484 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



is important for us here is the statement that light absorption by chloro- 

 phyll in solution may lead to the formation of long-lived active products. 

 Weil and Malherbe (1944) suggested that strong self-quenching may ex- 

 plain the nonquenching of fluorescence by sensitization substrates; but 

 this explanation is not applicable when the quantum yield of sensitization 

 is high. 



1. Long-Lived Activated States 



In the photochemistry of simple molecules in the gas phase, the only 

 alternative to short-lived electronic excitation (duration ~ 10"'^ sec.) is 

 primary photochemical dissociation. If this dissociation is reversible, and 

 the dissociation products are capable of catalyzing autoxidations, this 

 mechanism can account for the lack of relationship between fluorescence 

 and sensitization, since the "long-lived activation state" can be identified 

 with the state of dissociation. However, in a complex molecule in solution, 

 several other processes — e. g., tautomerization, or reversible photochemical 

 reaction with the solvent — also may lead to "long-lived activation." We 



shall discuss these alternatives presently. 



(a) Primary Dissociation 



Excited dyestuff molecules may dissociate, e. g., lose a hydrogen 

 atom (Franck and Wood 1936). 



(18.1) Chi* > H + oChl 



(o for oxidized, e. g., dehydrogenated chlorophyll). One may object 

 that, if such dissociation would occur, chlorophyll solutions would not 

 fluoresce at all; in diatomic gases (like iodine vapor), direct photo- 

 chemical dissociation takes place within one vibrational period of the 

 molecule (~ 10~^^ sec), thus reducing fluorescence to zero. However, 

 in polyatomic molecules, photochemical dissociation may be delayed 

 until sufficient thermal energy has accumulated (by energy fluctuation 

 within the molecule) in the degree of freedom where it is needed for 

 decomposition (Franck and Herzfeld 1937); and the delay may give to 

 some excited molecules the chance to fluoresce. 



However, a direct photochemical dissociation of chlorophyll appears 

 unlikely for a difl"erent reason, the insufficiency of available energy. In 

 the lowest fluorescent state, Y, reached by absorption of red light (c/. 

 Vol. II, Chapter 21), chlorophyll contains only about 40 kcal per mole of 

 excitation energy, hardly sufficient to break a carbon-hydrogen bond. 

 (According to table 9. II, a standard R — H bond has a strength of about 

 100 kcal; stabilization of radical R by resonance could bring it to 70, 

 perhaps even to 60 kcal — as in the case of viologens, mentioned on page 

 233 — but hardly any lower.) 



In the nonfluorescent excited state B which is reached by the ab- 



