10 PHOTOCHEMICAL PRINCIPLES 



cally ceases because of the concentration quenching of the triplet state, 

 which I mentioned earUer. One might wonder how any photochemical 

 reaction can take place with chlorophyll in the plant since the chloro- 

 phyll concentration in chloroplasts may be as high as 0.2M (see for 

 example, Hill and Whittingham, 1955, p. 27). The answer to this 

 apparent anomaly, I feel, lies in the fact that in nature the chlorophyll 

 molecules are bound to the substrate. I have already indicated some 

 changes in properties that dyes undergo when bound to high poly- 

 mers. Most striking are the differences between the photochemical 

 properties of free and of bound dyes. In the bound state the quantum 

 yield of photoreduction increases with increasing dye concentration 

 (Bellin and Oster, 1957), in marked contrast to the behavior of the 

 free dye. Even certain dyes which do not undergo photoreduction in 

 the free state will do so in the bound state (Oster and Bellin, 1957). 

 Another characteristic of bound dyes, at least in the limited number of 

 cases that we have studied, is that certain impurities (e.g., nitro- 

 benzene) in trace amounts inhibit rather than merely retard the photo- 

 reduction. The induction period is proportional to the concentration 

 of the inhibitor. One inhibitor molecule can affect many dye molecules 

 in analogy with the concept of the photosynthetic unit which has been 

 postulated for plants (see, for example. Hill and Whittingham, 1955, 

 pp. 60-62). Our kinetic analysis indicates that an excited bound dye 

 exchanges its energy with a neighboring ground state bound dye to 

 give the chemically reactive triplet state species. The inhibitor mole- 

 cules react with some intermediate reduction species of the dye and are 

 thereby destroyed. It is amusing to note that although triphenylmeth- 

 ane dyes are more difficult to reduce in the dark when bound, the op- 

 posite is true for photoreduction of these dyes (Oster and Bellin, 

 1957). 



PHOTOOXIDATION 



It has been known since 1900 that organisms containing certain 

 dyes are killed if exposed to visible light (Raab, 1900; for reviews, see 

 Blum, 1941, and Clare, 1956). This phenomenon, referred to as 

 "photodynamic action," requires oxygen and is clearly a dye-sensitized 



