Molecular assembly in ordered mesoporosity: A new class of highly functional nanoscale materials

Assembly of organized molecular structures in ordered mesoporosity has shown to be a very powerful approach to synthesize novel functional nanoscale materials. This approach allows rational design of a wide range of material properties, such as pore dimension, surface chemistry, stereochemistry, spatial distribution of functionality, etc. This paper discusses molecular conformations and assembly mechanisms and illustrates the principles involved in fabricating sophisticated molecular structures in the pore channels. First, an introduction highlights the important progress in synthesizing and understanding ordered mesoporous materials and in incorporating functional molecules and groups in these mesoporous materials. Next, the molecular conformations of simple alkyl chains are discussed as related to chain lengths and pore geometry. The pore size, as well as the uniformity of the porosity, can affect how the long-chain molecules are assembled. Homogeneous molecular layers can be formed in 10 nm pores. Smaller pore sizes cause pore clogging and chain entanglement. Larger pore sizes increase the degree of pore irregularity and produce disordered multilayer coating. Molecules with intermediate chain lengths form better molecular layer structures. Detailed mechanisms of monolayer formation are studied, and a stepwise growth model is proposed. The step-growth mechanism is due to the surface roughness of the pore channels and is believed to be universal in forming "monolayers" involving surfaces that are not atomically smooth. Finally, the development of multifunctional nanoporous materials is described. Examples include multifunctionalized catalysts, hierarchical size-and-shape selective nanoporous materials with tunable micropatterns and microcavities. The assembly of multifunctional groups and structures will allow us to develop sophisticated nanoscale materials with "enzyme mimic" and "biomimic" properties.