TRANSITION TO LONG-LIVED Af'TIVE STATES 789 



})y chlorophyll, c. q., isoamylamino and allylthiourea, do not cause any 

 weakening of chlorophyll fluorescence. According to Kautsky, chloro- 

 phyll solutions in acetone, saturated with isoamylamine, fluoresce brightly; 

 chlorophyll solutions in pure isoamylamine are not only fluorescent, but 

 also show a red afterglow, lasting for about 0.01 sec. According to Franck 

 and Livingston, the fluorescence of a 10-* M chlorophyll sohition in ace- 

 tone, saturated with air and containing 0.5 mole/l. of allylthiourea, is only 

 15-20% weaker than the fluorescence of the same solution free of both oxy- 

 gen and allylthiourea — despite the fact that the quantum yield of sensi- 

 tized photoxidation is, under these conditions, of the order of unity. This 

 shows that the sensitization of the reaction between allylthiourea (and 

 similar oxidizable substrates) and oxygen cannot be attributed to the inter- 

 action of excited chlorophyll molecules in the fluorescent state with either 

 oxygen or the oxidation substrate. In other words, sensitization is brought 

 about — in this particular case — predominantly or exclusively by molecules 

 whose energy would otherwise be dissipated without fluorescence (for 

 similar observations with other sensitizers, see Shpolskij and Sheremetev 

 1936). 



To explain this phenomenon, one has to assume that the majority of 

 excited chlorophyll molecules do not fluoresce because they undergo trans- 

 formation into a still energy-rich but comparatively long-lived form. In 

 this form, they retain some of their original excitation energy as electronic 

 or chemical energy. Because of their long life, these activated molecules 

 have a good change of encountering oxygen molecules (or substrate mole- 

 cules, A), even when the latter are present in a very low concentration. 

 This explains why a high yield of sensitized autoxidation was sometimes 

 observed even at very low values of [O2] and [A]. 



Thus, experiments on the quenching of chlorophyll fluorescence by 

 oxygen and autoxidizable substrates bring us back to the problem of long- 

 lived activation state (or states) of chlorophyll, which we have discussed 

 once before when dealing with the mechanism of photochemical sensitiza- 

 tion by chlorophyll in vitro and in vivo (cf. Vol. I, chapters 18, page 483, 

 and 19, page 544). 



Weiss and Weil-Malherbe (1944) suggested that self -quenching may ex- 

 plain the nonquenching of chlorophyll fluorescence by isoamylamine, with- 

 out the assumption of long-lived active states. On page 774, we noted 

 that the high efficiency of self-quenching, reported by these observers, 

 turned out to be an error, caused by self-absorption. But even if it 

 were as high as they suggested, it could not prevent sensitization from 

 competing with fluorescence. If, in the absence of the sensitization sub- 

 strate, 90% of excited chlorophyll molecules undergo self-quenching by 

 encounters with nonexcited chlorophyll molecules (Chi* + Chi), and 10% 



