HENRY LINSCHITZ 181 



than those resulting in non-radical piothu ts only. The ultimate break- 

 clown of the dye may residt Irom molecules ot (M+ — OH~) which 

 are diverted from the reaction secjuence of Mechanism A, or from 

 the simple chain mechanisms. Alternatively, the dye breakdown may 

 proceed by free-radical reactions not directly related to those which 

 result in excited catalyst molecules. 



The excitation process itself may perhaps be mulerstood more 

 simply by saying that the electron, transferring from OH- to M+, 

 drops back into an excited orbital instead of the ground state. In 

 classical terms, we may consider that the electronic oscillator (the 

 harp-string of the porphyrin!) is set into vibration by the electron 

 displacement and return. In fact, if sufficient energy is available, one 

 might expect that quite generally the returning electron will fall 

 into another orbital, since the reaction is accompanied by movemerits 

 of the heavy nuclei, and the electronic energies of the various orbitals 

 are coupled to the nuclear configuration. Charge-transfer will thus 

 be accompanied by a change in molecular shape, and provide the op- 

 portunity for transition into a new electronic state during the back- 

 transfer. 



The frequent association of peroxide-breakdown reactions with 

 chemiluminescence of complex organic molecules suggests that the 

 mechanism in all such cases may be essentially the same as that pro- 

 posed here for the jDorphyrin reaction — that is, the molecule being ex- 

 cited, or its precursor, functions always as a catalyst for peroxide de- 

 composition, the excited state being reached following charge-transfer 

 processes involved in this catalysis. However, the general charge- 

 transfer mechanism for excitation need not, of course, specifically 

 require peroxides. 



Recent work on quenching of excited (triplet) states of aromatic 

 molecules (including porphyrins) by heavy metal ions indicates that 

 charge-transfer intermediates also are involved here (10) . Thus, if 

 R* is an excited molecule and / is the quenching ion, we would have 

 (direction of charge-transfer is arbitrary) formation of the complex 



R* -\- I -^ (R+~I-) ^ /? + / 4- heat 



followed by radiationless transition to the ground state. It thus 

 appears that ionic intermediates play an important role in catalyzing 

 transitions among the electronic states of the separated donor or 

 acceptor. This is not to say that charge-tiansfer processes are the only 

 ones which lead to chemiluminescence. In cases where atom rather 

 than charge transfer may appear to be the reaction mechanism, we 

 may, of course, utilize the coupling of electronic and vibrational 



