Effect of Conjugation on λmax
The n→π* transition for methyl vinyl ketone is at 324 nm, and the π→π* transition is at 219 nm. Both λmax values are at longer wavelengths than the corresponding λmax values of acetone because methyl vinyl ketone has two conjugated double bonds.
Figure 8.7 Conjugation raises the energy of the HOMO and lowers the energy of the LUMO.
Conjugation raises the energy of the HOMO and lowers the energy of the LUMO, so less energy is required for an electronic transition in a conjugated system than in a nonconjugated system (Figure 8.7). The more conjugated double bonds there are in a compound, the less energy is required for the electronic transition, and therefore the longer is the wavelength at which the electronic transition occurs.
The λmax values of the π→π* transition for several conjugated dienes are shown in Table 8.3. Notice that both the λmax and the molar absorptivity increase as the number of conjugated double bonds increases. Thus, the λmax of a compound can be used to predict the number of conjugated double bonds in the compound.
“The λmax increases as the number of conjugated double bonds increases.”
If a compound has enough conjugated double bonds, it will absorb visible light (λmax > 400 nm) and the compound will be colored. -Carotene, a precursor of vitamin A, is an orange substance found in carrots, apricots, and sweet potatoes. Lycopene is red and is found in tomatoes, watermelon, and pink grapefruit.
An auxochrome is a substituent that when attached to a chromophore, alters the λmax and the intensity of the absorption, usually increasing both; OH and NH2 groups are auxochromes. The lone-pair electrons on oxygen and nitrogen are available for interaction with the π electron cloud of the benzene ring, and such an interaction increases λmax. Because the anilinium ion does not have an auxochrome, its λmax is similar to that of benzene.
Removing a proton from phenol and thereby forming phenoxide ion (also called phenolate ion) increases the λmax because the resulting ion has an additional lone pair. Protonating aniline (and thereby forming the anilinium ion) decreases the λmax, because the lone pair is no longer available to interact with the π cloud of the benzene ring. Because wavelengths of red light are longer than those of blue light, a shift to a longer wavelength is called a red shift, and a shift to a shorter wavelength is called a blue shift. Deprotonation of phenol results in a red shift, whereas protonation of aniline produces a blue shift.