Influence of Positional Isomerism on the Chiroptical Properties of Functional Aromatic Oligoamide Foldamers

Abstract

A series of functionalized quinoline-based aromatic oligoamide foldamers were prepared in their two enantiomeric forms, comprising an enantiopure terminal camphanyl chiral inducer, which governed the adjacent (P-/M-) helical-handedness. Hierarchical chirality transfer was further investigated in chromophore-appended variants via a range of electronic and vibrational spectroscopic techniques, including circularly polarized luminescence, vibrational circular dichroism and fluorescence. Intense total and polarized photoluminescence (up to Φ lum = 0.39, g lum = 1.5 × 10 À 3) was observed in the visible region from these modular multicomponent architectures and a significant influence of positional isomerism was evidenced. The optimal position of a fluorophore substituent on the quinoline hexamers was determined as being position 2 over position 6, as stronger chiroptical features were systematically observed with the 2-positioned derivatives.

CPL-active water-soluble aromatic oligoamide foldamers

“…The functionalized quinoline Q Mor was introduced at position 2 of the sequence, based on previous studies related to the positional isomerism of such functionalized oligoquinoline foldamers. 17 In addition to the four water-soluble hexameric sequences, Fig. 1 represents previously reported sequence 5 , 17 composed of Q Leu monomers, i.e.…”

“…17 In addition to the four water-soluble hexameric sequences, Fig. 1 represents previously reported sequence 5 , 17 composed of Q Leu monomers, i.e. bearing isobutoxy chains that make it highly soluble in chlorinated solvents.…”

“…The slightly lower CPL intensity observed for the functionalized sequence 4 compared to the hexamers without additional substituents is consistent with previous observations in organic solvent and stems from the perturbation of the excited states related to the presence of the achiral fluorophore on the chiral oligomeric helices. 14,17 A correlation between the absorption and emission dissymmetry factors (g abs and g lum ) was established by Mori et al considering several categories of chiral small organic molecules. 23 Although the supramolecular helices reported in the present study do not fall into the categories described in this Journal Name…”

“…13 In parallel, we and others have reported on the CPL properties of related classes of quinoline oligomers in organic media. [14][15][16][17][18] Notably, functional oligoamide foldamers appended with different fluorophores were shown to display strong CPL emission at adjustable wavelengths. 14 Herein, we report on the preparation of a series of enantiomerically pure water-soluble quinoline oligomers and the evaluation of their optical and chiroptical properties in water.…”

“…13 In parallel, we and others have reported on the CPL properties of related classes of quinoline oligomers in organic media. 14–18 Notably, functional oligoamide foldamers appended with different fluorophores were shown to display strong CPL emission at adjustable wavelengths. 14…”