746 



FLUORESCENCE OF PIGMENTS IN VITRO 



CHAP. 23 



Table 23. IC compares the most reliable figures in Table 23. IB — those of 

 Biermacher, and Zscheile and Harris — with the wave lengths of the ab- 

 sorption bands as listed in Table 21. IV. This comparison shows that the 

 fluorescence maximum remains on the long-wave side of the absorption 

 maximum in all solvents, but that the distance between the two maxima 

 sometimes falls to as little as 1 m/x. 



^^— Fluorescence absorbed by minimum layer 

 of chlorophyll solution 



Fluorescence obsorbed by V4mm. loyer 



of chlorophyll solution 



Fluorescence absorbed by 3 V* mm. layer 



of chlorophyll solution 



Fluorescence obsorbed by 10mm. layer 



of chlorophyll solution 



>////^^. Absorption (from fig. 21.1) 



620 



780 



660 700 740 



WAVE LENGTH, m/i 



Fig. 23.3. Effect of self-absorption on fluorescence spectrum of ciilorophj'll a in 

 ether (after Zscheile and Harris 1943). 



It is difficult to say whether any of the variations in the shift, AX, indi- 

 cated by Table 23. IC, are significant; in particular, whether the conspicu- 

 ous difference in the order of solvents found for the two chlorophylls, a and 

 &, is real. 



Seybold and Egle (1940) suggested that AX is abnormally large (^^15 myu) in chloro- 

 phyll solutions in Hpides such as lecithin. (This assumption was necessary for their 

 "two-phase theory" of the state of chlorophyll in vivo; cf. Vol. I, page 393). This sug- 

 gestion is not plausible in itself, and therefore cannot be accepted without confirmation 

 by reliable measurements. 



As to the reason for the "red shift" of the fluorescence bands compared 

 with the absorption bands, the explanation must lie in the loss of vibra- 

 tional quanta in the interval between excitation and re-emission, or after 

 re-emission. Quite generally, the molecule has a somewhat different nu- 

 clear configuration in the excited and in the normal state. Therefore, ac- 

 cording to the so-called Franck-Condon principle, electronic excitation is 

 accompanied by the excitation of a certain amount of vibrations. A large 

 part if not all these vibrational quanta are dissipated before re-emission of 



