62 



LIGHT AND LIFE 



sensitivity of the optical properties of the chlorophylls to wet sol- 

 vents. Fig. 20 shows their absorption curves for chlorophyll-b in 

 anhydrous benzene and wet benzene. A long-wavelength shoulder at 

 6700 A is present in anhydrous benzene, but disappears when water 

 or alcohols are added. Simultaneously, the fluorescence makes its 

 appearance. Both of these phenomena are analogous to the behavior 

 of some of the simpler heterocyclics. 



Piatt (31) interpreted these fluorescence activation experiments in 

 terms of an interchange of electronic states of n,Tr* and tt,tt* type, 

 but did not consider that the absorption spectral changes were di- 

 rectly related. This is possible, since the spectra of the porphyrins 

 are rather complex in the region of the long-wavelength absorption 

 band. Nevertheless, Becker and Kasha (2) advanced the same ex- 



-z 



UJ 



o 



_J 

 <: 

 o 



\- 



0- 



o 



4500 



5500 Q500 



WAVELENGTH, A 



Fig. 20. Visible region absorption curves of chlorophyll b at room temperature. 

 Open circles in anhydrous benzene; black circles, in wet benzene. (After Livingston. 

 Watson, and Mc.\rdle, 19). 



