FACTORS LIMITING THE YIELD 757 



As to the specific nature of the quenching reactions, two types appear 

 most hkely: Oxidation-reductions and complex formation. The first 

 type can be represented by equation (23.1 A or B) : 



(23.1 A) Chi* + Q > oChl + rQ 



(23.1B) Chi* + Q > rChl + oQ 



(where o stands for oxidized, r for reduced, Q for quencher and Chi* for 

 excited chlorophyll). The second tj^pe is described by equation (23.2) : 



(23.2) Chi* + Q > Chl*Q * ChlQ > Chi + Q 



the quenching effect being due to accelerated internal conversion of ex- 

 citation energy into vibrational energy in the complex Chl*Q. 



If Q is the solvent, only reactions (23.1 A and B) can be classified as 

 "chemical quenching," while reaction (23.2) becomes identical with "phys- 

 ical" energy dissipation in the solvated pigment molecule. 



Photochemical reactions with foreign molecules interfere with fluores- 

 cence only if they take place directly, i. e., by encounters of electronicallj'' 

 excited molecules with the quencher. If, on the other hand, reactions 

 of this kind are preceded by monomolecular steps such as isomerization 

 (or dissociation) of the excited molecule, their effect on fluorescence may 

 become negligible (since the molecules that take part in the photochemical 

 reaction are the ones lost for fluorescence anyhow; (c/. second scheme on 

 p. 483, Vol. I). We will use this concept below; cf. page 788 in interpreting 

 the nonquenching of chlorophyll fluorescence by certain compounds whose 

 autoxidation is sensitized by this pigment. 



Sometimes the isomeric molecule, formed after light absorption, has a 

 certain chance of reverting into the original electronicalh' excited state 

 (e. g., with the help of thermal energy) . Similarly, photochemical dissocia- 

 tion products may have a certain chance of forming an electronically ex- 

 cited molecule by recombination. If this exact reversal of the primarj^ 

 photochemical process occurs after a period that is long compared to the 

 duration of ordinary fluorescence (10~'^ sec), we obsei-i'e the emission of 

 "delayed fluorescence" or "phosphorescence." Chemical reactions of the 

 metastable, isomeric photoproduct (or of the dissociation products) will 

 cause quenching of this delayed emission. This is the mechanism of the 

 strong quenching of phosphorescence of many dyestuffs by oxygen {cf. 

 page 789). 



{d) Bulk Transfer of Electronic Energy 



Transfer of electronic excitation energy "in bulk" to another type of 

 molecules can lead to the quenching of the fluorescence of the originally 

 excited molecular species, either by substitution of a secondary (stronger 



