ENERGY MIGRATION AND THE PHOTOSYNTHETIC UNIT 1291 



possible for this state to contribute significantly to resonance energy migration. In 

 other words, the number of energy transfers that occur while the molecule is in the meta- 

 stable state is smaller than those occurring in the brief period (4 X 10 "i", or 4 X 10 ~" 

 sec.) the molecule spends in the original excited state. 



In chapter 23 (p. 795) we mentioned the— inconclusive — attempts by Calvin and 

 Dorough to identify a long-Uved, infrared fluorescence of chlorophyll in vitro; no similar 

 experiments have been made with chlorophyll in vivo. 



Terenin and Ermolaev (1952) saw evidence of energy exchange in the metastable 

 state in benzaldehyde-sensitized phosphorescence of naphthalene in a frozen mixture of 

 these two compounds. The energy content of the excited singlet state of benzaldehyde 

 is too small to lift naphthalene into its excited singlet state. The authors suggested that 

 excited benzaldehyde is first converted into the metastable triplet state, and that the 

 energy of the latter is then transferred to naphthalene. The triplet state of naphtha- 

 lene, resulting from this transfer, slowly decays by phosphorescence. 



It was suggested long ago, from the study of sensitized fluorescence in gases, that, 

 for the resonance transfer to be a "permitted" process, the total electronic spin of the 

 system must remain constant — and that this condition can be fulfilled by the excitation 

 of a "prohibited" singlet -^ triplet transition at the cost of another "prohibited" triplet 

 -> singlet transition, as well as by the more trivial replacement of one "permitted" tran- 

 sition by ajiother. There is a certain contradiction between this statement and the argu- 

 ment used above in discussing the probability of energy migration in the metastable 

 state, since in the latter the low oscillator strengths of the "prohibited" transitions were 

 supposed to be unaffected by the mutual approach of the two partners in the exchange. 

 Which of the two conclusions is correct must depend on the strength of the coupling be- 

 tween the electron spins of the two molecules at the distances over which the energy ex- 

 change occurs. 



We will now consider whether the "fast" exchange mechanism ("com- 

 munal absorption") could operate in the chloroplasts, and whether the 

 spectroscopic properties of chlorophyll in the Hving cell are consistent 

 with such an exchange. Since chlorophjdl is not distributed uniformly in 

 the granum (instead, it probably is arranged in monomolecular layers, cj. 

 chapter 37 A), resonance exchange may be considerably faster than was 

 calculated by Forster from the average concentration of chlorophyll in the 

 granum. If we attribute the total red shift of the absorption band in vivo 

 (about 10 m^ from its position in ethereal solution, and about 25 m/i from 

 its extrapolated position in vacuum), to such an exchange, we calculate 

 for the exchange process a wave number of from 250 to 625 cm.~^ and a 

 frequency of from 7.5 to 20 X lO^^ sec.-^; this would permit 300-1200 

 exchanges, enough to satisfy the requirements of the "photosynthetic 

 unit" during an excitation period as short as 4 X 10"^^ sec. (not to speak 

 of the 4 X 10-'° sec. available if Duysens' revised estimate of the fluores- 

 cence yield in vivo is correct). 



A difficulty arises, however, when we consider the sha'pe of the absorp- 

 tion band. As pointed out before, this shape is determined, in solution, 

 by the coupling of electronic excitation with intramolecular vibrations; 

 and one can expect this coupling to be destroyed if the excitation does not 

 stay with each visited molecule »10-i^ sec, to permit molecular vibra- 

 tions to get excited according to the Franck-Condon mechanism. With 

 a transfer occurring each 5-15 X lO"'* sec. this is impossible; excitation 



