Controlling the Adsorption-Induced Reaction Selectivity of Biphenyl-Based Molecules on Ag(111)

On-surface synthesis (OSS) has huge potential for the fabrication of functional molecules and covalent nanostructures because the reaction pathways of adsorbates can be manipulated by confining the molecular diffusion and utilizing different metal substrates. However, the influence of the adsorbates’ adsorption behavior on the reaction pathway is usually neglected in OSS due to the lack of knowledge with regard to the underlying reaction mechanism. Here we combine low-temperature scanning tunneling microscopy (LT-STM), synchrotron radiation photoelectron spectroscopy (SRPES) and density functional theory-based (DFT) calculations to illustrate how molecular adsorption on a metal surface can influence the reaction pathways of biphenyl-based molecules with two and four bromine substituents on a Ag(111) surface. We find that after the debrominations, 2,2’,6,6’-tetrabromo-1,1'-biphenyl (TBBP) transforms into a biphenylene dimer because of a spontaneous intramolecular [2+2] cycloaddition of four radicals of biphenyl when coordinated with surface Ag atoms. The reaction pathway of TBBP is shown in Figure 1. This pathway is different to what was previously studied for 2,2’-dibromo-biphenyl (DBBP), which transforms into a dibenzo[e,l]pyrene dimer. We rationalize that this is caused by a different adsorption-induced precursor state. These experimental and theoretical results confirm the expectation that the strategy of manipulating OSS by changing the adsorption state of molecules on a metal surface is an effective way in controlling the selectivity of a chemical reaction in a complex reaction environment.