PHOTOREDUCTION OF SYNTHETIC DYES 53 



DYES IN THE BOUND STATE 



Most dyes exhibit a spectral shift to long wavelengths when small 

 amounts of a high polymeric material to which the dye binds are 

 added to the solution. In the case of diphenyl and triphenyl methane 

 dyes, the fluorescence of the bound dye is considerably greater than 

 that of the unbound dye (12). Such dyes also exhibit a greater fluo- 

 rescence when in highly viscous media, and it appears that under such 

 conditions the dye molecules in the excited state spend a longer time 

 in the planar configuration (a necessary condition for fluorescence) 

 than they do in low-viscosity media (13). 



Dyes in the bound state have profoundly different photochemical 

 properties from those of free dyes. For example, dyes of the fluores- 

 cein family are photooxidized irreversibly with a quantum yield in the 

 neighborhood of 10"'* to 10 ""^ On binding, however, the quantum 

 yield is decreased by a factor of about one thousand. Acriflavine 

 under conditions where it is not photoreduced in the unbound state, 

 is readily photoreduced when bound to polymeric acids (14). Tri- 

 phenyl methane dyes when bound to polymeric acids resist reduc- 

 tion in the dark on treatment with strong reducing agents which 

 readily reduce the free dye. With light, however, the situation is 

 reversed. Now the bound dye, in the presence of a mild reducing 

 agent, is photoreduced w^hile the unbound dye remains colored (15). 



Introduction of certain substances such as nitrobenzene to an 

 aqueous system containing the dye in the bound state and an elec- 

 tron donor inhibits the photoreduction of the dye, the induction 

 period during the irradiation being proportional to the concentra- 

 tion of inhibitor. After the induction period is complete, the rate of 

 the reaction becomes that of the original uninhibited system. It 

 appears that the dye molecules on each polymeric substrate molecule 

 (the local concentration is very high) are acting as a single photo- 

 chemical unit and a few inhibitor molecules can affect several hundred 

 dye molecules. 



Discussion 



Rabinowitch: In the case of methylene blue, you also had no quenching of 

 fluorescence? 



Oster : No, we used too small amounts of allylthiourea. 



Rabinowitch: What is the quantum yield of fluorescence of methylene blue? 

 Ten per cent? 



