PHOTOCHEMISTRY 15 



excitation energy is present as oscillational energy (i.e., if the energy-rich 

 molecule is in its electronic ground state), the solvent will increase the 

 probability of this energy being lost to the surroundings as heat and so 

 reduce the quantum yield. ^Nlany dyes and other complex molecules go 

 (by internal conversion) from their excited singlet states to a relatively 

 long-lived (triplet) energetic state, which is chemically an activated state. 

 Under these latter conditions, deactivation by collisions of the second kind 

 has little, if any, effect on the probability of reaction. 



CAGE EFFECT 



When a dissolved molecule dissociates, its fragments (atoms or radicals) 

 are surrounded by a barrier of solvent molecules. Held in this cage, they 

 will make a number of collisions with one another before they can move 

 out of the cage. This increases the probability that they will recombine 

 and so reduces the efficiency of the photochemical reaction. There are 

 several factors which influence the importance of this "cage effect." 

 When the absorbed photon has greater energy than is required to dis- 

 sociate the molecule, the excess kinetic energy of the resulting atoms will 

 increase their chance of escaping from the cage. This chance of escape is 

 greater for small atoms (especially hydrogen atoms) than it is for larger 

 radicals. In the case of large radicals, there is a compensating factor. 

 The recombination of large radicals requires some (small) energy of acti- 

 vation and demands that very strict conditions of relative orientation be 

 fulfilled. Both these requirements greatly reduce the probability of 

 recombination at a collision and correspondingly decrease the importance 

 of the cage effect. Experimental investigation (Rollefson and Burton, 

 1939, Chap. XIV) of the cage effect demonstrates that it is real and can be 

 of importance in photochemical reactions. However, it appears to be 

 unexpectedly specific. 



A solvent may change the nature of the reaction products either by 

 reacting (in one or more secondary steps) with the primary products or by 

 altering the relative probabilities of alternative primary reactions. An 

 excited complex molecule can dissociate either into two radicals or into 

 two stable molecules. It has been reasonably substantiated (Bamford 

 and Norrish, 1938) that certain ketones undergo both types of dissociation 

 to a comparable extent in the gas phase but, in solution, dissociate only 

 into stable molecules. This difference in products has been attributed to 

 the action of the solvent, which cages in the free radicals and so brings 

 about their recombination. 



PHOTOCHEMICAL TRANSFER OF ELECTRONS OR PROTONS TO THE 



SOLVENT 



In condensed systems the absorption of a photon of ultraviolet or even 

 visible light can lead directly to the formation of an ion by the ejection of 



