HtNR) I.ISSCHIIA 17!) 



question ol ihc luniiiicsccnre alone, the evidence of Fig. .''> that the 

 luinines(ent molecule is the excited inuhanged porphyrin itsell also 

 indicates that the dye is excited in the coinse ol its catalytic function, 

 rather than its breakdown. The proposed mechanisms are based on 

 tiiis reasonable assumption. 



The metal requirement suggests that the first step in the reaction is 

 complexing of the peroxide at the central metal ion. By analogy 

 with the classic mechanism of Haber and Weiss (4, 1) , we then as- 

 siniie an initial oxidation of the metallo-porphyrin. This may occur 

 via hydrogen atom or electron transfer (9) . In tetraphenyl porphyrins 

 and phthalocyanines, it is evidently much more likely that electron 

 transfer is involved. We therefore Avrite (Af ^ metallo-porphyrin): 



H H 



Rr-COOn + M -^ R.—C~0- + M+ + OH- 



Particularly in non-polar solvents, the driving force of this reaction 

 \\ill be increased by binding OH^ to the central metal ion, giving 

 a type of charge-transfer complex: 



H H 



I I 



/?2— COOH -\- M ^ R2~C~0' + (M+— OH-) (7) 



X'arious reactions may now ensue. However, in all of these it is 

 proposed that electronic excitation of the metallo-porphyrin results 

 from removal of 'OH from the charge-transfer complex to form 

 HjO, and the return of the electron into the ir-system of the porphyrin. 



(A) Excitation in bimolecular reaction. Following reaction (/) , we 

 may have {M* = excited metallo-porphyrin) : 



H 



/?..— C— O- + (iU+— OH-) -^ 



R.2~C=0 + M* + HoO (2) 



In this case, reactions (/) and (2) may be considered to occur 

 without separation of the intermediate products. However, the kinetic 

 complications of this reaction and of peroxide chemistry in general 

 (12) indicate that chain processes are involved. Reasonable mech- 

 anisms may be written in which the excitation process may occur in 

 chain-starting, propagating, or ending steps, as follows: 



