28 LUMINESCENCE SPECTROSCOPY OF MOLECULES 



states are possible for the molecule. However, in accordance with the 

 above discussion of internal conversion, only the lowest triplet state 

 will be of any importance in photochemical and other energy utiliza- 

 tion processes. 



We shall be interested in what role the lowest triplet state may 

 play in the photosynthetic reaction. Outstanding in its importance in 

 photochemical and energy transfer mechanisms is the long lifetime of 

 the triplet state. In general, a triplet state will be longer lived by a 

 factor of one million over the lifetime of a corresponding singlet 

 state. This places the range of intrinsic triplet state lifetimes between 

 10-^ second and 1 second for "normal" cases. There are several mech- 

 anisms by which intrinsic triplet state lifetimes may be varied con- 

 siderably without cjuenching of excitation energy. These phenomena 

 will be discussed mainly in Sections V and VI. It is likely, however, 

 that if a triplet state of a chlorophyll may be excited easily, it may 

 play an important part in any energy utilization process, on the basis 

 of lifetime alone. 



It is now generally recognized among spectroscopists that the low- 

 est triplet state can be detected readily in most molecules by the 

 study of low-temperature phosphorescence spectrum (Lewis and 

 Kasha, 1944; Kasha, 1947; Kasha, 1950). The intrinsically long Hfe- 

 time of phosphorescence generally makes its observation in fluid 

 systems impossible, since in such cases the quantum yield and life- 

 time are reduced to immeasurable limits by the competitive collisional 

 deactivations. Consequently, the phosphorescence emission of a mole- 

 cule is always sought in rigid glass solvents. The high viscosity in 

 such systems allows the long-lived phosphorescence to be observed. 

 At lower viscosities, quenching will occur, and any phosphorescence 

 emission observed will have a shorter lifetime and a diminished quan- 

 tum yield compared with the intrinsic valves. 



In Section V we shall discuss the expected wavelength range and 

 other characteristics of the lowest triplet states of the chlorophylls. 



III. Basic Quantitative Relationships 



(1) Intrinsic Lifetime of an Excited State 



In the previous section we have argued that an important criterion 

 of utilizability of excitation energy is the intrinsic lifetime of the excited 



