FACTORS LIMITING THE YIELD 759 



due, in some cases, to resonance transfer of- excitation energy not requiring 

 molecular contact. Examples will be found on p. 778 (quenching of dye 

 fluorescence by other dyes), p. 790 (chlorophyll q fluorescence sensitized 

 by b) and in chapters 24 and 32 (energy transfer between pigments in 

 vivo). Self-quenching, too, may be caused by resonance (p. 797). 



(e) Self-Quenching 



Experience shows that quenching by molecules identical with the ex- 

 cited one often is particularly strong. This is revealed by rapid decrease 

 in the yield of fluorescence of many substances with increasing concentra- 

 tion of the fluorescent pigment. This strong self -quenching probably is 

 due to very close resonance between the fluorescent molecule and the 

 quencher. However, resonance transfer of electronic energy does not in 

 itself explain self-quenching, because, from the point of view of the yield 

 of fluorescence, it should be irrelevant whether the excitation energy 

 stays with the originally excited molecule or is transferred to another 

 molecule of the same kind. Nevertheless, self-quenching can result from 

 resonance, if some additional phenomena are taken into account. Effective 

 energy dissipation can result either from kinetic encounters of excited and 

 normal pigment molecules ("kinetic self-quenching"), or from their close 

 average proximity ("static self-quenching"). In the first case, we can pos- 

 tulate transient formation of dimeric molecules during the encounter. 

 Resonance between the two structures, D*D ^ DD*, creates an attraction 

 force, leading to a more intimate contact of the two electronic systems 

 than that established in an encounter of two nonresonating molecules. 

 This can bring about accelerated conversion of electronic into vibrational 

 energy; the molecules which met as D* -|- D will then separate as D + D. 



Resonance transfer of excitation energy over distances wider than a 

 collision diameter also can explain self-quenching, if one makes certain 

 auxiliary hypotheses. Forster (1947, 1948) suggested, for example, that 

 even dyestuff solutions which reveal no equilibrium dimerization (i. e., 

 show no effect of concentration on the absorption spectrum; cf. below), 

 contain a small proportion of nonfluorescent, dimeric molecules. If the 

 resonance exchange of excitation energy is so fast that this energy visits. 

 during its life time, a considerable number (say > 100) pigment molecules, 

 the presence of even a single dimer in this series of "hosts" may suffice to 

 "trap" the excitation energy, and dissipate it into heat. (Of course, for 

 this mechanism to be effective, the absorption band of the dimer and the 

 fluorescence band of the monomer must overlap sufficiently to permit reso- 

 nance exchange.) 



Franck and Livingston (1949) suggested another possibility — that 



