Photonenergy-controlled symmetry breaking with circularly polarized light.

Circularly polarized light (CPL) is known to be a true chiral entity capable of generating absolute molecular asymmetry. However, the degree of inducible optical activity depends on the λ of the incident CPL. Exposure of amorphous films of rac-alanine to tunable CPL led to enantiomeric excesses (ee) which not only follow the helicity but also the energy of driving electromagnetic radiation. Postirradiation analyses using enantioselective multidimensional GC revealed energy-controlled ee values of up to 4.2 %, which correlate with theoretical predictions based on newly recorded anisotropy spectra g(λ). The tunability of asymmetric photochemical induction implies that both magnitude and sign can be fully controlled by CPL. Such stereocontrol provides novel insights into the wavelength and polarization dependence of asymmetric photochemical reactions and are highly relevant for absolute asymmetric molecular synthesis and for understanding the origins of homochirality in living matter.