Probing the Nonlinear Chiroptical Activity of Multiturn Aromatic Helices Using Linearly Polarized Light

Four quinoline-based oligoamide foldamers of varying lengths, from 6 to 28 Å, were synthesized in both of their enantiomeric forms, exhibiting between 1.6 and 6 helical turns. The helically chiral structures were analyzed using hyper-Rayleigh scattering (HRS) in solution to evaluate their chiroptical nonlinear activity in second-harmonic generation, exclusively employing linear polarization of the incident light. To achieve this, we completely overhauled our homemade equipment setup. By applying a nonlinear chiroptical model developed by us, we were able to distinguish between the achiral and chiral contributions to the nonlinear response of the samples. Surprisingly, the effect of helix elongation on the chiral response, as monitored by HRS, exhibited a nonmonotonic trend, which contrasts with the behavior observed with classical linear optical techniques. A detailed investigation revealed that the number of helical turns in the foldamers governs the magnitude of the nonlinear optical (NLO) response, exhibiting a periodic evolution rather than a linear increase with length.