R. S. BECKER AND M. KASHA 27 



bands in the visible and ultraviolet regions (cf. Coulson, 1947, and 

 Piatt, 1951, for general discussions). A general study of luminescence 

 properties of molecules (Kasha, 1950) shows that if the 7r-electronic 

 states are the lowest singlet states of an excited series, fluorescence 

 generally will be observed. If, however, the lowest excited singlet level 

 is of an n,Tr type (cf. Section IV), the molecule will be generally a 

 nonfluorescent one. In Section IV we shall interpret the behavior of 

 chlorophyll fluorescence activation on this basis. 



The intrinsic lifetime of the lowest singlet excited state will limit 

 the probability of utilization of the excitation energy of that state. 

 If the state is very short-lived, it may be that no energy transfer or 

 photochemical process will be rapid enough to compete with spon- 

 taneous fluorescence emission. If a molecule has a high intrinsic 

 quantum yield of fluorescence (or phosphorescence, see below) in 

 an undisturbed (isolated) system, this property indicates excitation 

 energy availability. However, if strong fluorescence actually is ob- 

 served for the molecule in any reacting system, the lifetimes of 

 fluorescence must be shorter than those for any other process and 

 the observed quantum yield of fluorescence then indicates energy 

 wasted. In other words, the spontaneous emission rate in such a case 

 was comparable to the rates for some other energy utilization process. 

 A few quantitative relations governing lifetimes and quantum yields 

 will be given in the next section. 



(3) Lowest Triplet State of the Molecule 



All normal molecules studied thus far, barring understandable 

 exceptions, have exhibited lowest triplet excited states (Lewis and 

 Kasha, 1944; Kasha, 1947). By normal molecules is meant here that 

 an even number of electrons is present in the molecule, with electron 

 pairing, so that diamagnetic, singlet ground electronic states result. 

 The common electronic excitation is that in which the electron pairing 

 is maintained: this gives rise to all the commonly observed absorption 

 bands and the fluorescence phenomenon. Such transitions are labeled 

 singlet — > singlet. 



On the other hand, if electron pair uncoupling takes place, states 

 of higher multiplicity are observed, the common state of this type 

 being a triplet state in normal molecules, the singlet-triplet transition 

 being designated an intercombination. Of course, many such triplet 



