ABSORPTION AND EMISSION CENTERS. 



13 



is brownish red. But the compounds NH : C G H 4 : NH and : C a H 4 : NH are 

 colorless. This can easily be explained if the latter compounds are assumed 

 to have the superoxide formula similar to that of o-benzoquinone. The qui- 

 none chromophore enters into the composition of quite a large number of 

 colored compounds. 



The space relations of the chromophores seem to affect the color of 

 compounds. For instance, the ethylene group can have two isomeric config- 

 urations : 



R C H R C H 



or 



R' C H 



H C R' 



This geometric isomerism may explain the following facts: 



1. Diethoxynaphthostilbenes, C 2 H 5 C 10 H 6 .CH : CH.C^He.OC.H.. 1 

 The form which has the highest melting-point is colorless, while the lower 

 melting form is yellow. 



2. Dibenzoylethylenes, C 6 H 5 CO.CH : CH.COC 6 H 5 . 2 The higher melt- 

 ing form is colorless, while the lower melting form possesses a deep yellow 

 color. 



That the color-producing power of the chromophores is due to the double 

 bonds seems quite certain. When these bonds are saturated the resulting 

 compounds are colorless. 



Colored. 

 C 6 H 5 . CO. CO. CO. C 6 H 5 , diphenyltriketone. 

 C 6 H 5 N = N C 6 H 5 , azobenzene. 

 C 6 H 6 N = 0, nitrosobenzene. 

 = C 6 H 4 = 0, quinone. 



Colorless. 



CH 



H . CO. CH. CO. C 6 H 5 , dibenzoylmethane. 

 C 6 H S NH NHC 6 H 5 , hydrazobenzene. 

 C 6 H 5 N. H. OH, phenylhydroxylamine. 

 HO. C 6 H 4 . OH, hydroquinone. 



The different di- and tri-substitution products usually give rise to dif- 

 ferent absorption bands when the absorption is selective. Ortho- and meta- 

 xylene have one band, whereas paraxylene has two. The following gives the 

 value of the limit of absorption when light is passed through a gram molecule 

 of the substance : 



When a chromophore is introduced into a compound the bands may be 

 shifted towards the red or towards the violet. The former effect is batho- 

 chromous, the latter hypsochromous. The effect of joining chromophores is 

 usually bathochromous. For example: 



1 Elbs: Journ. prakt. Chem., 47, 72 (1893). 



2 Paal and Schulze: Ber. d. chem. Ges., 33, 3795 (1900); 35, 168 (1902). 



