Self-assembly

Fc2(BrPh)2P does not possess any functional groups which drive directional self-

assembly. Halogens can participate in hydrogen bonds, or can form halogen bonds,508 _{neither of which are expected to be very strong or directional for this}

0 20 40 60 80 100 120 0 2 4 6 8 10 X (Å) Z (Å) (a) (b) (c) a drow dFc b

Figure 7.2: Self-assembled islands of Fc2(BrPh)2P on Au(111). The large area

scan in (a) shows both the ordered and disordered areas of the surface. A close- up of a molecular island is shown in (b), with the molecule and surface unit cell marked in. (c) topographic profiles along the lines indicated in (b). The unit cell parameters are a=17Å, b =26.5Å. The b parameter has dual periodicity stem- ming from the distance between the Fc groups within the moleculedFc =11Å and

the distance between nearest Fc groups in adjacent rows drow =15.5Å. Imaging

conditions: (a) 15 pA,₋2.5 V; (b) 20 pA, ₋1.25 V.

Waals interactions is expected to play a role in self-organisation of Fc2(BrPh)2P

on the surface. The lack of a strongly preferential packing structure, coupled with the non-trivial amount of impurities present due to ESD, explains why most of the surface is disordered, and only small well-organised islands are present, as shown in figure 7.2(a). Molecular ordering is increased after gentle annealing.

When the molecules do self-assemble, they first form lines at an angle of∼15° to the⟨11¯2⟩directions, as determined from the Au(111) herringbone ridges, with a repeat unit a measuring approximately 17Å. Di(bromophenyl)-di(iodophenyl) porphyrin (Br₂I₂TPP) self-assembles with the Br atoms next to the porphyrinic

β-hydrogens, giving a nearest neighbour distance of approximately15Å.500 _Sim-

driver for self-assembly is attributed to the phenyl π-π interaction, as well as

to the Br_···H₋C hydrogen bonds.509 _{We believe that the same interactions drive}

self-assembly of Fc2(BrPh)2P into lines. However, since the Fc groups obscure the

rest of the molecule, it is not possible to confirm this without a relaxed molecular model.

The molecular rows pack with an inter-row periodicity of 26.5Å. Adjacent rows are offset by half a unit cell length, giving a 60° angle between the surface unit cell vectors. The surface unit cell is indicated relative to the centre of the porphyrin core in figure 7.2(b). We note here that the unit cell periodicities are within the margin of error of distances corresponding to 6 and 9 Au(111) nearest neighbour distances (17.3Å and 25.9Å, respectively), indicating a commensu- rate unit cell. The red line in figure 7.2(b) has dual periodicity representing the intramolecular Fc-Fc distance and the spacing between adjacent rows, as can be seen in figure 7.2(c). The intramolecular Fc-Fc distance is 11Å, while adja- cent rows are separated by 15.5Å. The expected intramolecular Fc-Fc distance is approximately 12Å, and the small discrepancy observed could be either due to the inherent experimental error, or due to conformational relaxations upon adsorption.

The origin of the feature between the Fc2(BrPh)2P rows is less clear. From

the structure we have assigned to the islands, it does not appear to arise from any part of the molecule, in terms of geometry alone. It is present both before and after annealing up to 200◦C, which suggests that neither Au(111) adatoms nor residual solvent atoms are the cause. This does not rule out interaction with some less volatile impurity, however the regularity of the structure and its apparent thermodynamic stability make this very unlikely.

The fact that we see breaks in the inter-row feature at the island edges where packing is imperfect, as well as in the middle of the island, such as at the bottom left-hand corner of Fig. 7.2b, suggests that the feature is due to some easily removed part of the molecule. It is possible that the feature stems from an electronic interaction between the Br atoms in adjacent molecular rows. Close inspection of a Br₄TPP island reported by Grill et al.212 _{shows a feature}

reminiscent of our zigzag feature between the Br atoms on adjacent molecules. This same feature can be seen more clearly in the cobalt 5,10,15,20-tetra(4-bro- mophenyl)porphyrin (CoBr₄TPP) islands in ref. 347. However, seeing as the distance between the molecular rows is significantly larger than the Br-Br dis- tance in Br₄TPP/CoBr₄TPP islands, and since the Br atoms seem not to be pointing directly into the gap between the rows, it is unlikely that this feature

Z (Å) 0 20 40 60 80 100 120 140 0 2 4 6 8 X (Å) b (a) (b) c a

Figure 7.3: The minority packing phase of Fc2(BrPh)2P on Au(111). (a) shows a

transition between the majority phase and the minority phase, with the unit cells marked in pink and yellow, respectively. Imaging conditions: 20 pA, −1.8 V. (b) shows the apparent height profiles along the lines indicated in (a). The unit cell parameters for this structure are a = 17.5Å and b = 23.5Å. The period along the blue line is c= 32.5Å.

can be attributed to the Br_···Br interaction.

Other groups have observed a similar feature which they attribute to a per- turbation in the surface charge density in the pores and around the edges of a covalently-bonded porphyrin network.510 _{The topographic feature resulting from}

this perturbation looks very similar to our inter-row feature. However, we do not observe any such feature at the island edges, only between the molecular rows. Additionally, no such feature was observed for islands of unreacted Br₄TPP.212,347

It is possible that the small intermolecular spacing obscures the feature in the Br₄TPP islands.

We also observed a minority phase where this feature does not appear. The row structure is identical to that in the majority phase, however the rows are shifted relative to each other so that Fc groups from molecules in different rows are beside each other, rather than interdigitated. This structure is shown in Fig. 7.3, and is described by the unit cell vectors a = 17.5Å, which represents the intermolecular spacing along the row and is within the experimental error of that in the majority phase, and, b = 23.5Å, representing the inter-row periodicity. The angle between the unit vectors is approximately 95°.

The fact that at the top of Fig. 7.3 we see the majority phase shifting to the minority phase with no disruption to the island, supports our assertion that the zigzag feature between molecular rows is not due to any species other than Fc2(BrPh)2P. The closer inter-row spacing may suppress the appearance of the

zigzag feature, consistent with it originating in a surface charge density perturba- tion. However, we remain unable to definitively assign the origin of this feature.

0 20 40 60 80 100 -2 -1 0 1 2 Sample Bias (V) 0 20 40 60 80 100 -10 1 X (Å) Z (Å) X (Å) (a) (b)

Figure 7.4: STS across a molecular row on Au(111). (b) shows a dI/dV colour map compiled from Z(Vb)spectra measured along the yellow line in (a). Imaging conditions for (a): 20 pA, −1.3 V.