ANALYSIS OF CHLOROPHYLL SPECTRUM 



631 



X -^ Ai, and the band X -^ Ao must be considered missing because of low 

 intensity. However, this interpretation is made untenable by the observa- 

 tion — to be discussed in chapter 23— that both in porphins and in dihydro- 

 porphins the main fluorescence bands are close to the first absorption band 

 in the red — whether this band is the weakest of all absorption bands, as in 

 porphin, or the strongest one, as in chlorophyll. This shows that in both 

 cases, the first observed absorption bands lead to the vibration-free upper 

 level Ao. If the first absorption band of porphin were X -> Ai (as tenta- 

 tively suggested above), we would expect to find the main fluorescence 



B 



«3 



^1 



Yo 





o 

 > 



a> 



m 



Fig. 21.20. Hypothetical term system of the chlorins. 

 Band X -♦ Ao is submerged by band X — »■ Yi. 



band some distance toward the red from it — in the approximate position of 

 the "invisible" X -^ Ao absorption band (Rabinowitch 1944). 



Since this is not the case, an alternative hypothesis must be considered. 

 It is represented in figure 21.20, and suggests that the hydrogenation of 

 one pyrrole nucleus creates a new low electronic excitation state Y, situated 

 a little lower than the level Ao, "inherited" from the nonhydrogenated 

 system.* Figure 21.12 makes one suspect that the second chlorin band 

 (612.5 m/i in chlorophyll), which is stronger than the corresponding band 

 in the nonhydrogenated compound, may also belong to the system X — > F 

 (as a second band of this system, X ^ Yi, . . .), and that it masks the weak 

 l)and A^ -^ Ao. A similar interpretation can be suggested for the infra- 

 red band of bacteriochlorophyll and other tetrahydroporphin dei'ivatives : 

 We can attribute the strongest infrared band of bacteriochlorophyll (fig. 



* Another possibiHty — suggostetl by the spectra of protochlorophyll and chloro- 

 phyll c — iR that the band A' -* I'o exists also before hydrogenation, but is strongly 

 enhanced by the latter. 



