180 LIGHT AND LIFE 



(B). Excitation in (liain-enclin<> step. Reaction (/) would be followed 

 by the chain: 



H H 



i I 



Rr—C—O- + i?2— C— OOH -^ 



H 



I 

 /?2— C=0 + R2—CO' + HoO, (5) 



and finally by reaction (2) . 



(C) . Excitation in chain-starting step. Reaction (/) would be fol- 

 lowed by: 



H 



1 

 (A/+— OH-) + /?>— COOH -^ 



/?2— COOH + M* + H2O {4) 



Ri—COOn ^ /?2— C=0 + -OH (5) 



H 



I 

 •OH + /?2— COOH -^ chain 



(D) . Excitation in chain-propagating step. Depending on the relative 

 reactivity of peroxide and porphyrin toward hydroxyl radicals, we 

 may have, after reaction (5) , 



M + -OH -^ (A/+— OH-), {6) 



followed again by reaction (V) . Hydroxyl radicals in this mechanism 

 may also arise originally from the sinij^le thermal cleavage of the 

 peroxide. 



Other possible sequences may evidently be written. 



The close parallel between intensity of hmiinescence and rate of 

 peroxide decomposition is consistent with all these mechanisms, as 

 is the overall second-order character of the respective constants. In 

 mechanism B, for example, any other competing chain-ending step 

 (due j)erhaps to intermediates in dye breakdown) \vould diminish 

 both the peroxide decomposition rate and the luminescence. Mechan- 

 isms for excitation involving reac tions of type (■/) , in \\hich the 

 prodiuts include botJi free radicals (oid excited molecules, are en- 

 eigctically feasible, l)ut would seem to be intrinsically less probable 



