Optical purification of a mixture of chiral forms by dimer formation

We introduce a readily executable method for the optical purification of scalemic (non 50%-50%) mixtures of chiral molecules of opposite handedness (enantiomers). The method relies on the formation of two types of dimers, (R-R or S-S) homodimers and (R-S) heterodimers. The selectivity is linked to the difference in sign recently discovered by us to exist between certain transition-dipole matrix elements of opposite enantiomers. This sign difference results in differences in spectral propensity rules: In homodimers, transitions from the ground state can only take place to inversion symmetric excited states, while in the heterodimer the transitions are much more likely to proceed to antisymmetric excited states (although for heterodimers weak transitions to symmetric states might exist). These opposing propensity rules fully explain the observed large differences in the spectra of homodimers vs. heterodimers, which exist despite the almost identical energy levels positions. We illustrate the general concepts by computationally demonstrating the optically induced enantio-purification of scalemic mixtures of the hydropropionic C ³H⁶O³ (lactic) acid.