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Chain-length dependence of the 2D self-assembly of alkylated-triphenylenes on gold
N. Katsonis, A. Marchenko, D. Fichou
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Chain-length dependence of the 2D self-assembly of alkylated-triphenylenes on gold

Figure 1: Molecular structures of 2,3,6,7,10,11-hexakis-dodecalkoxy-triphenylene, T11

Among the many types of liquid crystals (LCs), planar disk-like molecules such as triphenylenes are known to exhibit columnar mesophases. An example of such compounds are 2,3,6,7,10,11-hexakis-alkoxy-substituted triphenylenes, hereafter noted Tn where n indicates the number of carbon atoms in the alkyl chains. Tn molecules consist in flat polyaromatic cores symmetrically surrounded by flexible n-carbon side chains (Figure 1). When deposited on a surface, Tn molecules stack upon each other to form hexagonally packed columns with a p-p stacking configuration. Triphenylene LC mesophases present a quasi-1D transport of charge carriers along these vertical columns which make them attractive for electronic devices applications.

 

In order to control the 3D arrangement of Tn molecules on a metal surface, it is necessary to control the structure of the first Tn monolayer. We report here on in-situ STM investigations of T11 (Figure 2) self-assembled monolayers (SAMs) at the n-tetradecane/Au(111) interface with the essential aim to elucidate the interplay between substrate-molecule and molecule-molecule interactions.

T11 molecules self-organize as twin rows with molecules lying flat on the surface in an anti-parallel position (see Ref.1). Three alkyl chains of each T11 molecule align along the <110> direction of Au(111). The association of T11 in molecular pairs  (Figure 3) appears to result from a substrate-induced mechanism governed by the strong anisotropic interaction between T11 alkyl chains and Au(111).

 
Chain-length dependence of the 2D self-assembly of alkylated-triphenylenes on gold

Figure 2: Middle: STM image (17×17 nm2, Ut= 0.26 V, It= 96 pA) of a self-organized domain of T11 on Au(111)

Chain-length dependence of the 2D self-assembly of alkylated-triphenylenes on gold

Figure 3: Possible model for the packing of T11 on Au(111)

Shortening the length of the six peripheral alkyl chains strongly affects the 2D arrangement of Tn on gold. For chains as short as T5 and T7, molecules adopt a hexagonal packing instead of twin rows. Surprisingly, for an intermediate chain length such as T9, both types of packing coexist in the same monolayer, a clear case of symmetry breaking which we attribute to a lever arm effect of the chains (see Ref.2). In conclusion, tuning the length of the alkyl chain appears as an efficient way to precisely control the 3D arrangement of triphenylenes Tn in columnar discotic on conducting surfaces.

REFERENCES:

1. N. Katsonis, A. Marchenko, D. Fichou, J. Am. Chem. Soc. 2003.

2. N. Katsonis, A. Marchenko, D. Fichou, Eur. Phys. J. B submitted.

 
#852 - Màj : 12/06/2007

 

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