Photochromic Atropisomer Generation and Conformation Determination in a Ruthenium Bis(bipyridine) Phosphonite γ-Cyclodextrin System

Irradiation of rac-[Ru(bpy)2(PhP(OMe)2)(Cl)]Cl (2) at λ > 460 nm results in the photochromic
generation of a new atropisomer and chirality inversion, via rotation of the PhP(OMe)2 moiety around the
Ru−P bond. However, since the energetic barrier to rotation is low resulting in racemization, it was found that
the formation of a supramolecular complex between 2 and γ-cyclodextrin (γ-CDx) facilitated the stabilization
of the new atropisomeric conformation. On irradiation the bisignate signals in the circular dichroism spectrum
of the 2:γ-CDx complex were converted to an entirely new and distinct circular dichroism spectrum, as a
result of a different spatial orientation of the phenyl electronic transition in the PhP(OMe)2 moiety (the active
circular dichroism spectra were found to arise from different g factor values of the Δ-2:γ-CDx and Λ-2:γ-CDx complexes). The new atropisomer formation and subsequent thermally induced interconversion could be
further detected by conventional and variable temperature 1H NMR studies. Determination of the conformation
of the new atropisomer was achieved by combining analysis of the changes in the circular dichroism spectra
by exciton coupling theory with molecular modeling and DFT calculations.