REVERSIBLE PHOTOCHEMICAL 

 PROPERTIES OF DYES 



GERALD OSTER 



Polytechnic Institute of Brooklyn, Brooklyn, New York 



Photoperiodic phenomena in plants appear to be confined to the 

 neighborhood of 650 m/x (red) and to be reversed by longer wave- 

 lenaths in the neidiborhood of 730 m/x (far red). Light of these 

 wavelengths is more penetrating in plant tissues than, say, blue light, 

 yet it is sufficiently energetic to affect chemical transformations that 

 require 40 kcal per mole. 



The impression I gather from the majority of papers presented at 

 this symposium is that despite the variety of phenomena manifested by 

 organisms exhibiting photoperiodism, there is a common photochemi- 

 cal reaction that triggers the reaction and this reaction is reversed by 

 the far red. Obviously two pigments are involved, yet they have eluded 

 detection by ordinary spectrophotometric techniques. It would perhaps 

 be presumptuous to propose a mechanism for the phenomena on the 

 basis of such meager information. As a chemist who has had some 

 experience with the photochemistry of dyes, the most that I dare do 

 is to acquaint you with certain systems that exhibit reversible photo- 

 chemistry. By reversible, in this context, I mean photochemical 

 changes obtained with one wavelength of light and reversed by an- 

 other wavelength of light. First of all, however, I should like to con- 

 sider the possible nature of what Dr. French calls "this pigment of the 

 imagination." 



LIGHT RECEPTORS 



In order for the pigments to be absorbing in the red region of the 

 spectrum they should be highly conjugated. If they were carotenes 

 (which, by the way, I doubt because of their photochemical inertness), 

 they would have to consist of 14 or 15 conjugated double bonds. The 



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