Development of Supramolecular Polymer Using Ferrocene as a Modulator

Abstract

Supramolecular polymers (SPs) are an important class of soft materials where the small molecular monomers are associated by some directional noncovalent forces leading to the high degree of internal order in the resulting one-dimensional array of molecules, aggregates. In order to construct efficient SP, it is essential to design suitable monomers that can support high directionality and strength of the noncovalent interactions between the monomers. Herein, we have designed a molecule 1, 1,n’-disubstituted ferrocene–azobenzene–long alkyl chains [Fc(CONH-Azo-TDP)2] with a high probability of forming SP. The flexibility of ferrocene with the “molecular ball-bearing’’ property may allow the two azobenzene arms to rotate freely to find the correct conformation for assembly. To study the role of ferrocene, we also designed three reference molecules; molecule 2 [Fc(CONH-Azo-TDP)] with one azobenzene arm, molecule 3 [Ad(CONH-Azo-TDP)¬] with a flexible aliphatic linker and molecule 4 [Tp(CONH-Azo-TDP)2] containing a rigid benzenoid linker. The idea behind the reference compounds was that molecule 1 is likely to support higher enthalpy gain with lower entropy loss due to its flexibility and restricted rotation, whereas molecule 2 will neither support higher enthalpy gain nor low entropy loss. Molecule 3 might support enough enthalpy gain but not lower entropy loss due to higher molecular flexibility, whereas molecule 4, with a large conjugated π surface, may support a much higher enthalpy gain compared to 3 but not the entropy loss. The gelation studies show a super gel for 1, whereas 2 and 3 did not form gels or ordered SP. But, molecule 4 also forms an SP gel. However, 1 shows more efficient gelation than 4, suggesting the large flat π -surface could not beat the non-π-stackable ferrocene, which was surprising and might be attributed to the conformational flexibility of ferrocene in 1, giving rise to better orientation for assembly. NMR shows the presence of stronger H-bonding of the amide of 1 than other reference compounds and UV-Vis showed higher π-π stacking of azobenzene in 1 than 4, likely due to the rotational flexibility of ferrocene. Further, we used UV-Vis, SAXS, and TEM to characterize the resulting supramolecular assemblies. The study shows that ferrocene can act as a modulator for SP formation when the monomer is designed properly. Moreover, due to the presence of two responsive units, redox-responsive ferrocene, and photo-responsive azobenzene, this type of coupled monomers can be imporartnt for the construction of potential soft material with multiple stimuli-responsiveness.