Shining chiral light (red helices) onto molecules arranged on a scaffold of gold nanohelices (white dots) produces a new optical effect that could be used to probe the chiral structures of molecules. [Image: Ventsislav Valev and Kylian Valev]
An international research team has reportedly shown for the first time that an optical process is able to transfer chiral properties to an organic molecule that has no overall chirality (Nat. Photonics, doi: 10.1038/s41566-024-01486-z). In doing so, the researchers have provided the first experimental demonstration of an optical phenomenon that was predicted 45 years ago, called hyper-Raman optical activity, that could be used to probe the chiral configurations of complex molecules that are important for applications such as drug discovery, synthetic biology and the design of self-assembling nanostructures.
Digging into chirality
Scientists are currently investigating new ways to control and harness the chiral nature of molecules, which could help in the design of more effective pharmaceuticals or improved materials for solar cells. While chiral properties are frequently transferred between molecules through chemical reactions, recent studies have shown that chirality can also be conferred to organic molecules from synthetic nanostructures, and vice versa.
In these new experiments, the researchers sought to induce chirality in molecules of crystal violet through the electromagnetic field produced by gold nanohelices when illuminated by circularly polarized light. These crystal violet molecules can take both left-handed and right-handed forms, but constant conversion between the two means that on average they are considered to be achiral.
To analyze the chiral properties of the molecules, the researchers exploited hyper-Raman scattering, which is sensitive to some vibrational modes that cannot be seen in the Raman spectra. Since hyper-Raman requires two photons at different frequencies to produce a single scattered photon, the researchers boosted the signal by designing a doubly resonant system: The nanohelices produce strong Raman scattering at the laser wavelength of 1064 nm, while the crystal violet exhibits a resonance at half that wavelength.
To analyze the chiral properties of the molecules, the researchers exploited hyper-Raman scattering, which is sensitive to some vibrational modes that cannot be seen in the Raman spectra.
“The gold nanohelices serve as tiny antennas that focus light onto the molecules," explains Ventsislav Valev of the University of Bath, UK, who led the study. "This process augments the hyper-Raman signal and helped us to detect it."
From Raman to hyper-Raman
In their experiments, the researchers show that the intensity of hyper-Raman scattering from the crystal violet depends on whether the polarization of the light is clockwise or counterclockwise. This divergence, called the circular intensity difference (CID), also changes sign when the handedness of the nanohelices is switched from left to right. Such behavior is the hallmark of Raman optical activity, which is already used to study chiral structures in molecules, and these results provide the first experimental demonstration of its hyper-Raman equivalent.
Further experiments confirm that the data reflect a genuine chiral phenomenon, rather than similar effects that could be induced by the ordered structure of the nanohelices. Theoretical analysis also shows that the observed CID in the hyper-Raman signals is caused by the coupling between the electromagnetic field generated by the nanohelices and the crystal violet, confirming the role of the light field in conferring chirality to the organic molecules.
"This is the very first observation of a fundamental physical mechanism," comments Valev. "There is a long way ahead until the effect can be implemented as a standard analytical tool that other scientists can adopt, but we look forward to taking that journey."