THE EVOLUTION OF THINKING ON THE MECHANISM OF SPECTRAL SENSITIZATION

The evolution of thinking on the mechanism of spectral sensitization h as been traced back to its discovery by Hermann Wilhelm Vogel in 1873, who outlined the possibilities of both energy transfer and electron t ransfer in the sensitization process. However, it took more than 50 ye ars until more serious theories were formulated, while in the meantime a sound experimental basis was developed. Although in 1938 Gurney and Mott suggested a modern concept of the electron transfer mechanism, t he energy transfer mechanism was believed to be the more important one until 1967, when the Symposium on Spectral Sensitization took place a t Bressanone. Since then convincing proofs of the electron transfer me chanism have been provided by evaluation of basic relationships betwee n the energy levels of sensitizing dyes and their photographic activit ies. This happened despite the fact that spectral sensitization proces ses are without doubt kinetically controlled. How the kinetic and ther modynamic quantities are related to one another was shown by Shapiro i n 1977, aided by the Marcus theory on electron transfer processes. How ever, only recently has time-resolved laser spectroscopy made it possi ble to test those relationships experimentally. Therefore, future deve lopment will be mainly engaged with such methods.