26 LUMINESCENCE SPECTROSCOPY OF MOLECULES 



The main points of our discussion will involve (1) the role of 

 n,7r-transitions in the chlorophylls, (2) the importance of intercom- 

 binations in these molecules, and (3) the possible interaction of 

 the ethylenic potential function with the electronic transitions in the 

 chlorophylls. Our interpretations will be applicable rigorously to the 

 properties of the chlorophylls as physically isolated molecules in vitro. 

 However, by discussing expected changes in properties of the mole- 

 cules upon change in environment, we shall be able to extrapolate 

 occasionally to the behavior of the molecule in vivo as a photosyn- 

 thetic agent. 



Omitted from our discussion will be any discussion of chemilumi- 

 nescences, of the mode of energy transfer involving chlorophylls, 

 and of other problems which are involved mainly in the biological 

 system, such as accounting for the difference in the chlorophyll ab- 

 sorption bands in the living plant and in vitro. 



II. Importance of Lowest Excited Electronic States in 

 Utilizability of Excitation Energy 



( 1 ) Internal Conversion 



Extensive studies in the field of molecular luminescence have re- 

 vealed that only the lowest electronic state of a given series of elec- 

 tronic states in a molecule is capable of re-emitting its excitation 

 energy. Upper electronic states of the series, upon excitation, lose 

 energy thermally (by collisions) and radiationlessly go over into the 

 lowest electronic excited state of the series. By series of states is 

 meant states of one electronic multiplicity, or mode of electronic 

 pairing (see Section V). This phenomenon of radiationless combina- 

 tion of excited states is known generally in the field of spectroscopy 

 as internal conversion (for additional discussion, see Kasha, 1950). 

 Its simplest consequence is that only the lowest excited singlet state 

 is important in any energy transfer or energy utilization process: the 

 higher excited states decay far more rapidly and generally cannot be 

 expected to participate in any photochemical process. 



(2) Observation of Fluorescence 



In unsaturated molecules, including the chlorophylls, the w-electrons 

 give rise to most of the low-lying states corresponding to absorption 



