Laser-induced enantioenrichment of tartaric acid via a multiphoton absorption process

Abstract

It has been found that tartaric acid (TA) undergoes efficient enantioenrichment when it is irradiated with highly intense circularly polarized light (CPL) from an XeF (351 nm) excimer laser. The concentration of the L-form during the irradiation of racemic TA with a focused right CPL was shown to decrease dramatically with increasing absorbed energy; in contrast, the concentration of the D-form remains almost constant throughout the irradiation, demonstrating that the right CPL can induce the selective enrichment of the D-form. The maximum ee of the D-form was found to be ca. 7.5%. In sharp contrast, irradiation with the left CPL resulted in selective enrichment of the L-form through a decrease of the D-form. Thus, it was shown that high-intensity CPL irradiation can induce the enantiodifferentiation of racemic TA with reasonable enantiocontrol merely by switching the sense of the CPL. Furthermore, the fluence dependence studies revealed that this enantiodifferentiation using the CPL from an XeF laser proceeds by the selective excitation of the carboxyl chromophore in TA by two-photon absorption, followed by photodecomposition (e.g. decarboxylation).