32 



LUMINESCENCE SPECTROSCOPY OF MOLECULES 



The study of fluorescence activation of the chlorophylls by Living- 

 ston, Watson, and McArdle (1949) can be interpreted spectroscopi- 

 cally in terms of the above discussion. These workers found that 

 chlorophyll a and chlorophyll b in hydrocarbon solvents showed very 

 little fluorescence. However, especially in hydroxylic solvents such as 

 alcohols and water, strong fluorescence was observed. 



^(iT;ir/ 





t 



UJ 



(triplet) 



HVPROCARBON HYDROGEN BONDING 



Fig. 1. Effect of hydrocarbon and hydrogen-bonding solvents on the frequencies 

 of electronic transitions of a molecule with tt.tt and n,7r levels close together. 

 Ground states are equalized arbitrarily, and excited vibrational and triplet 

 electronic levels are omitted. 



Moreover, the lowest frequency absorption band of these two chlo- 

 rophylls was more complex in hydrocarbon solvents (cf. their Figs. 

 4 and 5), since it is lower in apparent intensity in the latter (though 

 not necessarily in integrated absorption) and, notably, shows evi- 

 dence of a shoulder on the long-wavelength side of the main peak. 



These results may be interpreted according to the diagram shown 

 in Fig. 1. On the left side of the diagram we indicate the energy 

 relationships which we picture for chlorophyll a. We assume that the 

 shght shoulder shown in Livingston et al. (1949) is an indication 

 of an n,7r-transition just slightly lower in energy than the stronger, 

 7r,7r-transition. Upon changing to a hydrogen-bonding solvent, the 

 latter transition would be expected to undergo a normal red shift. 



