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Two-dimensional self-assembly of benzotriazole on an inert substrate

It is well-known that some physicochemical properties of a material may depend upon the dimensionality of the material itself; a classic example is that of gold, considered unreactive as a sizable lump of metal, but showing exceptional catalytic properties when in form of nanoparticles. Similarly, for organic molecules intermolecular interactions present in the 3D solid may differ from those exhibited by a 2D or a 1D system. Benzotriazole (BTAH, C6H5N3) is a heterocyclic compound used in a variety of fields spanning from organic synthesis, to photostabilizer in photographic emulsions and antimicrobial. However, one of its most important applications is in the field of protection of copper artefacts form atmospheric and underwater corrosion. Following adsorption of BTAH on the surface of the copper item, its passivating action is due to the growth of a water insoluble layer, in the form of a Cu-BTA complex of, as yet, controversial structure. A major part is thought to be played by portions of this layer comprised of purely hydrogen-bonded BTAH.
This paper focuses on the identification and description of BTAH lateral intermolecular interactions occurring when the dimensionality of the moiety is reduced by carefully tuning the experimental conditions. The use of an essentially unreactive substrate, Au(111), allows one to highlight how specific directional interactions are characteristic of a system of a specific dimensionality. NEXAFS and XPS measurements were undertaken at the SuperESCA beamline of Elettra, while HREELS, LT-STM measurements and theoretical modelling were performed at the School of Chemistry, University of St Andrews, UK.

Experimental evidence indicates that BTAH lies flat on a Au(111) surface, as seen by angular dependent NEXAFS (figure 1a), and retains its chemical structure, as shown via XPS (figure 1b), where the C 1s and N 1s relative surface concentrations agree with those expected for an intact BTAH molecule. LT-STM measurements (figure 1c), necessary condition as the molecular ad-layer diffuses too fast to be imaged at room temperature, show a variety of structures within a narrow temperature range. Those structures are characterised by an unusual vibrational spectrum (figure 2a, left), indicating that a combination of intermolecular interactions has to occur. By modelling dimeric species, vibrational spectra are best fitted including contributions deriving from three different intermolecular interactions (figure 2a, right). Modelling extended up to hexameric chains, the closer approximation to an infinite chain, non-commensurate with the substrate (figure 2b), allows rationalisation of the very low contribution exerted by the surface. The very good match with the STM images indicates type I to be the most relevant intermolecular interaction. The features observed are best described as an alternation of pro-R and pro-S, mirror image, molecules to form racemic chains. The varying periodicity observed is due to adsorption energy and hydrogen-bonding being of similar magnitude. Kinks in the chains are likely due to a different type of tautomer. Qualitative information on BTAH balanced gas-phase tautomeric equilibriumis also obtained.
These results provide an important molecular level insight into complex phenomena, describing how a bulk organic material is reduced to single molecular species, which are at the basis of chemical and physical interactions between molecules, and how a variety of intermolecular interactions can occur within a very limited temperature interval. Moreover, this study also contributes to understand further the surface chemistry of benzotriazole, a well-known corrosion inhibitor for copper and its alloys.

Figure 1. a) N K-edge NEXAFS spectra of an as prepared saturated BTAH layer on Au(111); b) N 1s and C 1s XP spectra of an as prepared saturated BTAH layer on Au(111) and after annealing to the indicated temperatures; c) STM images of an as prepared saturated BTAH layer on Au(111) and after annealing to the indicated temperatures, collected at 77 K; scanning parameters: 293 K (9.3 × 9.3 nm2, 1.04 V, 0.07 nA); 298 K (9.3 × 9.3 nm2, 1.07 V, 0.1 nA); 308 K (9.3 × 9.3 nm2, 1.03 V, 0.04 nA); 323 K (18.7 × 18.7 nm2, 0.551 V, 0.02 nA).
 

Figure 2. a) Comparison between the HREEL spectrum collected for an as prepared saturated BTAH layer on Au(111) and those calculated for the three dimers types indicated; b) electron densities of type I BTAH chains of different lengths projected on a plane at 2 Å above the surface.
 

 

This research was conducted by the following research team:


Federico Grillo1, José Antonio Garrido Torres1, Michael-John Treanor1, Christian Rodriguez Larrea1, Jan Philipp Götze1, Paolo Lacovig2, Herbert Anton Früchtl1, Renald Schaub1, Neville Vincent Richardson1

EaStCHEM and School of Chemistry, University of St Andrews, St Andrews, United Kingdom.
Elettra – Sincrotrone Trieste, Trieste, Italy.




Contact person:

Federico Grillo, email: federico.grillo@st-andrews.ac.uk
 

Reference

F. Grillo, J. A. Garrido Torres, M.-J. Treanor, C. R. Larrea, J. P. Götze, P. Lacovig, H. A. Früchtl, R. Schaub and N. V. Richardson, “Two-dimensional self-assembly of benzotriazole on an inert substrate”, Nanoscale, 8, 9167-9177 (2016); DOI: 10.1039/C6NR00821F

This paper is part of the themed collection: 2016 Hot Articles in Nanoscale


Last Updated on Thursday, 30 June 2016 16:52