50 PHOTOCHEMICAL PRINCIPLES 



number of molecules over which transfer occurs, is also inversely pro- 

 portional to the sixth power of the distance between the molecules and 

 thus directly proportional to the square of the concentration. In gen- 

 eral, the absorption and emission spectra of the components will not 

 be changed by resonance transfer. This mechanism is generally con- 

 sidered to be the most important one in visible or UV-induced energy 

 transfer phenomena. 



Experimental Aspects of Energy Transfer 



Efficient energy transfer has been demonstrated thus far only from 

 aromatic compounds, with one exception, 1,4-dioxane (Kallman et ah, 

 1956). Energy transfer from many other types of organic molecules, 

 both saturated and unsaturated, has been observed, but only with 

 lower efficiency. This is probably due to the fact that the excited state 

 lifetimes of these molecules are much shorter than those of the aro- 

 matic solvents. Thus, a higher concentration of acceptor molecules is 

 necessary. With such poor solvents it is possible to increase the yield 

 of energy transfer by adding a small amount, say 10%, of an efficient 

 energy acceptor. This acceptor will then mediate the transfer of energy 

 from solvent to solute. 



The general types of observable phenomena which can be inter- 

 preted in terms of energy transfer are as follows: 



1. Sensitized emission. In such measurements one dissolves a small 

 amount of an emitting substance in a solvent which absorbs energy at 

 somewhat shorter wavelengths (i.e., higher energy) than does the 

 solute. Upon excitation of such a system with radiation absorbed only 

 by the solvent, the resulting emitted light is that characteristic of the 

 solute. The solvent emission is normally almost completely absent. 



2. Concentration depolarization. The fluorescence emitted by dilute 

 solutions of organic molecules in viscous solvents is normally partially 

 polarized owing to a limited amount of orientation of the molecules. 

 As the concentration is increased, however, the extent of polarization 

 decreases. This is due to a transfer of excitation energy between mole- 

 cules in different orientations. 



3. Self-quenching of fluorescence. The quantum yield of fluores- 

 cence of solutions of organic molecules generally decreases with in- 

 creasing concentration. This phenomenon is made somewhat more 



