SOLVENT EFFECT 



647 



energy, of complex formation of chlorophyll with different organic mole- 

 cules. Livingston, Watson and McArdle (1949), in a study devoted pri- 

 marily to the strong effect of small admixtures of polar solvents on the 

 fluorescence of chlorophyll solutions in hydrocarbons, noted that these ad- 

 mixtures also changed the absorption spectrum. As an example, figure 

 21.26 B shows the effect of traces of water on the absorption spectrum of 

 chlorophyll b in benzene. In the dry solution, both main peaks are lower 

 and a shoulder appears at about 670 m^t on the long-wave side of the red 



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 WAVE LENGTH, m/x 



Fig. 21.26B. Absorption spectra of chlorophyll 6 in dry and wet benzene. 



peak. Figure 21.26 A shows the effects of two other polar solvents, benzyl- 

 amine and benzyl alcohol, on the absorption spectrum of chlorophyll a in 

 dry benzene. In this case, the main absorption peaks are higher in dry 

 nonpolar solvent, and no "shoulder" appears on the long-wave end of the 

 spectrum. A remarkable fact shown by this figure is that the spectrum of 

 the activated solution (the term "activated" refers to fluorescence, which 

 is absent in pure benzene), is the same whether activation is due to amine 

 or to alcohol, although the absorption spectra of chlorophyll a in pure 

 benzylamine and in pure benzyl alcohol are quite different. This could 

 mean that polar molecules associate preferentially with a certain tautomeric 

 form of chlorophyll, and in this way stabilize it; the presence of a small 



