GEORGE PORTER 71 



photographic methods (11)- Subsequently more accurate kinetic 

 studies have been carried out spectrophotometrically, and the results 

 of them will now be briefly reviewed. 



Kinetics of the Triplet State in Solution 



The triplet state of a molecule is a species with its own structure 

 and its own physical and chemical properties which may be very 

 different from the properties of the same molecule in its ground state. 

 These properties have not yet been studied in a wide enough range 

 of molecules for secure generalization to be possible. Most detailed 

 studies have been carried out on aromatic hydrocarbons and their 

 derivatives and, in this series of compounds, certain modes of be- 

 havior appear to be rather general and probably applicable to most 

 triplet states. Since most of these results have already been published 

 in full, all that will be given here is an outline of the principal 

 processes and references to the original papers. 



Radiative and Radiationless Decay to Ground State 



In the absence of specific physical or chemical quenching processes, 

 the triplet state disappears by a first-order conversion to the ground 

 state. In rigid media the decay is usually radiative, and the lifetime, 

 which is determined by the transition probability between the two 

 states, is usually between 10-^ seconds and 10 seconds for molecules 

 composed only of light atoms. 



In fluid solution and in the vapor phase the lifetime is usually 

 found to be between K)-"' and 10-^ seconds, and there is little re- 

 lation between this lifetime and the radiative one. The radiationless 

 decay is partly second order — brought about by collisions between two 

 triplet molecules — and partly first order in triplet concentration. 

 Both processes are viscosity-dependent and the nature of the first- 

 order process is still not fully understood. It is thought to be a true 

 first-order radiationless conversion — at least in part — but the possi- 

 bility that pseudo-first-order processes involving some unidentified 

 quenching species may play a part is still under discussion. 



Quenching by Paramagnetic Substances 



The second-order decay of the triplet state mentioned above is 

 only a special example of the rather general finding that all para- 

 magnetic species quench the triplet state. The first quencher to be 

 found (10) , and the most common, was oxygen, and quenching by 

 this molecule is so efficient that very careful outgassing of solutions 



