GEORGE PORTER 73 



Quenching by Chemical Reaction 



Chemical rcaciioii ol t.hc triplet state will clearly lead to its destruc- 

 tion. Photochemical reactions may proceed from the lowest excited 

 singlet or triplet states (and possibly in some cases from higher singlet 

 states) , but only in a very few cases has the state responsible for 

 reaction been definitely identified. Examples of reactions involving 

 the triplet state are hydrogen abstraction by benzophenone (9), elec- 

 tron transfer between thionine dyes and ferrous salts (8) , and addi- 

 tion of oxygen to anthracene (6) . On the other hand similar re- 

 actions, such as hydrogen abstraction by duroquinone, appear to occur 

 through the singlet state (2) . 



Physico-Chemical Properties 



Properties such as redox potential and acidity are often entirely 

 different in the excited state. An example of the difference in redox 

 potential between ground state and first triplet state is given by methy- 

 lene blue, whose triplet state oxidizes ferrous to ferric, whilst the leuco 

 form of the dye so formed is itself oxidized, in the ground state by 

 ferric iron. 



The acidity constants in excited states are particularly illuminat- 

 ing, since they provide a rather direct measure of electron density 

 distribution. Fluorescence measurements have shown that in mole- 

 cules such as naphthols, naphthylamines, and acridine the first ex- 

 cited singlet state has an acidity constant differing from that of the 

 ground state by as much as six pK units (15) . Recent measurements 

 of the acidity constants of the corresponding triplet states have shown 

 (4), rather surprisingly, that these differ very little from those of the 

 ground state. It follows that the singlet transition in these molecules 

 have considerably more charge transfer character than the transition 

 to the triplet state. 



The triplet state in solution is clearly a chemical species in its 

 own right, with its own physical structure and chemical properties. 

 Very little can be predicted about these properties from a knowledge 

 of the ground state alone, but direct observation and measurement of 

 its properties is now conveniently carried out by flash photolysis 

 methods. 



REFERENCES 



1. Backstrom, L. J., and Sandros, K., Acta. Cliem. Scand., 12. 823 (19r)8). 



2. Bridge. X. K.. and Porter, G.. Froc. Ro\. Soc. A244, 2r)9, 276 (1958). 



3. Evans, D. F.. ./. Chem. Soc, 1351, 3885 (1957). 



