PHOTOCHEMISTRY OF CHLOROPHYLL ill vUrO 7 



(10,11). The probability of the transfer of excitation from h to a 

 leading to the fluorescence of a reaches a maximum of 0.5 at about 

 10~' m, when the concentrations of a and b are equal (10,11). Con 

 centration quenching becomes dominant at higher concentration 

 and reduces the probability of sensitized fluorescence to a negligible 

 value at 10"^ m. Sensitized fluorescence is a well-known phenomenon 

 in the intact plant. 



The fluorescence of chlorophyll solutions is strongly quenched by 

 oxidizing agents (such as quinones, aryl nitro compounds, nitroso 

 compounds, and oxygen) but is not detectably quenched by a wide 

 variety of reducing agents (including thiourea, urethane, hydro- 

 quinone, ascorbic acid, etc.). Quenching by the oxidizing agents is 

 chiefly a dift'usional process. The polarization of fluorescence increases 

 with the degree of quenching in accord with Perrin's equation (4). 

 The rate constants corresponding to the bimolecular quenching reac- 

 tion can be calculated from the ratio of the Stern- Volmer quenching 

 constant, kg, to the actual mean life of the excited state, r. If the rate 

 of the reaction is determined by diffusion, kg/r should be approxi- 

 mately equal to ko = 8RT/S0v, where rj is the viscosity of the solvent 

 expressed in centipoise. A few such values for chlorophyll a at 25° 

 C. are listed in Table II. In these calculations a value of 5 X 10~^ 

 second has been assumed for r. The agreement is fairly good, but 

 there is probably some static quenchhig, which is more apparent 

 in viscous solvents (4). The data are not sufficiently extensive to 

 justify a more extensive analysis. 



TABLE II. Bimolecular Quenching Constants 



PROPERTIES OF CHLOROPHYLL IN ITS TRIPLET STATE 



Kinetics studies of photochemical reactions sensitized by chloro- 

 phyll demonstrate that it is usually not the fluorescent (i.e., first 

 excited singlet) state but rather a metastable state of chlorophyll 

 which is directly responsible for the photochemical action. When the 

 reactant is present at high concentration in a fluid medium or exists 



