12 RADIATION BIOLOGY 



that tlie half Hfe of the excitod ruhrene molerule is very long (possibly 1 

 sec or more). This is very mwh {greater than the maximum half life of 

 the directly excited, sinj^let state. 1 ii this case, as in those sensitized reac- 

 tions previously referred to, a long-lived excited state must be an inter- 

 mediate in the photochemical reaction. 



At the present time, there appear to be no data availal)l(' from which it 

 can be concluded that the long-lived state recpiired by photochemical evi- 

 dence is identical with the phosphorescent state, usually studied at low 

 temperatures and in rigid media. However, in the absence of information 

 to the contrary, this identification is commonly made as a simplifying 

 hypothesis. In one case, that of fluorescein in boric acid glass, direct 

 measurements of the paramagnetism of the excited substance (Lewis et al., 

 1949) have demonstrated that the phosphorescent state is a triplet one. 

 Comparison (McClure, 1949) of the measured half lives of the long-lived 

 fluorescence of a large number of aromatic compounds and of a few ali- 

 phatic ketones makes it appear probable that the phosphorescent states 

 of these compounds are likewise triplet states. 



It should not be assumed that all photochemical reactions involve either 

 dissociation or a long-lived excited state. Benzene and some of its methyl 

 derivatives react photochemical ly with molecular oxygen to form perox- 

 ides. Their limiting fluorescence yields are high, and their fluorescence 

 and photochemical reactions are complementary functions of oxygen con- 

 centration (Bowen and Williams, 1939). It is probable, therefore, that 

 the photochemical reaction in these cases goes by way of a direct inter- 

 action between an oxygen molecule and the singlet excited, i.e., fluores- 

 cence, level of the hydrocarbon molecule (Kasha and Xauman, 1949). 



QUENCHING OF EXCITED STATES 



A low quantum yield for a photochemical reaction may, under some con- 

 ditions, be the result of the quenching of the excited molecule by impuri- 

 ties, reaction products, the solvent, or even one of the reactants. Such 

 quenching brings about a decrease in the fluorescence yield, which is not 

 accompanied by an ol^servable chemical reaction. The mechanism of the 

 process has been the subject of mu(;h speculation (Pringsheim, 1949, p. 

 335), and it appears probable that no one explanation is consistent with 

 all the experimental facts. In some cases the quenching appears to be 

 the result of the reversible formation of nonfluorescent complexes, 

 whereas in others it may be due to collision (or the close approach) of a 

 quencher and excited molecule. It is possible (Rollefson and Stoughton, 

 1941; Livingston and Ke, 1950) that the quencher acts by inducing a 

 transition of the potentially fluorescent molecule from its excited singlet 

 state to its lowest triplet state. Fluorescence (juenching of this type 

 v/ould not necessarily be accompanied by a corresponding reduction in 

 the quantum yield of a photochemical reaction. 



