794 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



In the case of chlorophyll solutions in organic solvent, the same authors 

 could find no afterglow, except when isoamylamine was used as solvent. The 

 interpretation of this result is uncertain. Perhaps, the tautomeric state, 

 T, does occur in chlorophyll, as in other dyestuffs, but the relative prob- 

 ability of its termination by direct transition into the ground state X 

 (shown by broken line in scheme 23. IB) is much larger than that of the 

 return into the fluorescent state A. Thus, the intensity of red phos- 

 phorescence is practically zero— except in certain cases (such as that of 

 solution in isoamylamine) where, for an as yet unknown reason, the rela- 

 tive probability of the two competing processes is changed in favor of 

 phosphorescence . 



It is hardly a coincidence that isoamylamine — the only medium in 

 which red chlorophyll afterglow was observed— is a compound whose 

 photoxidation is sensitized by chlorophyll with a high quantum yield. It 

 was pointed out above that the sensitization process does not compete with 

 fluorescence (since neither isoamylamine nor oxygen, in the low concen- 

 trations used, has a marked quenching effect on chlorophyll fluorescence). 

 Therefore, sensitization must be brought about by long-lived active forms 

 of chlorophyll. If the same forms were also responsible for (red) phosphores- 

 cence, then clearly this phosphorescence and sensitization would be com- 

 peting; this is obviously not the case, since red phosphorescence occurs only 

 in a medium (isoamylamine) where sensitization also takes place. This is 

 indeed a paradoxical result! One may try to find a solution by assuming 

 that the phosphorescence of chlorophyll in isoamylamine is a photochemi- 

 luminescence, with the light-emitting pigment molecules being formed in 

 the process of restoration of chlorophyll after its reversible oxidation (or 

 reduction). For example, using the sensitization mechanism (18.40) 

 one could write (with A for amine, and t for tautomer) : 



(23.17a) Chi + hf > Chi* ( > tChl) 



(23.17b) Chi* (or tChl) + A > oA + rChl 



(23.17c) rChl + M O2 > Chl*(+ I H2O) 



(23.17d) Chi* > Chi + hf 



(23.17) A + O2 ) oA( + ^H20) 



Reaction (23.17c) is an exception from the rule that back reactions in 

 photochemical sensitizations occur unthout chemiluminescence. In chapter 

 24, we will quote evidence of a chemiluminescence accompanying the back 

 reactions in photosynthesis. 



