Steering Effect of Bromine on Intermolecular Dehydrogenation Coupling of Poly(<i>p</i>-phenylene) on Cu(111)

Abstract

Among the multitudinous methodologies to steer on-surface reactions, less attention has been paid to the effect of externally introduced halogen atoms. Herein, highly selective <i>trans</i>-dehydrogenation coupling at the specific <i>meta</i>-C-H site of two poly(<i>p</i>-phenylene) molecules, <i>p</i>-quaterphenyl (Ph₄) and <i>p</i>-quinquephenyl (Ph₅), is achieved on Cu(111) by externally introduced bromine atoms. Scanning tunneling microscopy/spectroscopy experiments reveal that the formed molecular assembly structure at a stoichiometric ratio of 4:1 for Br to Ph₄ or 5:1 for Br to Ph₅ can efficiently promote the reactive collision probability to trigger the <i>trans</i>-coupling reaction at the <i>meta</i>-C-H site between two neighboring Ph₄ or Ph₅ molecules, leading to an increase in the coupling selectivity. Such Br atoms can also affect the electronic structure and adsorption stability of the reacting molecules. It is conceptually demonstrated that externally introduced halogen atoms, which can provide an adjustable halogen-to-precursor stoichiometry, can be employed to efficiently steer on-surface reactions.