Insight into organometallic intermediate and its evolution to covalent bonding in surface-confined Ullmann polymerization

Novel nanostructured low-dimensional materials have received marked interest in the last decade since they could be employed as active media in organic electronics devices.
Graphene-like 2D organic materials can be grown and confined onto suitable surfaces depositing and activating selected molecules with different strategies. In this respect, the surface-confined polymerization is a very promising bottom-up approach that allows the creation of layers with desired architectures and tunable properties of charge-transfer: polymers with different shape can be formed changing the molecules used as precursor, tuning the final properties according to the request, unlike the rigid graphene sheet.
The most successful method used to obtain ordered polymers on surfaces in ultra-high vacuum (UHV) is based on the Ullmann coupling reaction, producing covalently linked networks starting from aryl halide precursor molecules. However, the assessment of the occurred polymerization has been derived to observation of scanning tunneling microscopy (STM) images only, thus indirectly.
Our work reports a detailed insight into the Ullmann reaction, obtained by complementary spectroscopic measurements of occupied and unoccupied states performed at the ALOISA beamline of Elettra, STM analysis and first principles calculations (DFT).
The STM image in Figure 1 shows an ordered superstructure produced by depositing 1,4-dibromobenzene (dBB) precursor molecules on a Cu(110) surface at room temperature (RT).

Figure 1. Scheme of the Ullmann coupling at the different reaction steps (upper panel) and corresponding STM images (lower panel) showing the organometallic phase (at RT, 5.5×5.5 nm², I_t=0.77 nA, V_b=-0.76 V) and the polymeric phase (at 500 K, 6.8×6.8 nm², I_t=0.60 nA, V_b=-0.40 V).

According to the reaction scheme in Figure 1, the molecules dehalogenate upon deposition at RT resulting in copper-bound bromine atoms and an organometallic structure, which consists of phenylene groups bonded at each end to Cu atoms. This organometallic phase is different from that obtained using iodine in the precursor molecules witnessing the importance of the halogen in driving a specific structure. DFT calculations unveil the configuration of the organometallic chains (not shown here).
The topography changes by heating the surface up to 500 K: the chains are all oriented along the direction and the spacing between the protrusion is reduced. However, only through surface spectroscopic analysis at the different reaction steps it is possible to assign the different topographies to organometallic or polymeric phase. The X-ray Photoelectron Spectroscopy (XPS) results in Figure 2b prove that the molecules are completely dehalogenated (compared to the signals from the intact molecules, obtained at low temperature (LT), i.e. 100 K) and show the presence of the organometallic structure, related to the peak at 283.2 eV. After the annealing at 500 K this peak disappears and the C 1s signal is in agreement with the presence of poly(para-phenylene) (PPP) polymers.

Figure 2. C-K edge NEXAFS (a), C 1s and Br 3d XPS (b), and C 1s Fast-XPS (c) analysis of the system at the various reaction steps (a, b) and during the transition from organometallic to polymeric phase (c). The inset in c shows some profiles of the plot in the transition region (between the yellow dotted lines).

NEXAFS measurements in Figure 2a provide another proof of the polymerization. The disappearance of the transition after the annealing is related to a recovery of planarity of the organic phase (i.e. aromatic rings) on the substrate: the only way to obtain a planar system from the distorted organometallic structure is to form polymers.
Fast-XPS measurements (Figure 2c) at the C 1s core level performed during the annealing of the surface (one spectrum each 2 s, heating rate: 2 K/s) provided real-time information about the transition from organometallic to polymeric phase: we found it to occur at 460 ± 10 K. At such temperature the component at 283.2 eV (well visible in the lowest curves of the inset in Figure 2c) vanishes, witnessing the breaking of C-Cu bond breaks. The polymers remain stable up to 600 K, since the lineshape remain unchanged.

This research was conducted by the following research team:

M. Di Giovannantonio, G. Contini, Istituto di Struttura della Materia, CNR, Roma, Italy
M. El Garah, J. Lipton-Duffin, L. Cardenas, F. Rosei, Centre EMT, INRS, Varennes, Canada
V. Meunier, Dept. of Phys., Appl. Phys., and Astronomy, Rensselaer Polytechnic Inst., Troy, New York, US
Y. Fagot-Revurat, Inst. Jean Lamour, Univ. Lorraine/CNRS, Vandoeuvre-les-Nancy, France
A. Cossaro, A. Verdini, IOM-CNR, Laboratorio TASC, Trieste, Italy
D. F. Perepichka, Dept. of Chemistry, McGill Univ., Montreal, Canada

Contact person:
Giorgio Contini:

Reference

M. Di Giovannantonio, M. El Garah, J. Lipton-Duffin, V. Meunier, L. Cardenas, Y. Fagot Revurat, A. Cossaro, A. Verdini, D.F. Perepichka, F. Rosei and G. Contini, “Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization”, ACS Nano 7, 8190 (2013), DOI: 10.1021/nn4035684.

Last Updated on Monday, 09 December 2013 09:46