516 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



As mentioned on page 491, a tautomerization of the sensitizer may 

 be replaced by a reversible reaction with the solvent, as, for example: 



(18.34a) Chi* + S V rChl + oS 



(18.34b) rChl + i O2 > Chi 



(18.34c) oS + A > oA + S 



(18.34) A + 102 J'oA 



A mechanism similar to (18.33) may account also for the sensitized 

 oxidoreductions listed in the last part of table 18.1: 



(18.35a) Chi* > tChl 



(18.35b) tChl + Ox > oChl + rOx 



(18.35c) oChl + Red > Chi + oRed 



(18.35) Ox + Red > rOx + oRed 



In this case, too, a primary reaction with the solvent can be substituted 

 for tautomerization as the initial step. 



(c) Energy Transfer to the Reductant (Mechanism Aa2) 



It has been mentioned on page 500 that Gaffron (1927, 1933, 1937) 

 postulated a transfer of the excitation energy of chlorophyll to the oxida- 

 tion substrate, A (c/. Eq. 18.22a). To explain the efficiency of sensitized 

 oxidation at low oxygen pressures, Gaffron had to assume that these 

 substrates, e. g., amines or rubrene, are transferred into long-lived 

 activated states. Kautsky objected to Gaffron's mechanism because 

 many sensitization substrates — including allyl thiourea, used by Gaffron 

 — do not quench the fluorescence of chlorophyll. However, this objection 

 loses its strength if one assumes, with Franck and Livingston (1941), 

 that sensitization is brought about by a long-lived active modification of 

 chlorophyll, rather than by the short-lived fluorescent chlorophyll 

 molecules. 



If we assume tautomerization (or a reversible reaction with the 

 solvent) as the first step, the next logical step is oxidation-reduction 

 (transfer of electrons or hydrogen atoms), rather than a transfer of 

 energy (as assumed by Gaffron), because the active product, being 

 chemically different from normal chlorophyll, cannot return to the 

 normal state without a shift in the position of the atomic nuclei (e. g., 

 an intramolecular or intermolecular transfer of a hydrogen atom). We 

 have then to consider, instead of mechanism Aa2, mechanism A^2, that 

 is, an oxidation-reduction reaction between sensitizer and reductant. 



Franck and Levy (1934) suggested that collisions with excited dyestuff molecules 

 may induce a dissociation of the acceptor (which we formulate for this purpose as RH) : 



(18.36) Chi* + RH > Chi + R + H 



