752 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



It appears that all porphyrins, chlorins and phorbins, when brought 

 into an electronic state with an energy higher than that of their lowest ex- 

 cited state, rapidly lose this excess energy, and revert into state A (por- 

 phyrins), Y (chlorins and phorbins) or Z (bacteriochlorophyll and its 

 derivatives). Differences in the yield of red fluorescence among various 

 compounds of these three classes must then be due to variations in the 

 longevity of the lowest excited states {A, Y or Z). In ''nonfluorescent" 

 compounds (e. g., copper pheophorbide, wdth <p <0.01%) the energy of the 

 lowest excited state must be dissipated within 10 ~^^ sec. ; while in "strongly 

 fluorescent" compounds, such as chlorophyll itself, this state must persist 

 for 10~^ or 10~^ sec. to allow at least several per cent of the excitation 

 energy to be re-emitted as fluorescence. 



Prins (1934) mentioned that the quantum yield of the fluorescence of 

 chlorophyll a in ethanol is smaller if excited by blue than if excited by red 

 light — an observation that seemed to indicate that the conversion of 

 chlorophyll molecules in state B into those in state Y is less than 100% ef- 

 ficient, perhaps due to competition on the part of a photochemical reac- 

 tion with the solvent (cf. page 756). However, this result needed confirma- 

 tion; Button, Mannmg and Duggar (1943), who worked with acetonic 

 solutions of chlorophylls a and b, found identical yields of fluorescence when 

 using excitation by violet light (436 m/x) or yellow light (578 mju). More 

 recently, measurements of the relative yield of the fluorescence of chloro- 

 phyll, excited by the Hg lines 435.8 and 577-579 m;u, and by narrow bands 

 centered at 645 and 681 m/x (isolated by means of Farrand's interference 

 filters) were made by Livingston and co-workers (1949). Table 23.IIC 

 shows their results. 



Table 23.IIC 



Effect of Wavelength of Exciting Light on Relative Quantum Yield 



OP Chlorophyll Fluorescence 



Table 23.IIC indicates that the yield of fluorescence of both chloro- 

 phylls is much higher when it is excited in the orange- red system, than 

 when it is excited in the blue-violet band. Furthermore, the yield drops 

 sharply in the far red — a result which recalls the drop in the quantum yield 

 of photosynthesis observed in the same range by Emerson and Lewis 



