niYRICAL AND OHEMK'AL QUENCHING 781 



Li^•ingston and co-workers (Livingston 1948, Livingston and Ke, 1949) 

 made the first systematic investigation of changes in the intensity of 

 chlorophyll fluorescence, produced by small admixtures. They used 

 various organic compounds, certain salts and several gases. 



As long as the fluorescence remains the only property measured, all 

 the observed changes can be described as quenching (if fluorescence be- 

 comes weaker) or stimulation (if fluorescence becomes stronger — as it 

 actually does upon addition of traces of an alcohol or amine to a chlorophyll 

 solution in dry hydrocarbon, or upon the addition of iodine to an alcoholic 

 solution of chlorophyll h) . It would be best, however, to restrict the terms 

 "(luenching" and "stimulation" to cases in which the admixture does not 

 affect the composition or state of the light-absorbing molecules in the 

 dark, but acts only on molecules which have been excited by the absorption 

 of hght. We may refer to these phenomena as "true quenching" (or 

 "true stimulation"— if the latter does exist at all, which is doubtful). 

 True quenching can be due to physical or chemical processes. In the first 

 case (physical quenching), kinetic encounters of light-excited, fluorescent 

 molecules with the molecules of the quencher, or mutual proximity of these 

 molecules, lead to accelerated conversion of electronic excitation energy into 

 vibrational energy and, ultimately, into heat. The accelerated dissipa- 

 tion can occur within the excited molecule itself (because its configuration 

 or charge distribution change under the influence of the quencher), or in a 

 complex formed by the excited molecule and the quencher, or even within 

 the quencher molecule alone — which in this case, must first take over the 

 electronic excitation energy "in bulk" and then dissipate it, by internal 

 conversion to vibrational energy. (It is also possible for this energy to be 

 re-emitted by the quencher as sensitized fluorescence.) In the second 

 case (chemical quenching), either the excited molecule or the molecule of 

 the quencher (or both) are changed chemically in the process of quenching. 

 In this case, a kinetic encounter of the two molecules is needed. If the 

 photochemical reaction responsible for quenching is completely reversible 

 by a dark reaction, the net result is the same as in physical quenching— 

 conversion of light energy into heat. Otherwise, a net photochemical 

 change remains; and only if this change does not involve the fluorescent 

 species is a steady yield of fluorescence observable in the presence of the 

 quencher. 



Contrasted to true quenching can be the changes in the yield of fluores- 

 cence which are caused by alterations in the composition or structure of 

 the light-absorbing molecules produced by the addition of the admixture. 

 (AH cases of "stimulation" probably belong to this class.) These processes 

 can be distinguished from true quenching by the fact that the absorption 

 spectrum of the solution also is changed by the presence of the quencher 



