January 22, 2026 | UR Gate
Effects of Conjugation, Aromaticity, and Auxochromes on UV–Vis Absorption (λmax Shifts)
conjugation, π→π* transition, bathochromic/red shift, aromaticity, hyperchromic shift, auxochrome, chromophore, λmax, β-carotene, lycopene, UV–Vis, polyenes
1) Effect of conjugation on absorption
The absorption coefficient is affected by conjugation, where the value of the absorption coefficient increases as the degree of conjugation in the molecule increases. This is due to the additional stabilization of π*, which leads to a decrease in the transition energy from π → π*. This, in turn, produces a bathochromic (red) shift.
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Figure1: UV–Vis absorption bands for R(CH=CH)_nR (n = 3, 4, 5) showing increased intensity and a red shift with increasing conjugation.
It is possible to consider the following pairs of compounds and compare the change in wavelength with conjugation:
%20and%20lycopene%20(%CE%BBmax%20=%20474%20nm).jpg)
Figure2: Structures of β-carotene (λmax = 455 nm) and lycopene (λmax = 474 nm).
Also:
%20and%202,4-hexadiene%20(%CE%BBmax%20=%20227%20nm).jpg)
Figure3: 1,5-hexadiene (λmax = 178 nm) and 2,4-hexadiene (λmax = 227 nm).
2) Effect of aromaticity
It is known that aromaticity confers a very high stability on the compound, where the energy of π* decreases noticeably. This leads to a decrease in the π → π* transition energy, which in turn produces a bathochromic (red) shift. It also leads to a higher absorptivity (hyperchromic shift). The wavelength increases with increasing number of aromatic rings, as in the following figure:
Figure4: Spectral curves showing increasing λ and higher absorptivity with more aromatic rings.
Note that the number of π bonds in this type of molecules (or even the conjugation between them) cannot explain the high absorptivity that distinguishes these compounds. For example, benzene contains three π bonds, and they are also conjugated; therefore, we would expect an absorption coefficient close to 33000. However, we observe that the absorption coefficient value reaches approximately 60000, which cannot be explained except in light of the stability resulting from the aromatic nature of benzene.
3) Effect of auxochromes
In general, the part of a compound responsible for absorption (which contains π bonds) is called a chromophore. The groups attached to the chromophore that do not absorb by themselves, but whose attachment to the chromophore improves absorption and increases the wavelength, are called auxochromes. Examples include groups such as –OH, –NH₂, –OCH₃, and similar groups.
Figure5: Diagram labeling chromophore and auxochrome.
The presence of such groups increases the stability of π* and therefore causes a decrease in the energy required for the transition from π to π*, producing a red shift (bathochromic shift).
,%20phenol%20(270%20nm),%20phenoxide%20ion%20(287%20nm),%20aniline%20(280%20nm).jpg)
Figure 6: Examples and λmax values—benzene (255 nm), phenol (270 nm), phenoxide ion (287 nm), aniline (280 nm).
