764 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



Table 23.III 



Changes of Fluorescence Intensity of Chlorophyll with Time in Different 



Solvents" (after Zscheile and Harris 1943) 



» According to Franck and Levi (1934), the fluorescence of chlorophyll is about 

 twice as strong in ethanol as in aniline. 



More recently, Livingston and co-workers found that fluorescence is 

 very weak or entirely absent in nonpolar solvents if these are free from 

 traces of water, alcohols or amines. 



An effect of solvent on the yield of fluorescence is to be expected what- 

 ever the mechanism of quenching. Even the probability of "monomolecu- 

 lar" chemical quenching by dissociation (or tautomerization) of the ex- 

 cited molecule probably depends on the degree of stabilization of the dis- 

 sociation products (or of the tautomeric form) by solvation. (Well known 

 is the effect of solvent on enolization — a tautomeric transformation that 

 can occur in chlorophyll; cf. Vol. I, page 444.) The occurrence of self- 

 quenching, too, may differ strongly depending on the solvent, as indicated, 

 e. g., by the vastly different stabilities of dimeric dyestuff molecules in 

 water and alcohol. The velocity of physical dissipation of excitation en- 

 ergy by "internal conversion" also must depend on the nature of the me- 

 dium to which the vibrational quanta are to be transferred in order to make 

 dissipation irreversible. Finally, the probability of "bimolecular" chemi- 

 cal quenching by reaction with the medium obviously is a function of the 

 nature and purity of the solvent. 



The "physical" quenching of fluorescence by the solvent can be ex- 

 pected to be least efficient in solvents of symmetric, nonpolar nature, such 

 as cyclohexane or carbon tetrachloride. However, recent evidence shows 

 that chlorophyll solutions of this type fluoresce less strongly than those 

 in more polar or polarizable solvents, and this makes it probable that 

 chemical interactions of the pigment molecules may often be at least as 

 important as physical dissipation. The rapid decline of fluorescence with 

 time, found in many chlorophyll solutions (e. g., those in benzene and car- 

 bon tetrachloride), also points to chemical interaction; but whether it in- 

 volves the solvent or impurities (e. g., dissolved oxygen) remains to be es- 

 tablished. It is also uncertain whether the rapid decline of fluorescence is 

 caused by a chemical transformation of chlorophyll, or by chlorophyll- 

 sensitized formation of substances with strong quenching properties. 



