THE TRIPLET STATE IN FLUID SOLUTION 



George Porter 



Cheinistty Drixn Imcnt. VniTcisilv of Sliefficlil . F.iioJ(ni(l 



In SO tar as excited electronic states are oi importance in photo- 

 biology two such states deserve primary consideration: the lowest 

 excited singlet state and the lowest triplet state. The lormer deter- 

 mines the long wavelength absorption spectrum and is the first state 

 reached by absorption of light in this region, the second is generally 

 reached only indirectly by intersystem crossing from the singlet state 

 l)ut is the excited state of lowest energy and, being of different 

 midtiplicity from the ground state, it has a relatively long lifetime 

 and a consequently greater probability of entering into reaction. 



There are three spectroscopic methods by means of which the lowest 

 triplet state of a molecule may be studied. 



(1) By direct absorption jrom the ground state. This requires 

 long path-lengths owing to the forbidden nature of the transition, 

 although the transition probability may be increased by perturbation 

 techniques, e.g., by observation in a solution in equilibrium with a 

 high pressure of oxygen (3) . However, the only information obtained 

 from such measurements is the energy separation between the lowest 

 triplet and the ground state and also the extinction coefficients which 

 are related to the transition probability and lifetime. 



(2) Emission spectrum jrom the triplet to the ground state. This 

 is known as phosphorescence and yields the energy of the lowest 

 triplet level and its lifetime (5). Since the transition arises from the 

 triplet state, phosphorescence may also be used, in principle, to study 

 the lifetime of the triplet state as a function of conditions and hence 

 to study its reactions. In practice this is not very useful, since phos- 

 phorescence is usually only observed in rigid glasses or in very viscous 

 solutions, whereas as far as the reactions of the triplet state are con- 

 cerned, we are normally interested in gases or in fluid solutions. 



(3) Absorption from the triplet state resulting in excitation to a 

 higher triplet level. This transition is spin-allowed and may therefore 

 be very intense. It tells us nothing about the energy difference be- 

 tween the lowest triplet and the ground state, but this information is 



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