Circular dichroism of oriented molecules. Magnetic dipole and electric quadrupole contribution to the ACD of chirally substituted diaminoanthraquinones

Abstract

Measurements of the anisotropy of the circular dichroism, the ACD, yield the decomposition of the tensor of circular dichroism Δϵ ij or the rotation tensor R 0 n ij into an electric dipole/magnetic dipole contribution Δϵ μ m ij or R μ m ij and an electric dipole/electric quadrupole contribution Δϵ μ Q ij or R μ Q ij . Furthermore, the transformation behaviour of Δϵ ij and R 0 n ij is analyzed in order to make the decomposition unequivocal by choosing a suitable molecule-fixed coordinate system. From the temperature dependence of the ACD of 1,4-bis-(R-1-phenylethylamino)-9,10-anthraquinone ( 1) and 1,8-bis-(R or S-1-phenylethylamino)-9,10-anthraquinone ( 2) Δϵ ii and R 0 n ii are determined and analyzed with respect to the decomposition into Δϵ μ m ii , R μ m ii and Δϵ μ Q ii , R μ Q ii ( i=1, 2, 3). For the CT transition both quantities have the same order of magnitude but different sign for 1 whereas for 2 they are of equal sign. The electric quadrupole tarnsition moments are estimated for the CT transitions.