Gold-linked strings of donor–acceptor dyads: on-surface formation and mutual orientation ChemComm COMMUNICATION

Strings of gold-organic oligomers of polar units have been formed by on-surface synthesis and investigated with non-contact atomic force microscopy. The mutual alignment of dipoles within the strings is analyzed. While an alternating head-to-tail alignment might be expected from dipolar interactions, a more complicated alignment order is observed. The data suggests that coordination bonding to additional gold adatoms leads to stabilization of parallel pairs of molecules, suppressing a head-to-tail alignment order. polymers comprising electron donor and acceptor units

directionality in molecular self-assembly via weak non-covalent interactions such as hydrogen bonding, 12,13 electrostatic interactions, [14][15][16][17] and metal-ligand coordination. [18][19][20][21] We synthesize a precursor by annulation of an electron donor and an electron acceptor, 3,22,23 leading to an intrinsic dipole moment of B5 Debye 23 along the molecular axis that is expected to steer the relative alignment of adjacent monomers. The low-energy configuration of dipoles would suggest an antiparallel head-to-tail ordering of monomers. 24 Here we demonstrate on-surface synthesis of gold-organic hybrids comprising highly polarizable tetrathiafulvalene-benzothiadiazole (TTF-BTD) dyads exhibiting a complicated short-range order that is lost within a few molecular units. The parallel and antiparallel arrangements of neighboring monomers appear to be correlated with the existence or absence of additional gold coordination bonds, as identified directly by means of noncontact atomic force microscopy (nc-AFM) with sub-angstrom lateral resolution. Formation of aryl-aryl bonds between molecules is not observed, presumably owing to Coulomb repulsion between lone-pair electrons.
Dibromo-TTF-BTD dyads (inset of Fig. 1a) were synthesized (see details in ESI †) via a reaction of 4,8-dibromo [1,3]dithiolo-[4,5-f ]-2,1,3-benzothiadiazol-6-one with vinylene trithiocarbonate in presence of triethylphosphite in 35% yield. The molecules were sublimated onto Au(111) substrates at temperatures below B12 K and later annealed at elevated temperatures to induce debromination. Prior to annealing, we investigated the surface to exclude dissociation of molecules upon deposition, and ensured the purity of the precursors. Experiments were performed with a qPlus-based 25 combined scanning tunneling and atomic force microscope operated at B9 K under ultra-high vacuum. A PtIr wire was used as the tip, treated in situ by indenting into the substrate. CO-terminated tips were used for acquiring frequency shift (Df ) images. 26 Fig . 1a shows chains of TTF-BTD molecules formed upon annealing on Au(111) at 490 K. The chains run preferentially along fcc regions of herringbone reconstructions. Single molecules are observed to be trapped at the reactive elbow sites of the reconstructed surface. Already at first glance, the chains seem to exhibit some short range order, but also pronounced irregularities. Closer inspections (Fig. 1b) using scanning tunneling microscopy (STM) show features related presumably to monomer units, but the detailed internal structure remains elusive.
To discern the atomistic details, the chains were investigated with nc-AFM using a CO-terminated tip 26 which enables direct visualization of the chemical structures of surfacesupported adsorbates. Fig. 2a shows frequency shift (Df ) image of a chain acquired at a constant tip height. Parallel pairs of molecules are readily identified that are bonded in an ordered array. Inter-monomer bonds are observed where they were expected, namely at the sites that can be thermally activated by debromination. The observed center-to-center spacing of B0.72 AE 0.03 nm between adjacent molecules (denoted L in Fig. 2a and b) is too large to be compatible with a covalent arylaryl (for which L would have been B0.43 AE 0.02 nm) coupling that was expected in analogy to other on-surface Ullmann coupling reaction. [28][29][30] Instead, it suggests formation of linear aryl-gold-aryl bonding (i.e., C-Au-C, Fig. 2b) with the separation between two neighboring aryl rings being consistent with previous reports. [31][32][33][34] It is expected that surface atoms diffuse in large numbers, likely with enhanced mobility in the presence of bromines, 35 at temperature to which the sample is annealed, which eventually leads to the formation of organometallic strings. The structural reordering of herringbone reconstruction caused by the adsorption of molecules may act also to release gold adatoms. [36][37][38][39] A direct coupling between carbon atoms through reductive elimination is presumably hindered owing to Coulomb repulsion between lone pairs of electrons among neighboring monomers. We note that reports on direct observation and geometric characterization of stable goldorganic hybrids 31,39,40 are scarce due to relatively low activation barrier of aryl-aryl bond formation between chemisorbed aryl radicals. 40 The Df signal in the AFM image is inhomogeneous across some of the molecules (indicated with black arrows in Fig. 2a), suggesting tilt of molecules with respect to the surface plane. It may arise from a repulsion among lone pairs of electrons on the nitrogen and sulfur atoms of identical neighboring moieties. Moreover, an incommensurability with the substrate lattice may explain why the tilt of molecules is not homogeneous. The influence of surface corrugation due to herringbone reconstruction may be excluded since segments of the chains lain within fcc regions of the reconstructed surface were investigated.
Between pairs of monomers oriented in parallel, small features of repulsive contrast are observed (indicated with red arrows). Such a feature is absent between molecules with antiparallel alignment. We speculate that these features could originate from gold atoms that may be trapped between molecules via coordination bonds [41][42][43] involving nitrogen and sulfur atoms (Fig. 2c). This would straightforwardly explain why these features are only observed between pairs of parallel monomers. Otherwise, one would not expect to observe a stable pair of TTF-BTD molecules arranged in a parallel head-to-head fashion because Coulomb repulsion between equal moieties should Fig. 1 (a) Large-scale STM image shows strings of molecules on Au(111). A ball and stick model of the precursor molecule (dibromo-TTF-BTD) is shown as an inset, where grey, blue, yellow, and red represent carbon, nitrogen, sulphur, and bromine, respectively. The p-conjugated molecular backbone comprises a tetrathiafulvalene (TTF) and a 2,1,3-benzothiadiazole (BTD) moiety fused in a nearly-planar geometry. 24   favor a head-to-tail arrangement. 24 Bending of the CO molecule at the tip apex causes image distortions 44,45 and thereby prevents a direct accurate measurement of bond distances. However, the positions of centers of aromatic rings can be determined relatively precisely, such that the bond distances could be extracted by making use of the known geometry of the precursor molecules. Thereby, we estimate the distance between observed additional features and the closest atomic sites of the molecules to B0.35 AE 0.07 nm. This seems rather long for a coordinative bond, but appears still compatible with a stabilization mechanism via (weak) coordination bonds. 41 Note that the bond lengths may vary with different coordination geometries and the adatoms involved in the coordination may not always exhibit a clear Df contrast. 20, 46 We do not exclude the possibility of bromine or carbon monoxide being responsible for or involved in the additional coordination.
We analyze segments of different chains, two of which are presented in Fig. 3a and b showing aperiodic alignments of molecules. While parallel pairs are still seen as majority, segments with more than two neighboring molecules in the same orientation or adjacent antiparallel orientations are also observed. An insight into the order of molecular orientations is provided by an autocorrelation analysis (Fig. 3c) extracted from the segments of different chains containing in total of 166 monomer units. The gray area encompasses values within the standard deviation (E0.08). The nearest and the next-nearest neighbors appear to be significantly anti-correlated, while every third (and, only very weakly, every sixth) neighbor is found to be in weak correlation. The anticorrelation of next-nearest neighbors reflects a higher probability of finding molecules in pairs (e.g., Fig. 2a). To better comprehend the degree of correlation, a hypothetical autocorrelation for an infinite chain of perfect pairwise ordering of the type -up-up-down-down-up-up-down-down-is shown as an inset of Fig. 3c, which exhibits no nearest-neighbor correlation (because one of the neighbors is always parallel and the other antiparallel) and a perfect anticorrelation of À1 for the next-nearest neighbors (because all next-nearest neighbors are antiparallel). The experimentally observed autocorrelation is far from such a perfect order.
In a very simple model, one could assume that the mutual alignment of two adjacent units is given by a certain probability p for parallel and a probability q = 1 À p for antiparallel alignment. This model implies that the probability of parallel or antiparallel alignment is only governed by local interactions between the nearest neighbors. We tested this assumption and observed that this simple model cannot capture the observed autocorrelation irrespective of the value of the parameter (see details in ESI †). Instead assuming that the alignment of the next unit does not only depend on the direct neighbor, but also on the second-nearest neighbor, does qualitatively reproduce the autocorrelation trace (Fig. S1b in ESI †). From this we conclude that the alignment of molecules observed in the experiment must be partially driven by next-nearest-neighbor effects. The slightly canted azimuthal orientation of two neighboring molecules with the same orientation hints towards a locally developed compressive strain in the polymeric chain.
The release of such strain may be the driving force for the observed next-nearest-neighbor correlation effects. We note that the number of additional atoms that can be coordinated between each two adjacent monomers depends on their mutual alignment. This mechanism will therefore contribute to the nearest-neighbor correlation effects. However, it is confined to the interaction of direct neighbors and can therefore not explain the observed next-nearest-neighbor correlation effects.
Finally, to address the role of the substrate, experiments were repeated on a more reactive Cu(111) surface. The substrate with sub-monolayer coverages of precursor molecules was annealed to 315 K to facilitate catalytic reaction, 47 resulting in dehalogenation and subsequent polymerization of molecular radicals (Fig. 4). Aperiodic 1D networks are observed and bromine atoms, dissociated from the precursor monomers, are found either in closepacked islands or to get trapped at the periphery of the networks. Further annealing of the sample to 385 K did not give rise to appreciable changes in the structures. These results suggest that, indeed, a less reactive surface like gold is more likely to allow for ordered arrangements in on-surface structures, in agreement with previous work. 48 In conclusion, 1D strings comprising fused donor-acceptor dyads were prepared by on-surface synthesis on Au(111), with the prospect of a controlled implementation of electric dipoles in surface-supported polymers. The molecules are observed to couple via aryl-gold bonds into hybrid wires, while coordination with additional adatoms inhibits the long-range order in the mutual alignment of adjacent units. A direct aryl-aryl coupling reaction has not occurred, probably owing to Coulomb repulsion between lone pairs of electrons among neighboring monomers. While dipole-dipole interactions would favor an antiparallel orientation of neighboring units, the experimentally observed alignment of molecular dipoles in the chains shows a weak local correlation but no long-range order. We hope that our study will trigger new strategies to facilitate mutual alignment of adjacent . ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi n À jkj p , only values larger than that may be considered significant. In the inset a hypothetical autocorrelation for an infinite chain of perfect pairwise ordering (u-u-d-d-u-u-d-d-) is presented for comparison. units in on-surface synthesis from non-symmetric monomer units in the future.
Financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project IDs SFB 689 and 314695032 SFB 1277 -B01 is gratefully acknowledged. Open Access funding is provided by the Max Planck Society.

Conflicts of interest
There are no conflicts to declare.