PHOTOCHEMISTRY 285 



2C14H10 ?=^ (Cl4Hio)2 



The de-polymerization of the double compound proceeds as a thermal 

 reaction. Anthracene shows a brilliant fluorescence when excited by 

 ultra-violet light. The di-anthracene is not affected by the ultra-violet 

 light, giving neither fluorescence nor dissociation. 



These facts lead to an understanding of the photochemical reaction. 

 When an electron is displaced in the anthracene molecule by the absorp- 

 tion of a photon of ultra-violet light, the molecule becomes excited and, 

 on collision with a second anthracene molecule, produces the di-anthra- 

 cene. If the excited molecule cannot collide with the second molecule 

 immediately, the electron returns to a lower energy level and emits visible 

 light. The fluorescence decreases in intensity as the concentration of 

 anthracene molecules increases. At ordinary light intensities there is a 

 sufficient number of molecules to give reaction, and the rate of the reac- 

 tion is independent of the concentration, i.e., it is a zero-order reaction. 

 The photochemical polymerization, as in the case of other photochemical 

 reactions, has a small temperature coefficient; the reverse reaction, the 

 de-polymerization, is an ordinary thermal reaction and as such has a 

 high temperature coefficient. The rate of polymerization to di-anthra- 

 cene is directly proportional to the intensity of the light, and the rate of 

 decomposition is independent of the light intensity and directly pro- 

 portional to the concentration of di-anthracene. These facts can be 

 expressed by the following equation, in which x represents the con- 

 centration of di-anthracene, / the intensity of the light absorbed, k a 

 velocity constant for the light reaction, and k' a velocity constant for the 

 dark reaction: 



dx/dt = kl — k'x 



When the rate of de-polymerization exactly offsets the rate of poly- 

 merization, dx/dt = and an equilibrium or stationary state exists, 

 given by the following equations: 



kl = k'x and x = kl/k' 



It is evident that the concentration of di-anthracene in this stationary 

 state will depend on the intensity of light and on temperature. The 

 temperature change for the photochemical reaction (k) for a 10° rise is 

 1.1; and the temperature change for the thermal de-polymerization (A;') 

 is 2.8 for a 10° rise. The ratio of these two values, 1.1 divided by 2.8, 

 gives directly the influence of a 10° rise in temperature on this stationary 

 state and permits a simple experimental check of these conclusions. The 

 experimental facts agree well with these calculations (55, 56). 



This reaction is an interesting one in showing how the quantum yield 

 changes with concentration and with other variables. At the very 

 beginning of the illumination, very little di-anthracene has accumulated 

 and the reverse reaction is negligible. The quantum yield should then 



