a R. LIVINGSTON 



as a thermally stable complex with the chlorophyll, the fluorescent 

 state of the chlorophyll molecule can play a direct part in the photo- 

 (ihemical reaction. Such reactions are, of course, accompanied by 

 marked quenching of the fluorescence. 



By analogy with fluorescein and with aryl hydrocarbons, it is com- 

 monly assumed that the metastable state is the lowest triplet state 

 of the molecule. Most unsaturated compounds exhibit phosphores- 

 cence when they are dissolved in glassy media. Recently Becker and 

 Kasha (12) have confirmed the existence of a weak, near infrared 

 phosphorescence of chlorophyll h. The present status of the subject 

 may be summarized as follows. No temperature-dependent phos- 

 phorescence (with the same wavelength distribution as fluorescence) 

 has been observed for either chlorophyll under any conditions. No 

 phosphorescence of any type has been observed for chlorophyll a. 

 Chlorophyll b shows a temperature-independent phosphorescence 

 (i.e., long-lived fluorescence) at 8650 A, which has a half-time of 

 about 3 X 10 ~2 second and a quantum yield less than 10~'. The ob- 

 served wavelength corresponds to an energy diff"erence between 

 the ground and the metastable states of 33.0 kcal. 



Organic molecules are phosphorescent only when they are dis- 

 solved in rigid media or adsorbed on solids. Therefore, the measure- 

 ment of phosphorescence is not well adapted to the investigation of 

 the chemistry of the triplet states of such molecules. This is especially 

 true for chlorophyll, whose phosphorescence is very weak. The 

 flash-photolytic method, which was brought to its present state of 

 development by Dr. George Porter (13) at Cambridge, is a much 

 more effective tool for such studies. 



Preliminary studies (14), using a flash of actinic light and a steady 

 monochromatic beam for analysis, were made by Dr. V. Ryan. 

 He succeeded in demonstrating that a large fraction of the chloro- 

 phyll in a dilute solution is raised to its triplet state by a single in- 

 tense flash. Under his experimental conditions the half-life of this 

 state was about 3 X lO"'* second. Dr. Linschitz (15) who used an 

 improved apparatus of this type, confirmed in general Dr. Ryan's 

 findings. He also obtained the absorption spectrum of chlorophyll 

 a in its triplet state, and showed that it has a maximum on the 

 long-wavelength side of the red band of normal chlorophjdl. Pre- 

 liminary absorption spectra of the triplet states of the chlorophylls 

 and related compounds were obtained by Livingston (7) using the 



