REVERSIBLE PHOTOCHEMICAL PROPERTIES OF DYES 11 



autoxidation of some constituents of the organism. We have been able 

 to dupHcate this reaction with synthetic systems by using p-phenylene- 

 diamine as the substrate. This substance (or hs tolyl analog) is a 

 convenient substrate since it becomes highly colored when oxidized, 

 and hence the course of the oxidation can be followed colorimetrically. 

 Kinetic analysis of our data (Schrader, 1955) shows that the dye in 

 the triplet state reacts with oxygen to form a photoperoxide, which in 

 turn attacks the substrate to give oxidized substrate and regenerated 

 dye. Thus the dye can be used over and over again, and oxidized sub- 

 strate is continually being formed as long as the system is illuminated 

 and oxygen is being continually supplied. 



We have further established that there is a one-to-one correspond- 

 ence between the photodynamic action for living organisms and the 

 dye-sensitized photauxidation of p-phenylenediamine (Oster and 

 Kimball, to be pubhshed). Retarders (e.g., certain reducing agents) 

 for one process are retarders for the other. Still further, we have shown 

 that the dyes which are effective for these two processes are also the 

 dyes which are capable of being photoreduced in the presence of suit- 

 able electron donors. The connection between the ability of certain 

 dyes to be photoreduced and their ability to be sensitizers for photo- 

 oxidation lies in the fact that these particular dyes are readily con- 

 verted to the triplet excited state. 



I doubt that dye-sensitized photauxidation is a factor in photo- 

 periodism simply because of the presence in plants of reducing agents 

 which would prevent such a process. For example, chlorophyll in vitro 

 is a powerful sensitizer for the photodynamic action. Obviously, 

 chlorophyll does not function in this manner in the natural state; other- 

 wise any plant would be destroyed in a few minutes with exposure to 

 full sunhght. The in vitro sensitizing action of chlorophyll can be 

 stopped by adding a small amount of ascorbic acid or glutathione to 

 the system. The presence of reducing agents together with the fact that 

 chlorophyll is bound in the plant precludes any appreciable photo- 

 dynamic action in vivo. Photooxidation in recovery of leuco dyes is 

 another matter, of course. Here oxidation of substrate takes place with 

 light but, unlike photauxidation, no oxygen is involved and complete 

 reversibility is possible. 



