754 FLUORESCENCE OF PIGMENTS 7A" VITRO CHAP. 23 



mat ion of chlorophyll into a long-lived active (tautomeric, isomeric or 

 metastable electronic) state. This transformation may perhaps occur not 

 only from state A, but also directly from state B (fig. 21.9 and scheme 

 23.1). Molecules excited to the higher state B, have the choice of going 

 directly to T, or first to A and thence to T. Consequently, the total prob- 

 ability of conversion to a metastable state is higher for molecules excited 

 by blue-violet, than for those excited by red light. In the numerical ex- 

 ample indicated in scheme 23.1, this probability is 95% for molecules in 

 state B and 90% for molecules in state A — leaving for fluorescence, 5% 

 in the first case and 10% in the second. 



More measurements of the yield of fluorescence of chlorophyll under 

 different conditions are greatly needed for better understanding of the 

 photochemistry of this compound. At present, our knowledge of the 

 absolute yield of chlorophyll fluorescence in solution is limited to a single 

 estimate by Prins (1934), who found it to be of the order of 10% (in a 10"^ 

 M solution of chlorophyll a in ethanol). He did not take into account 

 self-absorption (c/. fig. 23.3), and, not knowing the exact experimental ar- 

 rangement, it is impossible to estimate its influence on the yield. 



Perrin (1929) calculated 3 X 10"^ sec. for the hfe-time of chlorophyll 

 fluorescence, from polarization measurements in four solvents of different 



viscosity. 



If we accept the Prins estimate as substantially correct, it follows that, 

 even in so-called "strongly fluorescent" chlorophyll solutions, 90% of the 

 excited pigment molecules lose their excitation energy before they have an 

 opportunity to fluoresce. The actual mean life-time of the excited state Y 

 under these conditions must be of the order of 0.1 X 8 X 10-^ = 8 X 10"^ 

 sec. (cf. equation 22.3). Not all of the excitation energy needs to be lost 

 during this period. The fate of the residual excitation energy is not defi- 

 nitely established, but in the next section we will discuss indications (men- 

 tioned in Volume I, page 483) that the chlorophyll molecules in state A, 

 which fail to emit fluorescence, are converted into a long-lived active form 

 (level T in scheme 23.1), which may represent a metastable triplet electronic 

 state, or a tautomer, or an oxidized or reduced molecular species. 



The fluorescence yield of allomerized chlorophyll a in methanol is, ac- 

 cording to Livingston (1949) about one-half, and that of allomerized chloro- 

 phyll h about twice, that of the intact pigment. The change is the same 

 when the "allomerization" of the pigments is caused by iodine, or acceler- 

 ated by catalysts such as LaCls. 



This confirms that all these reagents lead to the formation of the same 

 product — which, according to Fischer (Volume I, page 461), is chlorophyll 

 oxidized at the carbon atom C(10). 



