PHOTOCHEMISTRY OF CHLOROPHYLL in vUrO 5 



is an ion, formed by the loss of a proton from Cio it appears plausible 

 that the stable electronic state of this ion is a triplet. 



PROPERTIES OF CHLOROPHYLL IN ITS SINGLET EXCITED STATES 



Chlorophyll, excited by the absorption of blue light, emits only the 

 usual red fluorescence. The molecule goes from its second to its first 

 excited singlet state by a radiationless process of high probability 

 (internal conversion). Since visible fluorescence having a yield as 

 high as 10~' is readily detectable and no blue or green fluorescence 

 has been obser^'ed, the quantum yield, of this fluorescence is less than 

 10~'. The actual mean life, r = ^r°, where t° is the intrinsic mean 

 life, must be less than 10"' X 10^^ = 10-^^ second. It is, therefore, 

 most improbable that the second excited singlet state pla3^s any 

 direct role in the photochemistry of chlorophyll. In other words, the 

 photochemical action of visible light should be independent of wave- 

 length whenever chlorophyll is the only light-absorbing entity pres- 

 ent. This does not necessarily apply to the absorption of ultraviolet 

 light. Higher excited states may possibly enter directly into photo- 

 chemical reactions, but such processes would not be related to normal 

 photosjTithesis. 



The intrinsic mean life, t°, of the first excited state of chlorophyll 

 may be calculated (4) from its integrated extinction coefficient for 

 "red" fight. The actual mean life, r, may be obtained (4) from meas- 

 urements of the degree of polarization of fluorescent light in viscous 

 solvents, by the use of Perrin's equation. Since the maximum quan- 

 tum jdelds of fluorescence, (p°, are known (8) for chlorophylls a and 

 h, the intrinsic fife may be obtained from the actual life, as t° = 

 t/(p°. The presently available values are listed in Table I. Weil's 

 value for t° (h) is based upon measurements made with benzyl 

 alcohol as the solvent, assuming that <p° = 0.10. Disregarding the 

 values which Rabino^dtch calculated from the early data of Prins, the 

 values of the intrinsic mean lives of chlorophylls a and h fafi in the 

 ranges 3 to 1.3 X 10"^ and 4 to l.G X 10~^ second, respectively. 

 The maximum quantum yield, <p°, for the fluorescence of chlorophyll 

 a is 0.24, and it is independent of the solvent (8). The yield for chloro- 

 phyll b varies with the solvent, being 0.11 for ether and 0.06 for 

 methanol. The corresponding values for the actual mean lives are 

 r'^Ca) ^ 5 X 10-" and r°(6) ^ 3 X 10"' second. 



