1292 



THE PIGMENT FACTOR 



CHAP. 32 



shoots through the molecules without setting them in vibration, like a 

 bullet passes through a glass pane without shattering it. One therefore 

 expects this type of energy exchange to strip the absorption band of its 

 vibrational structure and, by preventing electronic energy dissipation into 

 vibrations, to produce a high yield of fluorescence of the "resonance" 

 type {i. e., fluorescence with a wave length practically identical with that 

 of the absorption band). 



Absorption and fluorescence phenomena of this type have been ob- 

 served by Scheibe et at. (1936-41; see also Katheder 1940; Jelley 1936) in 

 concentrated solutions of certain dyestuffs. These investigators found 

 that the absorption spectra of isocyanine, pinacyanol and certain other 

 dyestuffs change radically when their concentration in aqueous solution is 

 increased above a certain value. The original absorption band loses its 

 intensity and a new sharp and narrow band appears on the long-wave side 



22,000 



20,000 



18.000 



cm 



Fig. 32.7. Absorption spectrum of pseudo-isocyanine at three different concen- 

 trations, showing the formation of dimors at 5 X 10"' mole/1, and of polymers 

 with a sharp absorption band at 1 X 10 "^ mole /I. (after Scheibe 1938). 



of the original band. Figure 32.7 shows this phenomenon for pseudo- 

 isocyanine. This change is due to linear 'polymerization of the dyestuff ; it 

 disappears sharply upon heating to a certain temperature. The extinction 

 coefficient of the polymer per single link is of the same order of magnitude 

 as that of the monomeric form, thus showing that the "super-molecule" 

 acts as the sum of as many chromophores as it contains monometric mole- 

 cules. 



Figure 32.7 shows that, in a typical case, the polymer band is shifted 



