Studies on π Interactions in Liquid-Phase Separations

π interactions have recently received considerable attentions due to principal factor governing molecular recognitions and self-assemble abilities, as accumulated in the database on proximate arrangements in structures of biological systems and organic functional materials. Therefore, further deep understanding and control of π interactions will greatly facilitate the development of new functional materials. Despite the importance of π interactions, they are still challenging to study because π interactions are much weaker than most other molecular interactions, such as hydrophobic interaction, hydrogen bonding, and electrostatic bonding. On the other hand, liquid chromatography (LC) is a powerful separation technique, which is able to distinguish the partition coefficients of solutes between the mobile and stationary phases, and can sensitively reflect the strength of molecular interactions. We investigated the properties of π interactions by developing new silica-monolithic capillary columns modified with carbon materials providing strong π interactions. In this focusing review, we introduce a few specific π interactions by columns modified with fullerenes and polycyclic aromatic hydrocarbons (PAHs), which showed strong π-π interactions due to spherical recognition and multiple CH-π interactions. Furthermore, π interactions can contribute to the separation of various samples, which are difficult to achieve by the available retention mechanisms. Briefly, we applied various π interactions to specific separation analyses, and we succeeded in separating halogenated compounds, H/D isotopologue pairs, and saccharides by effective π interactions. These results indicate that π interactions contribute to practical separation science, such as removal of environmental pollutants and quantitative determination of medicinal compounds.