44 



could not be accounted for by deactivating collisions only and had 

 to be, partly, a resonance quenching which takes place without a 

 collision, through the resonance transfer of E*. Fluorescence, on 

 the whole, is rather insensitive to quenchers, which can easily be 

 explained by the brevity of the lifetime of the singlet excitation. 

 But fluorescence, as such, is of little direct interest for our prob- 

 lem. If E* plays a role in biology it has to be in the form of trip- 

 lets, as shown in the previous chapters. I found triplets most sen- 

 sitive to quenchers, some of v^hich suppress phosphorescence in 



Fig. 8. Phenothiazine. 



high dilution. The insensitivity of fluorescence can be used with 

 advantage to show that the quenching of phosphorescence ob- 

 served was a "true quenching" and the light emission did not 

 simply disappear because the quencher absorbed the exciting light. 

 If this were the case, then fluorescence would have to be quenched 

 as well. The quenchers to be described, in the concentrations ap- 

 plied, did not diminish fluorescence appreciably and so their 

 quenching had to be a "true" one. A quenching activity can have 

 different mechanisms and so for the time being we'd better re- 

 frain from an interpretation of the mechanisms involved and con- 

 tent ourselves with the fact that a quenching, that is a suppression 

 of light emission, must involve an interference with E* and an 

 interference with JB* must mean interference with biological func- 

 tion if £* has an important role to play in it. 



One of the strongest inorganic quenchers is SQSF" which sup- 

 presses the phosphorescence of riboflavine in 1 0"'* M concentration. 

 The atom combination SQSJ plays no role in organic chemistry. 

 However, the closely related SC=CN is the core of phenothiazine 

 (Fig. 8), in which it is a center of an extensive system of con- 

 jugated double bonds. Phenothiazine itself is unsuitable for ex- 



