LONG-LIVED ACTIVE STATES AND AFTERGLOW 795 



Another possible cause of phosphorescence — with a spectrum somewhat 

 different from that of instantaneous fluorescence — can be the direct, radia- 

 tive return of the metastable molecules from state T into the ground state 

 X, with the emission of a quantum. This is an alternative to the above- 

 mentioned, nonradiative return by internal conversion of electronic into 

 vibrational energy. The assumption that this type of phosphorescence 

 alone limits the life-time of the metastable state is the basis of the above- 

 mentioned calculations of Lewis, Kasha and McClure. Long-lived lumi- 

 nescence, with a frequency 2000-20,000 cm.-^ lower than that of direct 

 fluorescence, actually is known for many dyestuffs and other fluorescent 

 organic compounds, particularly at low temperatures in glassy solvents. 

 In the case of chlorophyll, a phosphorescence of this type would have to be 

 sought in the infrared. 



Calvin and Dorough (1947) reported that in a mixture of chlorophyll a 

 and h, dissolved in a "rigid solvent" (EPA = mixture of ether, pentane 

 and alcohol solidified <-100° C. without crystalHzation), an afterglow 

 lasting 0.2 sec. can be observed after illumination. Spectroscopic obser- 

 vation revealed a band beginning at 780-800 m/z and stretching into the 

 infrared. Similar results were obtained with zinc tetraphenylchlorin, 

 but not with copper tetraphenylchlorin— a difference ascribed by Calvin 

 to weakening, by the paramagnetic Cu'^+ ion, of the metastability of the 

 triplet state. 



Livingston and co-workers (1948) found no such afterglow, at —180° or 

 -150° C. (2 X 10-^ mole/1, chlorophyll a in EPA). Experiments in other 

 solvents and at other temperatures also gave negative results— with 

 chlorophyll a + & as well as a, and at concentrations from 10 ~- to 5 X 

 10-^ mole/1., in air or in vacuum. According to Livingston, a personal 

 communication from the Berkeley group confirmed that the luminescence 

 of Chi (a + h) at 800 m^, reported by Calvin and Dorough, probably had 

 been due to impurities. However, Berkeley observers asserted that chloro- 

 phyll b does have a weak infrared afterglow (r = 0.02 sec), starting at 860 

 m/i. This phosphorescence, if it exists, should be in direct competition 

 with photochemical sensitization by chlorophyll (Weiss, 1948). 



8. Summary — A Scheme of Fluorescence and Sensitization 



In summing up the discussion, we may go back to Volume I (chapter 

 19) and reproduce again, in a somewhat amplified form, scheme 19.11 



