Global Aromaticity in Macrocyclic Polyradicaloids: HÃ¼ckel's Rule or

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/accounts

Global Aromaticity in Macrocyclic Polyradicaloids: Hückel’s Rule or Baird’s Rule? Chunchen Liu, Yong Ni, Xuefeng Lu, Guangwu Li, and Jishan Wu*

Downloaded via KEAN UNIV on July 18, 2019 at 02:11:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore CONSPECTUS: Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic πconjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel’s rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel’s rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird’s rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar’s aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel’s rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel’s rule and Baird’s rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure−radical character− aromaticity−properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed. charged [n]annulene analogues6−8 (2, Figure 1a). The concept can also go to the π-conjugated polycyclic systems such as porphyrionids9,10 if there is a dominant [n]annulene-like conjugation pathway (e.g., 3, Figure 1a). Topological change from a planar to a half-twisted Möbius strip will lead to an inverse of aromaticity; i.e., the molecules with [4N] ([4N + 2]) π electrons will be aromatic (antiaromatic).11 This rule has been experimentally validated by several π-conjugated

1. INTRODUCTION 1.1. Aromaticity Rules

Aromaticity is a key concept to govern the electronic properties of cyclic π-conjugated molecules. In 1931, Hückel proposed his famous [4N + 2] aromaticity rule to explain the unusual stability of planar monocyclic π-conjugated systems with [4N + 2] delocalized π-electrons (Figure 1a).1 In 1967, Breslow presented the concept of antiaromaticity for planar πconjugated monocycles with [4N] π electrons.2,3 Both rules have been well validated in [n]annulenes4,5 (1, Figure 1a) and © XXXX American Chemical Society

Received: May 15, 2019

A

DOI: 10.1021/acs.accounts.9b00257 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 1. Illustration of (a) Hückel’s rule, (b) Möbius’s rule, (c) Baird’s rule, (d) local aromaticity versus global aromaticity, and (e) Clar’s aromatic sextet rule with representative examples. Reproduced with permission from ref 13. Copyright 2007 WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim.

molecules with Möbius twist (e.g., 4-6, Figure 1b).12−16 In addition, Craig-type Mobius aromaticity17 has been reported recently in osma-complexes.18 In 1972, Baird predicted that the Hückel (anti)aromaticity observed for singlet (S0) groundstate systems would become reversed in the lowest triplet state (T1) (Figure 1c),19 which was very recently supported by some spectroscopic measurements.20−23 Using valence bond theory, the T1 state of benzene (7) with six π electrons can be described as a combination of a closed-shell Hü ckel antiaromatic benzene dication plus two π-electrons of the same spins (Figure 1c).24 Similarly, the T1 state of cyclooctatetraene (COT, 8) with eight π electrons can be described as a combination of a closed-shell Hückel aromatic COT dication plus two unpaired π-electrons (Figure 1c).24 Baird’s rule was well elaborated in pentafulvene (9), which exhibited a reverted dipole moment from the S0 state to the T1 state (Figure 1c).25 It is also worth to note that π-conjugated molecules with [4N] π-electrons could have a triplet ground state to satisfy Baird’s [4N] aromaticity rule.26

Strictly speaking, all the above-mentioned aromaticity rules are only applicable to monocyclic systems. While many porphyrinoids still follow these rules, all-benzenoid polycyclic aromatic hydrocarbons (BPAHs) usually display local aromaticity with π-electrons predominantly localized at certain benzene rings because the benzene ring has a larger resonance energy than the pyrrole ring does. For example, kekulene has two representative forms, one with six localized aromatic sextet rings (the hexagons shaded in blue color) (10a) and another with a delocalized [18]annulene-within-a-[30]annulene structure (10b) (Figure 1d).27 Experimentally, 10a was proved to be the dominant structure.28 For BPAHs with the same composition, the isomer with the largest number of aromatic sextet rings will show the highest stability and largest band gap according to Clar’s aromatic sextet rule.29 This was well illustrated by comparing the reactive tetracene (11, with one sextet ring) with its very stable isomer triphenylene (12, with three sextet rings) (Figure 1e). B


Article


Figure 2. (a) Representative resonance forms of Tschitschibabin’s hydrocarbon. (b) Ground-state electronic configuration of open-shell singlet diradicaloids and polyradicaloids. (c) Schematic singlet-to-triplet thermal population. (d) Representative benzenoid polycyclic hydrocarbon-based diradicaloids. (e) Macrocyclic and (f) cagelike polyradicaloids. Reproduced with permission from ref 39. Copyright 2014 American Chemical Society.

The concept of aromaticity also went to 3D π- or σconjugated systems, and one notable example is Hirsch’s [2(N + 1)2] spherical aromaticity rule for fullerenes.30,31 This rule later was extended for open-shell spherical compounds, which predicts that spherical species having [2N2 + 2N + 1] electrons and S = N + 1/2 will be aromatic.32 The spherical aromaticity rule was also used to explain the aromatic character of some metal- and boron-based clusters.33 In addition, spherical homoaromaticity was found in some highly symmetric hydrocarbons.34 It was also predicted that close stacking of antiaromatic π-systems could induce three-dimensional (3D) aromaticity as a result of strong frontier orbital interactions35 and this was recently validated by using a stacked antiaromatic norcorrole dimer.36 However, there were very limited studies on the global aromaticity of 3D π-conjugated organic molecules.37

the multiple diradical character index yi, defined as the occupation numbers of the LUNO + i (i = 0, 1,···), can be used as a measure of the diradical character.49 These molecules have open-shell singlet ground state with diradical-like behavior and thus they are called as open-shell singlet diradicaloids. The small singlet (S0)−triplet (T1) energy gap (ΔES−T) leads to the unique magnetic activity arising from thermal population from the diamagnetic S0 state to the paramagnetic T1 state (Figure 2c). In the past 10 years, a large number of relatively stable open-shell singlet diradicaloids have been synthesized by using different synthetic methods and thermodynamic/kinetic stabilizing strategies. Our research group has demonstrated that quinoidal polycyclic hydrocarbons such as extended zethrenes (14)50 and paraquinodimethanes (15)51 have intrinsic tendency to become diradicals by recovery of the aromaticity of the quinoid rings (Figure 2d). Interestingly, even the aromatic zigzag edged periacenes (16) show open-shell diradical character due to the gain of aromatic resonance energy in the diradical form.52 The detailed synthesis and fundamental structure−diradical character−electronic properties relationships have been reviewed in literature.38−42,50,51 Recently, much attention has been paid to the development of open-shell polyradicaloids in which more than two spins are antiferromagnetically (AFM) coupled with each other. It was found that moderate AFM coupling between the frontier radicals was critical to attain multiple diradical character,53,54 and several linear polyradicaloid molecules have been synthesized.55 We had particular interest in cyclic π-conjugated polyradicaloids (Figure 2e) because the frontier π-electrons on the macrocycles may behave similarly to the π-electrons in the typical closed-shell [n]annulenes. This raises the questions about their possible global aromaticity by electron delocalization along with the macrocycle backbone. The small singlet− triplet energy gap may allow spin flip and thus both Hückel’s rule and Baird’s rule could be applicable to these open-shell

1.2. Aromaticity and Diradical Character

So far, most π-conjugated molecules with even number of πelectrons have shown closed-shell ground state. However, recent studies demonstrated that certain types of molecules could exhibit open-shell singlet ground state.38−42 The history can be dated back to 1907 when Tschitschibabin reported a quinoidal conjugated hydrocarbon (13, Figure 2a).43 This molecule showed unusually high reactivity, which is believed to be due to its diradical character via recovery of two aromatic sextet rings in the open-shell diradical form (Figure 2a). After that, a few π-conjugated molecules were occasionally reported to show similar diradical-like behavior.44−47 Different from the traditional closed-shell molecules, the frontier π-electrons in these molecules are moderately coupled (paired), which results in a unique electronic configuration. These molecules usually have a small energy gap, and the HOMOs and LUMOs can admix in the ground state, leading to partial electron occupancy at the LUMOs (Figure 2b).48 Theoretically, the natural orbital occupation number (NOON) in the lowest unoccupied natural orbitals (LUNOs) can be calculated and C


Article


Figure 3. (a) Resonance structures of two fully fused carbazole based macrocycles. (b) VT 1H NMR spectra of 17 in CD2Cl2. (c) Calculated ACID plots of 17 and 18 in the singlet states. Reproduced with permission from ref 60. Copyright 2016 American Chemical Society.

2. MACROCYCLIC POLYRADICALOIDS WITH GLOBAL AROMATICITY

systems depending on the spin state. The concept could go further to the fully conjugated polyradicaloid cages (Figure 2f) which may lead to possible 3D global aromaticity which was rarely studied. In the past 5 years, our research group has successfully synthesized a number of stable macrocyclic polyradicaloids, which allowed us to systematically investigate their global aromaticity and electronic properties. We have used various experimental methods (e.g., NMR, X-ray crystallographic analysis, electronic absorption spectroscopy) and theoretical approaches (e.g., anisotropy of the induced current density (ACID),56 nucleus independent chemical shift (NICS),57 and iso-chemical shielding surface (ICSS)58,59 calculations to thoroughly understand their global aromatic/ antiaromatic character. Our studies revealed interesting aromaticity rules and physical properties for this type of open-shell cyclic conjugated molecules. In particular, the long sought-after annulene-within-an-annulene (AWA) super-ring structure was attained for the first time, and both Hückel’s rule and Baird’s rule can be applicable for a sole molecular system. Our studies also demonstrated the importance of global antiaromaticity at the transition state for the valence tautomerization process. We believe that it is time to summarize these studies in this Account.

2.1. Macrocyclic Polyradicaloids Showing Hückel (Anti)aromaticity

Synthesis of π-conjugated macrocycles with polyradical character is a challenging task as the molecules are usually highly reactive. We also need carefully tune the spin−spin interactions as too strong coupling will decrease the polyradical character. In 2016, we reported two fully fused carbazole macrocycles (17 and 18, Figure 3a), which contains four and six alternatingly arranged quinoidal and aromatic carbazole units, respectively.60 For these two molecules, two aromatic sextet rings are gained at each stage of transition from closedshell to open-shell diradical/tetraradical/hexaradical form, providing sufficient driving force toward higher polyradical forms (Figure 3a). Consequently, moderate tetraradical character (y1 = 0.19) and hexaradical character (y2 = 0.27) were calculated by restricted active space-spin flip (RAS-SF) method for 17 and 18, respectively. The variable-temperature (VT) 1H NMR spectra of 17 in CD2Cl2 revealed broadened signals at room temperature (rt) due to existence of thermally populated paramagnetic triplet species, while as the temperature was lowered, the signals became sharper (Figure 3b). D


Article

Accounts of Chemical Research Scheme 1. Representative Synthetic Methods for the Macrocyclic Polyradicaloids

Figure 4. (a) Chemical structures fluorenyl-based macrocyclic polyradicaloids. (b) Calculated ACID plots of 19 and 21 in the singlet states. Reproduced with permission from ref 55. Copyright 2016 American Chemical Society.

Notably, at 0 °C, the inner protons of the macrocycle backbone (d, d′) appeared at lower field (δ: 10.62 ppm) while

the outer protons (c, c′) at a higher field (δ: −3.2 ppm), indicating an antiaromatic character. The ACID plot of 17 E


Article


Figure 5. Representative resonance forms (A−C) of the macrocycles 22 and 23 (a), calculated ACID plots of backbones of 22 (b), 222+ (c), 23 (d), and 232+ (e), and calculated 2D ICSS maps of backbones of 22 (f), 222+ (g), 23 (h), and 232+ (i). Ar: 4-tert-butyl-2,6-dimethylphenyl. Reproduced with permission from ref 62. Copyright 2018 WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim.

clearly shows a counterclockwise ring current circuit along the periphery containing 36 ([4N]) π electrons (Figure 3c). Therefore, 17 was confirmed to show global antiaromaticity like antiaromatic cyclobutadiene (CBD). The VT 1H NMR spectra of 18 in CD2Cl2 were significantly broadened even at 183 K due to a larger radical character in comparison to 17, limiting an analysis of its global aromaticity. However, the ACID plot of 18 reveals a weak clockwise diatropic ring current circuit with 54 ([4N + 2]) π electrons (Figure 3c), suggesting a global aromatic character like benzene ring. Due to the large radical character, 17 and 18 exhibited a small optical energy gap (0.52 eV for both). Noticeably, both compounds showed small excitation energies from the singlet ground state to higher-spin excited states, which resulted in weak magnetization even at rt. Compounds 17 and 18 were synthesized in a three-step protocol: (1) Suzuki coupling between a dibromo-dialdehyde monomer (A) and a dibronic acid/ester monomer (B) to give macrocycles with different sizes; (2) nucleophilic addition of the aldehyde groups with arylmagnesium bromide followed by Friedel−Crafts alkylation to generate the cyclopenta ring-fused macrocycles; and (3) oxidative dehydrogenation by oxidants to afford the fully conjugated macrocycles (Scheme 1a). This will be also a general approach for other fully fused macrocyclic polyradicaloids (vide infra). Bulky aryl groups need be attached onto the head carbons with high spin densities to ensure sufficient stability. Compounds 17 and 18 have a high-lying HOMO energy level (−4.01 and −4.14 eV, respectively), which makes them unstable under ambient conditions. A 9-(3,5-di-tert-

butylphenyl)anthryl (An) blocked 9-fluorenyl radical was found to show good stability.55 Therefore, we synthesized a series of macrocyclic polyradicaloids such as 19-21 in which the An-substituted fluorenyl radicals are either directly linked (19/20) or linked by a C−C triple bond (21) (Figure 4a).61 The former two were synthesized by Yamamoto coupling of the dibromo-monomer (C) followed by reductive removal of the −OMe groups (Scheme 1b). The latter was synthesized by Stille coupling reaction, followed by reduction (Scheme 1c). Xray crystallographic analysis reveals that the backbone of 19 adopts a saddle shape conformation due to steric hindrance between the neighboring fluorenyl units. However, the ethynylene bridged 21 has a nearly planar π-conjugated backbone due to diminished steric repulsion. ACID plots of 19 and 21 both show a counterclockwise ring current circuit along the periphery containing 36 and 44 ([4N]) π electrons, respectively (Figure 4b), indicating a global antiaromatic character. In addition, 19 and 21 are calculated to have a positive NICS(0) value of +11.7 and +14.3 ppm, respectively, further demonstrating their moderate antiaromaticity. On the other hand, the cyclic hexamer 20 has a largely distorted figureof-eight geometry and there is no ring current along the periphery. Due to the moderate AFM coupling between the radicals and the antiaromatic character, 19 and 21 show a smaller energy gap and slightly larger two-photon absorption (TPA) cross sections compared with a linear tetramer counterpart. However, the cyclic hexamer 20 exhibited larger energy gap and smaller TPA cross section compared with a liner hexamer due to less effective π-conjugation. F


Article

Accounts of Chemical Research Instead of using a stable monoradical as the building block, we also designed and synthesized macrocycles 22 and 23 in which three/four open-shell diradicaloid units are directly linked together (Figure 5a).62 These macrocycles were synthesized by Yamamoto coupling of the building block C followed by reduction (Scheme 1b). The monomer dithieno[a,h]-s-indacene was demonstrated to have open-shell singlet ground state. The macrocycles can be drawn in different resonance forms (Figure 5a), for example, an open-shell hexa-/ octa-radical form with all benzene and thiophene rings being aromatic (form A), a closed-shell form in which only three/ four thiophene rings are aromatic (form B), and a closed-shell form in which only three/four benzenoid rings are aromatic (form C). The X-ray and VT NMR analysis clearly show that both 22 and 23 have a closed-shell ground state with global antiaromatic character. A counterclockwise ring current circuit along the cyclic backbone containing 36 and 48 ([4N]) π electronsis found for 22 and 23, respectively, in their ACID plots (Figure 5b,d), and the electron delocalization pathway follows the inner and outer rims alternatingly (Figure 5a). The NICS(0) value at the ring center was calculated to be +7.0 and +4.9 ppm for 22 and 23, respectively, further suggesting their global antiaromaticity. Coincidentally, the calculated 2D ICSS maps demonstrate that the inner cavity area is deshielded (negative values) while the outside area is shielded (positive values) (Figure 5f,h). The change from open-shell singlet ground state for the monomer to the closed-shell antiaromatic macrocycles implies that the AFM coupling (bonding) between the spins is more efficient through the dithiophene spacer (form C) than through the meta-benzenoid spacer (form B). Compounds 22 and 23 exhibit intense absorption band in the visible region, with a long tail to the near-infrared region which can be correlated to partially forbidden transitions for these antiaromatic macrocycles. In addition, they display small electrochemical energy gap and multiple reversible redox waves. Chemical oxidation gave their dications 222+ and 232+, and both turned out to be open-shell singlet diradical dications. NMR measurements reveal that both dications are globally aromatic and ACID plots shows a clockwise diatropic ring current circuit (Figure 5c,e). The NICS(0) values now become negative (−12.5/−12.6 ppm) and the 2D ICSS maps further confirm an inverse of aromaticity after losing two π electrons (Figure 5g,i). Indeed, a 34π/46π conjugation pathway can be drawn for the dication of 22/23, satisfying [4N + 2] Hückel aromaticity rule. Spin delocalization of unpaired electrons through a large πconjugated framework such as porphyrinoid would improve the thermodynamic stability. Therefore, a bithiophene bridged expanded porphycene 24 and a cyclopentabithiophene bridged expanded porphycene 25 (Figure 6a) were synthesized by McMurry coupling of the respective aldehyde precursor, followed by oxidative dehydrogenation (Scheme 1d).63 Compounds 24 and 25 have intrinsic tendency to become open-shell diradicals/tetradicals by recovery of two/four aromatic thiophene rings (Figure 6a). Magnetic measurements and theoretical calculations reveal that both 24 and 25 exhibit open-shell singlet ground state with significant radical character (y0 = 0.63 for 24; y0 = 0.68, y1 = 0.18 for 25) and small singlet−triplet energy gap (ΔES−T = −3.25 kcal/mol for 24 and ΔES−T = −0.92 kcal/mol for 25). Both compounds display exceptional stability under ambient air and light conditions due to effective delocalization of unpaired electrons. X-ray crystallographic analysis and NMR measurements

Figure 6. (a) Representative resonance forms of the expanded porphycenes 24 and 25. (b) Calculated ACID plots of backbones of 24 and 25. Reproduced with permission from ref 63. Copyright 2018 WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim.

suggest that 24 is globally aromatic, and its ACID plot displays a clockwise diatropic ring current circuit with a 34π ([4N + 2]) conjugation pathway along the periphery (Figure 6b). On the other hand, ACID plot of 25 exhibits a counterclockwise paratropic ring current circuit with a 32π ([4N]) conjugation pathway and thus it is antiaromatic (Figure 6b). This is further supported by the calculated NICS(0) value of −15.4 ppm and +9.8 ppm for 24 and 25, respectively. In accordance with their aromatic/antiaromatic character, the electronic absorption spectrum of 24 exhibits an intense Soret band along with a distinct Q-like band, but compound 25 only displays a broad band in the visible region, with a weak long tail extended beyond 2000 nm. 2.2. Macrocyclic Polyradicaloids Showing Baird Aromaticity and a Decoupled AWA Super-Ring Structure

The generally observed Hückel (anti)aromaticity for the above-mentioned macrocyclic polyradicaloids however encountered difficulty to explain the unusual aromaticity in two macrocyclic octaradicaloid 26 and decaradicaloid 27, in which the six- and five-membered rings are alternatively fused (Figure 7a).64 They were synthesized according to the general method shown in Scheme 1a and both are soluble and stable. X-ray structure analyses reveal that 26 has a bowl-shaped backbone while 27 has a nearly planar backbone. RAS-SF calculations predict that 26 has a moderate octaradical character (y3 = 0.45) and 27 has a moderate decaradical character (y4 = 0.46). Magnetic measurements reveal that 26 has open-shell singlet ground state while 27 has triplet ground state. The 1H NMR spectrum of 26 in THF-d8 at 233 K shows that the outer-rim protons on the backbone appear at very low field (δ = +11.37 ppm) and while the inner rim protons locate at very high field (δ = −12.08 ppm), indicating a strong aromatic character. G


Article


Figure 7. (a) Representative open-shell and closed-shell resonance forms of the macrocycles 26 and 27. (b) ACID plots of the individual inner/ outer rings and the backbone of 26 in the open-shell singlet ground state. (c) 2D NICSzz map of the backbone of 26 in the open-shell singlet ground state. (d) ACID plots of the individual inner/outer rings and the backbone of 27 in the triplet ground state. (e) 2D NICSzz map of the backbone of 27 in the triplet ground state. Reproduced with permission from ref 64. Copyright 2018, Elsevier Inc.

Surprisingly, ACID plot of 26 in ground singlet state displays a unique AWA super-ring structure with two decoupled diatropic ring current circuits (Figure 7b), which is against Hückel’s rule as both the inner rim (24π) and outer rim (32π) formally have [4N] π electrons (Figure 7a). The 2D NICS map shows that the inner protons on the macrocycle are highly shielded while the outer rim protons are highly deshielded (Figure 7c), further supporting its large aromatic character. Two alternative electronic structures as the source are considered for such unusual “anti-Hückel” global aromaticity: (1) one annulene gives two electrons to the other one (2−/2+ or 2+/2− for inner/outer rings), and (2) each annulene holds a “triplet” diradical character. Constrained DFT calculations predict that the 2−/2+ and 2+/2− states are much higher in energy than the triplet/triplet (TT) state and hence point toward option 2: the global aromaticity can be ascribed to the “triplet(in)-triplet(out)″ character of the singlet ground state, that is, two triplets coupled as a singlet. These results can be rationalized due to the large radical character in 26. ACID plots of individual inner/outer annulenes also suggest that

these [4N]annulenes show a clockwise diamagnetic current circuit in the triplet state (Figure 7b). Therefore, Baird’s rule should be applied to both the inner and outer annulenes, and as a result the whole molecule exhibits [4N] global aromaticity. Similarly, the lowest-energy contribution in the triplet ground state of 27 is the singlet/triplet (ST) state, and ACID plots (Figure 7d) and 2D NICS scan (Figure 7e) analysis again reveal a superaromatic AWA structure, with the inner ring being singlet and aromatic (Hückel’s rule) and the outer ring being triplet and aromatic (Baird’s rule). Due to the polyradical character and effective conjugation, both compounds show an optical energy gap smaller than 0.5 eV, and amphoteric redox behavior with at least 12 accessible redox waves. The AWA super-ring structure has been sought-after by chemists for more than 50 years but with limit success.65−67 However, it is now occasionally attained in 26 and 27, which deserves further investigation. We realize that only four Kekulé structures can be drawn for the backbone of 26 (denoted as 26′) and 27 in closed-shell form, with all radial bonds being H


Article


Figure 8. (a) Four possible resonance structures of 26′ in the closed-shell form. (b) Open-shell structure of 28. (c) Calculated ACID plot of the backbone of 28. (d) Two representative resonance structures of 28′ in the closed-shell form. Reproduced with permission from ref 68. Copyright 2018 WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim.

CBD (Figure 9a).70 Usually, [n]annulenes with [4N] π electrons prefer to form local double bonds resulting in two interconvertible valence isomers. However, it is very challenging to experimentally detect such a valence tautomerization process due to the very fast conversion.71,72 We envision that extension of the [4N]annulene carbon skeleton by incorporation of aromatic benzene rings could increase the valence tautomerization barrier as the process involves rupture and then reformation of aromatic benzene ring via a possible polyradicaloid transition state. X-ray crystallographic structure analysis at 100 K show a C2 symmetric structure with alternating aromatic/quinoidal biphenyl units and ACID plot also reveal two set of aromatic biphenyl units with local aromaticity on individual benzene rings (Figure 9b). Interestingly, a slow valence tautomerization process was observed in 29 by VT NMR measurements. At rt, only one set of signal was observed for the biphenyl units, but as the temperature was lowered below the coalescence temperature (263 K), two sets of peaks were observed, in accordance with a slow interconversion process between the valence tautomers. Careful line-shape and dynamics analysis gave a moderate interconversion barrier (11.22 kcal/mol at 263 K). Calculations suggest that the valence tautomerization process between the two ground-state (GS) isomers goes through a C4 symmetric tetraradicaloid transition state (TS), and ACID calculation based on the TS geometry shows a counterclockwise ring current circuit along the backbone (Figure 9b). In addition, a NICS(0) value of +0.6/+10.1 ppm is calculated for GS/TS geometry, respectively. The antiaromatic character of the TS is believed to raise the interconversion barrier and lead to an unusually slow valence tautomerization process.

single bonds (A-D, Figure 8a). That means, the inner rim and outer rim are almost electronically decoupled, which could be the origin for the decoupled AWA structure. To test this concept, another macrocyclic tetraradicaloid 28 (Figure 8b) was synthesized via the method shown in Scheme 1a.68 The backbone of 28 can be also drawn in an AWA structure, with 24 π electrons along the inner rim and 36 π electrons along the outer rim (A′). However, different from 26/27, another closed-shell form with four radial C−C double bonds can be drawn (B′), indicating possible electronic coupling between the inner rim and outer rim (Figure 8d). The ACID plot of 28 shows only one counterclockwise ring current flow along the periphery with a 36π ([4N]) conjugation pathway and no decoupled AWA structure is found (Figure 8c). A positive NICS(0) value of +19.8 ppm is calculated, indicating its antiaromatic character, which is also supported by NMR measurements. On the other hand, its dication 282+ and dianion 282− are globally aromatic due to the existence of a 34π/38π conjugation pathway. Compared with a smaller-size analogue,53 the longer spacer in 28 led to a moderate AFM coupling of four spins and moderate diradical and tetraradical character. Very recently, two fully conjugated carbon nanobelts in which six/eight cyclopenta- rings are fused onto a macrocycle containing three/four alternately linked 2,7-pyrenyl and 2,7-phenanthryl units were synthesized as analogues of 28.69 They both show local aromaticity in the neutral state, but their dications are globally aromatic with a 94π/126π [4N + 2] conjugation pathway.

3. GLOBAL ANTIAROMATICITY IN THE TRANSITION STATE The above discussion mainly focuses on the ground-state aromaticity. We recently demonstrated that global antiaromaticity in the transition state also played an important role in the valence tautomerization process of a macrocyclic tetraradicaloid 29, which can be regarded as analogue of the antiaromatic

4. CONCLUSION AND PERSPECTIVE In summary, a series of stable macrocyclic polyradicaloids have been successfully synthesized, which allow us to systematically understand the global aromaticity in both ground state and I


Article


Figure 9. (a) Schematic representation of valence tautomerization of two GS valence isomers of 29 via a tetraradicaloid TS. (b) Calculated ACID plots and NICS (0) value of backbone of 29 at GS and TS geometries. An is the same substituent shown in Figure 4. Reproduced with permission from ref 70. Copyright 2019 Elsevier Inc.

Figure 10. Three-dimensional π-conjugated PTM diradical cage (30) and its schematic representation. Reproduced with permission from ref 73. Copyright 2017 WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim.

transition state. It is found that aromaticity plays a very important role in determining their multiple diradical characters and electronic properties. Different from the traditional closed-shell systems, both Hückel’s rule and Baird’s

rule can be applicable for these ground-state open-shell molecules as the frontier π electrons are moderately coupled and spin flip is allowed due to a small energy barrier. Consequently, the molecules can adjust their geometry and J


Article

Accounts of Chemical Research spin state to become aromatic by following either [4N + 2] Hückel aromaticity rule in the singlet state or [4N] Baird aromaticity rule in the triplet state. Notably, for the first time, we attained a long sought-after AWA super-ring structure. The origin was rationalized, which will allow us to design such kind of molecules in the future. We also demonstrated that global antiaromaticity was very important for controlling the valence tautomerization process in the π-conjugated macrocycles with [4N] π electrons. Overall, these studies provide a good addition to the traditional aromaticity rules for 2D cyclic πconjugated molecules. However, the studies on the global aromaticity of 3D fully πconjugated organic molecules are still very limited mainly due to synthetic challenges. We can envision that aromaticity rules in the 3D conjugated cages shown in Figure 2f will be very interesting and it will depend on not only total number of πconjugated electrons, but also the molecular symmetry and spin state. Along this line, we have synthesized the first πconjugated diradical cage 30 in which two bridgehead perchlorotriphenylmethyl (PTM) radicals are linked by three bis(3,6-carbazolyl) spacers (Figure 10).73 Unfortunately, two PTM radicals in this molecule are weakly coupled due to the distorted structure and long distance between the bridgehead carbons. Synthetic effort toward 3D fully conjugated polyradicaloid cages with moderate spin−spin coupling is ongoing in our laboratory.

■

supervision of Prof. Jishan Wu. His research interests mainly focus on macrocyclic polyradicaloids. Jishan Wu received BSc degree from Wuhan University (1997), MSc degree from Changchun Institute of Applied Chemistry, CAS (2000), and PhD degree from Max-Planck Institute for Polymer Research (2004). He conducted postdoc study in UCLA (2005−2007) and then joined the Department of Chemistry of NUS as an assistant professor in 2007. He was promoted to a full professor in 2017. His major interests include novel π-conjugated systems and supramolecular chemistry.

■

ACKNOWLEDGMENTS We acknowledge financial support from the MOE Tier 3 program (MOE2014-T3-1-004) and NRF Investigatorship Award (NRF-NRFI05-2019-0005). We thank our major collaborators Professors David Casonova, Dongho Kim, and Juan Casado for their valuable contributions to this project.

■

REFERENCES

(1) Hückel, E. Quantentheoretische Beiträge zum Benzolproblem. Eur. Phys. J. A 1931, 70, 204−286. (2) Breslow, R.; Groves, J. T.; Ryan, G. Cyclopropenyl Cation. J. Am. Chem. Soc. 1967, 89, 5048. (3) Breslow, R. Antiaromaticity. Acc. Chem. Res. 1973, 6, 393−398. (4) Sondheimer, F. The Annulenes. Acc. Chem. Res. 1972, 5, 81−91. (5) Spitler, E. L.; Johnson, C. A.; Haley, M. M. Renaissance of Annulene Chemistry. Chem. Rev. 2006, 106, 5344−5386. (6) Von E. Doering, W.; Knox, L. H. The Cycloheptatrienylium (tropylium) Ion. J. Am. Chem. Soc. 1954, 76, 3203−3206. (7) Breslow, R.; Hover, H.; Chang, H. W. The Synthesis and Stability of Some Cyclopropenyl Cations with Alkyl Substituents. J. Am. Chem. Soc. 1962, 84, 3168−3174. (8) Wiberg, K. B. Antiaromaticity in Monocyclic Conjugated Carbon Rings. Chem. Rev. 2001, 101, 1317−1331. (9) Tanaka, T.; Osuka, A. Chemistry of meso-Aryl-substituted Expanded Porphyrins: Aromaticity and Molecular Twist. Chem. Rev. 2017, 117, 2584−2640. (10) Stępień, M.; Latos-Grażyński, L. Aromaticity and Tautomerism in Porphyrins and Porphyrinoids. Top. Heterocycl. Chem. 2008, 19, 82−153. (11) Heilbronner, E. Hückel Molecular Orbitals of Möbius-type Conformations of Annulenes. Tetrahedron Lett. 1964, 5, 1923−1928. (12) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Synthesis of a Möbius Aromatic Hydrocarbon. Nature 2003, 426, 819−821. (13) Stępień, M.; Latos-Grażyński, L.; Sprutta, N.; Chwalisz, P.; Szterenberg, L. Expanded Porphyrin with a Split Personality: A Hückel-Möbius Aromaticity Switch. Angew. Chem., Int. Ed. 2007, 46, 7869−7873. (14) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata, N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; et al. Metalation of Expanded Porphyrins: A Chemical Trigger Used to Produce Molecular Twisting and Möbius Aromaticity. Angew. Chem., Int. Ed. 2008, 47, 681−684. (15) Yoon, Z. S.; Osuka, A.; Kim, D. Möbius Aromaticity and Antiaromaticity in Expanded Porphyrins. Nat. Chem. 2009, 1, 113− 122. (16) Stępień, M.; Sprutta, N.; Latos-Grażyński, L. Figure eights, Mö bius bands, and more: Conformation and Aromaticity of Porphyrinoids. Angew. Chem., Int. Ed. 2011, 50, 4288−4340. (17) Craig, D. P.; Paddock, N. L. A Novel Type of Aromaticity. Nature 1958, 181, 1052−1053. (18) Zhu, C. Q.; Xia, H. P. Carbolong Chemistry: A Story of Carbon Chain Ligands and Transition Metals. Acc. Chem. Res. 2018, 51, 1691−1700.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jishan Wu: 0000-0002-8231-0437 Notes

The authors declare no competing financial interest. Biographies Chunchen Liu was born in Jiangxi province, China, in 1989. He received his BSc degree (2010) and PhD degree (2015) from the South China University of Technology. He worked as a research fellow in the National University of Singapore (NUS) under the supervision of Prof. Jishan Wu from 2015 to 2019. He mainly worked on novel open-shell macrocyclic polyradicaloids. He is now a research assistant professor in Southern University of Science and Technology. Yong Ni was born in Jiangxi province, China, in 1988. He received his BSc degree from Zhejiang University (2010) and PhD degree from NUS (2014) under the supervision of Prof. Jishan Wu. In 2014, he joined Agency for Science, Technology and Research (A* Star) as a research scientist. He is now working in Prof. Jishan Wu’s group at NUS as a research fellow and mainly worked on 2D and 3D polyradicaloids. Xuefeng Lu was born in Hunan province, China, in 1986. He received his BSc degree from Zhengzhou University (2010) and PhD degree from Fudan University (2015). Then, he conducted postdoctorate research in Prof. Jishan Wu’s group in NUS. He is now a tenure-track professor in the Department of Materials Science at Fudan University. His research interests mainly focus on novel π-conjugated molecules and their applications. Guangwu Li was born in Anhui province, China, in 1989. He received his BSc degree (2010) and PhD degree (2015) from Beijing Normal University. He is now working as a research fellow in NUS under the K


Article


(43) Tschitschibabin, A. E. Ü ber Einige Phenylierte Derivate des p, p-Ditolyls. Ber. Dtsch. Chem. Ges. 1907, 40, 1810−1819. (44) Müller, E.; Pfanz, H. Ü ber Biradikaloide Terphenylderivate. Ber. Dtsch. Chem. Ges. B 1941, 74, 1051−1074. (45) Dimroth, K.; Umbach, W.; Blocher, K. H. Bis-phenoxy-Radicals of the Polyphenyl Series. Angew. Chem., Int. Ed. Engl. 1963, 2, 620− 621. (46) Ohashi, K.; Kubo, T.; Masui, T.; Yamamoto, K.; Nakasuji, K.; Takui, T.; Kai, Y.; Murata, I. 4,8,12,16-Tetra-tert-butyl-s-indaceno[1,2,3-cd:5,6,7-c’d’]diphenalene: A Four-Stage Amphoteric Redox System. J. Am. Chem. Soc. 1998, 120, 2018−2027. (47) Takahashi, T.; Matsuoka, K. I.; Takimiya, K.; Otsubo, T.; Aso, Y. Extensive Quinoidal Oligothiophenes with Dicyanomethylene Groups at Terminal Positions as Highly Amphoteric Redox Molecules. J. Am. Chem. Soc. 2005, 127, 8928−8929. (48) Yamaguchi, K. In Self-Consistent Field: Theory and Applications; Carbo, R., Klobukowski, M., Eds.; Elsevier: Amsterdam, the Netherlands, 1990. (49) Nakano, M.; Fukui, H.; Minami, T.; Yoneda, K.; Shigeta, Y.; Kishi, R.; Champagne, B.; Botek, E.; Kubo, T.; Ohta, K.; Kamada, K. (Hyper)polarizability Density Analysis for Open-shell Molecular Systems Based on Natural Orbitals and Occupation Numbers. Theor. Chem. Acc. 2011, 130, 711−724. (50) Hu, P.; Wu, J. Modern Zethrene Chemistry. Can. J. Chem. 2017, 95, 223−233. (51) Zeng, Z.; Wu, J. Stable π-Extended p-Quinodimethanes: Synthesis and Tunable Ground State. Chem. Rec. 2015, 15, 322−328. (52) Ni, Y.; Gopalakrishna, T. Y.; Phan, H.; Herng, T. S.; Wu, S.; Han, Y.; Ding, J.; Wu, J. A Peri-tetracene Diradicaloid: Synthesis and Properties. Angew. Chem., Int. Ed. 2018, 57, 9697−9701. (53) Nobusue, S.; Miyoshi, H.; Shimizu, A.; Hisaki, I.; Fukuda, K.; Nakano, M.; Tobe, Y. Tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene: A Potential Tetraradicaloid Hydrocarbon. Angew. Chem., Int. Ed. 2015, 54, 2090−2094. (54) Hu, P.; Lee, S.; Herng, T. S.; Aratani, N.; Gonçalves, T. P.; Qi, Q.; Shi, X.; Yamada, H.; Huang, K.-W.; Ding, J.; Kim, D.; Wu, J. Towards Tetraradicaloid: The Effect of Fusion Mode on Radical Character and Chemical Reactivity. J. Am. Chem. Soc. 2016, 138, 1065−1077. (55) Lu, X.; Lee, S.; Kim, J. O.; Gopalakrishna, T. Y.; Phan, H.; Herng, T. S.; Lim, Z. L.; Zeng, Z.; Ding, J.; Kim, D.; Wu, J. Stable 3,6Linked Fluorenyl Radical Oligomers with Intramolecular Antiferromagnetic Coupling and Polyradical Characters. J. Am. Chem. Soc. 2016, 138, 13048−13058. (56) Geuenich, D.; Hess, K.; Köhler, F.; Herges, R. Anisotropy of the Induced Current Density (ACID), a General Method to Quantify and Visualize Electronic Delocalization. Chem. Rev. 2005, 105, 3758− 3772. (57) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. Nucleus-independent Chemical Shifts: A Simple and Efficient Aromaticity Probe. J. Am. Chem. Soc. 1996, 118, 6317−6318. (58) Klod, S.; Kleinpeter, E. Ab initio Calculation of the Anisotropy Effect of Multiple Bonds and the Ring Current Effect of Arenes Application in Conformational and Configurational Analysis. J. Chem. Soc., Perkin Trans. 2001, 2, 1893−1898. (59) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580−592. (60) Das, S.; Herng, T. S.; Zafra, J. L.; Burrezo, P. M.; Kitano, M.; Ishida, M.; Gopalakrishna, T. Y.; Hu, P.; Osuka, A.; Casado, J.; Ding, J.; Casanova, D.; Wu, J. Fully Fused Quinoidal/aromatic Carbazole Macrocycles with Poly-radical Characters. J. Am. Chem. Soc. 2016, 138, 7782−7790. (61) Lu, X.; Lee, S.; Hong, Y.; Phan, H.; Gopalakrishna, T. Y.; Herng, T. S.; Tanaka, T.; Sandoval-Salinas, M. E.; Zeng, W.; Ding, J.; Casanova, D.; Osuka, A.; Kim, D.; Wu, J. Fluorenyl Based Macrocyclic Polyradicaloids. J. Am. Chem. Soc. 2017, 139, 13173− 13183.

(19) Baird, N. C. Quantum Organic Photochemistry. II. Resonance and Aromaticity in The Lowest 3ππ* State of Cyclic Hydrocarbons. J. Am. Chem. Soc. 1972, 94, 4941−4948. (20) Wörner, H. J.; Merkt, F. Photoelectron Spectroscopic Study of The First Singlet and Triplet States of The Cyclopentadienyl Cation. Angew. Chem., Int. Ed. 2006, 45, 293−296. (21) Sung, Y. M.; Yoon, M.-C.; Lim, J. M.; Rath, H.; Naoda, K.; Osuka, A.; Kim, D. Reversal of Hückel (Anti)aromaticity in The Lowest Triplet States of Hexaphyrins and Spectroscopic Evidence for Baird’s Rule. Nat. Chem. 2015, 7, 418−422. (22) Rosenberg, M.; Dahlstrand, C.; Kilså, K.; Ottosson, H. Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations. Chem. Rev. 2014, 114, 5379−5425. (23) Ottosson, H. Organic Photochemistry: Exciting Excited-state Aromaticity. Nat. Chem. 2012, 4, 969−971. (24) Zilberg, S.; Haas, Y. Two-State Model of Antiaromaticity: The Low Lying Singlet States. J. Phys. Chem. A 1998, 102, 10843−10850. (25) Dreyer, J.; Klessinger, M. The photochemical Formation of Fulvene from Benzene via Prefulvene−a Theoretical Study. Chem. Eur. J. 1996, 2, 335−341. (26) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.; Breslow, R.; Hoffmann, J. M. Jr.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck, V. J. Unsubstituted Cyclopentadienyl Cation, a Ground-state Triplet. J. Am. Chem. Soc. 1973, 95, 3017−3018. (27) Aihara, J. Is Superaromaticity a Fact or an Artifact? The Kekulene Problem. J. Am. Chem. Soc. 1992, 114, 865−868. (28) Buttrick, J. C.; King, B. T. Kekulenes, Cycloarenes, and Heterocycloarenes: Addressing Electronic Structure and Aromaticity Through Experiments and Calculations. Chem. Soc. Rev. 2017, 46, 7− 20. (29) Clar, E. Aromatic Sextet; John Wiley, 1972. (30) Hirsch, A.; Chen, Z.; Jiao, H. Spherical Aromaticity in IhSymmetrical Fullerenes: The 2(N+1)2 Rule. Angew. Chem., Int. Ed. 2000, 39, 3915−3917. (31) Bühl, M.; Hirsch, A. Spherical Aromaticity of Fullerenes. Chem. Rev. 2001, 101, 1153−1183. (32) Poater, J.; Solà, M. Open-shell Spherical Aromaticity: The 2N2 + 2N + 1 (with S = N + 1/2) Rule. Chem. Commun. 2011, 47, 11647− 11649. (33) Chen, Z.; King, R. B. Spherical Aromaticity: Recent Work on Fullerenes, Polyhedral Boranes, and Related Structures. Chem. Rev. 2005, 105, 3613−3642. (34) Chen, Z.; Jiao, H.; Hirsch, A.; Schleyer, P. v. R. Spherical Homoaromaticity. Angew. Chem., Int. Ed. 2002, 41, 4309−4312. (35) Corminboeuf, C.; Schleyer, P. v. R.; Warner, P. Are Antiaromatic Rings Stacked Face-to-face Aromatic? Org. Lett. 2007, 9, 3263−3266. (36) Nozawa, R.; Tanaka, H.; Cha, W.-Y.; Hong, Y.; Hisaki, I.; Shimizu, S.; Shin, J.-Y.; Kowalczyk, T.; Irle, S.; Kim, D.; Shinokubo, H. Stacked Antiaromatic Porphyrins. Nat. Commun. 2016, 7, 13620. (37) Cha, W.-Y.; Kim, T.; Ghosh, A.; Zhang, Z.; Ke, X.-S.; Ali, R.; Lynch, V. M.; Jung, J.; Kim, W.; Lee, S.; Fukuzumi, S.; Park, J. S.; Sessler, J. L.; Chandrashekar, T. K.; Kim, D. Bicyclic Baird-type Aromaticity. Nat. Chem. 2017, 9, 1243−1248. (38) Abe, M. Diradicals. Chem. Rev. 2013, 113, 7011−7088. (39) Sun, Z.; Zeng, Z.; Wu, J. Zethrenes, Extended p-Quinodimethanes, and Periacenes with a Singlet Biradical Ground State. Acc. Chem. Res. 2014, 47, 2582−2591. (40) Zeng, Z.; Shi, X.; Chi, C.; López Navarrete, J. T.; Casado, J.; Wu, J. Pro-aromatic and Antiaromatic π-Conjugated Molecules: An Irresistible Wish to be Diradicals. Chem. Soc. Rev. 2015, 44, 6578− 6596. (41) Kubo, T. Recent Progress in Quinoidal Singlet Biradical Molecules. Chem. Lett. 2015, 44, 111−122. (42) Y. Gopalakrishna, T.; Zeng, W.; Lu, X.; Wu, J. From Open-shell Singlet Diradicaloids to Polyradicaloids. Chem. Commun. 2018, 54, 2186−2199. L


Article

Accounts of Chemical Research (62) Li, G.; Gopalakrishna, T. Y.; Phan, H.; Herng, T. S.; Ding, J.; Wu, J. From Open-shell Singlet Diradicaloid to Closed-shell Global Antiaromatic Macrocyles. Angew. Chem., Int. Ed. 2018, 57, 7166− 7170. (63) Rana, A.; Hong, Y.; Gopalakrishna, T. Y.; Phan, H.; Herng, T. S.; Yadav, P.; Ding, J.; Kim, D.; Wu, J. Stable Expanded Porphycenebased Diradicaloid and Tetraradicaloid. Angew. Chem., Int. Ed. 2018, 57, 12534−12537. (64) Liu, C.; Sandoval-Salinas, M. E.; Hong, Y.; Gopalakrishna, T. Y.; Phan, H.; Aratani, N.; Herng, T. S.; Ding, J.; Yamada, H.; Kim, D.; Casanova, D.; Wu, J. Macrocyclic Polyradicaloids with Unusual Super-ring Structure and Global Aromaticity. Chem. 2018, 4, 1586− 1595. (65) Barth, E. W.; Lawton, R. G. Dibenzo[ghi,mno]fluoranthene. J. Am. Chem. Soc. 1966, 88, 380−381. (66) Dickens, T. K.; Mallion, R. B. Ring-current Assessment of the Annulene-within-an-annulene Model for Some Large Coupled Superring Conjugated-systems. Croat. Chem. Acta 2014, 87, 221−232. (67) Dickens, T. K.; Mallion, R. B. π-Electron Ring-currents and Bond-currents in [10,5]-Coronene and Related Structures Conforming to the ‘Annulene-Within-an-Annulene’ model. Phys. Chem. Chem. Phys. 2013, 15, 8245−8253. (68) Lu, X.; Gopalakrishna, T. Y.; Phan, H.; Herng, T. S.; Jiang, Q.; Liu, C.; Li, G.; Ding, J.; Wu, J. Global Aromaticity in Macrocyclic Cyclopenta-fused Tetraphenanthrenylene Tetraradicaloid and Its Charged Species. Angew. Chem., Int. Ed. 2018, 57, 13052−13056. (69) Lu, X.; Gopalakrishna, T. Y.; Han, Y.; Ni, Y.; Zou, Y.; Wu, J. Bowl-shaped Carbon Nanobelts Showing Size Dependent Properties and Selective Encapsulation of C70. J. Am. Chem. Soc. 2019, 141, 5934−5941. (70) Ni, Y.; Sandoval-Salinas, M. E.; Tanaka, T.; Phan, H.; Herng, T. S.; Gopalakrishna, T. Y.; Ding, J.; Osuka, A.; Casanova, D.; Wu, J. [n]Cyclo-para-biphenylmethine Polyradicaloids: [n]Annulene Analogs and Unusual Valence Tautomerization. Chem. 2019, 5, 108−121. (71) Orendt, A. M.; Arnold, B. R.; Radziszewski, J. G.; Facelli, J. C.; Malsch, K. D.; Strub, H.; Grant, D. M.; Michl, J. 13C NMR and Polarized IR Spectra of Vicinally Labeled [13C2]cyclobutadiene in an Argon Matrix: Interconversion of Valence Tautomers. J. Am. Chem. Soc. 1988, 110, 2648−2650. (72) Pohl, M.; Schmickler, H.; Lex, J.; Vogel, E. Isophlorins: Molecules at the Crossroads of Porphyrin and Annulene Chemistry. Angew. Chem., Int. Ed. Engl. 1991, 30, 1693−1697. (73) Gu, X.; Gopalakrishna, T. Y.; Phan, H.; Ni, Y.; Herng, T. S.; Ding, J.; Wu, J. Three-dimensionally π-Conjugated Diradical Molecular Cage. Angew. Chem., Int. Ed. 2017, 56, 15383−15387.

M


Global Aromaticity in Macrocyclic Polyradicaloids: HÃ¼ckel's Rule or

Recommend Documents