LONG-LIVED ACTIVATED STATES 485 



sorption of blue or violet light, chlorophyll contains about 67 kcal of 

 excitation energy, an amount which may, with the assistance of thermal 

 energy, suffice to bring about dissociation. The lower yield of fluores- 

 cence (Prins, c/. Vol. II, Chapter 23) and the higher yield of bleaching 

 (Wurmser, c/. page 496) observed in chlorophyll solutions in blue light 

 (as compared with red light) may be symptoms of such a direct photo- 

 chemical dissociation; but these results need experimental confirmation. 



(&) Tautomerization 



An excited organic molecule may pass, by means of a nonradiant 

 internal transformation, into the ground state of a tautomeric form: 



(18.2) Chi* . tChl 



(t for tautomeric). Often, the result of tautomerization is "delayed 

 fluorescence," or phosphorescence (c/. Vol. II, Chapter 23). Chlorophyll 

 solutions, as a rule, do not show this effect. However, this does not prove 

 the absence of tautomerization; the energy of the tautomer may merely 

 be too low to allow its return into the normal form by a reversal of the 

 process by which it was formed (that is, by means of the back reaction 

 in 18.2, and emission of delayed fluorescence) while the direct trans- 

 formation of the tautomer back into the ordinary pigment, may occur by a 

 nonradiant rather than with the emission of phosphorescence process (c/. 

 Vol. II, Scheme 23.1). 



(c) Reversible Chemical Reaction 



Excited chlorophyll molecules may react with the solvent, the most 

 probable reaction being an oxidation-reduction. (An electronically ex- 

 cited molecule has an increased tendency for giving away an electron, 

 as well as a capacity for acquiring an electron to replace the one which 

 was removed from its normal level; cj. Weiss 1938). 



(18.3) Chi* + Ox. > oChl + rOx. or 



(18.4) Chi* + Red. > rChl + oRed. 



(Ox. for oxidant, Red. for reductant, o for oxidized, r for reduced.) In 

 impure, or concentrated solutions, an impuritij, or a second molecule of 

 the pigment, may replace the solvent as partner in the oxidation-reduction. 

 In the last case, the primary photochemical reaction is a photodismutation : 



(18.5) Chi* + Chi > rChl + oChl 



Since the quantum yield of the irreversible photochemical decomposition 

 of chlorophyll in solution is very small (c/. page 496), any of the above 

 reactions, if it occurs with a high quantum yield, must be almost com- 

 pletely reversible. 



To sum up — in considering the fate of excitation energy in illuminated 

 chlorophyll solutions, we may neglect the probability of monomolecular 



