Racemic mixtures do not rotate plane polarized light, but the equilibrium concentration of the two enantiomers can shift from unity in the presence of a strongly interacting chiral species.
Paul Pfeiffer, a student of Alfred Werner and inventor of the salen ligand, reported this phenomenon.
[1] The first example of the effect is credited to Eligio Perucca,[2] who observed optical rotations in the visible part of the spectrum when crystals of sodium chlorate, which are chiral and colourless, were stained with a racemic dye.
[3] The effect is attributed to the interaction of the optically pure substance with the second coordination sphere of the racemate.
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