Ferrocene: an Exciting Building Block for Designing Supramolecular Assemblies

Abstract

Ferrocene (Fc) possesses interesting molecular and electronic attributes, like shape, size, hydrophobicity, rotational flexibility, and redox responsiveness, rendering it a promising building block in developing various supramolecular architectures, including supramolecular polymers (SPs).1 SPs are one-dimensional molecular assemblies with a high degree of internal order formed through non-covalent interactions and are gaining prominence as functional soft materials. In SP formation, the design principle is pivotal, ensuring precise directionality and strength for the non-covalent interactions between monomers. Herein, molecules with Fc as the core component are used to leverage Fc’s distinctive “molecular ball-bearing” properties for optimal molecular conformation and robust intermolecular interaction through enhanced enthalpy gain.2 Molecule 1a-[Fc(CONH-Azo-TDP)2] featuring 1,n'-disubstituted ferrocene, with azobenzene and long alkyl chains was designed and synthesized, with the higher likelihood of forming SPs. We have also designed three reference molecules, 1b-[Fc(CONHAzo-TDP)], 1c-[Ad(CONH-Azo-TDP)2], and 1d-[Bz(CONH-Azo-TDP)2], to investigate and compare Fc’s role in SP formation. 1a formed a super gel, while 1b and 1c did not show gelation or ordered SPs. Surprisingly, despite Fc’s non-π-stackable nature, 1a displayed efficient gelation than 1d, having a larger π surface. 1a showed stronger π-π stacking and intermolecular H-bonding than the reference compounds investigated by UV-Vis spectroscopy and NMR. This hinted at Fc’s conformational flexibility for assisting in efficient SP formation. Additionally, azobenzene was exploited for creating photo-responsive soft material. Based on these findings, possibility of syn vs. anti-conformation toward forming SP is under investigation by introduction of amide bonds at strategic positions.