Studies of fluorescence and photosensitization in aqueous solution. III—Photosensitization and fluorescence

Abstract

It has been known for a long time that many photochemical reactions can be sensitized to a desired spectral region by the addition of a substance possessing an absorption band in that region. It is necessary that the quantum absorbed by the sensitizer is sufficient to bring about the chemical change in the reactant. Despite the wealth of experimental material and numerous theories offered in explanation, this class of reactions may be said to have found its first unequivocal interpretation in the classical experiment of Cario and Franck on the dissociation of hydrogen by optically excited mercury atoms. The complete study of the problem showed that a mercury atom in the 2 3 p1 state can either re-radiate its energy in the form of resonance radiation = 2537 Å. U., or by collision yield the energy to a hydrogen molecule and dissociate the latter into atoms. One consequence of this picture is that for a given concentration of mercury vapour and constant intensity of the radiation 2537 Å. U., the yield of atomic hydrogen should increase with increasing hydrogen pressure. Another consequence is that the amount of re-radiated resonance radiation should decrease with increased hydrogen pressure. Thus in the two reactions :— (1) Hg → Hg + hv2537 (2) Hg + H 2 → Hg + 2H the probability of reaction (1) taking place would be decreased and the probability of reaction (2) taking place would be increased by increased hydrogen pressure. This was found to be the case. Franck has pointed out that this situation is probably quite general, and offers a similar explanation for the sensitization of the photographic plate by means of dyes. It has been suggested that sensitized photochemical reactions occurring in solution might be treated from the same viewpoint. There are several sensitizers which afford means for a direct test of this viewpoint. These are the sensitizers which exhibit fluorescence in solution.