Multispectral Chiral Imaging with a Metalens.

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The title here is from an article on metalenses[1] which caught my eye.

Metalenses are planar and optically thin layers which can be manufactured using a single-step lithographic process. This contrasts with traditional lenses that are not flat and where the optical properties result from very accurately engineered curvatures, which in turn are expensive to manufacture. Metalenses can have built into them many interesting optical properties, including light polarisation and dispersion. Nanoengineering has now resulted[1] in a metalens which can simultaneously present two images of opposite helicity of an object within the same field of view.

What is the relevance to chemistry? Well, a well-known chiroptical technique is known as electronic circular dichroism (ECD). At its simplest, it probes the difference in absorption by a chiral molecule of UV and visible light with opposite circular polarisation. This difference plotted as a function of the wavelength of the light is known as the ECD response. Importantly, this response can also be calculated for either enantiomer of the chiral molecule and hence the absolute configuration can be assigned on the basis of which calculated response matches the observed spectrum. Because the difference in response to the two polarisations of the light (Δε) is actually very small, the ECD technique is intrinsically less sensitive than e.g. normal UV/Visible spectra and this requires the use of expensive instruments to record that small difference. Chiral metalenses offer an interesting future opportunity to create new forms of ECD instrument, perhaps ones that are far more sensitive. In turn, this could lower the costs of acquiring ECD functionality in the standard laboratory (see [2] for an application in teaching laboratories). Very possibly, the most expensive component would in fact then be the computational simulations required to match up with the experimental spectrum!

When metalenses were first introduced, they were only able to lens a limited range of wavelengths. In another article by the same group[3] they now announce a new generation of metalens that covers the region 470 to 670 nm. This excludes the UV regions (<300nm) or the IR regions (>1200nm). The latter covers another important chiroptical instrumental technique known as vibrational circular dichroism, or VCD. As with ECD, the VCD response of a chiral molecule can be pretty well calculated using quantum chemistry and indeed often the VCD method is the only one that can successfully be used to assign absolute molecular configurations.[4] Unfortunately, VCD instruments are even more expensive than ECD ones, again largely due to the intrinsic insensitivity and the need to accumulate data using Fourier Transform methods over many hours. Few chemistry departments have such an instrument. So I will keep an eye out for when an effective chiral metalens operating in infra-red regions is announced! The prospect of routine VCD analyses is tantalising! 

 

References

  1. M. Khorasaninejad, W.T. Chen, A.Y. Zhu, J. Oh, R.C. Devlin, D. Rousso, and F. Capasso, "Multispectral Chiral Imaging with a Metalens", Nano Letters, vol. 16, pp. 4595-4600, 2016. http://dx.doi.org/10.1021/acs.nanolett.6b01897
  2. K.K.(. Hii, H.S. Rzepa, and E.H. Smith, "Asymmetric Epoxidation: A Twinned Laboratory and Molecular Modeling Experiment for Upper-Level Organic Chemistry Students", Journal of Chemical Education, vol. 92, pp. 1385-1389, 2015. http://dx.doi.org/10.1021/ed500398e
  3. W.T. Chen, A.Y. Zhu, V. Sanjeev, M. Khorasaninejad, Z. Shi, E. Lee, and F. Capasso, "A broadband achromatic metalens for focusing and imaging in the visible", Nature Nanotechnology, 2018. http://dx.doi.org/10.1038/s41565-017-0034-6
  4. J.I. Murray, N.J. Flodén, A. Bauer, N.D. Fessner, D.L. Dunklemann, O. Bob-Egbe, H.S. Rzepa, T. Bürgi, J. Richardson, and A.C. Spivey, "Kinetic Resolution of 2-Substituted Indolines by N -Sulfonylation using an Atropisomeric 4-DMAP-N -oxide Organocatalyst", Angewandte Chemie International Edition, vol. 56, pp. 5760-5764, 2017. http://dx.doi.org/10.1002/anie.201700977

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