Discrimination between 3ππ * and 3nπ * states in organic molecules by circular polarization of phosphorescence

Abstract

The degree of circular polarization g in electronic transitions of chiral molecules is a measure of the ratio of the magnetic and electric dipole transition moments. From theory an upper bound for | g| as a function of dipole strength D can be found. This offers the possibility to distinguish 3ππ * from 3nπ * states in organic molecules. For molecules of approximate C 2v symmetry it can be shown that large absolute values of g lum (order 10 -1) can arise only in the case of 3ππ * phosphorescence, whereas emission from 3nπ * states yields low values of g lum (order 10 -3). This conclusion is substantiated with experimental data on three examples: α-santonin, (1S)-(-)-1-bromo-α-fenchocamphoronequinone and (+)-thicamphor.