ENERGY TRANSFER IN PHOTOCHEMICAL SYSTEMS 63 



observed by Arnold and Sherwood (1957) for quick-dried chloro- 

 plasts. 



While it is not possible to compare quantitatively the ESR results 

 with the luminescence results at this time, there are a number of quali- 

 tative similarities that are significant. These are summarized in Table L 



1. Both phenomena are excited by the same bands of wavelengths 

 and both are due to absorption by chlorophyll. 



2. The 25 °C decay times for wet chloroplasts are of the same order 

 of magnitude for both phenomena. Inasmuch as the ESR spectrometer 

 had a time constant of 2 sec, the ESR decay corresponding to the 

 shorter luminescence decay times could not have been detected. 



3. At — 140°C, the ESR decay times are of the order of hours and 

 no luminescence could be detected (a luminescence with a decay time 

 of the order of hours would be undetectable with the apparatus used in 

 the present studies). 



4. The decay time of the ESR at 25 °C for dried chloroplasts is of 

 the order of hours, and under similar conditions the chloroplasts did 

 not luminesce. 



5. At 60°C, the ESR of the dried chloroplasts had a decay time of 

 the order of seconds. At this same temperature, we have observed a 

 peak in the thermoluminescence of the dried chloroplasts. 



The above similarities strongly suggest that the 7000-9000 A light 

 emission of chloroplasts is at least in part the result of the decay of the 

 unpaired spins detected by the ESR experiments. A quantitative com- 

 parison of the quantum yields, action spectra, and kinetic constants of 

 these two phenomena is now being carried out. This should lead to a 

 more definitive assessment of the relationships between them. 



There are four possible mechanisms for the production of either 

 ESR or delayed light emission in systems of the type we are concerned 

 with here: (1) the production of radicals by the direct photodissocia- 

 tion of a single bond, followed by their recombination in the dark, (2) 

 the excitation and decay of a triplet state, (3) the reversible photo- 

 sensitization of chemical or enzymatic processes leading to the produc- 

 tion of free radicals, and (4) production of trapped electrons in a 

 quasi-ordered lattice. 



Mechanism (1) is incompatible with the following considerations. 

 No known stable naturally occurring chemical bond can be dissociated 



