322 



LIGHT AND LIFE 



ion to obtain a 10-15 per cent yield of the zinc porphyrin, with traces 

 of zinc chlorin in which one of the double bonds is reduced. These 

 substances can be separated by chromatography and fractional crystal- 

 lization, their properties determined independently and unequivocally, 

 and their photochemistry studied. Fig. 2 shows also the structure of 

 the zinc chlorin in which one of the pyrrole rings is in the dihydro 

 form, and Fig. 3 shows the absorption spectra of these two forms. 

 It is very easy to distinguish between the dihydroporphyrin, that is, 

 the chlorin, and the porphyrin, and the spectral difference between 

 the two substances has been used to study the kinetics of the photo- 

 chemical transformation of one to the other (21). 



Zn complex: TETRAPHENYL porpmin 



IN BENZENE 



Zn complex: TETRAPHENYL CHLORIN 

 IN BENZENE 



Fig. 3. Absorption spectra ot zinc telraphenylporphiii and zinc ictiaphenylchlorin. 



The first of these transformations (and the easiest to study) was 

 the photoinduced conversion of zinc dihydroporphyrin (chlorin) 

 into zinc porphyrin, using some hydrogen acceptor. A whole series 

 of hydrogen acceptors were used, most of them being quinones or 

 molecular oxygen (33) . It was easy to demonstrate a very clean 

 j)hotochemical conversion of dihydroporphyrin into porphyrin and 

 nothing else. Fig. 4 shows the relative rates (quantum yields) of the 

 transformations, and the relationship is clear between the rate of 

 hydrogen transfer from chlorin to quinone and the ability of the 

 quinone to hold the hydrogen, that is, the oxidation potential of 

 the quinone. The greater the oxidation potential of the quinone, 

 the faster is the transfer. Two series of experiments were done, one 



