782 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



(or stimulant). The absorption changes may be major or minor, depending 

 on the character of the interaction (complexing, tautomerization, dissocia- 

 tion, oxidation, reduction, etc.), but should never be entirely absent. 

 Furthermore, chemical changes in the dark will often (albeit not always) 

 proceed at a measurable rate, thus causing the yield of fluorescence to 

 depend on the length of time between the preparation of the mixture and 

 the illumination. Finally, the dependence of quenching on the concentra- 

 tion of the quencher should be different in the case when the absorbing 

 molecule and the quencher combine (or react) in the dark than when they 

 interact only after the absorption of light. 



Table 23.IIIC 



Quenching Data for Chlorophyll a in Methanol, Ethanol and 

 Acetone (after Livingston and Ke, 1949) 



Quencher Solvent X (excit.), m^i [0] 1/2 " *' '' 



mole/L 



Chloranil Me.CO 



Quinone Me2C0 



MeOH 



Methyl red MeOH 



Trinitrotoluene MeOH 



MeOH 

 m-Dinitrobenzene MeOH 



MeOH 



Duroquinone MeOH 



/3-Nitroso-a-naphthol MeOH 



/3-Nitrostyrene MeOH 



Nitric oxide EtOH 



/3-Nitro-i3-methylstyrene MeOH 



Oxygen EtOH 



Nitrobenzene MeOH 



/3-Nitro-)3,7-hexene MeOH 



o-Aminophenol MeOH 



Phenylhydrazine EtoO 



MeOH 



Dimethylaniline MeOH 



2-Phenyl-3-nitrobicyclo- 



[l,2,2]-heptene-5 MeOH 



2,6-Diaminopyridine MeOH 



"Half-quenching concentration. * Cf. equation 23.16C, p. 785. 



These general considerations should be kept in mind in the evaluation 

 of the results of quenching experiments. They make it particularly im- 

 portant that measurements of the intensity of fluorescence be combined 

 with measurements of the absorption spectrum (and if possible, also of the 

 fluorescence spectrum) of the light-absorbing species. 



