Enantioselective Magneto-Chiral Photochemistry Rediscovered

Abstract

Enantioselective magneto-chiral photochemistry (MChPh), which represents the ability of an unpolarized light beam k applied along a magnetic field B to produce an enantiomeric excess (ee), was experimentally demonstrated for the first time 25 years ago. Despite the relevance that this effect can have for the origin of molecular homochirality, no other experiment has been reported in the literature since then. With the aim of reexploring this enantioselective photochemical reactivity and quantitatively determining the ee achievable through MChPh as a function of the applied magnetic field and the laser irradiation wavelength, we report here on new magneto-chiral dichroism (MChD) studies and MChPh experiments on potassium tris-(oxalato)-chromate-(III). By irradiating a racemic mixture of enantiomers in solution (T = 5 °C) at λ = 695.5 nm (500 mW), the wavelength where MChD is maximum, under a magnetic field B = 30 T for 30 min, an ee of 0.50% has been obtained. We demonstrate that under the same experimental conditions, circularly polarized photochemistry (CPPh), the most accredited mechanism at the origin of molecular homochirality, yields a lower ee.

1

Absorption (black) and NCD (red) spectra of (Λ)-1 in DMSO solution (see legend) reported as extinction coefficients and differential extinction coefficients versus irradiation wavelength. The electronic transitions associated with the absorption and NCD spectra are indicated. Inset shows a zoom of the 690–708 nm region.

2

Absorption (a) and ΔA _MChD (b) spectra of (Λ)-1 dispersed in a KBr pellet at T = 4.0 K and B = 2.0 T. Inset of panel a shows the detail of the absorption between 650 and 750 nm. g _MChD plot in the 480–720 nm range (c) highlighting the most MChD-active electronic transitions and the associated g _MChD values.

3

Magnetic field (a) and temperature (b) dependence of the ΔA _MChD signal for (Λ)-1 dispersed in a KBr pellet. Insets show the linear dependence of ΔA _MChD over B and 1/T in the investigated ranges (0.0–2.0 T and 4.0–150 K) at λ = 696 nm.

4

(a) NCD spectra corresponding to the induced ee through MChPh as a function of the irradiation wavelength (see the legend) and (b) wavelength dependence of the Δε _NCD (λ = 552 nm) compared to the ΔA _MChD signal at T = 150 K in the same spectral region.

5

(a) Magnetic field dependence of the induced Δε _NCD (λ = 552 nm) through MChPh experiments performed at T = 5.0 and 18 °C (see legend) at magnetic fields B up to 30 T parallel and antiparallel applied with respect to the laser beam k (λ = 695.5 nm). (b) Wide range (350–800 nm) NCD spectra obtained for each experiment (see legend).