Interchain-linked Graphene Nanoribbons from Dibenzo[g,p]chrysene via Two-zone Chemical Vapor Deposition

Abstract

We report covalently interchain-linked graphene nanoribbons fabricated on Au(111) by a stepwise growth process via two-zone chemical vapor deposition. Prepolymer arrays were grown by surface-assisted polymerization of a precursor, 1,9-dibromodibenzo[g,p]chrysene at 250 °C. Further annealing at 450 °C converted the prepolymers into interchain-linked graphene nanoribbons via intramolecular and intermolecular dehydrogenation reactions.

Graphene nanoribbon, Two-zone chemical vapor deposition, Scanning tunneling microscopy

Organic two-dimensional (2D) materials have attracted widespread attention in the fields of material science.^1–5 Self-assembly of molecules on a metal surface enables to form the 2D structures by interactions with the substrate as well as those with the neighboring molecules.^6–8 Molecular 2D structures like covalent organic frameworks (COFs) are of great interest because the covalent bonding structures are preferable for various applications due to their thermal and chemical stability.^9–13 Various chemical reactions to form covalent bonds have been reported, such as Ullman coupling,^14,15 Glaser coupling,¹⁶ decarboxylative coupling,¹⁷ acylation reaction,¹⁸ imidization reaction,¹⁹ and condensation reaction.²⁰ Most of these materials reported have been based on sp³-hybridized carbons, which lack π-conjugation in 2D system. Therefore, the fully π-conjugated 2D organic structures, based on the sp²-hybridized covalent linkage, are desired.

Graphene nanoribbons (GNRs) are fully π-conjugated one-dimensional chains, in which the electronic characteristics are dependent on their widths and edges.²¹ On-surface synthesis of GNRs on metal surface, such as ultrahigh-vacuum (UHV) deposition²² and chemical vapor deposition^23–25 enables formation of GNRs with controlled widths and edges. Additionally, it has been reported that the GNRs could be interchain-linked via cross-dehydrogenative coupling.^26,27 These phenomena will be applicable to produce the covalently bonded π-conjugated 2D structures. However, the linked GNRs reported thus far have low density resulting in the undeveloped 2D structures. Since the UHV deposition methods might generate the low-density radicals of precursors due to the inseparable processes of radical generation from precursors, and GNR growth, resulting in inefficient inter-ribbon connection.

Here, we report a fabrication of sp²-covalently linked chiral GNRs with high density from a precursor of 1,9-dibromodibenzo[g,p]chrysene on Au(111) via a two-zone chemical vapor deposition (2Z-CVD). The 2Z-CVD is advantageous in that it can generate the high-density radicals of precursors resulting in a high yield of GNRs, because it allows the control of the independent temperature in the two zones 1 and 2, which are hypothesized to generate radicals for polymerization and grow GNRs, respectively.^23–25

The synthesis of precursor 1 is depicted in Scheme 1. First, 1,2-bis(2-bromophenyl)ethyne (4) was used as starting material, which was prepared according to the literature.²⁸ Suzuki coupling was performed using 4 and 2-methoxyphenylboronic acid to produce 1,2-bis(2′-methoxybiphenyl-2-yl)ethyne (5) in 73% yield. Afterwards, single side intramolecular cyclization was achieved by iodine chloride with cooling bath. Subsequently, oxidative cyclization was performed with PdCl₂(PPh₃)₂ as catalyst under nitrogen atmosphere at 120 °C to afford the dibenzo[g,p]chrysene backbone structure in 22% yield. Finally, the methoxy group was converted to bromo group via three steps, through hydroxy group and trifluoromethanesulfonate group as intermediates in an overall yield of 8%.

Scheme 1.

Synthetic route towards the precursor 1,9-dibromodibenzo[g,p]chrysene. Reagents and conditions: a) Pd(PPh₃)₄, Na₂CO₃, toluene, EtOH, H₂O, 110 °C, overnight, 5: 73%. b) ICl, CH₂Cl₂, −78 °C to −40 °C, 0.5 h, 6: 76%. c) PdCl₂(PPh₃)₂, CH₃COONa, N,N-dimethylacetamide, 120 °C, 4 h, 7: 22%. d) BBr₃, CH₂Cl₂, −78 °C, rt, 4.5 h. e) Tf₂O, pyridine, CH₂Cl₂, rt, 13 h, 7–9: 40%. f) LiBr, Ru(acac)₃, 3,4,7,8-Me₄-Phen, EtMgBr, 1,4-dioxane, 120 °C, 4 days, 1: 21%.

Figure 1a shows the schematic representation of the 2Z-CVD system. The 2Z-CVD system consists of a quartz tube (ϕ 26 mm, 86 cm) as a reactor, a rotary pump that can evacuate the system to below 7 × 10⁻⁴ Torr, an electric furnace with two independent temperature controllers, an Ar gas flow system with a mass flow controller, and a mantle heater for precursor evaporation. Ar gas was fed into the quartz tube at a flow rate of 500 sccm, resulting in a vacuum of 1 Torr. A precursor solution (100 µL of 0.20 mg mL⁻¹ in CHCl₃) was cast in a quartz boat, which was placed upstream of the quartz tube. A Au(111)-deposited mica substrate was placed in zone 2. This technique has successfully fabricated a series of armchair-edged²³ and acene-type GNRs.²⁴

Figure 1.

(a) Experimental setup of the 2Z-CVD. (b) Growth process of the linked GNRs though polymerization and dehydrogenation.

Scheme 2 depicts the growth process of the linked GNRs on Au(111). First, for the growth of prepolymer, the temperature of mantle heater was set to 325 °C for sublimation of precursors. The vaporized precursors can pass through the hot quartz tube (zone 1) heated to 350 °C, followed by deposition on Au(111) at 250 °C (Zone 2), as shown in Figure 1b. The sample was studied by scanning tunneling microscopy (STM). The alternatively aligned bright and dark dots are shown in Figures 2a and 2b. The magnified image (Figure 2c) shows the periodicity of a pair of bright and dark dots to be 1.25 nm, the length of the dot-pair to be 1.02 nm, and width of dots to be 0.91 nm. These results indicate that the aligned dots correspond to the prepolymers of polydibenzo[g,p]chrysene where the dot-pairs correspond to the precursor units of dibenzo[g,p]chrysene, because these are in good agreement with the molecular model (Figure 2d). Length histogram of polymer chains was obtained by statistical analysis of Figure 2a (Figure 2e). The ordinate shows the two-dimensional analogue of the weight-average molecular weight, N_L × L/a, where N_L refers to the number of prepolymer chains with length L (nm), and a corresponds to the precursor unit length of 1.02 nm.

Scheme 2.

Reaction scheme of surface-assisted bottom-up synthesis of linked GNRs through a three-step process via 2Z-CVD. Step 1: polymerization of the precursor to form prepolymer at 250 °C, Step 2: intramolecular dehydrogenation to form GNR at 450 °C, The colored arrows indicate the (3,1)-chiral vector. Step 3: intermolecular dehydrogenation to form linked GNRs via two ways which are referred to as “end-to-end” and “end-to-body” linkage.

Figure 2.

(a) and (b) STM images of prepolymers on Au(111) grown at 250 °C. (c) Magnified STM image and cross-sectional analysis of horizontal line (upper) and vertical line (lower). (d) Molecular model of prepolymer on Au(111). (e) Length histogram obtained from the STM image of (a).

The prepolymers on Au(111) were then further annealed at 450 °C. STM images after annealing reveal that the aligned dots are converted into the winding chains (Figures 3a and 3b). The magnified STM image of chains (Figure 3c) reveals a width of ca. 0.95 nm and an edge periodicity of 1.21 nm. Since the observed width and periodicity of chains are in good agreement with those of model of chiral GNR (Figure 3d), we conclude that the GNRs were formed via intramolecular dehydrogenation of prepolymers by thermal annealing. Additionally, the STM images of Figures 3a and 3b show that the high-density-linked GNRs were formed on Au(111) via intermolecular dehydrogenation, resulting in a well-developed 2D structure. Magnified STM images of linked GNRs (Figure 3e) shows that there are two kinds of GNR linkage modes of “end-to-body” and “end-to-end” linkages. Along the edges of chiral GNR, “bay regions” are supposed to be the geometries suitable for interchain coupling. These well-developed 2D-linked structures might be produced by 2Z-CVD, in which the high-density biradicals were generated via debromination of the precursor by collisions with the hot quartz tube wall of zone 1. Raman spectrum of the sample annealed at 450 °C (Figure 4a) showed a G band splitting into two peaks (ca. 1566 and 1588 cm⁻¹), a D band (ca. 1317 cm⁻¹), and a D′ band (ca. 1638 cm⁻¹), while negligible peaks were observed for those grown at 250 °C (Figure 4b). These results are in good agreement with previously reported chiral GNR.²⁶

Figure 3.

(a) and (b) STM images of linked GNRs on Au(111) annealed at 450 °C of prepolymers. (c) Magnified STM image and cross-sectional analysis of horizontal line (upper) and vertical line (lower). (d) Molecular model of GNR. (e) STM images with overlaid molecular model of linked GNRs in end-to-body and end-to-end manners.

Figure 4.

Raman spectra of linked GNRs (a) annealed at 450 °C and prepolymer (b) grown at 250 °C.

In summary, high-density interchain-linked GNRs from precursor 1,9-dibromodibenzo[g,p]chrysene were fabricated via 2Z-CVD. The fabrication process consists of polymerization, intramolecular- and intermolecular dehydrogenation. The sp² covalent bonding linkage of GNRs provides the potential applications for electronic devices.

Acknowledgment

This work was supported by JSPS KAKENHI grant numbers JP15H00995, JP16K13608, JP16K17893, JP16H00967, JP17K14471, JP17H05154, and JP17H02724; “Zero-Emission Energy Research” of IAE, Kyoto University.

Supporting Information is available on http://dx.doi.org/10.1246/cl.170614.

H. Sakaguchi

S. Song

T. Kojima

T. Nakae

H. Uno