44903-AC7
Surface Confined Polymerization: Towards 2-D Conjugated Organic Polymers
The original objective of this project was to use surface-confined polymerization in order to prepare hitherto unknown two-dimensional (2D) conjugated polymers. At the time we have started this research, only a few papers have reported surface-confined synthesis of epitaxially deposited (1D) conjugated polymers.
In the 1st year of the project we have explored three individual directions (1,4-addition polymerization; electrochemical and catalytic polymerization) and identified Ullmann catalytic coupling as the most promising approach.
In the 2nd year the effect of the monomer symmetry (1,4-diiodibenzene vs 1,3-diiodobenzene)[1] as well as the role of non-covalent monomer/surface interactions (strong Cu…S attraction; diiodobenzene vs. 2,5-diiodo-3,4-ethylenedioxythiophene[2]) were studied. The identified relationships allowed designing a tetradentate monomer that has been successfully polymerized into the first 2D polythiophene.
In developing the methodology for the above reactions, we have studied self-assembly of related aromatic building blocks (trimesic acid, rubrene) through hydrogen bonding and p…p interactions. The relations between the symmetry of the substrate and that of the molecule were used towards the main objective of the project.
Ullmann polymerization of isomeric diiodobenzenes.[1] The first attempt to achieve surface-confined Ullmann polymerization on crystal metal surfaces (Cu(111)) was performed by Weiss et al for 1,4-diiodobenzene.[3] However, under the conditions of the experiment no covalent polymer was formed, instead an unidentified “pre-polymer” (most likely, organocopper intermediate, -Ph-Cu-Ph-Cu-) was suggested. We have shown that a substantial activation energy is needed to ensure a covalent coupling and when the reaction was performed at elevated temperature (200°C), under Ultra High Vacuum (UHV), 1,4-diiodobenzene forms well-ordered poly(p-phenylene) lines, separated (insulated) by lines of CuI byproduct (Fig. 2). The most successful experimental runs were performed on Cu(110), exploiting the anisotropy of this surface, while Cu(001) and Cu(111) were much less effective. When the topology of the monomer is changed as is for 1,3-diiodobenzene, the resulting polymer/oligomer adopts a zigzag- or cyclic architecture with 120° kinks, as can be expected its symmetry (see images in the Nugget). In both cases, the polymer lines are oriented epitaxially on the surface.
Ullmann polymerization of 2,5-diiodo- and 2,5-dibromo(3,4-ethylenedioxy)thiophenes.[2] Studies of the same reaction with thiophene derivatives revealed that strong S…Cu interaction forces the intermediate oligomer to stand upright on the surface. The resulting cis linkages of thiophene rings distorts the polymer structure, and suppresses its growth, so only short-to-medium oligomers (2-15) were obtained in this reaction. This observation was taken into account in design of multidentate building blocks (see below).
Studying this building block, we have also shown that brominated thiophene monomer can be successfully polymerize on Cu, in lieu of diiodo-derivatives. Such substation will allow to decrease the molecular weight of the building blocks, which is important for low vapor pressure monomers.
The problem of byproduct formation (CuI) identified for diiodobenzene have reappeared, at a greated extent for thiophene polymerization. At high polymer coverage, CuI is aggregated in large domains which make the underlying surface “inactive”. The solution to the problem was found in using a different surface. Pd(110) can catalyze the dehalogenation in a similar manner, but the resulting PdI is volatile in UHV conditions and leaves the surface at 200°C.
2D polymer by Ullmann coupling of tetrabromotetrathienoanthracene. Based on the model 1D polymerization and DFT calculations, we have designed a tetradentate monomer for 2D polymerization. The desired tetrabromotetrathienoantracene has been synthesized, vacuum-deposition on Cu(111) surface and annealed at 200°C to form an ordered 2D polymeric structure, as predicted by calculations (see the Nugget).
In conclusion, the main objectives of the project have been successfully achieved. The electronic properties of 2D polymers are yet to be fully studied. The preliminary data generated with ACS-PRF grant allowed to attract additional funding from US-Air Force and Quebec agencies (FQRNT and MDEIE), which would allow to continue this new direction in the PIs research.[1] J. A. Lipton-Duffin, O. Ivasenko, D. F. Perepichka, F. Rosei, Surface-confined catalytic synthesis of polyphenylene wires, Angew. Chem. Int. Ed. submitted (anie.200803654)
[2] J. A. Lipton-Duffin, J. A. Miwa, F. Cicoira, V. Meunir, D. F. Perepichka, F. Rosei, Catalytic synthesis of epitaxial polythiophenes on noble metal surfaces, in preparation.
[3] G. S. McCarty, P. S. Weiss, P.S. J. Amer. Chem. Soc. 2004, 126, 16772
