Functional oligomeric helices as versatile chiral molecular materials for second-order nonlinear optics & chiroptics – NLOChiraMat

Chirality plays a crucial role at the molecular scale in a variety of research fields such as biochemistry, biology, catalysis, pharmacology, etc. In order to probe chirality, well-established techniques such as electronic circular dichroism (ECD) or circularly polarized luminescence (CPL) are necessary. Notably, ECD is a widespread technique used to determine the absolute configuration of chiral molecules, their enantiomeric purity, or the secondary structure of proteins. However, this technique is unable to reliably detect the chirality of monolayers of molecules or chiral surfaces, for which an alternative chiroptical technique is desirable. On the other hand, nonlinear optical (NLO) techniques such as second harmonic generation (SHG), are particularly well-suited to surfaces and interfaces characterization and offer a much greater sensitivity than their linear counterparts.
Using aromatic oligoamide foldamers as a chiral model, we have recently disclosed hyper-Rayleigh scattering (HRS), a 2nd-order NLO technique, as a powerful complementary chiroptical method, ideally suited for the analysis of chiral molecular and supramolecular systems in solution. Aromatic oligoamide foldamers are self-organized molecular helices that own intrinsic chirality, a prerequisite for both ECD and 2nd-order NLO-activity. Their exceptional modularity allows precise tailoring of the helical scaffold and tuning of their optoelectronic properties towards chiroptical and NLO signals amplification. Oligoamide foldamers can be readily handled in organic or aqueous media, or immobilized on surfaces. Moreover, they can be evolved as molecular capsules that are enantioselective receptors of chiral biologically relevant molecules such as carbohydrates. This type of chiral molecular recognition is of considerable significance in (bio)chemistry where chiroptical sensors with acute sensitivity are required to discriminate small enantiomeric excesses. However, the lack of an easily readable output of the binding events may limit their detection and understanding.
Consequently, aromatic oligoamide foldamers are ideally suited molecular models to be used for the development of novel nonlinear chiroptical probing techniques that are HRS-CD (in solution) and SHG-CD (on surfaces/interfaces).
In view of all this, the objectives of NLOChiraMat are: i) synthesis of novel chiral molecular architectures exhibiting strong 2nd-order NLO-activity; ii) rationalization of the HRS optical activity of oligoamide foldamers by in-depth experimental & theoretical study; iii) implementation of the molecular architectures into chiral surfaces and interfaces; iv) development of a versatile and highly sensitive nonlinear chiroptical probing technique, namely HRS-CD (in solution) and SHG-CD (on surfaces). Thereby, NLOChiraMat will promote these 2nd-order nonlinear chiroptical methods as an alternative approach in probing molecular chirality and potentially overcome the limitations of more conventional linear chiroptical methods, which are mostly saturated by achiral contributions. The nonlinear chiroptical effects will be studied from the molecular to material level.
Moreover, in this project, starting from molecular nonlinear chiroptical switches (i.e. the foldamer capsules) we will endeavour to build switchable chiroptical functional surfaces and perform NLO-resolved chiral molecular dynamics studies at interfaces.
The researchers involved in the project will bring their complementary skills in the fields of organic synthesis, supramolecular chemistry, chiral spectroscopies, 2nd-order nonlinear optics, surface chemistry and theoretical chemistry. Overall, the NLOChiraMat consortium by its organization and experience shall provide valuable information about nonlinear chiroptics of molecular systems, and any progress to be made in this almost uncharted area will be of great significance to the community.