Synthetic Molecular Sequences in Materials Science

Molecular sequences are composed of a variety of monomeric units that are linked together in a predetermined order. Their examples are ubiquitous in nature, demonstrating that it is the molecular design strategy of nature to create structures of great diversity effectively. In contrast, that strategy has not been popular for a long time in the design of synthetic compounds, owing to the difficulty in the actual synthesis of the designed structures. On the basis of the recent development of several sophisticated synthetic methodologies, however, a new trend has started to appear, where the design and preparation of novel synthetic materials have been attempted by focusing on the particular effects or roles of the sequenced structures at the molecular/supramolecular levels. Considering the on-going emergence of a new area of science, this monograph is written for readers to have a good understanding of the state-of-the-art structural accessibility, characteristic features, and future potentials of synthetic molecular sequences.

For the construction of precisely determined molecular sequences linked with static bonds such as covalent bonds and strong metal coordination, it would be generally important to control the order of the bond-forming reactions between the monomeric components. This requisite is typically fulfilled through the sequential addition of monomers to the reaction mixture, as exemplified in the solid-phase synthesis of biological and non-biological sequences. Another popular approach to achieve the fine control on the sequence is to use a template to predetermine the positions of monomers before their linkages, as in the case of the replication of DNA/RNA in nature. Moreover, polymerization of the corresponding pre-prepared short sequence has been proven to be effective for the synthesis of periodic sequences.

Construction of a molecular sequence linked with dynamic bonds is easier than that linked with static bonds if the targeted sequences are thermodynamically most stable, while the opposite is true when the construction needs to be controlled kinetically. One of the basic strategies to kinetically control the formation of sequences linked with dynamic bonds is the switching of the nature of these bonds, i.e., from being dynamic to static and vice versa, in a chemical or physical manner. Another approach is based on the simultaneous use of dynamic and static bonds for the sequence formation, which can lower the difficulty in the synthesis of target sequences by decreasing the number of dynamic bonds necessary in a single sequence. Considering the advantages of dynamic bonds to make the sequences stimuli-responsive, generally applicable directions for the flexible design of dynamically linked sequences that are comparable to the solid-phase synthesis concept for the statically linked sequences are awaited to be developed.

Although it is basically regarded to be in a preliminary stage, outcomes from synthetic molecular sequences have already been observed in both polymeric and discrete systems. Examples include improvement of the performance of charge-carrier transport materials by optimizing the sequences at the molecular and supramolecular levels, fabrication of porous materials with superior properties by sophisticatedly controlling the domain segregation in the assemblies of block sequences, and switching of physical properties of polymers by taking advantage of their dynamic nature.

Because materials science based on synthetic molecular sequences is still in its infancy of development, there is no satisfactory synthetic method to perfectly control every structural parameter of a molecular sequence, where one or more parameters have to be sacrificed to control other parameters. Valuable future innovations in the design of molecular sequences could be the development of methods to effectively exclude the wrong sequences from the target and efficient protocols of mutation–evaluation cycles to maximize their structural advantages. Examples of the attractive targets of sequences that are expected to bring specific outcomes include not only those working as tandem catalysts or molecular robotics, but also those having an unprecedented sequence pattern such as that of the Fibonacci sequence.