On-Surface Formation and Chemical Transformations of Indigo Coordination Polymers

Abstract

Coordination polymers are attractive candidates for miniaturized electronics. With unconventional electronic and magnetic properties posited, they are interesting for multiple applications including magnetic information storage and spintronics. Natural products are appealing to tailor advanced new materials, bearing inherent advantages in biodegradation. Among them, we distinguished indigo, a common natural dye with a distinctive blue color, used as a pigment since antiquity. Motivated by its optical properties and the reported metal complexes [1], we employed it as a molecular building block for the surface formation of coordination polymers with iron.We report on the on-surface formation of extended coordination polymers which controllably feature the cis and the transmonomers of the molecular building block. The reaction products of indigo and Fe were characterized atomically by a combination of state-of-the -art experimental and theoretical techniques. By a combination of X-ray photoelectron spectroscopy and scanning tunneling microscopy (STM), it is found that indigo deprotonates and coordinates with Fe atoms on silver surfaces. We identify unambiguously well-defined coordination polymers composed of (1 dehydroindigo : 1 Fe) repeat units with STM, bond-resolving atomic force microscopy and density functional theory (DFT) simulations. On both Ag(100) and on Ag(111), the trans configuration of dehydroindigo results in N,O-chelation in the polymer chains. On the more inert Ag(111) surface, the molecules undergo thermally induced isomerization from the trans to the cisconfiguration and afford N,N- plus O,O-chelation (see figure). This is enabled by the dehydrogenation and is preferential solely in the environment of the coordination chain on Ag(111). DFT calculations also confirm that the coordination polymers of the cis-isomers on Ag(111) and of the trans-isomers on Ag(100) are energetically favored.Our results demonstrate post-synthetic linker isomerization in interfacial metal-organic nanosystems. Both types of coordination polymers are predicted to be spin-crossover systems and the isomerization is anticipated to have a significant impact on their magnetic properties.