ENERGY EXCHANGE IN PHOTOREACTIONS 37 



Diabatic sensitization reactions are the primary concern of this section. 

 Most information on such reactions has come from studies of the quench- 

 ing of fluorescence. 



4-2. THE QUENCHING OF FLUORESCENCE 



Most quenching reactions follow some variant of a mechanism origi- 

 nally suggested by Stern and A^olmer (1919): 



k, 



A -f hp^ A*, (l-30a) 



A*-^A-\-hv, (1-306) 



A* -\- Q^A -h Q*. (l-30c) 



The efficiency of substances as quenchers is measured by the rate con- 

 stant ka, which is related to the light absorbed per second /„ and the 

 fluorescence intensity // by 



so that k2 values can be calculated once k-i is determined in other experi- 

 ments. More detailed discussions of quenching mechanisms can be found 

 in Rollefson and Stoughton (1941) and Rollefson and Boaz (1948). For a 

 majority of cases exact kinetic schemes have not been established. 



We may distinguish three types of quenching reactions, and hence three 

 interpretations of k2. In the first, every collision between excited and 

 quenching entities immediately destroys quenching entities. Diffusion is 

 rate-limiting. The value of k2 will vary with the viscosity of the solution 

 but not with the chemical properties of the quencher. When the primary 

 molecule is large with respect to the quencher, the rate constant may be 

 expressed approximately in terms of an expression due to Smoluchowski 

 (1918) for the coagulation of particles, 



k. = 4wDR, (1-32) 



in which R is the effective quenching radius of the excited molecules and 

 D is the diffusion constant, which can also be written as 



D = XH ^^ e-AFt/ier_ (1_33) 



In this expression X is the length of individual jumps of the quenching 

 molecules through solution and is equivalent to the mean free path in 

 a gas-phase reaction. The other symbols have their usual meanings 

 (Glasstone et at., 1941, p. 477). It is ordinarily necessary to employ a 

 time-dependent expression for the amount ^4 reacted in time r at con- 

 stant incident light intensity: 



R 



A = ^tRDco 



^ "^ (tDt) 



(1-34) 



