INFLUENCE OF SOLVENT ON YIELD 765 



Obsei-vations of Albers and Ivnorr (1934, 1935) and Kjiorr (1941) con- 

 cerning the changes in the fluorescence spectra of chlorophylls a and b 

 with time, in different solvents (ether, acetone, benzene and methanol), and 

 under different atmospheres (air, oxygen, carbon dioxide and nitrogen) 

 revealed a bewildering variety of shifts in positions, shapes and intensities 

 of the fluorescence bands, which do not lend themselves to easy mterpreta- 

 tion, but indicate complex chemical changes. Apparently, both the sol- 

 vent and the dissolved gases participated in chemical reactions with ex- 

 cited chlorophyll molecules. In some systems, these reactions led to a 

 complete disappearance of fluorescence after less than one hour of illumin- 

 ation. One reason for the complexity of the results of Knorr and Albers 

 may have been the use of unfiltered light from a powerful mercury arc. 

 Strong ultraviolet irradiation may have caused chlorophyll to react with 

 substances that would not have affected it in visible (particularly red) 

 light. 



One strange observation of Ivnorr and Albers is that the fluorescence of 

 chlorophyll a (but not that of chlorophyll h) in acetone (but not in other 

 solvents) is best preserved under an atmosphere of oxygen (where it dis- 

 appears only after twelve hours of illumination, whereas, in nitrogen or 

 carbon dioxide, it vanishes completely in less than one hour). As de- 

 scribed in chapter 18 (Vol. I, page 491), this may mean that the quenching 

 of fluorescence is caused, in acetone, by a reduction of the pigment (and 

 oxidation of the solvent) and that oxygen restores the reduced pigment to 

 its original fluorescent form (c/. chapter 36) . 



We have considered so far only those changes in the intensity of fluores- 

 cence which could follow from the interaction of the chlorophyll molecule 

 in the excited state with the medium. It w^as mentioned, however, in the 

 introduction that another type of fluorescence effects is possible — one in 

 which the state of the chlorophyll molecule is altered already in the dark, 

 prior to excitation. This alteration must manifest itself in a change of the 

 absorption spectrum. One possibility of this type is that chlorophyll may 

 dimerize (or polymerize) in some solvents, and remain monomeric in others. 

 Dimerization is known to cause the disappearance of fluorescence of many 

 dyestuffs (such as methylene Ijlue) in aciueous sohition. In the case of 

 chlorophyll, no similar effect has as yel Ix'on discoN^crod— unless owv con- 

 siders the nonfluorescence of colloidal solutions and solid chlorophyll as 

 the result of "quenching by polymerization." Another type of associa- 

 tion, how^ever, appears to be important in this case — association of chloro- 

 phyll molecules with hydroxyl groups or amine groups present in solvent 

 molecules; in contrast to the quenching effect of dimerization, association 

 of this type seems to be necessary to bring out the fluorescence of chloro- 

 phyll. 



