Clinton R. South

Bridged Coordination Polymer Multilayers with Tunable Properties

Coordination multilayers consisting of Pd(II) pincer-type complexes and poly(vinyl pyridine) were synthesized and characterized. Film properties were found to be dependent on and could be tuned by varying bath deposition concentrations, polymer molecular weight, and solution additives that compete with binding. Generally, smoother, thinner films were obtained with lower poly(vinyl pyridine) deposition bath concentrations. Likewise, film thickness and roughness could be reduced by employing a higher-molecular-weight poly(vinyl pyridine). Film properties could also be influenced by using acetonitrile as a solution additive, effectively driving the binding equilibrium slightly toward the free species.

Erasable Coordination Polymer Multilayers on Gold

Herein we report the use of reversible coordination chemistry to assemble polymer multilayers on gold surfaces. Such multilayers have potential applications ranging from drug delivery to electro-optics. Our system 1) provides for uniform film deposition and control of multilayer thickness, 2) allows for the integration of diverse polymer components embedded in alternating polymer bilayers, 3) can potentially be employed on a wide variety of surfaces, and 4) affords stable yet responsive multilayers that can be manipulated by chemical means using coordination chemistry. Current methods to assemble multilayers on surfaces rely predominantly on the layer-by-layer deposition of polycations and polyanions to produce polyelectrolyte multilayers (PEMs). While PEMs have been successfully applied to a variety of applications, their long-term stability as well as stability toward heat and other solution conditions, such as changes in salt concentrations or even mild changes in the pH value, is limited. To overcome these shortcomings, several groups have explored the use of covalently bound multilayers as a robust alternative to PEMs for use in organic light-emitting diodes (OLEDs), etch-resistant materials, dielectrics, and as feature replicants. Covalently bound multilayers offer additional stability toward heat, solvent changes, pH value, and other solution conditions, but the responsiveness afforded by PEMs is largely sacrificed. A significantly less studied area is the use of metal–ligand interactions to integrate components within polymer multilayer thin films with the goal of enhancing stability and adding functionality. Metal–ligand-assisted lateral film growth has been achieved through ruthenium–pyridine complexation, while iron–bipyridine complexes have been laterally integrated between poly(styrene sulfonate) and poly(ethylene imine) multilayers. Polyelectrolyte assembly has also been assisted by intermittent integration of metal cations, namely the Cu ion, allowing for the reductive formation of polymer–Cu nanocomposites. Similarly, polyoxometalate nanoclusters have been integrated between polycations within multilayered thin films. Polymer multilayers with embedded metal complexes have also been employed in biological applications. Much like covalent multilayers, metal-coordination multilayers tend to increase multilayer stability while sacrificing responsiveness. A methodology that allows for the formation of metal-coordination multilayers that are stable and that can be formed fully reversibly has not been demonstrated. Our system combines the advantages offered by covalent multilayers, traditional metal-bound multilayers, and PEMs by using strong yet reversible noncovalent metal–ligand interactions to create a new class of coordination polymer multilayers (CoPMs). The method is based on the coordination of weak bases to palladium complexes to create stable yet responsive CoPMs. We utilize Pd pincer-type complexes because of their association strength and inertness toward a variety of functionalities, including polar, nonpolar, charged, and even acidic groups. Furthermore, Pd pincer complexes are tolerant toward many reaction conditions, including organometallic reactions, yet responsive toward stronger coordinating ligands. In our study, the Pd pincer complexes are supported on poly(norbornene) derivatives, PNBE (Mw = 30000). The acetonitrile ligand coordinated to the Pd pincer complexes along the PNBE can be displaced quantitatively by pyridine (Scheme 1). Therefore, we employed commercially available poly(vinyl pyridine) (PVP, Mw = 20000) as the complementary macroligand. Upon exposure of a 4-mercaptopyridine-functionalized gold substrate to PNBE, the acetonitrile ligands on the Pd pincer complexes along the PNBE backbone are quantitatively and instantaneously displaced by surface pyridine units,