114 RADIATION BIOLOGY 



(1947) for the forbidden transition in benzene requires additional param- 

 eters to describe the wave-length shifts. His theory of this behavior can 

 undoubtedly be adapted also to the porphyrin wave-length changes. 



LIGHT EMISSION 



Many reports of fluorescences and phosphorescences of porphyrins in 

 the literature have been later refuted. It is hard to prove that positive 

 results are not due to luminescent impurities, or that negative results are 

 not due to quenching impurities. In fact it is known that 10~^ M con- 

 centrations of certain polar impurities can produce marked changes in 

 the spectra and fluorescence of chlorophyll solutions (Livingston et al, 

 1949). But our concern in this chapter is with the positive results — 

 with the location and properties of the energy levels so far as they can be 

 determined. 



Calvin and Dorough (1947) found a weak phosphorescence of chloro- 

 phyll b at 8600 A, about 4000 cm-^ to the red of its 'Q^ band and 6800 

 cm-i from ^Q,„ with a lifetime of 3 X lO"- sec. A more definite result 

 was obtained by the same authors (1948) on zinc tetraphenylchlorin, 

 which had a phosphorescence at 8000 A, 3500 cm^^ to the red of its ^Q^ 

 band and 5700 cm-^ from ^Q„ with a lifetime of about 10"^ sec. The 

 triplet upper state of these phosphorescences could be either ^Qx or ^Qy. 

 The lifetime given is so short that it would surely be related to a strong 

 singlet (Fig. 2-8) , were it not for the fact that the heavy zinc atom will 

 shorten the phosphorescence lifetime, as described earlier. In fact, the 

 difference in the weight of the metal atoms may account for the differ- 

 ences in the intensities and lifetimes of the phosphorescences in these two 

 compounds. 



In the Ci8 and C22 condensed-ring hydrocarbons, the lowest triplet is 

 always at an almost constant distance of about 9000 cm^^ below a "corre- 

 sponding" singlet (Klevens and Piatt, 1949), but whether this correspond- 

 ence means that both states belong to the same orbital wave function is 

 still being debated. In porphyrins the gap from the triplet to ^Qy is 

 more like this figure than the distance to ^Qx, so that we may say that 

 ^Qy is probably the "corresponding " singlet. Provisionally we may label 

 the triplet as ^Qy. However, the separation from ^Q^ is still within the 

 range of other triplet-singlet separations expected in condensed-ring sys- 

 tems. A determination of the phosphorescent-fluorescent^ separation in 

 a porphin, a dihydroporphin, and a tetrahydroporphin would settle this 

 question of the "correspondence." If the separation is constant, a corre- 

 spondence of the triplet with ^Q^ is indicated; if it gets smaller (by 2000- 

 4000 cm-i) in the hydroporphins, then the correspondence is with ^Qy. 



The changes in fluorescence found by Livingston and coworkers (1949) 

 are more peculiar. They established that strictly dry chlorophyll a or b 

 in a dry solvent probably has no fluorescence, but that minute quantities 



