34 



LUMINESCENCE SPECTROSCOPY OF MOLECULES 



we can state that we have obtained a triplet —> singlet emission for 

 chlorophyll b, a phosphorescence observed previously by other work- 

 ers under uncertain conditions. We shall give a thorough analysis of 

 the significance of this finding. 



(1) Singlet-Triplet Split: Expected Lowest Triplet Energy 



The energy difference between the lowest excited singlet state (S') 

 and the lowest triplet state (T) (cf. Fig. 2) is called the singlet-triplet 

 split'* in a molecule. This split is very variable (Kasha, 1947) from 



t 



>- 

 O 



i 



UJ 



Fig. 2. Electronic levels important in energy utilization processes in a molecule 

 (schematic). Only ir,ir levels are shown, and excited vibrational levels are 

 omitted. 



molecule to molecule. For example, in molecules like naphthalene and 

 anthracene the split is very large, of the order of 10,000 to 12,000 

 cm~^ On the other hand, in dyelike molecules, such as the cyanines 

 and many others, the singlet-triplet spht is of the order of 2000 cm~^ 

 If we classify a linearly or cyclically conjugated molecule like porphy- 

 rin as dyelike, then we might expect the singlet-triplet split to be of 

 the order of 2000 cm^^ ± 1000 cm^^ In zinc tetraphenylporphyrin 

 Dorough et al. (1951) have observed a singlet-triplet split of approxi- 

 mately 2500 cm-^ (cf. their Fig. 11). In recent experiments in our 



' This definition assumes that the singlet and triplet states correspond in 

 orbital configuration. If they do not, the energy difference given is called 

 the singlet-triplet separation. 



