Oligomeric chains made from I 2 DTE

As shown in ch. 2.4, the iodinated terfluorene (DITF) requires a lower activation energy which leads to the growth of extended chains already at room temperature. The omittance of the heating step allows the connection of these precursors yielding virtually defect-free structures. It was therefore attempted to apply this successful approach to the switch molecules by employing iodinated DTE precursors (labeled I2DTE (o) in Fig. 4.8) with the intention of avoiding both

the branching type of coupling as well as the modification to the functional unit. The structures that are observed after the deposition of I2DTE on Au(111) held at room temperature are shown

in Fig. 4.18. Different from the appearance of the brominated monomers forming interlocked islands, the molecules appear as chains of switches. Unlike the case shown above (Fig. 4.17) chain formation connects the central units via the legs of the switches and no signs of branching are observed. Equally as important, the central unit appears to be unchanged with regard to the appearance of the intact monomers (cf. section 4.2.1). A repeating unit is circled in the inset of Fig. 4.18. The connection between the units is homogeneous and no boundary is visible any more. The molecules’ central part displays the characteristic bright lobe with the surrounding brim as shown in Fig. 4.11. The bright feature originates from the thienyl group that is slightly tilted upwards located at the inside of the angle (the corresponding methyl groups are displayed as small wedges in the inset of Fig. 4.18). The DTE display an asymmetric shape and their orientation can be inferred by the brim, which displays a gap on the side where the methyl group is pointing inwards (cf. Fig. 4.9 (d)) on the leg that appears longer for the monomers. For this angled conformer there are three different types of connections possible: long-long, long-short, and short-short, as sketched at the bottom of Fig. 4.18. All three types of connections are represented in the high-res inset. The most frequently observed is the long-short combination with a distance of (13.8 ± 0.5) ˚A as measured from STM images (distance ’d’ in the sketch). This is compatible with a calculated chain that predicts a distance of 14.8 ˚A between neighboring units’ protruding methyl groups. (The calculation is for a molecular chain in the gas phase [234]). Most importantly for the corroboration of the intactness, recording differential conductance curves (cf. Fig. 4.19) show the same features in dI/dV curves as the intact DTE monomers shown in Fig. 4.16, whereas the stability of the newly formed bonds was asserted in many instances of manipulation with the STM tip. Both of the latter points will be discussed in detail in ch. 4.4.

The chains are mostly located on the fcc regions of the herringbone reconstruction and the largest part is observed in a zigzag conformation with a smaller number of linear segments in between.

d

l - l

l - s

s - s

Figure 4.18: Overview image of I2DTE deposited onto Au(111) at room temperature. Image size 130 × 140 nm2

, −500 mV, 0.1 nA. Several horizontal step edges separate the terraces occupied with molecular chain structures. The inset shows a high-resolution segment of an oligo-DTE chain with a sketch of the according chemical structure. Image size 9 × 9 nm2

, 500 mV, 0.1 nA. The arrow denotes the position of a gap in the brim (cf. main text). The dashed circles highlight lobes that likely are iodine adatoms from the activation process. Since the units are not symmetrical, there appear three different types of connections, as sketched below the STM images. All three types can be distinguished in the inset.

In some cases the gaps in the side of the chains appear filled with adatoms (marked by dashed circles in the inset of Fig. 4.18), which can lead to a slight reduction in contrast. As these atoms were not observed for the molecules without iodine, the identification suggests itself. At this low coverage, chains are observed isolated as well as arranged in islands.

bias voltage(V)

-1 -2

-3

dI/dV / (I / V) (arb. u.)

Figure 4.19: Normalized dI/dV-spectra recorded on a DTE unit in a homopolymer and a copoly- mer as indicated by the × in the STM images to the right. (top) Cyclic DTE trimer, 3.5 × 3.5 nm2

. (bottom) DTE contacted from both sides with TF units, 5 × 5 nm2

. Both images recorded at 500 mV, 0.1 nA. The HOMO (LUMO) is indicated by the down (up) arrows. Intervals of small differential con- ductance signal, i.e. mostly in the gap, are sensitive to the enhancement of noise by the normalization procedure, which is especially visible in the top graph. Therefore, a curve is added to the HOMO in this spectrum as guide to the eye.

The present findings point to the conclusion that the produced structures displayed in Fig. 4.18 are indeed the intact and covalently coupled DTE. This is the first realization of such a structure on a surface. Such a chain coupling several switches into one entity is very interesting in itself. For instance, thinking in terms of information storage eight units in one asymmetric compound such as the segment shown in the inset of Fig. 4.18 could be used to store one byte of information. (The shown area provides space for at least four such chain segments, i.e. 4 byte/100 nm2_{, which}

is equivalent to more than 25 Tbyte per square inch). It has to be noted that the predominant conformation is the angled one (as the ones shown in the inset) that was identified as the non- switching one above. This might be considered as a drawback. However, as will be shown below, for cyclization of individual units this poses a surmountable challenge, while for the parallel switching of a large number of such molecules the inclusion of additional groups during synthesis could be attempted to ensure a favorable, i.e. the linear, conformation on the surface. (This will be addressed below with the TM-I2DTE molecules). By heating the surface it could be shown

that these structures withstand a heating step to 100◦

C. Continued coupling is observed upon these annealing steps producing longer chains, whereas the characteristic features of the repeating units remains unchanged. This is analogous to what was observed for DBTF, cf. ch. 2.4.

Figure 4.20: STM image of a copolymer of terfluorene and dithienylethene on Au(111), parameters 500 mV, 0.1 nA, image size 55 × 65 nm2

. Solid and dashed arrows highlight homopolymer sections, the hollow arrow points at an intermixed segment.

The intact connection of the DTE by the above method paves the way for the contacting of individual switches with molecular wires that could not be realized using the brominated pre- cursors, because the switches sustained damage during the heating step. To that end I2DTE

was deposited together with DITF onto Au(111). Since both types of molecules activate at the same time and because the formation of block copolymers was to be avoided, the deposition had to take place simultaneously, much like the case of DITF and Br2I2DTE (cf. ch. 3.2). The

resulting structures are shown in Fig. 4.20. The coupling appears to have taken place efficiently and randomly as expected. While homopolymer segments as indicated in the figure by the solid (DTE) and dashed (TF) arrows are also present, there appears a sufficiently large number of intermixed chains that possess isolated units of one type contacted on both sides by the other (as pointed out by the hollow arrow).

While the poly-TF segments appear with a smoother curvature to accommodate the preferred fcc regions of the herringbone reconstruction, the switches predominantly display the character- istic angled shapes (cf. Fig. 4.21). The intramolecular appearance is in marked contrast to the one observed after the heating step that was required by the bromine-substituted monomers, cf. Fig. 4.17 and identical to the one observed for the individual monomers and inside the DTE homo-polymers, cf. Fig. 4.18. Examples for DTE contacted from both sides with terfluorene are shown in Fig. 4.21. As before, the linear DTE species is seldom found. The switch segments have been investigated with dI/dV-spectroscopy and their differential conductance signature (cf. Fig. 4.19) is in agreement with the one of the fluorenated monomers and the DTE units in ho-

mopolymer chains. Therefore it can be concluded that the switches covalently couple with the terfluorene units without damage or significant alteration of their conformation, adsorption or electronic structure. In all cases a binding of the thieno groups to the gold that has been sus- pected of causing a detrimental influence on the switching behavior [231] has not been observed.

a

b

Figure 4.21: High-resolution STM images of copolymer chains. (a) DTE-dimer contacted from both sides by long TF chains. The chemical structure is shown as a sketch (enlarged for better visibility). The box encloses the two switches. (b) Example of two switches inside a chain, but this time separated by five units of TF. One of the DTE units has been transformed into the linear conformation (circled). Parameters 500 mV, 0.1 nA, image sized 14 × 15 nm2

(a) and 10 × 15 nm2 (b).