Enantiospecific Desorption Triggered by Circularly Polarized Light

Abstract

The interest in enantioseparation and enantiopurification of chiral molecules has been drastically increasing over the past decades,since these are important steps in various disciplines such as pharmaceutical industry,asymmetric catalysis,a nd chiral sensing.B ye xposing racemic samples of BINOL (1,1'-bi-2-naphthol) coated onto achiral glass substrates to circularly polarized light, we unambiguously demonstrate that by controlling the handedness of circularly polarized light, preferential desorption of enantiomers can be achieved. There are currently no mechanisms knownt hat would describe this phenomenon. Our observation together with as implified phenomenological model suggests that the process of laser desorption needs to be further developed and the contribution of quantum mechanical processes should be revisited to account for these data. Asymmetric laser desorption provides us with ac ontamination-free technique for the enantioenrichment of chiral compounds.

Understanding laser desorption with circularly polarized light

“…Our earlier study showed that the emergence of optical activity from the racemate is temporally confined to the first 10 min of desorption. 16 After that, the measured g-value does not significantly change any more. This observation is reconfirmed by Figure 6B.…”

“…The experimental setup is very similar to the one reported previously. 16 Herein, we present the key information for convenience of the reader and in addition, we highlight the specific steps that have been taken for the work presented in this contribution. The setup depicted in Figure 1 is based on an amplified Ti-sapphire laser system (CPA-2001, Clark-MXR Inc.), which provides 150-fs pulses with a central wavelength of 775 nm at a repetition rate of 1 kHz.…”

“…The decay indicates successful removal of material, as the SHG response of the sample is depending on its thickness. 16,31 For further discussion, it must be noted that for semicrystallized ( Figure 6E) and crystallized samples ( Figure 6F), the SNR suffices to reliably determine the change in anisotropy factor over time Δg (t). Extrapolating the data from Figure 4B towards vanishing irradiation intensity, the nonlinear response from a completely noncrystallized sample is expectedly low.…”

“…The measured anisotropy factor matches the sign of CD at the given wavelength. 16 For measurements with SNR < 1.3 however, the sensitivity of the measurements is not high enough to reliably measure the induced anisotropy factor. In this part, we present a quantitative investigation of the influence of the pulse intensity I P on the induced anisotropy factor Δg, on the basis of the sample structure investigations and its influence on the nonlinear response and the optical activity generated in the sample.…”

“…Herein, we stick to the second harmonic generation (SHG) circular dichroism (SHG-CD) as our method of choice to probe the induced optical activity, as it presented itself as a sensitive and reliable method in our earlier contribution and elsewhere. [16][17][18][19][20]…”