Circular dichroism and absolute configuration of the tris-2,2′-biphenylylenephosphorus(V) ion

Abstract

The c.d. and electronic absorption spectra of (–)-tris-2,2′-biphenylylenephosphate(V) have been measured in order to determine the absolute stereochemical configuration of the complex ion. The spectra are analysed in terms of an exciton model in which the excitation moments of the three biphenyl chromophores of the complex ion couple to give helical charge displacements responsible for the observed optical activity. The results show that the (–)-isomer of the complex ion has the stereochemical form of a left-handed screw when viewed along the three-fold rotation axis, the M(C₃) configuration, although there are spectroscopic indications that the exciton treatment is approximate in this case, owing to π-electron delocalisation between the biphenyl chromophores of the complex ion.