338 LIGHT AND LIIE 



Fig. 17 shows a representation of the phthalocyanine negative ion, 

 phthalocyanine itself, phthalocyanine positive ion radical, o-quinone, 

 o-quinone negative ion radical, and o-quinone double negative ion, 

 in molecular orbital terms, to call to mind the way in which we have 

 been thinking about the 77-energy levels of these conjugated molecules. 



The actual processes which were seen in Fig. 16 can now be illus- 

 trated in terms of molecular energy levels, and Fig. 18 show^s that 

 interpretation. Reaction (1) is a transfer, in the dark, of an elec- 

 tron from the highest occupied phthalocyanine orbital to the lowest 

 unoccupied o-quinone orbital, leading to the formation of phthalo- 

 cyanine positive ions. This occurs in the crystal lattice so these are 

 conductivity holes, and the trapped electrons are in the o-quinone 



• • • • — • • 



• • — • • 



— • ♦ — •— • — — •— • — 



• _ _ 



0-Q o-Q o-Q~ 



Fig. 17. Schematic repiescntalion of the electronic encrg) Icnels of \arioiis I'c 

 and o-Q species (the \aiioiis species should not be compared). Pc, phthalocyanine; 

 o-Q, o-chloranil. 



negative ion. The lowest unoccupied cjuinone level is shown below 

 the highest phthalocyanine occupied level, and this reaction takes 

 place quite spontaneously in the dark. The j)hotochemical transforma- 

 tion, reaction (2) , involves, first the excitation of the phthalocyanine 

 itself, which could be represented by the raising of an electron from 

 the highest occupied vr-orbital (or from a N-n orbital) to the lowest 

 unoccupied TT-orbital, which must lie very nearly at the same level 

 as the singly-unoccupied orbital of the o-quinone negative ion. The 

 reason for this shift of relative levels is that, whereas in the first in- 

 stance transfer is occurring from one neutral molecide to another, 

 here the transfer is from a neutral molecule to a negatively-charged, 

 already singly-occupied orbital. 



The third jMocess, in Fig. 18, the one represented by the decrease 

 in photoconductivity by illumination at 4000 A, involves the excita- 



