Dipole strengths in chlorophyll-a and bacteriochlorophyll-a

Abstract

In principle, the dipole strength (D) of an optical transition of a chromophore in a given environment can be related to a vacuum dipole strength (d) of that transition. Such a vacuum strength refers to that of a completely unperturbed molecule and therefore to some unique theoretical value. In a given environment, characterized in practice by a refractive index n, it is possible to argue that D(n) = f(n)*d, where f(n) is a correction factor that is frequently taken to be that of Lorentz. By varying the solvent within broadly similar categories (polar or non-polar, for example), one might therefore expect to discover a unique value of d. However, it has been known for 70 years that even simple chromophores in simple solvents do not follow this prescription with a Lorentz factor. The chlorophylls also do not. More success has been achieved for the chlorophylls with an "empty-cavity" factor. The literature contains a confusing array of D values for the Qy transitions of both chlorophyll-a and bacteriochlorophyll-a. We analyze this problem and present a simplified procedure that should help to maintain consistency in comparing experimental results. In addition, we resolve some wide discrepancies among the predicted rates of Förster excitation transfer between chlorophyll-a molecules.