1828 SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



spectrophotometer. With chlorophyll b in ether, a curve very similar to 

 that in fig. 23.2 was obtained ; but with chlorophyll a the whole fluorescence 

 spectrum was displaced by about 7 mn towards the longer waves compared 

 to Zscheile and Harris' curve (X^ax. = 672 mju, as compared to 665 m/x in 

 fig. 23.2). 



(a) Quantum Yield 



In improvement of the measurements reported in Part 1 of Vol. II 

 (p. 752), Forster and Livingston (1952) re-determined the quantum yield 

 of fluorescence of the chlorophylls a and b, in different solvents and with 

 different exciting light. They used an integrating sphere, thus avoiding 

 errors that could be caused by polarized emission. Table 37C.IV sum- 

 marizes the results obtained in different solvents. In each case, the yield 

 was determined as function of chlorophyll concentration and extrapolated 

 to infinite dilution, to eliminate the effects of self-absorption (all concen- 

 trations used were too low for marked self -quenching, cf. section 4(c) below, 

 and chapter 23, p. 775). 



Table 37C.IV shows for chlorophyll a no differences in yield in the 

 several fluorescence-supporting solvents; at [Chi] —>■ 0, the values of <p 

 become practically the same (0.24 to 0.26) for methanol, ether, acetone, 

 benzene and cyclohexanol. 



The quantum yield is lower and solvent-dependent with chlorophyll 

 b (<po = 0.06 in methanol, 0.11 in ether). With pheophytin a, the yield in 

 10 ~^ AI solution is about equal to that of chlorophyll a of the same con- 

 centration ; but it fails to increase in the same proportion as that of chloro- 

 phyll when the concentration is reduced, because the re-absorption of 

 pheophytin fluorescence is relatively weak. (The same applies to meso- 

 porphyrin.) 



Table 37C.V compares the yields of fluorescence produced by excitation 

 in the blue band (X 436 mn), with those produced by excitation in the red 

 band (627.5, 644, 662 and 698 mju). Contrary to the earfier results shown 

 in table 23.IIC, (Vol. II, 1, p. 762), identical values were now found for (p, 

 whether the excitation was in the blue or in the red band. Clearly con- 

 firmed was the (as yet not adequately theoretically understood, cf. chapter 

 30) sharp drop of the fluorescence yield in the far red (cf. point at 698 m/x). 



It will be noted in table 37C.V that measurements have been made only in the 

 main blue-violet band (436 ni/x) and the main red band (628-698 m/t) of chlorophyll a; 

 and in the minor blue- violet band (436 m/x) and the main red band (644 mn) of chloro- 

 phyll b. In consideration of theoretical expectations (section 1) and experimental 

 observations of different polarizations of the several chlorophyll bands (section 4(6)), 

 more systematic measurements of the action spectrum of fluorescence of chlorophyll 

 (and bacteriochlorophyll) appear desirable. One would like to know (for example) 



