SOME PHOTOCHEMICAL CONSIDERATIONS 



25 



G 



© 



(Z) 



(Z) (Z) (Z) 



(D 



(D ® 



Fig. 8. Fluorescence, a: electron pair in the ground state, b and c: one electron in 

 the singlet state .S".) or Si. d: both electrons in the ground state again. A: absorbed radia- 

 tion. E: energy loss. F: fluorescence. 



T 

 T, 







Q) 



Ph 



0(7)0 











a bed 



Fig. 9. Phosphorescence, a: electron pair in the ground state, b and c: one electron 

 with opposite spin in the triplet state To or Ti. d: both electrons in the ground state again 

 (renewed change of spin). A: absorbed radiation. E: energy loss. Ph: phosphores- 

 cence. 





 s 







© 



D 





 







T 



Fig. 10. Multiplicity and free radicals. 

 S: singlet state, resulting spin = 0, mag- 

 neton excess = 0. D: doublet state, re- 

 sulting spin = '/o. magneton excess = 1. 

 T: triplet state, resulting spin = 1, mag- 

 neton excess = 2. The two unpaired elec- 

 trons are characteristic of a free biradical. 

 The one unpaired electron is characteristic 

 of a free monoradical. The horizontal 

 lines on the left represent energy levels. 



return via the singlet state to the ground state only by changing its spin 

 again. Such a triplet-singlet change, corresponding to a photoluminescence 

 of longer duration, is called phosphorescence (Fig. 9) in accordance with the 

 differentiation proposed by Lewis and Kasha (see footnote page 23). The 

 various states including the doublet state are represented in Figure 10. 

 The scheme shows the presence of a monoradical in the doublet state and of a 

 biradical in the triplet state.* Such molecules are paramagnetic owing to 

 the excess of magnetic momenta (see § 8). 



* Reid (40) points out that there is some tendency to identify the triplet state with the biradical. 

 This may often be justified but a biradical is not necessarily a triplet and triplet states may not be 

 biradicals. 



