On-Surface Synthesis of Spin Crossover Polymeric Chains

We report on the successful on-surface polymerization reaction of codeposited zwitterionic quinones with Fe atoms on Au(110) at an appropriate temperature. The resulting covalent one-dimensional polymer chains arrange in a well-ordered two-dimensional (2D) structure as proven by scanning tunneling microscopy and low-energy electron diffraction. The ordered regions can reach the micrometer size. Furthermore, the electronic and magnetic properties of the zwitterionic quinoidal polymers on Au(110) were investigated using the density functional theory with an explicit inclusion of the Hubbard U term. The spin-polarized generalized gradient approximation plus U method (SGGA + U ) has been used, and the freestanding isolated polymer chain, the 2D arrangement, and the adsorbed polymers have been calculated. From ab initio calculations, we predict the zwitterionic quinoidal polymer chains to be a one-dimensional spin crossover compound. For the freestanding chains, we find two local minima with comparable energies but different spin states: a high-spin state ( S = 2 per Fe) with an Fe–Fe distance of 7.9 Å and an intermediate-spin state ( S = 1) with an Fe–Fe distance of 7.72 Å. The experimental and theoretical results show that the substrate dictates the lattice constant, and the adsorbed polymer on Au(110) has an Fe–Fe distance of 8.16 Å and is in the high-spin state. The exchange coupling in the polymeric chain with the Au(110) lattice constants was found to be antiferromagnetic. The adsorption on Au(110) removes the surface reconstruction of a free surface, and the ab initio simulation gives the short-bridge position for Fe as the most stable one.