Pyridine Coordination Chemistry for Molecular Assemblies on Surfaces

ConspectusSince the first description of coordination complexes, many types of metal–ligand interactions have creatively been used in the chemical sciences. The rich coordination chemistry of pyridine-type ligands has contributed significantly to the incorporation of diverse metal ions into functional materials. Here we discuss molecular assemblies (MAs) formed with a variety of pyridine-type compounds and a metal containing cross-linker (e.g., PdCl2(PhCN2)). These MAs are formed using Layer-by-Layer (LbL) deposition from solution that allows for precise fitting of the assembly properties through molecular programming. The position of each component can be controlled by altering the assembly sequence, while the degree of intermolecular interactions can be varied by the level of π-conjugation and the availability of metal coordination sites. By setting the structural parameters (e.g., bond angles, number of coordination sites, geometry) of the ligand, control over MA structure was achieved, resulting in surface-confined metal–organic networks and oligomers. Unlike MAs that are constructed with organic ligands, MAs with polypyridyl complexes of ruthenium, osmium, and cobalt are active participants in their own formation and amplify the growth of the incoming molecular layer. Such a self-propagating behavior for molecular systems is rare, and the mechanism of their formation will be discussed. These exponentially growing MAs are capable of storing metal salts that can be used during the buildup of additional molecular layers. Various parameters influencing the film growth mechanism will be presented, including (i) the number of binding sites and geometry of the organic ligands, (ii) the metal and the structure of the polypyridyl complexes, (iii) the influence of the metal cross-linker (e.g., second or third row transition metals), and (iv) the deposition conditions. By systematic variation of these parameters, switching between linear and exponential growth could be demonstrated for MAs containing structurally well-defined polypyridyl complexes. The porosity of the MAs has been estimated by using electrochemically active probes. Incorporating multiple polypyridyl complexes of osmium and ruthenium into a single assembly give rise to composite materials that exhibit interesting electrochemical and electrochromic properties. These functional composites are especially attractive as they exhibit properties that neither of each metal complex possesses individually. Some of our MAs have very high coloration efficiencies, redox stability, fast responsive times and operate at voltages < 1.5 V. Moreover, their electrochemical properties are dependent on the deposition sequence of the polypyridyl complexes, resulting in MAs that possesses distinctive electron transfer pathways. Finally, some of these MAs are described in terms of their practical applications in electrochromic materials, storage-release chemistry, solar cells, and electron transport properties.