Effect of Substituents on Light Absorption

Introduction of methyl substituents into the starred NH3 groups produces a bathochromic effect. By using groups with varying inductive activity, the light absorption of such a dye can be controlled very closely. This is important in color photography, where the accuracy of color reproduction depends on the use of dyes with exactly the right shades. 

Rule 3. Introduction of a +/ substituent has qualitatively the same effect on light absorption as replacing the corresponding carbon atom by a heteroatom, while a — / substituent has the opposite effect. 

For the purpose of light absorption, electromeric substituents can be divided into two types. Those ( , -h ) with both filled and empty n MOs that can interact with n MOs of the substrate, and simple — substituents (NH3, OCH3, etc.) that have a pair of p electrons. 

FIGURE 6.17. Interaction between the HOMO and LUMO of a substituent S and the HOMO and LUMO of a substrate R (a) LUMO of substituent above LUMO of substrate and HOMO of substituent below HOMO of substrate (b) HOMO of substituent between HOMO and LUMO of substrate (c) LUMO of substituent between HOMO and LUMO of substrate. In all cases the transition energy SE for the substituted system is less than that SE for the unsubstituted system. 

The only place where this rule fails is in the case of simple derivatives of benzene such as aniline (PhNH2) or phenol (PhOH) that are isoconjugate with the benzyl anion (PhCH2 ). Thus while the nitrophenols do follow the rule, absorbing at longer wavelengths than phenol, the cyanoanilines do not namely (Aq in nm)  

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