6: Intramolecular Charge Transfer in Aromatic Free Radicals (CIT)
In 1958-1959 a number of investigators reported on the paramagnetic resonance spectra of the mono-negative ions of aromatic molecules each containing two chemically equivalent aromatic ring systems, but separated by one or more methylene groups, (CH2)n. The resonance spectra showed that the charge (and spin) transfer between the two aromatic rings was fast on the epr time scale when n < 2, 3, but slow otherwise. I worked out a theory that accounted for these results (74).
The main points of the theory are that (a) the polyethylene chain can be replaced by a pseudo potential that describes a direct transfer between the rings. (b) There is a strong tendency for self-trapping of the charge on one ring or the other due to bond distortions and solvent polarization effects. This self-trapping greatly reduces the rate of electron transfer between the rings. The intra-molecular charge transfer occurs when thermal fluctuations bring about a transient equivalence of the two ring systems. The rate of electron transfer decreases exponentially with of the length of the connecting polyethylene chains, the decrease being roughly a factor of ten for each methylene group (74). This calculation was extended by Evenson and Karplus.
These calculations are related to some of the ideas I had for my thesis work, and also related to some unpublished calculations I made in connection with the effects of solvent on cyanine dye spectra. See the work of Platt.