336 LIGHT AND LIFE 



Light off 

 -I Min- 



Fig. 14. Effect of illmnination on tlie election spin resonance signal of o-chloranil 

 "doped" metal-free phtiialocyanine. Curve represents unpaired spin concentration 

 \s. time. 



The conductivity, of course, goes in the reverse direction; when the 

 light is turned on, the conchictivity increases and when the light is 

 turned off, the conductivity decreases, with rates corresponding to 

 these spin signal changes. 



Fig. 15 shows the relationship between the kinetics of the spin 

 signal behavior and the several other associated phenomena in the 

 same lattice, such as conductivity, etc. Fig. 16 shows the interpre- 

 tation of these phenomena. They are interpreted in terms of electron 

 transfer from phthalocyanine molecules, in the lattice, to the o- 

 chloranil to give positive ion radicals of phthalocyanine which are 

 in a crystal lattice, and these are responsible for the conductivity 

 (Fig. 16-1). The electrons on the o-chloranil negative ion radical are 

 not mobile, and presimiably they are the things which the ESR 

 equipment sees. When the light is turned on, it is absorbed by the 

 phthalocyanine, and the exciton can migrate around in the phthalo- 

 cyanine until it is ionized (Fig. 16-2) . This ionization may take 

 place either at some unknown center, or the exciton may come di- 

 rectly in contact with the o-chloranil negative ion, transferring a 

 second electron to the o-chloranil negative ion, and thus reducing 

 the number of unpaired spins but increasing the conductivity. In 

 the dark the reverse reaction occurs. When light at 4000 A is used, 

 the reverse effect is observed, that is, there is a transfer of the elec- 

 trons from the o-chloranil negative ion radical into an unoccupied 

 orbital of the phthalocyanine crystal (Fig. 16-3) . This is also an 

 easily movable electron rather than a trapped one, and it will im- 

 mediately and rapidly recombinc with the conducting holes (positive 

 ion centers) originally present in the lattice, and thus result in a 

 decrease in conductivity but an increase in electron spin signal 

 (Fig. 16-4). 



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