Growth Study and electrical transport of thin film coordination polymers

Abstract

Porous Coordination Polymers (CPs), a class of hybrid organic-inorganic materials, with permanent porosity, popularly known as Metal-Organic Frameworks (MOFs), have garnered a lot of attention in the last two decades primarily due to their chemical tailorability and high porosity. This has allowed such materials to be used primarily for various applications pertaining to gas adsorption, separation and sensing. The insulating nature associated with such materials however, was a major drawback holding back such materials in applications requiring good charge transport properties. This shortcoming was overcome by adopting various strategies to impart electrical conductivity to CPs, among which, the use of linkers with extended conjugation and redox-active organic cores like hexa- substituted triphenylenes and benzenes, chloranilic acid and dihydroxybenzoquinone have been on the fore front. This results in frameworks having high porosity as well as high electrical conductivity, lending multi-functionality to such materials. Indeed, a class of materials based on the tetraoxolene ligand chloranilate, has been found to exhibit high electrical conductivity with inherent porosity, notable among which, the Fe based porous CP reported magnetism as well. Thus, processing such multi-functional porous CPs into thin film configuration becomes necessary for practical electronic applications. To this end, we have employed a simple and cost-effective solution processable Layer-by-Layer (LbL) technique with the aim of combining a paramagnetic metal copper (II) center with a redox active chloranilate ligand to generate for the first time a copper-chloranilate (Cu-CA) thin film exhibiting high electrical conductivity and the possibility of magnetic ordering. Successful fabrication of uniform and dense Cu-CA thin film on functionalized Au substrate was achieved exhibiting a high electrical conductance of ~10-3 S at room temperature. We anticipate that the judicious choice of metal center and ligand proposed in the current work will provide the rational basis for the generation of various other CP thin films with high electrical conductivity and intrinsic porosity.