FLUORESCENCE AND ABSORPTION BANDS 747 



light. After the emission, the molecule finds itself for a second time in a 

 deformed state, and, for a second time in the fluorescence cycle, some 

 energy is converted to vibrational energy. The magnitude of AX indicates 

 that the vibrational quanta concerned must be of the order of 100 cm.-^, 

 much smaller than the quanta (1000-1400 cm.-^) postulated on page 630 to 

 account for the sequence of the visible absorption bands of chlorophyll. 



Another explanation of the red shift could be derived from the hy- 

 pothesis (c/. page 631) that the main red absorption band, Xo -^ Yo {cf. fig. 

 21.20), conceals a weak band, Xo ^ Ao, which belongs to the yellow-orange 

 band system. If this is true, and if the red fluorescence band is the pure 

 Fo -^ Xo band, the somewhat different position of its maximum is under- 

 standable. This explanation is less likely because the displacement of the 

 fluorescence band toward the red is a general phenomenon, while the over- 

 lapping of the bands Xo -^ Fo and Xo ^ Ao, if it exists at all, can be only an 

 accidental occurrence. 



The influence of the solvent on the position of the fluorescence band must 

 be attributed to the same cause as its influence on the absorption spectrum, 

 i. e., to the difference in the solvation energy of the pigment in the ground 

 state and in the excited state. Table 23.IC shows that within a homolo- 

 gous group of solvents an approximate parallelism exists between the posi- 

 tion of the fluorescence band and the refractive index of the medium. This 

 regularity already was noted and discussed in chapter 21, when we dealt 

 with the absorption spectra of chlorophyll in different media. 



A new light on the effect of solvents on the fluorescence of chlorophyll 

 was thrown by the observations of Livingston, Watson and McArdle 

 (1949), which will be described further below. These experiments indicate 

 that the solvent effect is twofold: In the first place, the presence of at 

 least a small amount of solvent molecules of a certain type (Avater, alcohols, 

 amines) appears to be needed to bring out the fluorescence (presumably, 

 by converting chlorophyll from a nonfluorescent into a fluorescent tauto- 

 meric form). Alter the fluorescence had been "activated" in this Avay, its 

 spectrum and intensity are independent of the specific nature of the "ac- 

 tivator," and determined only by the nature of the bulk solvent. In other 

 words, whether the fluorescence of chlorophyll a in benzene is "activated" 

 by methanol, or piperidine, or water, its spectrum and intensity are char- 

 acteristic of benzene as medium. Of course, when larger quantities of the 

 "activator" are added, the spectrum must sooner or later approach that 

 characteristic of the chlorophyll solution in the pure activator; but these 

 transitions have not yet been studied. 



Like the two main chlorophylls, a and b, chlorophyll c {chlorofucin) also 

 has a red fluorescence band; its axis lies at 631.5 mju in ether (Dhere and 

 Fontaine 1931), and at 635 m/x in ethanol (Wilschke 1914). 



