1290 THE PIGMENT FACTOR CHAP. 32 



"chlorophyll c?" to divert quanta from the "reduction centers" would be 

 enhanced if the overlap integral for the transfer chlorophyll a -> "chloro- 

 phyll d" were larger than for the transfer chlorophyll a -»• "reduction 

 center" — as it may well be. (The "reduction centers" may be properly 

 located or complexed molecules of chlorophyll a itself.) 



One may ask whether all these estimates could be affected by the probable existence 

 of a long-lived, metastable state of the chlorophyll molecule (or of the pigments-protein- 

 lipide complex in the living cell). If the low yield of fluorescence of chlorophyll in vivo 

 is the result of the transfer of most of the excited chlorophyll molecules (or complexes) 

 into a metastable state containing considerable electronic energy, and not of total con- 

 version of this energy into heat (or chemical energy), why should it not be possible for 

 the electronic energy of the metastable state to be exchanged "at leisure," as it were, 

 between adjacent molecules? (The time available for the exchange could be, in this 

 case, a million — -or more^ — -times longer than the natural life time of the fluorescent 

 state.) If the metastable state is a tautomeric form of the molecule, no resonance trans- 

 fer of energy is possible, because it would require a displacement of the nuclei; but if the 

 long-lived state is an electronic mesomer (e. g., a triplet state, as envisaged by Terenin*) 

 and by Lewis and Kasha, cf. pp. 486, 790), energy transfer is feasible. However, the rate 

 of transfer by the "slow" mechanism is proportional to the fourth power of the transition 

 probability between the ground state and the excited state; while the natural life-time 

 of the excited state is inversely proportional to the square of the same probability. There- 

 fore, if the natural life-time of the excited state is tf, and that of the metastable state, 

 t„, the number of transfers during the full natural life-time of the latter will be smaller 

 (by a factor of t^/t„) — and not larger than the number of transfers during the full life- 

 time of the fluorescent state. If we assume that the low yield of chlorophyll fluorescence 

 in vivo (0.1%, or 1%) is caused by the transfer of 99 (or 99.9%) of the excited chloro- 

 phyll molecules from the fluorescent into the metastable state (rather than by the dissi- 

 pation of total energy of the fluorescent state by internal conversion), the number of 

 energy transfers during the actual excitation period will be increased in consequence of 

 this transfer, only if t„ is <103 (or 10") X tf. (We now compare the number of jumps 

 during the full natural life-time of the metastable state with that during one-thousandth 

 or one-hundredth of the natural life-time of the fluorescent state.) With <^ = 4 X 10~* 

 sec, this means that the existence of a metastable state could favor migration only if the 

 natural life-time of this state were <4 X 10 ~* (or 4 X 10"^) sec. If the first figure is 

 correct, a metastable state with a natural life-time of 4 X 10 ~« sec. would increase the 

 frequency — and thus also the range — of energy migration by a factor of 10, and a meta- 

 stable state with a life-time of 4 X 10 "^ sec, by a factor of 100 — presuming this state 

 actually survives for its full natural life-time. However, if this were the case, the meta- 

 stable state would produce a marked phosphorescence. Since this state is situated con- 

 siderably below the initial excitation state (as judged by the nonoccurrence of delayed 

 red fluorescence, which could be caused by return from metastable to the fluorescent 

 state by thermal energy fluctuations), an emission originating in this state should be 

 located in the infrared. Until evidence is presented that chlorophyll in vivo does emit 

 infrared phosphorescence, with a quantum yield higher than that of the known red 

 fluorescence, we have to assume that the metastable triplet state, if it occurs in vivo at 

 all, survives for only a small fraction of its "natural" life — and this should make it im- 



* Terenin (1940, 1941) had suggested the interpretation of the metastable state 

 of organic molecules as a triplet "biradical" state before this was proposed by Lewis 

 and Kasha (1945), but his work did not become known abroad, because of wartime con- 

 ditions, until considerably later. (For a review of the work by Terenin and co-workers, 

 see Terenin's Photochemistry of Dyes, 1947.) 



