750 



FLUORESCENCE OF PIGMENTS /iV VITRO 



CHAP. 23 



Table 23.11 

 A. Fluorescence Bands of Porphin in Dioxane 



B. Vibrational Quanta in Fluorescence Spectrum of Chlorophylls a and b 



Value 



V, cm. 



Chlorophyll a 



665 

 15,000 



736 

 13,580 



801 

 12,480 



1420 



1100 



Chlorophyll 6 



646 

 15,500 



713 

 14,020 



789 

 12,670 



1480 



1350 



The sequence of several bands observed in the fluorescence spectra of 

 chlorophyll and its derivatives (as well as in those of the porphyrins) prob- 

 ably corresponds to transitions from the excited electronic state ^o (or Fo) 

 to different vibrational levels of the ground state, Xo, Xi^ X2 . . . As an 

 example, we consider first the fluorescence spectrum of porphin, as ob- 

 served by Stern and Molvig (1936). It consists of the five bands listed 

 in Table 23.11. The band at 616.5 mju is the strongest. Comparison 

 with the absorption spectrum of the same compound (compare Table 

 21. V) shows that all five fluorescence bands probably are related to the one 

 weak band in the absorption spectrum at 613 m/x. According to the term 

 system in figure 21.9, this is the 0-^0 band of the A -^ X system. The 

 band designated as " — 1" in Table 23.11 must then be an "anti-Stokes 

 band," originating in the next-to-lowest vibrational level of the excited 

 electronic state A. According to Table 21. V, the level Ai is situated 1520 

 cm.~^ on the short-wave side of the 0-^0 band; while the fluorescence 

 band " — 1" is displaced only 700 cm. ~^ in the same direction. This differ- 

 ence can be interpreted by assuming that the state in which the " — 1" band 

 originates belongs to a vibrational sequence not excited by light absorption. 

 Or, one can postulate that the " — 1" band does originate in the vibrational 

 level A, but terminates in a vibrational level of the ground state situated 

 about 800 cm. ~^ above the Xo state. 



Bands 1, 2 and 3 probably all originate in the non vibrating excited 

 state Ao, and terminate in the successive levels, Xi, X2 and X3, of the ground 

 state. According to this interpretation, the vibrational quanta of the 

 ground state of porphin, which are excited by fluorescence, are compara- 

 tively small and rapidly declining in size (700-800, 550 and 350 cm.^). In 

 chlorophyll, on the other hand, the two sets of vibrational quanta, derived 

 from the absorption and the fluorescence spectrum, respectively, are of the 

 same order of magnitude (1100-1500 cm. ~^). This is shown by a compari- 

 son of the Av values in Table 21. VI with those in the Table 23.IIB. (For 



