Hard X-ray helical dichroism of disordered molecular media

Abstract

Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life to materials sciences. Chirality-sensitive spectroscopic methods, such as circular dichroism (CD), exhibit weak signal contributions on an achiral background. Helical dichroism (HD), which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an Å-size orbital. Here, we overcome this challenge using hard X-rays with spiral Fresnel zone plates, which can induce an OAM. We present the first HD spectra of a disordered powder sample of enantiopure salts of molecular complex of [Fe(4,4'-diMebpy)3] 2+ at the iron K-edge (7.1 keV) with OAM-carrying beams. The asymmetry ratios for the HD spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the orbital angular momentum of light.Chiral molecules exist in two mirror-image non-superimposable configurations that are called enantiomers 1 . Although chemically identical, the two enantiomers can have dramatically different chemical reactions and biological functions. Therefore, the asymmetric synthesis and the detection of a single type of enantiomer of chiral compounds are topics of crucial importance, due to their numerous applications, ranging from life to materials sciences.The two most commonly used methods to distinguish enantiomers are Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD). ORD is the rotation of the plane of linear polarization of visible-ultraviolet light induced by the propagation through a chiral medium, while CD denotes the difference in absorption of left-and right-handed circularly polarized light in chiral molecular systems. As CD vanishes in achiral media, it is a unique probe of molecular chirality. Both ORD and CD are based on interactions of the investigated chiral molecules with the spin angular momentum (SAM) of light. The SAM of light can take values of +1 or -1, corresponding to the left and right circular polarization states respectively. They

Molecular Chirality and Its Monitoring by Ultrafast X-ray Pulses

“…217 A demonstration of X-ray HD has been made at the synchrotron SLS on Fe-diMe(tpy) 3 at the Fe K-edge. 218 In the X-ray, spiral Fresnel zoneplates were used to generate vortex beams with OAM L = ±1 and ±3 and the X-ray absorption spectrum (XAS) was measured both in transmission and in Total Fluorescence Yield mode (TFY). Further experimental efforts at the carbon K-edge are ongoing, making the technique a promising chiral X-ray spectroscopy.…”

“…X-rays can readily overcome this difficulty thanks their ability to be focused down to a few nanometer spot size . A demonstration of X-ray HD has been made at the synchrotron SLS on Fe-diMe­(tpy) 3 at the Fe K-edge . In the X-ray, spiral Fresnel zone plates were used to generate vortex beams with OAM L = ±1 and ±3, and the X-ray absorption spectrum (XAS) was measured both in transmission and in total fluorescence yield (TFY) modes.…”

“…Setup to measure X-ray HD: spiral Fresnel zone plates are used to generate a helical wave front. The absorption signal can then be detected in transmission (detector D1) or in total fluorescence yield (TFY, detector D2).…”

Ultrafast chirality: the road to efficient chiral measurements

“…While standard CD signal relying on interaction with the magnetic field can be improved e.g. by using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption measurements, 39 or using short-wavelength light, [31][32][33]40,41 developing ultrafast and highly enantio-sensitive approaches, which track electronic/vibronic dynamics without relying on magnetic interactions, is an important challenge. This challenge has been taken on by the electric-dipole revolution in chiral measurements.…”

“…It has the same unfavourable scaling, 29,30 especially in the IR-UV range and for small and medium-size molecules. This makes time-resolved measurements very challenging, especially in optically thin media, unless one uses X-ray light to match the pitch of the helix to the molecular size 40,41,112,113 or very intense fields 36,[114][115][116][117][118] to generate and record high harmonic spectra. Both routes can enhance the chiral dichroism from 0.01% to a few percent level.…”