Potentially helical imidazole-containing conjugated oligomers: synthesis, optical properties, and conformation

Abstract

Three imidazole-containing conjugated oligomers (O1, O2, and O3) were synthesized by a Suzuki–Miyaura cross-coupling polymerization in combination with a post-polymerization reaction to clarify the structure–property–conformation relationship and ultimately develop a method for accessing helical secondary structures in the future. HR-ESI-MS analysis of the model reaction mixture revealed that debromination is responsible for the low molecular weight of these oligomers. The absorption and emission peak maxima gradually redshifted moving from O2 to O1 to O3, which can be rationalized by the number of intramolecular hydrogen bonds (IHBs). O2, with no IHBs, and O3, which forms multiple robust IHBs, were insensitive to the nature of the solvent, while O1, with weak IHBs, exhibited solvent-dependent absorption spectra. O3 was susceptible to both acid and base treatment, namely, the addition of HCl caused a blueshift in the absorption spectra, and the addition of NaOH resulted in a complex shift in the peak depending on the degree of deprotonation. A conformational study based on DFT calculations indicated that the formation of the IHB network potentially endows O3 with a helical conformation. The formation of multiple intramolecular hydrogen bonds (IHBs) is a reliable method for obtaining conjugated oligomers with helical secondary structures, and imidazole with both proton-accepting and proton-donating abilities plays an important role. On the basis of its optical properties in conjunction with DFT calculations, a conjugated oligomer composed of alternating imidazole and 1,4-dihydroxybenzene units was found to (1) form robust IHBs, (2) be susceptible to both acid and base treatment, and (3) potentially adopt a helical conformation.