30 LUMINESCENCE SPECTROSCOPY OF MOLECULES 



volved in a photochemical process, the limiting quantum yield of the 

 photochemical reaction would be 0.8. Conversely, if, say, an energy 

 transfer or photochemical reaction proceeds uniquely via the lowest 

 singlet excited state, and if ^f° = 0.2, then the limiting value of 



^photochem will bc 0.2. 



(3) Quenching: Lifetime and Quantum Yield 



If we define observed lifetime of an excited state by t, and intrinsic 

 lifetime by t°, then for collisional bimolecular quenching 





where $ is the observed quantum efficiency of a luminescence in the 

 presence of partial quenching, and $° is the intrinsic quantum yield 

 of fluorescence {or phosphorescence) as before. 



Consequently, for collisional bimolecular quenching 



For example, if <E>° = 0.2, and $ = 0.02, then the observed lifetime 

 will be 0.1 of the intrinsic lifetime of an excited state. In the older 

 literature, the incorrect expression t = *t° was used; for the example 

 given, the observed lifetime would appear to be 0.02 of the intrinsic 

 lifetime. This error can be traced to the neglect of the complementary 

 luminescence to the one observed, as the reader can confirm by the 

 study of the expression given in Section ( 2 ) . Obviously, the incorrect 

 expression sets 4)° = 1 for any luminescence. 



IV. Role of n,7r-Transitions in Spectra of Chlorophyll a 



and Chlorophyll b 



First we shall give a rough qualitative description of n,7r-transitions. 

 Such transitions have been described by McMurry and Mulliken 

 (1940), and more generally by Kasha (1950) and Piatt (1953). 



The designation n means nonbonding electron orbital. In addition to 

 pure TT-electron excitations, it is found that nonbonding electrons (re- 

 ferred to by chemists as "lone pairs") may be excited to unfilled 

 TT-electron orbitals in the molecule. 



Examples of n-» tt transitions are: (a) the excitation of a nonbond- 

 ing 2p orbital electron of the oxygen in formaldehyde H2C = O: to 



