Pyridine CoordinationChemistry for Molecular Assemblieson Surfaces.

Since the first description of coordination complexes, many typesof metal–ligand interactions have creatively been used in thechemical sciences. The rich coordination chemistry of pyridine-typeligands has contributed significantly to the incorporation of diversemetal ions into functional materials. Here we discuss molecular assemblies(MAs) formed with a variety of pyridine-type compounds and a metalcontaining cross-linker (e.g., PdCl2(PhCN2)).These MAs are formed using Layer-by-Layer (LbL) deposition from solutionthat allows for precise fitting of the assembly properties throughmolecular programming. The position of each component can be controlledby altering the assembly sequence, while the degree of intermolecularinteractions can be varied by the level of π-conjugation andthe availability of metal coordination sites. By setting the structuralparameters (e.g., bond angles, number of coordination sites, geometry)of the ligand, control over MA structure was achieved, resulting insurface-confined metal–organic networks and oligomers. UnlikeMAs that are constructed with organic ligands, MAs with polypyridylcomplexes of ruthenium, osmium, and cobalt are active participantsin their own formation and amplify the growth of the incoming molecularlayer. Such a self-propagating behavior for molecular systems is rare,and the mechanism of their formation will be discussed. These exponentiallygrowing MAs are capable of storing metal salts that can be used duringthe buildup of additional molecular layers. Various parameters influencingthe film growth mechanism will be presented, including (i) the numberof binding sites and geometry of the organic ligands, (ii) the metaland the structure of the polypyridyl complexes, (iii) the influenceof the metal cross-linker (e.g., second or third row transition metals),and (iv) the deposition conditions. By systematic variation of theseparameters, switching between linear and exponential growth couldbe demonstrated for MAs containing structurally well-defined polypyridylcomplexes. The porosity of the MAs has been estimated by using electrochemicallyactive probes. Incorporating multiple polypyridyl complexes of osmiumand ruthenium into a single assembly give rise to composite materialsthat exhibit interesting electrochemical and electrochromic properties.These functional composites are especially attractive as they exhibitproperties 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 sequenceof the polypyridyl complexes, resulting in MAs that possesses distinctiveelectron transfer pathways. Finally, some of these MAs are describedin terms of their practical applications in electrochromic materials,storage-release chemistry, solar cells, and electron transport properties. [ABSTRACT FROM AUTHOR]