the ground state. [88] Clar and Stewart made the first attempt to synthesize 30;h owever, only the polymerized compound was obtained, which indicates the thermodynamic instability of 30. [89,90] In 2001, Morita, Nakasuji, and co-workers introduced tert-butyl (t-Bu) groups onto the zigzag edges of triangulene to kinetically protect the reactive edges (t-Bu 30;Scheme 3). [91] However,t his compound was only confirmed by electron-spin resonance (ESR). In addition, this tert-butyl-protected triangulene was not stable in solution, with oligomers forming even at room temperature (Scheme 3).

Triangular-Shaped Nanographenes
In 2017, Pavliček, Gross et al. demonstrated the surfaceassisted synthesis of triangulene (30)u nder UHV conditions, [92] whereby dihydrotriangulenes (39; Figure 9a)w ere deposited on NaCl(100), Cu (111), and Xe(111) surfaces and 30 formed by atomic manipulation (Figure 9b-d). Thedifferential conductance dI/dV(V) of 30 was measured (Figure 9e). There are two clear peaks at V = À1.4 Vand V = 1.85 V, which correspond to the negative and positive ion resonances. Figure 9f-h shows the STM images of 30 at different voltages, with the image taken in the gap region (V = 0.1 V) appearing to be triangular-shaped. Very recently,F rederiksen and coworkers reported the on-surface synthesis of graphene flakes based on the [3]triangulene molecules,w hich possess as pin S = 1g round state. [93] Thenext generation in the family of PA Hs with triangular topology, [ 4]triangulene (31, C 33 H 15 )w ith three unpaired electrons,w as recently reported by the Fasel group and us (Scheme 4). [68] Thek ey precursor 42 with three strategically installed methyl substituents was achieved through ap hotocyclization reaction of 41.Subsequently, 42 was sublimated on the Au(111) surface,a nd then ring-closure reactions of the methyl groups were performed by annealing at 320 8 8Ct o afford 31.T he structure of 31 was confirmed by ultrahigh- Figure 8. The nomenclature of higher triangular-shaped nanographenes.
Scheme 3. Synthesis of triangulene t-Bu 30 with three tert-butyl groups. [91] [92] Angewandte Chemie resolution STM (Figure 10 a), which shows 31 adopts aplanar structure on the surface through adsorption onto Au (111), which is also in line with the DFT calculations (Figure 10 b). Thee lectronic properties of 31 were studied by STS (Figure 10 c), and the electronic band of 31 was estimated to be 1.55 eV.M oreover,c alculations based on density functional, tight-binding,a nd many-body perturbation theory were carried out, which indicated that [4]triangulene (31)m aintains an open-shell quartet ground state on the surface.
In 1972, Clar and MacKay predicted the bowtie-shaped PA H 53 (C 38 H 18 ;S cheme 6), [95] in which two [3]triangulenes are connected head-to-head. It is important to emphasize that 53 belongs to the family of the as then undiscovered concealed non-KekulØ structures. [96,97] Ac oncealed non-KekulØ molecule displays no KekulØ structure and possesses the same number of white and black (or unstarred and starred) vertices (Scheme 6a). [98] Like triangulene,aK ekulØ structure of 53 cannot be drawn with paired electrons.V ery recently,w ei nc ollaboration with the Fasel group demonstrated the first synthesis of Clarsg oblet PA H 53 through combined in-solution and on-surface synthesis. [99] Thes ynthesis of 53 involves the key precursor 52 (Scheme 6b), in which the four methyl groups serve as ring-closure units.T he generation of 52 involved amultistep synthesis (Scheme 6b). Precursor 52 was deposited onto Au (111) (111) surface. c) dI/dV spectrum of 31 (blue curve). [68] Scheme 5. The synthetic route towards [5]triangulene 32. [94]  1.07 V). [94] Angewandte Chemie Minireviews 23392 www.angewandte.org had as trong antiferromagnetic character with an exchange coupling of 23 meV (an effective exchange parameter J eff = 23 meV; Figure 12), which is larger than that of the Landauer limit of minimum energy dissipation at room temperature. Interestingly,t his electronic decoupling was confirmed when 53 linearly fused to form its dimer structure (di-53;F igure 13 a). This arises because the electron spins in di-53 are separated by al arge nonmagnetic unit. Thec hemical structure of di-53 was clearly confirmed by ultrahigh-resolution STM (Figure 13 b-d).    Scheme 7. Synthesis of 55 and 56. [102] Angewandte Chemie Them ethyl groups in 60 and 64 were transformed into dialdehyde and tetraaldehyde groups by bromination, esterification, hydrolysis,a nd Swern oxidation to afford 61 and 65.T he dihydro/tetrahydro precursors 62/66 were synthesized by treatment of aldehydes 61/65 with phenyllithium followed by Friedel-Crafts cyclization. Dehydrogenation of 62/66 using p-chloranil provided target compounds 55 and 56. Single-crystal analysis shows that both 55 and 56 have aflat pconjugated framework. Compared to [2 3]anthanthrene (54), the absorption maxima (l max )o f55 and 56 are bathochromicaly shifted from l = 449 nm to 561 nm and 702 nm, respectively (Figure 15 a). Accordingly,c ompared to periacenes,rhombic NGs 55 and 56 display somewhat larger band gaps and thus are much more stable under ambient conditions.

Rhombus Nanographenes
In addition, Wu and co-workers recently reported the synthesis of extended  (Figure 15 b). We also achieved the on-surface synthesis of [3 3]anthanthrene-based polymers,n amely,p oly(para-dibenzo-[bc,kl]coronenylene), which could be laterally fused to form zigzag-edge-extended GNRs. [104] Compared to the above-reported closed-shell [m,n]rhombus NGs,anopen-shell singlet ground state will emerge when m,n ! 5. Agapito et al. predicted that the [5,5]rhombus NG exhibits unique magnetic states that could be selectively tuned with the gate voltage. [105] Thus,the bottom-up synthesis of next-generation rhombus NGs with full zigzag edges is expected to provide exotic low-dimensional quantum phases of matter,s uch as magnetic exchange coupling behavior, in purely organic systems.

Heteroatom-Doped Nanographenes
In addition to edge topologies,t he employment of heteroatoms in sp 2 -carbon frameworks is another method to tune the intrinsic chemical and physical properties of PA Hs. [106][107][108] Thei mplementation of heteroatoms,s uch as an isoelectronic B-N unit (Figure 16), has asignificant influence on the electronic structures of PA Hs. [109,110] Moreover,c ompared with the nonpolar C = Cb ond, the B À Nb ond can be regarded as azwitterionic double bond in its neutral state.The redox behavior of the B-N unit has received significant interest recently (Figure 16). [111] Azomethine ylides (AMY 1;S cheme 9a)a re classic 1,3dipolar molecules. [112,113] Interestingly, AMY 1 possesses several resonance structures,s ince the negative charge on the allyl anion (AMY 1a and 1b)c an be distributed onto the neighboring carbon atoms.I na ddition to these two ionic structures, AMY 1 also displays diradical character (AMY 1c) in the ground state (Scheme 9a). [114,115] In 2014, we reported the synthesis of conjugated AMY-containing aromatic rings (PAMY), in which the C-N-C unit is installed at the zigzag edge.T his unprecedented PAMY can be used as ab uilding block to synthesize nitrogen-doped PA Hs (N-PAHs;Scheme 9b). [116,117] In general, the synthesis of zigzag-edged PAMY consists of three steps.F irst, 76 is synthesized through the Then, 77 with az igzag edge is obtained by HCl-induced cyclization of 76.T he final PAMY is synthesized by the treatment of 77 with basic conditions.R emarkably,z igzagedged PAMY enables the synthesis of extended N-PAHs by 1,3-dipolar cycloaddition. Fore xample, 78 was directly synthesized through a1 ,3-dipolar cycloaddition with dimethoxyacetylene dicarboxylate (Scheme 9b). Subsequently,the planar compound benzo [7,8]indolizino [6,5,4,3-def]phenanthridine (79)w as synthesized in 82 %y ield by the oxidative dehydrogenation of 78.F ollowing this strategy,a nd by using different dipolarophiles,d ifferent kinds of unprecedented inner nitrogen-doped NGs can be produced. [117][118][119][120] In addition to the cycloaddition reaction of PAMY, interestingly, 81 was produced through the dimerization of 80 in low yield (3 %; Scheme 10). Theyield of 81 (51 %) could be greatly improved by increasing the temperature.F urther oxidation of 81 a with DDQ provided pyrazine-incorporated hexabenzoperylene (HBP) 82.H owever, 82 is extremely unstable,l ikely because of the antiaromatic nature of the pyrazine-type core (Scheme 10). On the other hand, dimerized intermediate precursor 81 c was not observed when 80 c was annealed on the surface.I nstead, diaza-HBC 83 was directly formed (Scheme 10). [121] Moreover, 80 b substituted with a-CN group enabled the synthesis of polyaromatic azaullazine chain 84 (Scheme 10) on insulating layers,m etal substrates,and in the solid state through intermolecular headto-tail cycloaddition reactions. [122] Theintroduction of aN-B-N motif at the zigzag edge not only affords the synthesis of stable zigzag-edged NGs in solution but also provides the possibility to generate radical cations at the NBN-doped edge through selective oxidation (Scheme 11 a). [123][124][125][126] In 2016, our group demonstrated the first synthesis of 1,9-diaza-9a-boraphenalenes containing NBN zigzagged edges (88;S cheme 11 b). [123] NBN-edged 88 was synthesized from 87 through electrophilic borylation, in which trimethylsilyl (TMS) served as the leaving group to form the NBN unit. Interestingly, 88 was dimerized to 88-2 through chemical oxidation (Scheme 11 c). In addition, pextended dimer 94 was produced (Scheme 12), thus highlighting the potential of making GNRs containing NBN zigzag edges.

Zigzag-Edged GNRs
Theprocedures established for the synthesis of PA Hs can be further explored to construct GNRs from the corresponding polyphenylene polymers.I nc ontrast to armchair-edged GNRs (AGNRs) that display semiconducting behavior, zigzag-edged GNRs (ZGNRs) demonstrate unique electronic and magnetic properties,i ncluding narrow band gaps and localized edge states. [129,130] Therefore,a lthough the solutionbased synthesis of AGNRs has been successfully demonstrated, [131][132][133] GNRs with rich zigzag edges have been limited thus far to on-surface synthesis under UHV conditions because of their poor chemical stability.I n2 016, the first bottom-up synthesis of afull zigzag-edged GNR (6-ZGNR)onaAu (111) surface was realized by Fasel, Müllen, and us (Scheme 15). [69] This method relied on the rational design of the U-shaped dibenzoanthracene-based precursor 105,w hich allows onsurface polymerization to form as nake-type polymer (Polymer-1). Moreover,t wo additional methyl groups on phenyl ring Aa re preinstalled (Scheme 15), which are essential to bridge with neighboring phenyl rings to establish additional zigzag-edged rings.
Fort he on-surface synthesis,m onomer 105 was first sublimated at 150 8 8C. Then, Polymer-1 was synthesized by annealing at 200 8 8C ( Figure 17 a). Finally,t he fully zigzagedged 6-ZGNR was achieved by further annealing at 350 8 8C (Figure 17 b). Further structural details of 6-ZGNR could be unraveled by nc-AFM (Figure 17 c), which demonstrated that its edge topology and width correspond to the conceived 6-ZGNR.H owever,t he electronic edge states of zigzag edges are difficult to observe owing to the energetic electronic coupling between the gold surface and the ribbons.B y manipulating 6-ZGNR with an STM tip onto insulating NaCl islands,clear evidence of the edge states could be observed as  [124] . Scheme 14. Synthesis of BNB-edged 103 with mesityl groups. [126] Scheme 15. The synthetic route towards 6-ZGNR from the U-shaped monomer 105 with two preinstalledm ethyl groups. [69] Angewandte Chemie Minireviews 23396 www.angewandte.org ar esult of electronic decoupling from the gold substrate (Figure 17 d). [134] Edge states have also been detected on other carbon-based nanostructures, [135,136] such as graphene quantum dots (GQDs). Thel ess-defined zigzag edges resulted in their energy splitting being considerably smaller than that of 6-ZGNR.These results show that the magnetic and electronic properties of graphene nanostructures are very sensitive to their interactions with the underlying metal substrate and their edge roughness.
Following asimilar synthetic strategy,the surface-assisted synthesis of NBN-edged ZGNRs (ZGNR1 and ZGNR2; Scheme 16) from two U-shaped NBN-doped precursors (106 and 107)w as recently reported by us. [137] First, two iodofunctionalized monomers 106 were synthesized by multistep organic synthesis,inwhich the NBN motif was preinstalled on the zigzag periphery.Then, monomer 106 was deposited onto the gold substrate (Figure 18 a). Theswallow-shaped polymer poly-1 was synthesized by annealing at 200 8 8C ( Figure 18 b). Subsequently,t he target ZGNR1 could be obtained through intramolecular cyclodehydrogenation of poly-1 at 450 8 8C. The zigzag-edge topologies were evidently unveiled through STM and nc-AFM measurements (Figure 18 c,d). Monomer 107 containing one additional phenyl ring than 106 was further synthesized to afford ZGNR2 (Figure 18 e,f), in which the zigzag-edge proportion (57 %) was higher than that of ZGNR1 (37 %). However,asaconsequence of the substantial steric hindrance between the additional ring Cw ith the side rings (such as rings D; Scheme 16) in polymer poly-2,t he length of the corresponding ZGNR2 is shorter than that of ZGNR1.Acomparison of the electronic structures of allcarbon-based ZGNRs (PC-ZGNR1:0 .52 eV; PC-ZGNR2: 0.27 eV; Scheme 16) shows the energy band gaps of ZGNR1 (1.50 eV) and ZGNR2 (0.90 eV) are much higher, which indicates that NBN doping plays apivotal role in tailoring the electronic structures of graphene nanostructures.S ince the NBN unit can be selectively oxidized to form the radical cation (Scheme 11 a), which corresponds to ap ristine C 3 carbon segment, this strategy offers further chemical modification for NBN-doped ZGNRs.
Another significant breakthrough for GNRs was made in 2018 with the discovery of topological properties. [138,139] It was shown that zigzag edges provide ap latform to realize GNRs with exceptional physical properties.F or example,t opologically nontrivial (Z 2 = 1) and trivial (Z 2 = 0) GNRs were demonstrated by the introduction of short zigzag edge segments (pink segments) into 7-AGNR (Scheme 17). [139] First, GNR1 (7-AGNR-S(1,3)) was synthesized from monomer 108 on aA u(111) surface (Scheme 17 a). Them ethyl c) nc-AFM image of 6-ZGNR.d)STM image of 6-ZGNR on NaCl monolayer islands. [69] Scheme 16. Synthetic strategy for NBN-doped ZGNR1 and ZGNR2. [137] [137] groups play an essential role in the formation of the zigzag topology by bridging the neighboring rings.T he chemical structure of GNR1 was unambiguously confirmed using nc-AFM (Scheme 17 a, inset). STS measurements showed the band gap of GNR1 significantly decreased to 0.65 eV compared to pristine 7-AGNR (2.4 eV). Moreover,t od etermine whether GNR1 behaves similarly to the topologically trivial class (Z 2 = 0) or the nontrivial class (Z 2 = 1), sequential sublimation of 108 (for GNR1)a nd 109 (for 7-AGNR)w as carried out (Scheme 17 b). Thee xperimental results demonstrated that the resulting GNR1 could be classified as topologically trivial, with Z 2 = 0, which is consistent with the tight-binding prediction. In addition, by sequential sublimation of precursors 110 and 109, GNR3 (named 7-AGNR-I(1,3)) and 7-AGNR could be produced (Scheme 17 c). The inset in Scheme 17 cd isplays the nc-AFM image of GNR3 (with 5units), which is laterally expanded on both sides by 7-AGNR units.The dI/dV analysis shows that, in contrast to the trivial (Z 2 = 0) GNR1,t he resulting GNR3 belongs to the topologically nontrivial class (Z 2 = 1) and possesses topological end states.

Conclusions and Perspectives
This Minireview offers an overview of the recent developments in the synthesis of graphene nanostructures with dominant zigzag-edged topologies.B oth bottom-up solution-based and surface-assisted synthetic strategies have been established and have achieved significant breakthroughs in the past few years.F or example, peri-tetracene with its biradical character in the ground state,w hich had been pursued for more than 70 years,w as successfully synthesized in solution. p-Extended triangulenes and full zigzag-edged GNRs (6-ZGNR)h ave been successfully synthesized by surface-assisted routes.T hese atomically precise Z-NGs and ZGNRs open up tremendous opportunities for the exploration and regulation of their fundamental physicochemical properties.Aprominent example is the switching of the magnetic ground state from S = 1 = 2 to S = 0, which was recently reported for the concealed non-KekulØ structure of Clarsg oblet molecule,w ith quenched spins through atomic manipulation. [99] Despite the incredible advances over the last few years, the development of Z-NGs and ZGNRs is still in its infancy. Many challenges and opportunities remain for the synthetic exploration of this elusive type of graphene material, some of which are listed in this Minireview.A lthough the surfaceassisted synthesis of several prominent types of Z-NGs and ZGNRs has been demonstrated, their solution-based chemistry still lags behind because of their poor chemical stability. Thei ntroduction of bulky groups for kinetic protection or electron-deficient groups (such as fluorine atoms) for thermodynamic stabilization can be at radeoff strategy to obtain access to some stable Z-NGs or ZGNRs in solution. Moreover, the introduction of nonplanarity by the incorporation of nonhexagonal rings into sp 2 -carbon frameworks may provide an alternative pathway. [140][141][142][143][144] Only by overcoming the stability obstacle will further integration of these exotic materials in carbon-based nanoelectronic devices become possible.
Heteroatom doping has been demonstrated as an efficient strategy for synthesizing stable Z-NGs and ZGNRs with extended zigzag edges,which also enables chemical tuning of their electronic and magnetic properties.T herefore,f urther studies,both in solution and in on-surface synthesis,focusing on the specific concentrations and positions of the heteroatoms in the zigzag-edged graphene nanostructures will be essential for understanding the effects of substitutional doping on physicochemical properties.Inaddition to heteroatom doping,defect engineering,together with the structural design of zigzag edges,m ay provide an attractive method to tune the electronic and magnetic properties of Z-NGs and ZGNRs. [145,146] Our recent work shows that af ive-membered ring incorporated at the zigzag edge of nanographene can break the bipartite character of the sp 2 -carbon lattice and induce as ingle net spin of S = 1 = 2 . [147,148] On the other hand, concealed non-KekulØ NGs with intrinsic magnetism remain ac lass of less-developed graphene nanostructures,a nd more effort will be needed for both solution and on-surface synthesis.Moreover,given the high-spin ground states,other attractive fundamental and technical prospects can be realized by the synthesis of one-dimensional polymers,r ibbons, and two-dimensional networks by incorporating magnetic graphene molecules as building blocks. [149] We hope that this Minireview will inspire new ideas in the design and synthesis of novel and stable zigzag-edged graphene nanostructures,as well as the development of carbon-based nanoelectronic devices. Scheme 17. a) On-surface synthesis of GNR1.b )7-AGNR-extended GNR1.c)7-AGNR-extended GNR3. [139] Angewandte Chemie Minireviews 23398 www.angewandte.org