54 



of 2,4D, while the 2,4-dichlorophenol cannot be made to touch 

 the two N's of rhodamin with its OH and one of its Cl's. 



The activity of the iodo-substituted phenols (30, 31) should 

 also be noted. Here, evidently, the activity is connected with the 

 high atomic number of the iodine and not the dipole moment of 

 the molecule, since the corresponding chloro compounds, though 

 having a stronger dipole moment, are less active. A heavier chloro 

 substitution, as in pentachlorophenol (27), also renders the mole- 

 cule active, which deserves mention with regard to the insecticide 

 action of substances of this group. 



RHODAMIN COMPLEXES 



During these experiments, a fascinating reaction of rhodamin 

 with nitro- and chlorophenols was observed. If a chloro- or nitro- 

 phenol is added to an acid watery solution of rhodamin, a striking 

 change takes place; the fluorescence which, according to its maxi- 

 mum at 578 rcifx was orange, disappears and the solution assumes 

 the appearance of strong turbidity, without changing color. Look- 

 ing, however, at the light source through the solution, the latter 

 appears to be limpid and deep blue. The change is due to the 

 formation of an insoluble complex in colloidal dispersion which 

 settles slowly to the bottom of the test tube in a day or so. R. Steele 

 (unpublished) studied this compound — evidently a "molecular 

 complex." He found that an undissociated phenolic group and the 

 presence of water are necessary for its formation. On rendering 

 the solution alkaline or dissolving the complex in anhydrous sol- 

 vents, such as methanol or acetone, he found that it readily dis- 

 sociated. There is one quality which may lend biological interest 

 to this reaction — its sluggishness. The formation of the complex, 

 being accompanied by a decrease and a shift of the absorption 

 maximum, can be followed in the spectroscope. Figure 11 (bor- 

 rowed from Steele) illustrates this point. The first eight curves 

 went through an isobestic point, showing that it was actually the 

 rate of the formation of the complex and not its sedimentation 

 which caused the gradual change, the reaction having taken hours 



