Hyper-Rayleigh Scattering as a New Chiroptical Method: Uncovering the Nonlinear Optical Activity of Aromatic Oligoamide Foldamers

Abstract

Molecular helices based on self-organized aromatic oligoamide foldamers have been designed and prepared in their two enantiomeric forms in order to probe their second-order nonlinear chiroptical properties in solution. The quinoline oligoamides were rationally functionalized by electron-donating and electron-withdrawing groups to afford a gradual increase of the electronic polarization of the helical architectures. Their hyper-Rayleigh scattering (HRS) responses in solution were accordingly assessed, using either linearly polarized or circularly polarized incident light. Both methods allowed to observe nonlinear optical activity that was quantified, for the first time for molecular systems, through circular differential scattering intensity ratios. The hyper-Rayleigh optical activity (HROA) study reveals important charge transfer differences within the aromatic oligomers, depending on the helix handedness and on the extent of electronic polarization induced by the appended substituents. The origin of the enantiomeric difference is discussed considering both achiral and chiral contributions. Overall, we disclose the capabilities of HRS as a complementary chiroptical method, ideally suited for the analysis of chiral molecular and supramolecular systems in solution. Implementation of auxiliary polarization/detection techniques on our measurement setup is discussed to further increase the chiral discrimination sensitivity and extend the study to a wide rande of small molecular systems.

Single Nanoparticle Chiroptics in a Liquid: Optical Activity in Hyper-Rayleigh Scattering from Au Helicoids

“… 20 This effect had been predicted 40 years ago, 21 and its general significance was recently confirmed by a reported occurrence in molecules. 22 It should be noted that the molecular concentration reported was ≈10 9 times higher than that of our gold helicoids. The effect is illustrated in Figure 1 (a).…”

“…For comparison, similar HRS chiroptical values have been recently reported from molecules, whose concentration is ≈10 9 times higher. 22 These results indicate that the helicoids have an extremely large nonlinear chiroptical response, which could be even larger at wavelengths beyond those investigated here. As such, the helicoids could be an excellent model system, comparable to Pendry’s “chiral spheres”, 24 to precisely evidence the roles of chirality parameters, such as geometric chirality and optical chirality, 25 , 26 in the nonlinear optical regime.…”

Quantum electrodynamics in modern optics and photonics: tutorial

“…[162][163][164]. Another phenomenon first predicted by QED is hyper-Rayleigh (and Raman) optical activity [165], which has only recently been observed experimentally [86,166].…”

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

“…[20] Monomers based on an 8-amino-2-quinolinecarboxylic acid backbone and functionalized at position 4 were employed. [11,15] The quinoline monomer bearing the bis(9,9-dimethyl-9H-fluoren-2-yl)amine (DFA) substituent is hereafter named Q DFA while the quinoline analogue equipped with 1-ethynyl-3,5-bis (trifluoro-methyl)benzene is named Q CF3 . In order to evaluate the effect of the number of functional groups and their position on the helical scaffold, while preventing mutual steric interactions, positions 2 and 6 of the hexameric sequence were preferred for functionalization.…”

“…In particular their capacity for hole-or electron-transport was previously investigated, [13] as well as their fluorescence switching [14] and second-order nonlinear optical activity. [15] The strategy to finely tune the chiroptical properties of functionalized aromatic foldamers utilizes oligomers of 8amino-2-quinolinecarboxylic acid that self-organize into highly stable canonical helical conformations in solution. Moreover, the introduction of a camphanyl chiral inducer at the Nterminus of the oligomers allows absolute control of the helical handedness.…”