Coupling of Triamines with Diisocyanates on Au(111) Leads to the Formation of Polyurea Networks

The surface-confined urea coupling reaction between melamine (1,3,5-triazine-2,4,6-triamine) and 1,4-phenylene diisocyanate has been investigated on Au(111) by scanning tunneling microscopy (STM). The initial coupling reaction to produce small oligomers occurs at room temperature. These oligomers are incorporated into the two-dimensional melamine array. Annealing accelerates the formation of larger oligomers with multiple urea linkages. The oligomers can themselves form ordered 2-D structures stablised by intermolecular H-bonding. At higher annealing temperatures, oligomers containing as many seven or eight urea linkages were identified. These oligomers were able to form 2-D porous structures via inter-oligomer H-bonding interactions. We discuss the composition of all of the phases observed and identify how covalent and non-covalent interactions stabilize each phase.