ENERGY TRANSFER IN PHOTOCHEMICAL SYSTEMS 55 



the crystal. However, all real crystals contain imperfections in the 

 lattice structure resulting from dislocations, vacancies, impurities, etc. 

 Such imperfections will cause some of the excitons to be ionized, i.e., 

 the electron and the hole will no longer be constrained to migrate as a 

 unit, but rather each will be capable of moving independently of the 

 other. Furthermore, the crystal imperfections will give rise to trapping 

 centers which are capable of immobilizing the electrons or the holes or 

 both. These traps may be considered as energy levels lying somewhat 

 below the lowest level of the conduction band. Ultimately, the electrons 

 and holes will recombine with each other, but it is quite possible for 

 them to have very different histories before recombination, spending 

 various amounts of time trapped in impurities and imperfections in 

 different parts of the crystal. In general, the most mobile entities in 

 organic crystals are the holes (Kallman and Silver, 1956). 



There is some controversy in the literature (Birks, 1953) as to 

 whether exciton migration or resonance transfer is the most significant 

 mechanism for energy transfer in ordered systems. The objections of 

 Franck and Livingston (1949) and of Livingston (1957) to the 

 exciton theory, in the case of anthracene crystals, are primarily that 

 one cannot account for the absorption and emission spectra of the 

 pure crystals in terms of it. However, it must be pointed out that an 

 adequate quantum mechanical interpretation of the absorption spec- 

 trum of crystalline anthracene has been given by Davydov (1948) and 

 by Craig and Hobbins (1955) on the basis of exciton theory. Further- 

 more, recent work (Compton et ai, 1957) on the photoconductivity 

 (see below) of anthracene crystals and on the sensitized fluorescence 

 of impurities in anthracene crystals suggests that both of these phe- 

 nomena represent alternative pathways for the degradation of an 

 exciton, which probably takes place at a dislocation in the crystal. 



Experimental Aspects 



The above discussion, while greatly oversimplified, enables one to 

 obtain insight into many of the electronic properties of organic crystals, 

 some of which include: 



1. Photoconductivity. The electrons and holes formed as a result 

 of the absorption of light, being free to migrate throughout the crystal 

 endow the crystal with the property of conducting an electric current. 



