On-surface synthesis of disilabenzene-bridged covalent organic frameworks

Substituting carbon with silicon in organic molecules and materials has long been an attractive way to modify their electronic structure and properties. Silicon-doped graphene-based materials are known to exhibit exotic properties, yet conjugated organic materials with atomically precise Si substitution have remained difficult to prepare. Here we present the on-surface synthesis of one- and two-dimensional covalent organic frameworks whose backbones contain 1,4-disilabenzene (C4Si2) linkers. Silicon atoms were first deposited on a Au(111) surface, forming a AuSix film on annealing. The subsequent deposition and annealing of a bromo-substituted polyaromatic hydrocarbon precursor (triphenylene or pyrene) on this surface led to the formation of the C4Si2-bridged networks, which were characterized by a combination of high-resolution scanning tunnelling microscopy and photoelectron spectroscopy supported by density functional theory calculations. Each Si in a hexagonal C4Si2 ring was found to be covalently linked to one terminal Br atom. For the linear structure obtained with the pyrene-based precursor, the C4Si2 rings were converted into C4Si pentagonal siloles by further annealing.


Introduction
Covalent organic frameworks (COFs), as a large class of porous organic materials have attracted intense research in past few decades due to the great potential for applications in the fields of, for example, gas storage and separation (1,2), catalysis (3,4) and optoelectronics (5,6). Since the seminal work of in-solution COF synthesis by Yaghi and his co-workers in 2005 (7), various kinds of COFs, composed of light elements (B, C, N, O, H), have been synthesized by Schiff base reactions, self-condensation of boronic acid, as well as coupling between boronic acid and catechol (1,(7)(8)(9)(10). If the heavy elements are substituted in the precursor molecules, the elemental variety drastically enhances (11).
Silicon is an element of group 14 in the periodic table. Its four valence electrons at the outermost shell give it similar properties to carbon, yet the longer bond length and possible higher bonding states leads to unique chemical properties (12). Silicon incorporated organic functional molecules and novel nanostructures have attracted much attention in recent decades. For instance, silicates (Si-O) are some of the most studied compounds (13,14) and the successful synthesis of the silicate-COF was recently demonstrated (15). Silyl group commonly used as a protecting group in solution (16) was found to induce coupling reactions on surface (17,18). In contrast, silabenzene having unique heterocyclic rings with C-Si bonds (19) is still studied as a challenging target for the organic synthesis due to their high reactivities (19)(20)(21). Therefore, fabrication of twodimensional COFs based on silabenzene has yet to be achieved. Here, we present silabenzeneincorporated COFs by linking brominated poly aromatic hydrocarbons (PAHs) and silicon atoms on Au(111). Their structures and chemical properties are analyzed by a combination of bondresolved scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), photoelectron spectroscopy and density functional theory (DFT) calculations. The synthesis of the COFs by on-surface coupling of Si atoms and PAHs may pave a way for fabrication of novel lowdimensional nanostructures.

Results and Discussion
We employed on-surface synthesis to realize the silabenzene-incorporated COF under ultrahigh vacuum conditions (22,23). This strategy has proven to be an important bottom-up method in the successful synthesis of graphene nanoribbons (GNR) with different edges, including different atomic species (24,25). Once GNRs are fused with each other at their edges, two dimensional COFs can be synthesized (26,27). Alternatively, COFs have also been synthesized with predefined and small precursors (28,29). However, the structure of the precursor plays such a decisive role in this bottom-up approach that when no suitable precursor molecules are available, as is the case for unstable silabenzene, it becomes impossible to proceed.
Here, we overcome this limitation by combining conventional surface science techniques and on-surface chemistry. We employed 2,3,6,7,10,11-hexabromotriphenylene (HBTP) as a building block to fabricate two-dimensional Si-incorporated COFs (Fig. 1A). Firstly, a submonolayer AuxSi film was formed on Au(111) by depositing Si atoms at room temperature with a post-anneal to 420 K (Fig. S1). HBTP molecules were deposited on the substrate held at 420K, causing debromination.
Consequently, the sample was further annealed at a higher temperature of 580 K. We found the formation of hexagonal porous structures in the STM topography (Fig. 1B). The pores surrounded by six bright spots are separated by 1.75 ± 0.02 nm (Fig. 1C). The contrast of the nodal site and surrounding six bright spots changes with respect to the bias voltage (Fig. S2). Since following a similar growth procedure method, but without pre-deposited Si atoms on a clean Au(111) resulted in formation of disordered films (Fig. S3), we conclude that Si atoms play a decisive role in the synthesis of the porous nanostructure. Before the final step of annealing at 580 K, we also found the formation of SiBrx (x = 1, 2, 3) compounds on the Au terrace as well as on the porous structure ( Fig. S4)these can be desorbed as SiBr4 molecules from the surface by annealing at 450 K (30).
In order to resolve the inner structures of the porous nanostructure, the tip apex was terminated by a CO molecule (31,32). The bond-resolved STM image taken at a constant height mode indicates that the porous structure was composed of six triphenylene backbones at the nodal site ( Fig. 1D and Fig. S5), which are inter-connected by two different types of bonds. The length of the longer one (310 ± 20 pm) is far more than the typical length of a covalent bond, indicating it is composed of more than two atoms. We tentatively assigned this longer line as C-Si-X (X= Br or H) bonds, where Br atoms are from HBTP molecules, while H atoms are possibly from the chamber environment. In our previous study, Si and Br atoms can easily form a covalent bond on Au(111) ( Fig. 1A) (30). The central bond between two neighbouring triphenylene backbones has a shorter length (280 ± 20 pm), and we suggest that this is due to deflection of the CO tip during scanning and is not a physical bond (33). In order to investigate the structure behind these images, we undertook an extensive DFT analysis of possible molecular assemblies, including a wide variety of configurations and atoms in the network. The best agreement is shown in Fig. 1E, suggesting the Si atoms in the C4Si2 ring are passivated by Br atoms (Si atoms at the edge of porous structure can also bond to two Br atoms (Fig. S6)). We see a very good agreement with experiment in the STM simulated topography (Fig. 1F), with pores also separated by 1.75 nm. We also see good agreement in the comparison between the high resolution, CO tip STM imagesthe simulated image in Fig. 1G reproduces the sharp contrast over the central triphenylene backbone, shows a bond between neighbouring triphenylenes (293 pm) and also a long bond over C-Si-Br (333 pm).
Overall, this confirms that the triphenylene blocks are connected via the planar C4Si2 ring resulting in 1,4-disilabenzene-linked COFs (Si-COF). The electronic properties of Si-COF were measured in more detail by scanning tunneling spectroscopy (STS) ( Fig. 2A). We found occupied and empty states around -670 mV and +1.4 V, respectively, resulting in a band gap of 2.1 ± 0.1 eV. Si-Br assigned spots are brighter than triphenylene blocks at occupied state (Fig. 2B), while the opposite is observed at unoccupied state  In order to investigate the chemical properties of the Si-COF, we carried out synchrotron photoemission spectroscopic measurements after each reaction step. The clean surface of Au(111) substrate was first ensured by the presence of well-defined spin-orbit doublet peaks of Au 4f7/2 (83.8eV) and Au 4f5/2 separated by 3.69eV, which is in excellent agreement with established numbers (Fig. 3A). (34)(35)(36). After depositing Si atoms on a clean Au(111) surface, Au 4f doublet peaks( Fig. 3A) were significantly broadened by other components with a higher BE of 0.51 eV, which corresponds to the AuSix alloy (37-39) (Fig. S9). After the synthesis of the COFs, the Au 4f spectra became comparable to that of clean Au 4f doublet peaks (Fig.3A). As observed in the STM measurements, the dissociated Br atoms from HBTP molecules tend to react with Si atoms and consequently remove the AuSiX layer from the surface by forming highly-volatile SiBr4 molecules (30). This phenomenon was also identified in the Br 3d spectra (Fig. S11) where we observed a significant reduction in the signals of Br 3d after further annealing at higher temperature.
The corresponding Si 2p spectra of the AuSix layer on Au(111) shows that the characteristic Si 2p doublet peaks with a small separation from p-orbitals spin-orbit coupling (0.61 eV) as the Si 2p3/2 peak locates at 99.75 eV (Fig.3B, Fig. S11). We assume that charge transfer from Si to Au is responsible for the small shift to higher energy, compared with a value measured in the bulk (99.3 eV) (40). After the synthesis of the COFs, the Si 2p spectrum became complex. Our best fitting is consistent with four sets of doublet peak components, which can be associated to three different charge states of the Si atom (in our case, those components locate 0.71 eV, 2.0 eV and 3.18 eV higher in BE than elemental Si respectively). Although these numbers differ slightly from those of integer charge states of Si 1+ (1 eV), Si 2+ (1.81 eV), and Si 3+ (2.63 eV) measured in bulk inorganic form (41,42), the Si-COF exhibits two major species with relatively large peak area that locates 0.71 eV and 2.0 eV higher BE than the elemental Si position. Since the component shifted by 2 eV is almost comparable to a Si 2+ charge state (1.81eV), it is reasonable to assign it with the structure proposed in illustration (Fig.1A) where the Si atoms covalently bonded with two carbon atoms and as well as a bromine atom as indicated from the bond-resolved STM image (Fig. 1D) and theoretical calculations (Fig. 1E), as both carbon and bromine have a higher electronegativity than Si, therefore a slightly higher charge state than Si 2+ , but less than Si 3+ , can be expected for these C4Si2Br2 linker units. For the other major component that shifted by 0.71 eV in BE, we attributed it to the Si atoms that are incorporated to the edges of the Si-COF, bonded with one carbon and one Br atoms (Fig. S6). For the peak at 3.18 eV, it should be still from the Si at the edges, which bonded with one carbon and two Br atoms (Fig. S6). In order to demonstrate a generality of the Si-C bond formation by co-deposition of Si atoms and bromo-substituted molecules, we employed 4,5,9,10-tetrabromopyrene (TBP) molecules, in which two groups of ortho-bromine atoms are introduced at both sides of pyrene backbone (Fig.   4A). After TBP molecules were deposited on the partially-covered AuSix layer on Au(111) held at 420 K, short oligomers appeared (Fig. 4B). The close-up view (Fig. 4C) shows that bright dots are located at the edges of the longitudinal axis. A constant height dI/dV image taken with a COterminated tip (Fig. 4D) and the corresponding Laplace filtered image for enhancement of bond features (Fig. 4E) reveal the detailed structures, in which the pyrene backbones are connected via disilabenzene. The bright dots indicated by arrows in Fig. 4E correspond to Br atoms. Comparison to the simulated structure and STM images (Fig. 4F) support that a Si-doped cove-edge graphene nanoribbon (Si-cove GNRs) was synthesized. Note that the brighter dots indicated by circles in Fig. 4B can be assigned to the second Br atoms bonded with Si after C-Si-C coupling (Fig. S12), which also can be removed by tip manipulation (Fig. S13).
After annealing at a higher temperature of 580 K, the structure of oligomers further changed as the bright dots at the edges form a zigzag arrangement along the longitudinal axis (Fig. 4G,H).
The high-resolution constant height dI/dV image (Fig. 4I) and the corresponding Laplace filtered image (Fig. 4J) show that the C4Si2 six-membered rings were transformed into the C4Si fivemembered rings. Unlike the C4Si2 rings in Si-COF, the C4Si2 six-membered rings in Si-cove GNRs are not stabilized within the network structure of COF, and thus sequential cyclization from desilicification of the disilabenzene and subsequent dehydrogenation of the pyrene backbones proceed upon thermal activations. Again, the simulated structure and STM images support our analysis (Fig. 4K). Hence, the high reproducibility of the Si-incorporated COF structure is unambiguously proven.

Discussion
In summary, we synthesize silabenzene-incorporated COFs by reacting bromo-substituted molecules HBTP and Si atoms on Au(111). The linked structure of C4Si2 rings passivated by Br atoms can be determined by bond-resolved STM images combined with DFT calculations as well as XPS measurements. The TBP molecules also can form the C4Si2 ring after reacted with Si atoms, which even was transformed into the C4Si ring after desilicification and dehydrogenation.
These results demonstrate a high-generality of the C-Si on-surface coupling by co-depositing Si atoms and PAHs on Au(111), which may further extend syntheses of various low-dimensional nano nanostructures.

Experimental: Scanning Tunneling microscopic measurement
All the experiments were conducted in a low temperature scanning tunneling microscopy (STM) system (home-made) at 4.3 K under high-vacuum environment (< 1 × 10 -10 mbar).
The bias voltage was applied to the sample while the tip was electrically grounded. A Au(111) surface was cleaned through cyclic sputtering (Ar + , 10 min) and annealing (720 K, 15 min). Si atoms were deposited on a clean Au(111) surface with an electron beam evaporator (SPECS GmbH). 2,3,6,7,10,11-hexabromotriphenylene (HBTP) was purchased from Sigma-Aldrich and further purified by recrystallization from o-dichlorobenzene. The deposition and annealing parameters were the same as those in the STM measurement.
In Fig. S9b, upon Si deposition onto Au substrate, a discernable broadening of each Au 4f doublet peaks indicates the presence of two chemical components. The second set of doublet peaks were shifted by 0.51 eV to high binding energy compared to the bulk gold.
Shifting to a higher BE for the gold-silicide (AuSiX) alloy on the Au(111) was commonly reported. However, one might find it a bit contradictory that if charge flow from Si into Au as mentioned in the main text, yet both Au 4f and Si 2p spectra are shifting to the higher BE side. In fact, this phenomenon was well explained before and proposed with a delectron depletion model (3). It is understood that on the account of Au-Si interaction, Si transfers s charge to Au, but Au loses more localized d-electrons forming a s-d hybrid bond, in which the key factor is that the Coulomb interaction between Au 4f and the 6s conduction electron is quite different from that of 4f and 5d interaction, with the latter one is actually significantly larger (by ~3eV) and affecting the overall binding states of Au.
In Fig. S10, the lower binding energy set (Br 3d5/2 locate at 68.76 eV) were assigned to disassociated Br atoms/clusters (4) and higher BE position (3d5/2 locate at 69.71 eV) was in good agreement with previous studies (5)(6)(7). Compared to the C-Br bonding related Br 3d energy positions in these reports, our component at higher BE is highly accountable for Si-Br bond forming a silicon tetrabromide (SiBr4) compound which can be easily desorbed with higher temperature annealing. In addition, it is worth mentioning that the bromine signal essentially vanished after annealing at high temperature of 580 K, which is possibly due to different experimental setups (e.g. higher annealing temperature) at beamline.

Theoretical calculations
All first-principles calculations on the gold substrate in this work were performed using the periodic plane-wave basis VASP code (8,9) implementing the spin-polarized Density Functional Theory. To accurately include van der Waals interactions in this system, we used the DFT-D3 method with Becke-Jonson damping (10,11)various other van der Waals functionals were tested and no significant differences were observed. Projected augmented wave potentials were used to describe the core electrons (12) (19). The CO tip was approximated by 13% of the signal coming from the s orbital and 87% originating from the pxy orbitals on the Probe Particle, which gave a good agreement with close-by CO-STM and CO-dI/dV images (20). Since for the further away CO-STM images also 50/50 s/pxy ratio was also reported (21), we show additional comparison of s/pxy ratios for all the calculated voltage dI/dV images in Fig S.8. Density of states analysis was made using the VASPKIT package (22).