YIELD OF CHLOROPHYLL FLUORESCENCE 



753 



(c/. chapt. 30). Livingston mentioned that a similar decline of fluorescence 

 in the long-wave region was noted by Solomin with other dyestuffs. vSuch 

 a drop would be understandable if the long-wave wing of the red absorption 

 band were covering another band, leading to a different excited electronic 

 state. If, however, the long-wave wing is due to transitions originating in 

 vibrational levels of the normal state and leading to the same excited state 

 as that reached in the peak of the band (and this is what one would think 

 offhand), then the lower quantum yield of fluorescence and sensitization is 

 peculiar and awaits interpretation. 



Transition to 

 fluorescent stote 



Fluorescence 

 10% 



Scheme 23.1. 



The metastable state of chlorophyll {T may mean "triplet" 

 or'' tautomeric"). 



In contrast to the observations in the far red the confirmation by Liv- 

 ingston of Prins's observation— that the fluorescence yield of chlorophyll 

 in solution is considerably smaller when fluorescence is excited by blue- 

 violet than when it is excited by red light— has no parallel in the wave- 

 length dependence of the quantum yield of photosynthesis in live cells 

 {cf. chapter 30,), and (what seems to be even more remarkable) in the 

 quantum yield of chlorophyll-sensitized autoxidations in the same 

 medium (alcohol) (cf. chapter 18, page 513). This discrepancy con- 

 firms the supposition that photochemical sensitization by chlorophyll does 

 not compete directly with fluorescence, but is brought about by transfor- 



