Recent Advances in Various Metal-Organic Channels for Photochemistry beyond Confined Spaces

Tailor-made molecular channel architectures are a hot issue in the fields of nanotechnology, molecular sieves, ion sensors, recognition, confined space reactors, and fluidic transport systems. Carbon nanotubes have been a particular focus, though they cannot easily be synthesized to predefined structures and sizes. Rational design and construction of molecular channel structures based on coordination chemistry has been recognized as a useful approach. Metal-organic channel (MOC) structures can be generated by utilizing, at least in principle, molecular self-assembly of metal ions as angular units with designed ligands as spacers. Recent developments in molecular channel chemistry include exciting advances in photochemical applications and supramolecular material functionality, in addition to general applications such as transport, diffusion and exchange, separation, gas storage, catalysis, and simple encapsulation. In order to carry out the diverse channel functionalities, a large number of studies have been conducted on the synthesis of robust and stable 3D coordination polymers, which show permanent porosity without any guest molecules within the channels, in that the robustness of the channel structures after removal of the solvate/guest molecules is of interest because the structural integrity of the extended structures must be maintained during the reactions. These compounds can be regarded as analogous to zeolites. This Account highlights advances in the construction, from metal cations and multidentate pyridyl ligands, of various MOCs and useful molecular materials as photoreaction platforms. We begin by discussing the fact that detailed proof-of-concept construction of various systematic MOC structures has been introduced mainly in terms of the metal ions as angular components and the pyridyl ligands as spacers. This approach leads to structural complexity of assembled MOC motifs such as metallamacrocycles, helical and cylindrical coordination polymers, vertical arrays of 1D coordination polymers, interweaving and eclipsed stacking of 2D coordination polymers, and typical 3D coordination polymers. Notwithstanding the diversity of their skeletal structures, confined spaces of the channels are suitable for the study of photochemical performances including radical trapping, photocyclopropanation, dye inclusion and energy transfer, and guest-to-host structural transmission. The key aspect of their utility is not only the preparation of photoresistant MOCs but also channel tuning for inclusion of photoactive guests. Guest molecules, which are compatible with the size, shape, and polarity of the channels, can be incorporated into the crystals, instead of simple organic solvents, thus giving rise to host-guest complexes in the solid state. Such MOC studies could facilitate the development of chemical sensors, new photocatalytic systems, and useful molecular photochemical reactors.