The driving force for such self-assembly is thus the formation of type I halogen bonds. In contrast, on Au(110) the molecules are uniaxially aligned parallel to the 2×1 surface reconstruction. The intact molecules adsorb preferentially on top of the reconstruction's Au rows, with the halogenated sides facing each other, although the opposite halogen-to-methyl alignment can also be observed occasionally (Fig. ). Carbon-halogen bonds display a lower activation temperature than that corresponding to carbonhydrogen bonds, causing the initial reactions to be dominated by Ullmann coupling, whose main products are depicted in Fig. (completed with additional minority products shown in Fig. ). Only at higher temperatures in the range of 525 K do the methyl groups become activated, leading to the intermolecular coupling of the previously formed Ullmann coupling products (Fig. ). Interestingly, as shown in Fig. the preferred molecules' relative alignment on either substrate is perpetuated as the intermolecular covalent coupling reactions are thermally activated. That is, the one-and three-fold symmetry of the Au(110) and Au(111) substrates, respectively, is preserved in the self-assembled non-covalent structures of the as-deposited reactants, as well as in the predominant reaction products. In the following, we analyze in detail the initial reactions observed after annealing to only 425 K. A representative large-scale image after a sub-monolayer deposition of m1 on Au(111) followed by an annealing treatment is shown in Fig. . The compendium of all observed covalently coupled products is presented in Fig. -f (some apparently more complex structures observed in Fig. are just some of these products interacting non-covalently with additional unreacted monomers), and their relative abundance is shown in the histograms of Fig. . Among the products we observe dimers linked by a single covalent bond that is either on the same molecular side, resulting in a structure with the two acene moieties at an angle of approximately 120 degrees (d1, Fig. ), or on opposite sides, resulting in a staggered structure of two collinearly oriented acene moieties (d3, Fig. ). The other observed dimer presents two bonds linking the two anthracene moieties, forming a four-membered ring (d2, Fig. ). This product may evolve from d1 by a second bond C-C bond formation that quenches its remaining radicals, or be the result of a concerted formation of two bonds from two activated m1 monomers. 17 Whereas the two new bonds formed with d2 quench all the four radicals generated with the homolytic cleavage of the C-Br bonds of two reactant molecules, the single-bond formation of d1 and d3 quenches only two of them. Whether in the observed dimers the remaining radicals are passivated by H of unknown origin (e.g. residual H from the UHV chamber) or not remains unknown, but at least no obvious bonding of the -radical to the Au substrate is apparent from a molecular tilt that would in principle be associated with it. In any case, structures d1 and d3 with remaining radicals are very reactive species that presumably act as precursors for the formation of trimers. Taking the most common dimer d1 as starting point, the natural addition of a third activated m1 monomer quenching all radicals would form the three-fold symmetric hexamethyl [10]-starphene (t1, Fig. ). However, an alternative scenario including a hydrogen migration may result in an asymmetric triphenylene derivative (t2, Fig. ). The latter may also be obtained from d3 under the same conditions: the addition of an activated m1 monomer and an associated H migration (Fig. ). The product's relative abundance, however, shows that among the dimers the most common is d1, while in total numbers there is a dramatic preference for t1. The stepwise evolution of m1 to d1 and to t1 thus appears as the presumably most favorable pathway.