Chapter 18 



THE PHOTOCHEMISTRY OF PIGMENTS IN VITRO 



A. The Primary Photochemical Process* 



The division of this treatise into a "chemical" and a "physical" 

 part makes necessary a discussion of the photochemical reactions of 

 chlorophyll before the description of its spectrum (c/. Vol. II, Chapters 

 21 and 22) and fluorescence {cf. Vol. II, Chapters 23 and 24). A close 

 relation exists, however, between these phenomena, in particular between 

 fluorescence and the primary photochemical process, which often represent 

 two alternative ways of utilization of light energy: 



_______—> fluorescence 



Excitation or 



-* photochemical reaction 

 (direct or sensitized) 



However, it will be shown in chapter 23 (Vol. II) that the fluorescence 

 of chlorophyll in solution is not affected by certain compounds (e. g., iso- 

 amylamine or thiourea) whose autoxidation is sensitized by this pigment; 

 and that oxygen affects it only when its partial pressure exceeds 100 mm. 

 — while a much lower concentration is sufficient to obtain a full efficiency 

 of sensitized autoxidation. This demonstrates that, in the case of 

 chlorophyll, the primary process of sensitized autoxidation does not 

 compete with fluorescence. To explain this rather unusual relationship (for 

 other similar cases, see Shpolskij and Sheremetev 1936), one can assume 

 that excited chlorophyll molecules have a choice between fluorescence 

 and transformation into long-lived, active products, whose secondary 

 reactions can bring about sensitization: 



_______—). fluorescence 



Excitation or » sensitization 



' * transformation or 



into long-lived ' ' — * secondary reactions 

 active products which lead to deactivation 



The comparatively long life of these active products explains why 

 sensitized autoxidation can have a high quantum yield even at very low 

 concentrations of the autoxidation substrates and of oxygen. These 

 relations will be discussed in more detail elsewhere {cf. Chapter 19); what 



* Bibliography, page 523. 



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