Vapor-induced chiroptical switching in chiral cyclometalated platinum(II) complexes with pinene functionalized C^N^N ligands

A new couple of enantiomeric chiral cyclometalated platinum(II) complexes Pt(L_a)(CC–Ph) (1a) and Pt(L_b)(CC–Ph) (1b) (L_a = (−)-4,5-pinene-6′-phenyl-2,2′-bipyridine, L_b = (+)-4,5-pinene-6′-phenyl-2,2′-bipyridine) have been designed and synthesized. These complexes were found to undergo crystal transformation upon exposure to dichloromethane vapor, resulting in slight changes of vapochromism/vapoluminescence but very distinct chiroptical switching behaviour. Single crystal X-ray diffraction studies revealed that the two polymorphs (Form-Y and Form-O) of complexes 1a/1b display dissimilar molecular arrangements as well as inter-molecular interactions, which are responsible for the switching behaviour in electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) spectra. The vapor-induced chiroptical switching behaviour in these platinum(II) complexes is unique and may lead to novel chiroptical sensing applications.