792 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



ironic states. The difference between the two hypotheses is that, in the 

 second one, the position of the atomic nuclei in the metastable state is 

 supposed to be the same as in the normal state — or, at least, not separated 

 from the normal position by a potential barrier. In the tautomeric state, 

 on the other hand, the atomic nuclei are rearranged so that a return into 

 the ground state is prevented by a potential barrier, and thus requires a 

 certain activation energy. For the rest, the tautomeric state, too, may 



(o) Excitation 



(d) Emission -hv '^ -.%,- (b) Toutomerizotion 



(c ) Thermol excitotion 

 Scheme 23.IB 



have the chemical nature of a biradical, with two free valencies, and the 

 spectroscopical nature of a triplet state, with a corresponding paramagnetic 

 moment. 



The following two elementary transformations illustrate the difference 

 between pure electronic excitation to a triplet state and electronic excita- 

 tion coupled with tautomeric rearrangement: 



(23.16E) H H H H 



Ri — C=C — R2 ^ ^ Ri — C — C — R-. (electronic tautomerization ) 



I I 



(23.16F) H H H I 



Ri — C=C — R2 ^ ^ Ri — C — C — Ro (atomic tautomerization) 



H I 



The return into the normal state requires, in the second case, that the H 

 atom moves over to a neighboring carbon atom, swinging from one poten- 

 tial minimum into another over a barrier. This type of metastability 

 may therefore be longer lived than the first one, in which the return into 

 the ground state can be achieved by electronic rearrangement alone. 



The concept of metastable triplet states as the origin of long-lived 

 fluorescence (phosphorescence) of dyestuff solutions has been fiu'ther de- 

 veloped in several papers from the Berkeley laboratories by Lewis, Kasha, 

 McClure and Calvin (1945, 1947, 1948). One interesting result was the 

 experimental confirmation of (lie paramagnetism of the ph()si)horescent 



