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Direct Formation of C-C Double-Bonded Structural Motifs by On-Surface Dehalogenative Homocouplings of gem-Dibromomethyl Molecules LiangLiang Cai, Xin Yu, Mengxi Liu, Qiang Sun, Meiling Bao, Zeqi Zha, Jinliang Pan, Honghong Ma, Huanxin Ju, Shanwei Hu, Liang Xu, Jiacheng Zou, Chunxue Yuan, Timo Jacob, Jonas Björk, Junfa Zhu, Xiaohui Qiu, and Wei Xu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02459 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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Direct Formation of C-C Double-Bonded Structural Motifs by On-Surface Dehalogenative Homocouplings of gem-Dibromomethyl Molecules Liangliang Cai‡,a Xin Yu‡,a Mengxi Liu,b Qiang Sun,a Meiling Bao,a Zeqi Zha,b,c Jinliang Pan,b,c Honghong Ma,a Huanxin Ju,d Shanwei Hu,d Liang Xu,a Jiacheng Zou,a Chunxue Yuan,a Timo Jacob,e Jonas Björk,f Junfa Zhu,d Xiaohui Qiub,c* and Wei Xua* a

Interdisciplinary Materials Research Center, College of Materials Science and Engineering, Tongji University, Caoan Road 4800, Shanghai 201804, P. R. China b CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China c University of Chinese Academy of Sciences, Beijing, 100049, China. d National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China e Institute of Electrochemistry, Ulm University, Albert Einstein Allee 47, 89069 Ulm, Germany Department of Physics, Chemistry and Biology, IFM, Linköping University, 581 83 Linköping, Sweden f

*

To whom correspondence may be addressed. E-mail: [email protected], [email protected]

ABSTRACT Conductive polymers are of great importance in a variety of chemistry related disciplines and applications. The recently developed bottom-up on-surface synthesis strategy provides us with opportunities in the bottom-up fabrication of various nanostructures in a flexible and facile manner, which could be investigated by high-resolution microscopic techniques in real space. Herein, we designed and synthesized molecular precursors functionalized with benzal gemdibromomethyl groups. A combination of scanning tunneling microscopy, non-contact atomic force microscopy, high resolution synchrotron radiation photoemission spectroscopy and density functional theory calculations demonstrated that it is feasible to achieve the direct formation of C-C double bonded structural motifs via on-surface dehalogenative homocoupling reactions on the Au(111) surface. Correspondingly, we convert the sp3 hybridized state to sp2 hybridized state of carbon atoms, i.e., from an alkyl group to an alkenyl one. Moreover, by such a bottom-up strategy, we have successfully fabricated poly(phenylenevinylene) (PPV) chains on the surface, which is anticipated to inspire further studies towards understanding the nature of conductive polymers at the atomic scale.

Keywords: Dehalogenative homocoupling ·gem-dibromomethyl ·scanning tunneling microscopy ·non-contact atomic force microscopy ·surface chemistry

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Conductive polymers are intrinsic organic semiconductors involving π electrons, which are delocalized along the chain.1 Among others, poly(phenylenevinylene) (PPV) is one of the most intriguing electroactive polymers demonstrating high thermal stability, nonlinear optical activity, electroluminescence and photoluminescence.2-5 Owing to its unique properties, PPV has been considered for a wide variety of applications in light-emitting diodes, field-effect transistors, photovoltaic devices, etc.6-9 Numerous approaches have been developed to synthesize PPV, such as the Wittig-type coupling, Knoevenagel condensation, Heck reaction, and so forth.10-18 In comparison with conventional solution chemistry, on the other hand, the recently developed on-surface synthesis approach is particularly attractive owing to its following characters: i) it is relatively easy to trigger the reactions (just by heating) from rationally designed molecular precursors on surfaces; ii) the confinement and catalytic effects of surfaces19-30 may largely reduce the reaction energy barriers resulting in the occurrence of some unexpected reactions inhibited in solution chemistry.3143

The on-surface synthesis route has thus opened a gate to the fabrication of a plethora of novel nanostruc-

tures/nanomaterials and, more importantly, the products can be investigated by in-situ surface sensitive characterization methods, e.g., scanning tunneling microscopy/ spectroscopy (STM/STS), non-contact atomic force microscopy (nc-AFM), high resolution synchrotron radiation photoemission spectroscopy (SRPES) and angle resolved photoemission spectroscopy (ARPES), with which real-space atomic-scale structural assignments and detailed electronic properties could be unraveled.44-49 It is therefore of general interest to employ such an on-surface synthesis strategy to achieve the bottom-up fabrication of conductive polymers like PPVs.

On-surface dehalogenative homocoupling reactions offer a relatively elegant and efficient route for incorporating various carbon scaffolds. To the best of our knowledge, most of the employed halide precursors have only one halogen attached to a carbon atom. In order to incorporate increasingly complicated carbon scaffolds into the formed nanostructures, we recently introduced a molecular precursor functionalized with an alkenyl gem-dibromide group on a sp2 carbon, which results in the formation of a cumulene moiety by the dehalogenative homocoupling reaction on the Au(111) surface.50 In light of this previous work, to generate conjugated carbon backbones with alternate C-C single and double bonds as in a conductive polymer, we now attempt at designing a molecular precursor functionalized with a gemdibromomethyl group on a sp3 carbon, and exploring the possibility of direct formation of C-C double bonds from sp3 hybridized carbons by dehalogenative homocoupling reactions on the Au(111) surface. As shown in Figure 1, we have designed and synthesized a molecule functionalized with the benzal gem-dibromomethyl group (that is, 4-

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(dibromomethyl)-1,1'-biphenyl, abbreviated DBMBP).51 From the interplay of STM/STS, nc-AFM, XPS and DFT calculations, we have investigated dehalogenative homocouplings of DBMBP molecules on the surface. Interestingly, it is found that the formation of trans-4,4-diphenylstilbene products (i.e., involving C-C double bonds) occurred at room temperature (RT). Furthermore, we designed a ditopic molecular precursor (that is, 1,4-bis(dibromomethyl)benzene, named BDBMB) with the aim of forming a one-dimensional chain, that is, PPV with alternate vinylene linkages and phenyl groups. By activating the dehalogenative homocoupling reaction of BDBMB on the surface, we have successfully fabricated PPV chains (cf. Figure 1). This study once more exhibits the versatility of on-surface dehalogenative C−C homocoupling reactions, and more importantly, it provides us with a facile manner for the fabrication of conductive polymers, which would inspire further fundamental characterizations towards understanding the nature of conductive polymers at the atomic scale.

dehalogenative homocoupling

trans-4,4-diphenylstilbene

DBMBP

dehalogenative homocoupling BDBMB

n

PPV

Figure 1. Upper: Schematic illustration showing the direct formation of a vinylene group through dehalogenative C−C homocoupling of a gem-dibromomethyl molecule. Lower: Schematic illustration showing dehalogenative homocoupling of the ditopic molecular precursor, which results in the formation of poly(phenylenevinylene) (PPV).

Results and Discussion

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(b)

(a)

2 nm

(c)

5Å

(d)

Figure 2. (a) Large-scale and (b) close-up STM images showing the formation of an ordered island structure after deposition of DBMBP molecules on Au(111) held at RT. The STM topography of a dimer product is indicated by the white contour. (c) The high-resolution STM image of the dimer structure. (d) The corresponding simulated STM image overlaid by an equally scaled DFT relaxed structure on Au(111).

After the deposition of DBMBP molecules on Au(111) held at room temperature (RT), we have observed the formation of ordered islands as shown in Figure 2a, which consists of round protrusions and curved structures. According to the previous studies,29,52 the round protrusions are attributed to dissociated bromine atoms on the surface. The curved structure highlighted by the white contour (Figure 2b) is composed of two lobes and a dim contrast in the center. Moreover, a closer inspection of the dimer structure allowed us to identify that the two bright lobes were not coaxial and exhibited a staggered arrangement. To further identify the atomic scale structure, we performed DFT calculations. From a detailed comparison of the experimental topology and dimensions with the molecular model and the simulated STM image (cf. Figure 2c and 2d), we could identify that the curved structure should be assigned to a C−C double bonded dimer. Remarkably, the staggered arrangement of two lobes in a formed dimer together with the characteristic STM contrast (i.e., the middle part is smooth and seamless, and apparently lower than those of the phenyl groups) also implied the formation of the C−C double bond. Furthermore, the DBMBP molecules couple to give trans-isomers with a quite high yield (>90%) indicating that the trans-isomer is thermodynamically favored over the cis-isomer.

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(a)

- Br

- Br

0.16 eV

0.75 eV

TS1

IS

Int

TS2

FS 0.30 eV

0.16 eV

E

0.00 eV

-0.02 eV -0.45 eV

(b)

homocoupling

+

0.47 eV

TS

IS

FS

0.47 eV

E

0.00 eV

-3.26 eV

Figure 3. (a) The DFT-calculated reaction pathway for the successive C–Br bond activations of the DBMBP molecule on Au(111). The structural models of the initial (IS), transition (TS), intermediate (Int) and final states (FS) along the pathway are also shown. (b) The DFT-calculated reaction pathway for the homocoupling from the debrominated intermediates to the dimer product, together with the corresponding structural models along the pathways.

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To further understand the reaction mechanism on the formation of the C-C double bonded dimer structure, we have calculated the reaction pathway from the DBMBP molecular precursor to the dimer product through successive C–Br bond activations and subsequent C-C homocoupling, which is in analogy to the dehalogenative homocoupling reaction of alkenyl gem-dibromides.50 The DBMBP molecule is evaporated from the crucible at RT, so it is unlikely for the molecules to be debrominated in the crucible or in the gas phase during evaporation. As shown in Figure 3a, the barriers for the successive debromination processes are determined to be 0.16 eV and 0.75 eV, respectively. The abstraction of the first Br atom is nearly spontaneous on the Au(111) surface, reminiscent of the debromination of the bromomethyl group.39 While the activation of the second C-Br bond is significantly larger than the first one, it is still smaller than the debromination of aryl-halides (~1 eV).53 We also considered the pathway of the subsequent C-C homocoupling, starting from a configuration where the debrominated species are adsorbed to adjacent atoms of the Au(111) surface (Figure 3b). Here, the energy barrier was calculated to be 0.47 eV, thus smaller than the second debromination barrier, corroborating why no debrominated intermediate structures were observed in the experiments. Furthermore, the coupling reaction is exothermic by 3.26 eV, reflecting on the irreversibility of the C-C bond formation. While a detailed discussion of the theoretical studies and analysis will be subject of a separate work, the calculated pathways presented here well account for the formation of the C-C double bonded dimer product under the experimental conditions.

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72

70

STM image

(c)

Br 3d hv= 160 eV

68

66

(d)

STM simulation

B.E.(eV)

2Å -3.2 Hz

(a)

intensity (arb. units)

5 nm

-5.3 Hz

nc-AFM image

-2.6 Hz

(e)

(f) -5.5 Hz

(b)

nc-AFM simulation

(g)

-8.2 Hz

DFT model

-5.8 Hz

5Å

5Å

(h)

(i) 1

×

×

2

×

3

×

4

dI/dV offset (a. u.)

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5 ×

7Å

1 2 3 4 Au(111)

5 -1.5

-1.0

-0.5

0.0

0.5

1.0

Sample SampleBias Bias(V) (V)

Figure 4. (a) Large-scale and (b) close-up STM images showing the formation of PPV chains after deposition of BDBMB molecules on Au(111) held at RT and annealing to ~400 K. The XPS data of Br 3d is inserted in (a). (c) Equally scaled STM image, STM simulation, nc-AFM image recorded by CO-functionalized tip/with model overlaid, nc-AFM simulation, and DFT-optimized model of a single PPV chain on Au(111). (d) Close-up STM image of PPV chain and (e) The corresponding nc-AFM image highlighting the C-C double bond configuration. (f) Close-up STM image of PPV chain with the linkages in two directions and (g) The corresponding nc-AFM image. (h) STM image of a PPV chain on the Au(111) surface. (i) dI/dV spectra recorded at different sites (1, PPV edge; 2-4: PPV chain; 5:

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Au(111) substrate) as marked in (h). The dI/dV spectra were acquired by a lock-in amplifier while the sample bias was modulated by a 553 Hz, 30 mV (r.m.s.) sinusoidal signal under open-feedback conditions.

In view of the successful formation of the C−C double bond from the sp3 hybridized carbon, we have further introduced a ditopic molecular precursor (i.e., BDBMB) onto the Au(111) surface (as shown in Figure 4a) with the aim of fabricating polymer chains, that is, PPVs. As shown in Figure 4a, after deposition of BDBMB molecules on Au(111) held at RT and annealing to ~400 K, we indeed observed the formation of well-ordered chain structures on the surface (short chains have already formed at RT, and post-annealing of the sample is needed to improve the quality of the chain structures). The Br 3d core level spectrum of the chain structures (the inset shown in Figure 4a) was fitted with one doublet: 68.6 eV (Br 3d3/2) and 67.6 eV (Br 3d5/2), assigned to detached bromine atoms (from carbon atoms) chemisorbed on the Au(111) surface.54-57 The SRPES data unambiguously demonstrate the complete debromination of BDBMB molecules on Au(111). From the close-up STM image (Figure 4b), the beaded chains have a period of 6.9 ± 0.2 Å between adjacent protrusions, and the detached bromine atoms are observed as dim protrusions between the chains. From a detailed comparison of the equally scaled high-resolution STM image, STM simulation, nc-AFM image, nc-AFM simulation, and DFT-optimized model of a PPV chain on Au(111) (as shown in Figure 4c), we determined the dimension and topography of the PPV chain, and identify that the experimental periodicity corresponds well to the theoretical value of 6.73Å. Furthermore, the nc-AFM images allow us to identify the bonding configurations between the linked phenyl groups. From the close-up high-resolution STM image (Figure 4d) and the corresponding nc-AFM image (Figure 4e) of a section of the molecular chain, the staggered line with crossing angle ~120°can be observed between two adjacent phenylenes, which is attributed to the formation of C-C double bond. Such a staggered line is dramatically different from the case on the formation of a cumulene moiety (i.e., having three consecutive C-C double bonds) where a straight line with a homogeneous contrast was imaged.50 Therefore, we conclude that the dehalogenative homocoupling of BDBMB molecules as depicted in Figure 1 has been realized on the Au(111) surface. The ncAFM image (Figure 4g) also proves that the formed C-C double bond in the middle of the adjacent phenylenes could have two trans-configurations (owing to the rotation along a C-C single bond), which is also reflected from STM images (Figure 4f and Figure S1). We have also carried out STS measurements on Au(111)-supported PPV chains. The dI/dV spectrum on the Au(111) substrate shows typical Shockley surface state at approximately – 400 mV (curve 5 in Figure 4i).58 In contrast, the intrinsic HOMO and LUMO states of PPV chains were not observed from dI/dV spectra

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(curve 1-4 recorded on different sites of a PPV chain), which could be attributed to doping effect from the metal surface and the Br adsorbents,59,60 as well as the weak contribution to density of states (DOS) relative to that from Au substrate.61

Conclusion

In conclusion, from a combination of high-resolution UHV-STM, nc-AFM imaging, SRPES and DFT calculations, we have demonstrated the feasibility of direct formation of C-C double-bonded structural motifs by dehalogenative homocoupling of gem-dibromomethyl molecules on the Au(111) surface. Notably, in this case, we have converted the sp3 to the sp2 hybridized state of carbon atoms, i.e., from an alkyl group to an alkenyl one. As a consequence, this bottom-up on-surface synthesis strategy allows manufacturing functional nanostructures with vinylene scaffoldings by rational design of molecular precursors, which we exemplify by the fabrication of PPV chains. It is anticipated that our study will facilitate further fundamental characterizations towards understanding the intrinsic nature of conductive polymers, or other effects (like doping) at the atomic limit. Methods and Materials

STM/AFM characterization and sample preparation. The STM experiments were carried out in a UHV chamber with a base pressure of 1 × 10–10 mbar. The whole system was equipped with a SPECS variable-temperature “Aarhustype” STM,62,63 a molecular evaporator and standard facilities for sample preparation. The nc-AFM experiments were performed with an Omicron low temperature AFM, operating at 4.8 K in UHV. The AFM is equipped with a qPlus sensor with a tungsten tip mounted on a quartz tuning fork (spring constant: 3600 N/m, resonance frequency of f0=40.7 kHz, Q ≈ 5.6×104, oscillation amplitude ≈ 100 pm). The Au(111) substrate was prepared by several cycles of 1.5 keV Ar+ sputtering followed by annealing to 850 K, resulting in clean and flat terraces separated by monatomic steps. After the system was thoroughly degassed, the DBMBP molecule (synthesized) and BDBMB molecule (purchased from Tokyo Chemical Industry Co., Ltd. with purity >98%) were sublimated from the molecular evaporator onto the substrate. The sample was thereafter transferred within the UHV chamber to the SPECS STM, where measurements were performed in a typical temperature range of 100 K−150 K, and the typical scanning parameters were: It = 0.5 ~ 1.0 nA, Vt = ±1000 ~ 2000 mV.

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DFT calculations. The calculations were performed within the framework of DFT using Vienna Ab Initio Simulation Package (VASP) code,64,65 with projector augmented wave method to describe the interaction between ions and electrons,66,67 and with a plane waves expanded to an energy cutoff of 400 eV. Exchange-correlation effects were described by the van der Waals density functional (vdW-DF)68 using the version by Hamada denoted as rev-vdWDF2,69 which has shown to describe the adsorption of aromatic hydrocarbons on Au(111) accurately.70 The Au(111) surface was modeled as a four-layer slab vertically separated by a vacuum region of 15 Å. For the debromination calculations we used a p(7 * 6) surface unit cell, while for the coupling reaction a p(10 * 9) surface unit cell was used, together with a 3*3*1 k-point grid in both cases. Reaction pathways were calculated with a combination of the climbing image nudge elastic band (CI-NEB)71 and Dimer72 methods, where CI-NEB was used to find an initial guess of a transition state, which was then refined by the Dimer method. The atomic structures were geometrically optimized until the residual forces on all atoms, except the two bottom layers of the Au(111) slab (kept fixed), were smaller than 0.01 eV/Å. The simulated STM image was obtained using the Tersoff-Hamann method,73 in which the local density of states (LDOS) is used to approximate the tunneling current.

Synchrotron radiation photoemission spectroscopy. The SRPES experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The core level spectra of Br 3d were recorded with a VG Scienta R4000 analyzer using a monochromatic Al Kα X-ray source. Br 3d core level spectra were taken with a photon energy of 160 eV. The peak fitting was performed using the XPS Peak 41 program with Gaussian functions after subtraction of a Shirley background. The photon energies were calibrated and referenced to the Au binding energy of a sputter-cleaned Au substrate. Before every scan of the Br 3d spectra, we also measured the Au binding energy of the Au substrate for calibration.

Synthesis. All commercially available chemicals were purchased from Adamas-beta, Aldrich and TCI, and used as received without further purification. 1H NMR spectra were recorded on a Bruker AVANCE 400 spectrometer. The chemical shifts are reported in  ppm with reference to residual protons of CDCl3 (7.26 ppm in 1H NMR and 77.16 ppm in 13C NMR). Thin layer chromatography (TLC) was performed on glass plates coated with 0.20 mm thickness of silica gel. Column chromatography was performed using neutral silica gel PSQ100B. ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. Additional STM images and synthesis data.

The authors declare no competing ﬁnancial interests. AUTHOR INFORMATION Corresponding Author * [email protected]

* [email protected] Author Contributions ‡These authors contributed equally to this work.

ACKNOWLEDGMENTS. The authors acknowledge the ﬁnancial support from the National Natural Science Foundation of China (21473123, 21622307, 21790351, 51403157, 21603045, 21425310, 91427301), the Fundamental Research Funds for the Central Universities of China and International Cooperation Training Project Funding for Postgraduate of Tonggji University (2018XKJC-009), TJ and JB acknowledge funding from the Alexander von Humboldt Foundation. Computational resources were allocated by the National Supercomputer Centre, Sweden, through SNAC.

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TOC dehalogenative homocoupling

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STM image

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Direct Formation of CC Double-Bonded Structural ... - ACS Publications

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