Investigating the edge state of graphene nanoribbons by a chemical approach: Synthesis and magnetic properties of zigzag-edged nanographene molecules

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Investigating the edge state of graphene nanoribbons by a chemical approach: Synthesis and magnetic properties of zigzag-edged nanographene molecules Akihito Konishi, Yasukazu Hirao, Hiroyuki Kurata, Takashi Kubo

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Received date: 15 April 2013 Revised date: 2 July 2013 Accepted date: 7 July 2013 Cite this article as: Akihito Konishi, Yasukazu Hirao, Hiroyuki Kurata, Takashi Kubo, Investigating the edge state of graphene nanoribbons by a chemical approach: Synthesis and magnetic properties of zigzag-edged nanographene molecules, Solid State Communications, http://dx.doi.org/10.1016/j.ssc.2013.07.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Investigating the edge state of graphene nanoribbons by a chemical approach: Synthesis and magnetic properties of zigzag-edged nanographene molecules Akihito Konishi, Yasukazu Hirao, Hiroyuki Kurata, and Takashi Kubo* Department Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan *

Corresponding author. E-mail address: [email protected]

ABSTRACT The edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (GNRs) originating from an electron–electron correlation in an edge-localized π-state, has promising applications for magnetic and spintronics devices and has attracted much attention of physicists, chemists, and engineers. For deeper understanding the edge state, precise fabrication of edge structures in GNRs has been highly demanded. We focus on [a.b]periacene, which are polycyclic aromatic hydrocarbons (PAHs) that have zigzag and armchair edges on molecular periphery, as a model compound for the understanding and actually prepare and characterize them. This review summarizes our recent studies on the origin of the edge state by investigating [a.b]periacene in terms of the relationship between the molecular structure and spin-localizing character. Keywords: A. Graphene Nanoribbons; B. Edge State; C. Polycyclic Aromatic Hydrocarbons

1. Introduction Since its first characterization in 2004 by Geim and Novoselov [1] as a new member of nanocarbons, graphene has developed into booming research topics in physics, chemistry, and electronic device applications [2]. In comparison with other nanocarbons, such as C60 and carbon nanotube, the definitively different feature of graphene is the existence of open edges. A central concern in the field of graphene-related research is how graphene behaves when it is cut into nanoscale ribbons or flakes with distinct open edges. An essential structure relevant to this concern is the graphene nanoribbons (GNRs), a narrow tape of nanographene, which have peculiar electronic properties strongly influenced by its shape of edges [3, 4]. Zigzag-edged GNRs (ZGNRs) have been predicted to possess a localized nonbonding π-state around zigzag edges, whereas armchair-edged GNRs not (Figure 1). The localized nonbonding π-state, which is the spin-polarized state caused by multielectron correlation among unpaired electrons on the zigzag edges, is believed

to be associated with magnetic activities of GNRs. This peculiar electron localization in ZGNRs is referred to as “edge state” [5, 6], and the edge state has been attempted to be observed by various experimental measurements [7−16]. Especially, scanning tunneling microscopy and spectroscopy (STM/S) have achieved to propose the good evidence of the edge state around the zigzag edges in GNRs [7, 15, 16]. These broad theoretical and experimental studies on the edge state have clearly demonstrated that precise fabrication of edge structures in GNRs is a key factor to understand various properties of the edge state [5, 6, 15, 17, 18]. Although GNRs can be prepared by chemical [19−22], and lithographic methods [23, 24], as well as unzipping of carbon nanotubes [25−28], the synthesis of perfectly terminated and reliable size-controlled GNRs has been in high demand, and therefore, the elucidation of the edge state at the molecular level is growing in importance. Polycyclic aromatic hydrocarbons (PAHs) have been regarded as suitable model compounds for elucidating the fundamental structure−property relationships of nanographenes and GNRs [29−32]. Anthenes [33−35] and [a.b]periacene [36−39] (Figure 2), in which two and more anthracenes or acenes are condensed in the peri-directions, have been good candidates for small segments of GNRs, because they have well-defined two edge structures; zigzag and armchair edges. Recent theoretical and experimental studies reveal that anthenes and [a.b]periacenes can inherently possess the same electronic structure as the edge state of ZGNRs, which depends on the edge shape and the molecular size [33−42]. The edge state of GNRs is closely related to the singlet biradical (spin-polarized) character from the perspective of molecular science. We focus on anthenes (that is, peri-condensed anthracenes), because all anthenes can be written as the resonance hybrid of the Kekulé and biradical forms (Scheme 1). In the resonance formula, all anthenes are subjected to loss of only one double bond when the structure is written with a maximum number of Clar sextets. In these biradical forms, Clar sextets, the isolated double bonds, and the unpaired electrons have fixed positions, that is, the influence of sextet migration or spin delocalization is limited. Therefore, the discussion of the biradical character can be concentrated on the energy balance between the formal loss of the double bond and the aromatic sextet formation. This review focuses on the recent our studies on the origin of the edge state through the observation of magnetic, optical, and chemical behaviors of PAHs having zigzag-edges; bisanthene 1, teranthene 2, and quateranthene 3.

2. Synthesis Unsubstituted bisanthene 1 was isolated as an air-sensitive crystalline powder [43−45]. UV-Vis [46, 47], photoelectron [48], fluorescence [49], and vibrational spectra [50] of this labile compound have also been measured, and very recently, its crystal structure was determined by us

[51]. Wu et al. investigated the stability and electrochemical properties of meso-substituted derivatives [52]. An important study on the reactivity of bisanthene comes from Scott et al., who found that the Diels–Alder cycloaddition of acetylene to a bay region could be a suitable method to form cylindrical hydrocarbons [53−56]. We prepared a tetra-tert-butyl 1a derivative of bisanthene [51] for investigating its biradical character experimentally. The synthetic procedure for 1a is basically the same as that of unsubstituted bisanthene (Scheme 2). The final compound 1a was obtained as a deeply blue solid and was found to be moderately stable. Its half-life in a toluene solution is 19 days at room temperature open to air under room light. The larger anthenes than bisanthene have not been explored experimentally before our reports [33, 34]. We designed the teranthene derivative 2a having mesityl and tert-butyl groups, because of the improvement of solubility and stability. Scheme 3 shows the synthetic procedure for 2a. The final product 2a was obtained as dark green crystals. A toluene solution of 2a showed gradual decomposition with a half-life period of three days open to air under room light at room temperature. Furthermore, we also prepared quateranthene derivatives 3a and 3b having mesityl or 4-tert-butyl-m-xylyl and tert-butyl groups [35]. The synthetic procedure is summarized in Scheme 4. The synthetic strategy for 3a and 3b is basically the same as that of teranthene 2a. Fortunately, we could obtain single crystals of 3a suitable for X-ray crystallographic analysis by a careful recrystallization from an o-dichlorobenzene/mesitylene solution in a degassed sealed tube. The half-life of 3a at room temperature was only 15 hours when exposed to air under room light.

3. Molecular geometry Figure 3 shows the crystal structures of 1a, 2a, and 3a, and mean bond lengths of them are summarized in Figure 4. As shown in Scheme 1, the biradical resonance contribution enforces shortening of the a bonds due to their double bond character. The X-ray determined geometry of 3a strongly suggests the significant localization of electrons at the zigzag edges. The mean length of the a bond in 3a (C56−C63 and C70−C70* in Figure 3) is 1.416(3) Å, which is considerably shorter than that of the corresponding bonds in teranthene 2a (1.424(2) Å) and bisanthene 1a (1.451(2) Å) (Figure 4). The harmonic oscillator model of aromaticity (HOMA) values [57, 58], which is the geometry-based aromaticity index, indicate large benzenoid character for peripheral six-membered rings (the bold benzene rings in the biradical form in scheme 4), as shown in Figure 5. The aromatic stabilization energy of benzene based on the homodesmic stabilization energy is ca. 90 kJ/mol [59, 60], whereas the destabilization energy due to C−C π-bond cleavage is ca. 270 kJ/mol [61].

Destabilization energy during the transformation from the Kekulé form to the biradical form is very large. For bisanthene 1, the destabilization energy of the π-bond cleavage is not compensated by the formation of additional sextets since the additional sextets in the biradical form are only two, thus the Kekulé form has dominant contribution to the ground state in bisanthene 1. For larger anthenes such as teranthene 2 and quateranthene 3, the difference of the sextets upon the transformation increases with increasing molecular size; three for teranthene and four for quateranthene. More sextets in the biradical forms result in more dominant contribution of the biradical form to the ground state. The trend in the bond shortening is in line with the increase of singlet biradical character (y) estimated by the natural orbital analysis occupation number (NOON) of the LUMO at a broken-symmetry UBHandHLYP/6-31G* calculation, which gave a significantly large LUMO occupation number of 0.84 for 3 in contrast to 0.59 for 2 and 0.12 for 1 [62 and see also Table 1]. These geometric findings suggest that the formation of aromatic sextets overwhelms the penalty for breaking one π-bond and pushes the resulting unpaired electrons out to the zigzag edges.

4. Magnetic properties Magnetic properties of singlet biradical species cannot be investigated directly by the methods of electron spin resonance (ESR) and superconducting quantum interference device (SQUID), because all electrons are covalently coupled within the molecules. Instead, the presence of thermally accessible triplet species would be a good criterion for the singlet biradical ground state. The CD2Cl2 solution of 2a showed no 1H-NMR signals from the teranthene core at room temperature, while upon cooling, progressive line sharpening was observed (Figure 6 (A)). This behavior is associated with the presence of thermally excited triplet species at elevated temperatures. The influence of the thermally excited triplet species was more distinctly observed in the 1H-NMR measurements of quateranthene. Even at −92 °C, the CD2Cl2 solution of 3b gave no 1H-NMR signals in the aromatic region (Figure 6 (B)), which suggests that 3b possesses the large population of the thermally excited triplet species even at low temperatures, due to a smaller singlet−triplet energy gap (ΔES−T). In contrast, 1a gave sharp 1H-NMR signals even at +110 °C, indicating a negligible influence of the triplet species due to a large ΔES–T. The ΔES-T of 1 was estimated to be 6300 K at the B3LYP/6-31G* level of calculation. The SQUID measurements afford the extent of the magnetic coupling of the edge-localized electrons. The powdered 2a and 3a showed the increasing χpT values above 200K and 50 K, and from the curve fitting, the magnetic coupling strength was estimated to be 1920 K and 347 K, respectively. It is notable that the value of 3a is comparable to the theoretical coupling strength of 290 K calculated by a first principle method for 15 Å wide ZGNR [63]. Thus, the two unpaired

electrons in 2a and 3a couple weakly and are easily activated to a thermally excited triplet state. Especially, the population of the triplet species of 3a amounts to ~50% at room temperature. This peculiar magnetic activity of 3a originating from the strong electron−electron correlation between the edge-localized electrons well represents the magnetic behavior of the edge state of ZGNRs. The UB3LYP/6-31G* calculation of quateranthene (Figure 7) gives similar edge-localized distribution (except for the spin orientation) of the unpaired electrons in both singlet and triplet states. These magnetic behaviors strongly indicate that the edge state becomes obvious from the molecular size of teranthene.

5. Optical properties The edge localized electrons also give peculiar optical property. One signature of the biradicaloid character is the presence of a low-lying excited singlet state dominated by the doubly excited configuration. Negri et al. reported that a weak low-energy band was observed for thienoquinoid compounds with large biradical character [64]. Computational analysis indicates that this weak band is assignable to a doubly excited band. As shown in Figure 8 (A), teranthene 2a and quateranthene 3a afford a weak low-energy band centered at 1054 nm and 1147 nm, respectively, whereas non-magnetic 1a gives an intense band at 686 nm whose profile is quite similar to that of rylenes (that is, peri-condensed naphthalenes) (Figure 8 (B)) [65]. Basically, rylenes have small singlet biradical character, as shown in Table 1. The feature of the lowest-energy bands of 1a and rylenes is the similarity of the shape, that is, well-structured vibrational bands. It is interesting that all these non-magnetic PAHs have a common feature in the band shape. However, antiferromagnetic PAHs 2a and 3a give totally different band shape and feature very weak low-energy bands. These weak bands would come from double excitation of electrons, which is generally one-photon-forbidden process. In order to assign the weak bands of 2a and 3a, we performed a strongly contracted second-order n-electron valence state perturbation theory (NEVPT2) calculation that allows multielectron excitation [66]. The NEVPT2(8,8)/6-31G* calculation revealed that the first excited singlet states (S1) of quateranthene 3 is a 2Ag state dominated by a HOMO, HOMO → LUMO, LUMO double excitation. For quateranthene 3, the excitation energy from the ground state (S0) to S1 was estimated to be 1.32 eV (940 nm). The S0 → S1 transition is one-photon-forbidden because of the parity (g–g transition), and consequently, the weak band of 3a centered at around 1150 nm (1.08 eV) could be assigned to the transition to the forbidden state. This assignment was confirmed by measuring the two-photon absorption (TPA), since a two-photon excitation into an Ag state is an allowed process. We could identify a broad band at around 2300 nm, which is the same excitation energy as that of the one-photon band at around 1150 nm, in TPA measurements of a CS2 solution

of 3b. Thus, the weak band at around 1150 nm is associated with the simultaneous excitation of the edge-localized electrons.

6. Chemical reactivity at the zigzag edges The edge-localized electrons would affect the reactivity of anthenes. A toluene solution of 3b was allowed to stand exposed to air under dark conditions at 3°C for several days (Scheme 5), which afforded an oxygen-adduct one-dimensional polymer (4) in a crystalline form. The structure of 4 could be determined by X-ray crystallographic analysis (Figure 9). The oxygen molecule attacks a less-protected carbon atom with a large spin density around the zigzag edge, presumably via a radical mechanism, and forms a peroxide bond connecting the neighboring molecules. The HOMA analysis of 4 (Figure 10) clarified that the zigzag edges disappear after the oxygen attack, and instead, a more stable PAH skeleton that is completely surrounded by armchair edges is generated. The zigzag edges in nanographene are well known to be thermodynamically unstable [67, 68] and can be easily transformed into the more stable armchair edges by attacks by oxygen and various small molecules. The degradation process of the zigzag edges in 4 may be a key element for clarifying the degradation mechanisms of zigzag-edged GNRs.

7. Spin state of larger [a.b]periacene Recent theoretical calculations [36−39] predict that not only a singlet biradical ground state but also a singlet multi-radical ground state becomes more likely in larger systems of [a.b]periacene. We calculated the singlet open-shell character of larger [a.b]periacene using the index defined by Yamaguchi [69, 70] coupled to the symmetry-broken UBHandHLYP/6-31G* calculation [71]. The degree of the singlet open-shell character, yi, can be determined from the following equations: yi = 1 − 2Ti / (1 + Ti2), Ti = (nHOMO–i – nLUMO+i) / 2, where nHOMO–i and nLUMO+i represent natural orbital occupation numbers of HOMO–i and LUMO+i, respectively. The y0 and y1, which are determined from the HOMO–LUMO pair and the HOMO–1–LUMO+1 pair, are related to first and second π-bond cleavages, respectively, and vary continuously from zero to unity. A perfect biradical molecule has y0 of unity and y1 of zero, but for the molecule with large y1 the biradical description is inaccurate and tetraradical character should be taken into account. Table 1 shows the y0 and y1 values of acenes and [a.b]periacene. With the increase in the molecular size, the y0 of [a.b]periacene increases. The y0 of [3.7]periacene (quateranthene), [4.b] (b = 5, 7), and [5.b] (b = 3, 5, 7) are close to unity, whereas y1 remains quite small. These open-shell degrees strongly suggest that the ground state of these periacenes should be biradical. A multi-spin ground state becomes more likely in larger systems of [a.b]periacene. The y1 of [6.7]periacene (peri-condensed hexacene) is found to be large (0.44),

suggesting an appreciable contribution of tetraradical structure to the ground state. The largest [a.b]periacene in this paper ([7.7]periacene) has very large y0 (1.00) and y1 (0.77), indicating the singlet tetraradical ground state.

8. Conclusion Based on the valence bond model shown in Scheme 1, anthenes are expected to behave as singlet biradical (spin-polarized) species, since the biradical canonical structures are reasonably stabilized by the formation of aromatic sextets when the Kekulé structures are transformed to the biradical structures. The contribution weight of the biradical structure in the ground state increases with the molecular size as a result of the cumulative increase in the number of aromatic sextets. The studies on bisanthene 1 and teranthene 2 revealed that bisanthene 1 can be categorized as a nonmagnetic species, while teranthene 2 lies at the onset of the edge state. More distinct behaviors relevant to the edge state are observed in quateranthene 3, whose physical and chemical properties are well explained by the edge localization of the unpaired electrons. Table 2 summarizes the selected data of 1a, 2a, and 3a,b. Through a series of the studies on anthenes, from nonmagnetic bisanthene 1 to anti-ferromagnetic quateranthene 3, we clearly demonstrate the exteriorization of the edge state at the molecular level. Furthermore, the quantum chemical study for wider periacenes implies the magnetic state beyond the biradical state. The multi-spin correlation in zigzag-edged GNRs, which is antiferromagnetic across the ribbon and ferromagnetic along the ribbon, could be understood through a bottom-up chemical approach.

9. Acknowledgement This work was supported in part by the Grant-in-Aid for Scientific Research on Innovative Areas “Reaction Integration” (No. 2105) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the ALCA Program of the Japan Science and Technology Agency (JST). The authors thank R. Kishi and M. Nakano (Graduate School of Engineering Science, Osaka University) for quantum chemical calculations, and K. Kamada (National Institute of Advanced Industrial Science and Technology, AIST) for TPA measurements.

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Figure 1.

(a) Zigzag- (15.4 Å ribbon width) and (b) armchair-edged (14.5 Å ribbon width)

GNRs. The bold line in the zigzag-edged GNR represents a quateranthene (3) skeleton.

Figure 2.

Structure of anthenes1−3 and [a.b]periacenes.

Figure 3.

The crystal structures of (left) 1a (center) 2a and (right) 3a.

1 .3

98 .426 1

1 .3 68

13 82 1 .4 1 . 1.3 43 3

1.451 a

13 1 .4 1 .4 1 .38 24 31 1 .4 1 .4

1.412 a

1 .4 32

1 .4

6

08 1 .4 14

18 1 .4

1 .4 09 33

1.4 1

1.4

4

18

1.419 a

1a 1.4 1

0 1.41

18 1.4 1. 41

0 1 .3 7 5

1 .3

94

7

1.424 a 17 1 .4 1.4 1.3 19 95

3a

2a

Figure 4.

The mean bond lengths of 1a, 2a and 3a.

0.786

0.707

0.524

0.823

0.580

0.262

0.470 1a

0.746

0.602

0.730

0.854

0.611 0.683

0.832 3a

2a

Figure 5.

The HOMA values of 1a, 2a and 3a.

a

b

Figure 6. Variable temperature 1H-NMR spectra of (a) 2a and (b) 3b in CD2Cl2.

a

b

Figure 7. Spin density maps of 3. (a) Singlet state and (b) triplet state are calculated at the UB3LYP/6-31G* level (black: up-spin, white: down-spin).

a

b

Figure 8. (a) Electronic absorption spectra of bisanthene 1a, teranthene 2a, and quateranthene 3a. (b) Electronic absorption and fluorescence spectra of rylenes, taken from ref. [65]. 2b, perylene ([2.3]periacene), 3b, terrylene ([2.5]periacene), 4b, quaterrylene ([2.7]periacene), and 5b, pentarylene ([2.9]periacene).

Figure 9.

X-ray structure of 4. The Ar groups and H atoms are omitted for clarity.

HOMA values H

Clar structure

0.468 0.872

H

O

O 0.787 0.558 0.642 0.487 0.765 0.671

O

O

Hn

H n

Figure 10. HOMA values and the corresponding Clar structure of 4.

Scheme 1.

Resonance structures of anthenes. The amplitudes of the biradical character (y) were

calculated at the CASSCF(2,2)/6-31G level. The six-membered rings depicted by bold lines represent the Clar sextets.

Scheme 2.

Synthetic route to 1a.

Scheme 3.

Synthetic route to 2a.

Scheme 4. Synthetic route to 3a and 3b.

Scheme 5. Generation of oxygen-adduct one-dimensional polymer 4.

TABLE 1. The degrees of open-shell character y0 and y1 of acenes and [a.b]periacenes. b

a

2

3

4

5

6

7

b a

2

3

4

5

6

7

1

0.00

0.00

0.01

0.07

0.20

0.40

1

0.00

0.00

0.00

0.00

0.01

0.02

3

0.00

0.12

0.60

0.84

0.94

0.98

3

0.00

0.00

0.01

0.02

0.07

0.20

y0

y1 5

0.01

0.59

0.91

0.98

0.99

1.00

5

0.00

0.00

0.01

0.05

0.23

0.54

7

0.05

0.84

0.98

1.00

1.00

1.00

7

0.00

0.00

0.01

0.10

0.44

0.77

TABLE 2. Summary of the data of anthenes (1a, 2a, and 3a,b) methods and conditions

1a

2a

3a,b

12

59

84

BHandHLYP/6-31G*

1.451(2)

1.424(2)

1.416(3)

X-ray crystallography

ΔES–T / K

6300

1920

347

B3LYP/6-31G* (1a) and SQUID (2a and 3a)

HOMO–LUMO gap / eV

1.87

1.41

1.35

UV-Vis-NIR

HOMO–LUMO gap / eV

1.68

1.05

1.06

cyclic voltammetry

19

3

0.6

at room temperature in air under room light

biradical character (y) / % length of the bond a / Å

half-life (t1/2) / day

Highlights Periacenes are investigated to elucidate the origin of the edge state of ZGNR. The edge state becomes obvious from the molecular size of teranthene. The edge state is strongly associated with the formation of aromatic sextets.