The role of defects in the properties of functional coordination polymers

Abstract

Coordination Polymers (CPs) are periodic architectures defined by the assembly of metal entities and ligands through coordination bonds. They can be designed to present porous architectures, known as Metal-Organic Frameworks (MOFs). They can be prepared as bulk materials and at the nanoscale using bottom-up synthesis or top-down approaches. Nanoscale CPs are a subject of high current interest due to the new physico-chemical properties that they can show because of confinement effects as well as their material process-ability. We present several selected CP samples with different electronic properties, electrical conductivity and/or emission, as well as ways to down-size their scale to obtain CP nanostructures. Studies on their physical properties at the nanoscale show the relevance of the confinement effects and the presence of defects, which can be controlled during the preparation process. Indeed, defect engineering is an extremely relevant tool to manipulate the material crystal quality and its specific properties. Therefore, engineered defects are gaining attention in both CPs and their porous version, MOFs, because of their implications for both physical properties and properties affecting catalysis and sorption. However, this is still a very poorly developed field. The main scope of this chapter is to provide a general overview of the impact that defects may have on the chemical, physical, electrical and/or optical properties, as well as in catalysis and sorption capabilities of some CPs, and the way to gain control of the defect production. We have selected two relevant types of 1D CP families, MMX and Cu2I2 double chains, to discuss their electronic properties and the influence that the incorporation of defects has on them. MMX chains based on the assembly of two dimetallic entities: [Pt2(dta)4] (dta = ditiocarboxylate) and [Pt2(dta)4I2], behave as metallic conductors at room temperature in bulk and at the nanoscale, but the weakness of the PtI coordination bonds facilitates the occurrence of defects such as iodine vacancies, which significantly alter their conductivity. Their preparation at the nanoscale is feasible based on the reversible process between [Pt2(dta)4] and [Pt2(dta)4I2]. Thus, the polymer [Pt2(dta)4I2]n can be solubilized or sublimated to produce [Pt2(dta)4] and [Pt2(dta)4I2] and rearrange back to [Pt2(dta)4I2]n even on-surfaces. The exquisite control of the assembling process gives rise to a number of structural defects present along the MMX chains. In any case, electrical measurements in single chains of [Pt2(dta)4I2]n currently postulated these CPs among the best molecular wires. We also describe several examples of Cu2I2 double chains grafted with N-aromatic terminal ligands. They show interesting electronic properties as emission and semi-conductivity. The general structural core of these 1D-CPs is based on a Cu2I2 double chain which is very sensitive to chemical and physical stimuli. We discuss the use of these CPs as stimuli-response materials, their bottom-up preparation using fast precipitation and the use of these nanostructures to prepare novel composites as multifunctional ultra-thin films. Moreover, we also show the possibility of modulating their physical properties upon the creation of structural defects. Finally, we describe different synthetic pathways (pre- and post-synthesis) to harness the incorporation of both local- and long-range defects in MOFs, resulting in altered chemistry and structures without compromising the porous scaffold. The role of defects in MOF properties related to catalysis, sorption and conductivity is widely discussed in this chapter, highlighting the importance of using advanced scanning probe microscopy, synchrotron X-rays and neutron techniques to achieve a better understanding of these functional nanomaterials.