Induced photoelectron circular dichroism onto an achiral chromophore

Abstract

An achiral chromophore can acquire a chiral spectroscopic signature when interacting with a chiral environment. This so-called induced chirality is documented in electronic or vibrational circular dichroism, which arises from the coupling between electric and magnetic transition dipoles. Here, we demonstrate that a chiroptical response is also induced within the electric dipole approximation by observing the asymmetric scattering of a photoelectron ejected from an achiral chromophore in interaction with a chiral host. In a phenol-methyloxirane complex, removing an electron from an achiral aromatic π orbital localised on the phenol moiety results in an intense and opposite photoelectron circular dichroism (PECD) for the two enantiomeric complexes with (R) and (S) methyloxirane, evidencing the long-range effect (~5 Å) of the scattering chiral potential. This induced chirality has important structural and analytical implications, discussed here in the context of growing interest in laser-based PECD, for in situ, real time enantiomer determination.

Fig. 1. Structure of the phenol (Phe) (S) methyloxirane (MOx) complex.

a Schematic view of the complex. The stereogenic centre is indicated by an asterisk. b Most stable structure of the Phe-MOx complex calculated at the B3LYP-D3BJ/6-311 + + G(d,p) level of theory. The hydrogen bond is indicated by a dotted line. c Electronic spectrum of the Phe-MOx complex recorded by using the resonance-enhanced multi photon ionisation technique. The bands marked with asterisk (*) were probed in IR-UV experiments. d Vibrational spectrum of isolated phenol and of the Phe-MOx complex recorded with the IR-UV double resonance technique by setting the probe on the transition origin of bare phenol and of the Phe:MOx complex, respectively. Source data are provided as a Source data file.

Fig. 2. Photoelectron spectra.

Comparison between the photoelectron spectrum of bare phenol (red dashed line), MOx (blue dotted line) and the Phe:MOx complex (black line), recorded at a photon energy of 10.4 eV, slightly above the adiabatic ionisation energy of MOx (10.24 eV). Source data are provided as a Source data file.

Fig. 3. Localisation of the frontier orbitals Frontiers orbitals starting from the highest occupied molecular orbital (HOMO) calculated for the Phe:MOx complex (a) as well as bare phenol (b) and bare methyloxirane (c) at the MP2/6-31 + + G(d,p) level.

The electronic density was plotted with an isodensity value of 0.07. Calculated vertical ionisation energies (IE) are obtained with the outer valence Green’s function (OVGF) method and cc-pVTZ basis set. Source data are provided as a Source data file.

Fig. 4. PES and dichroic parameter at 10.4 eV and 8.5 eV for the complex of phenol with the two enantiomers of methyloxirane.

a The photoelectron spectra (PES) (solid line) and photoelectron circular dichroism (PECD) for the complex of phenol with (S) MOx (red circles) and (R) MOx (blue triangles) are recorded in coincidence with the ions at m/z 152 at a photon energy of 10.4 eV. The zones corresponding to the HOMO and HOMO−1 of the complex are highlighted in orange and pink, respectively. The insert shows raw (left part) and Abel-inverted (right part) difference between left (LCP) and right (RCP) circularly polarised light (LCP–RCP) images, corresponding respectively to the 2D projection and an equatorial slice of the 3D velocity distribution filtered at the mass of the Phe:(R)MOx complex, obtained with the velocity map imaging (VMI) spectrometer. The direction of the light is vertical, from the bottom to the top of the image and is indicated by an arrow. b Same for bare phenol, recorded in coincidence with the ions at m/z 94. The zones corresponding to the HOMO and HOMO−1 of bare phenol are highlighted in green and grey, respectively. As expected for an isolated achiral chromophore, the value of the PECD is zero. c Same for the complex at a photon energy of 8.5 eV. Neither bare phenol nor MOx can be ionised at this energy value. d Mean values of the PECD weighted by the PES intensity of the two highest occupied orbitals HOMO and HOMO−1 of the Phe:(R) MOx complex as a function of the electron kinetic energy recorded at the photon energies of 8.5, 9.0, 9.7 and 10.4 eV. The shown values are half the difference between those obtained for the Phe:(R)MOx and Phe:(S)MOx complexes. The colour code is the same as that used in a–c, namely, green full circles and grey empty circles for the HOMO and HOMO-1 of phenol, respectively, and orange full squares and pink empty squares for the HOMO and HOMO-1 of the complex, respectively. Error bars correspond to statistical (standard deviation) errors. Source data are provided as a Source data file.