Photochromic atropisomer generation and conformation determination in a ruthenium bis(bipyridine) phosphonite...

Irradiation of rac-[Ru(bpy)(2)(PhP(OMe)(2))(Cl)]Cl (2) at lambda > 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 gamma-cyclodextrin (gamma-CDx) facilitated the stabilization of the new atropisomeric conformation. On irradiation the bisignate signals in the circular dichroism spectrum of the 2:gamma-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 Delta-2:gamma-CDx and Lambda-2:gamma-CDx complexes). The new atropisomer formation and subsequent thermally induced interconversion could be further detected by conventional and variable temperature (1)H 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.