Frontiers of solvent-free functional molecular liquids.

The chemistry of functional molecular liquids (FMLs) is a recently developed research area that describes the unique properties of the liquid physical state of the materials under solvent-free conditions. FMLs are generally composed of uncharged molecular units and exist in free-flowing as well as amorphous states in nature under ambient conditions. Attachment of flexible and bulky alkyl chains to a π-conjugated chromophore is the most common method for producing such molecular liquids. FMLs exhibit many uncommon phenomena, including inherent molecular properties which have rarely been observed for conventional organic condensed materials. In the last decade, these optoelectronically active alkylated-π liquids have attracted significant attention because of their applicability to flexible/foldable electronic devices with high stability towards photo-oxidation and thermal decomposition, as well as facile processability into devices. Besides these π-conjugated chromophore-based liquids, a few other types of fluidic materials such as liquid biomaterials and liquid metals also exhibit many attractive functions in their solvent-free state. The breakthrough of FMLs in cutting-edge research and their fundamental liquid features on the basis of intermolecular architectures are highlighted in this Feature Article. It is expected that this Feature Article will encourage the understanding of liquid properties as a consequence of intermolecular interactions and inspire the development of molecular architectures that could be useful for designing more attractive FMLs with specific functions in the future.