Clarification on bathochromic shift

Thread starter pisluca99
Start date Jun 1, 2023

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A bathochromic shift, or red shift, occurs when there is increased conjugation among chromophore groups, leading to a higher absorption lambda max. The discussion highlights that this shift is often associated with the HOMO-LUMO transition, specifically the π2 --> π*1 transition in butadiene compared to ethene. However, a hypsochromic shift is noted for the π1 --> π*2 transition in butadiene, indicating that not all transitions experience the same shift. The conversation raises the question of whether the red shift is exclusively linked to the HOMO-LUMO transition or if it can also apply to other orbital transitions. The example of ethene and vinyl chloride illustrates how the addition of an auxochrome, like chlorine, enhances conjugation, resulting in a higher absorption lambda max and a red shift. This prompts further inquiry into whether increases in lambda max consistently relate to the HOMO-LUMO transition or extend to other orbitals as well.

Jun 1, 2023

pisluca99

Generally, a bathochromic shift is observed when there is conjugation between more chromophore groups, that is, an increase in the absorption lambda max is observed. But what lambda max is being referred to?

For example, comparing ethene and butadiene there is a batochromic shift only if we consider the π2 --> π*1 transition (see image), that is, the HOMO-LUMO transition.
In contrast, if we go to consider the π1 --> π*2 transition of butadiene, a hypsochromic shift is observed, as the lambda max relative to this transition is smaller than the lambda max relative to the π --> π* transition of ethene.

That said, does the red shift, then, ALWAYS refer to the HOMO-LUMO transition or does it affect other orbitals?

Another example is that of ethene and vinyl chloride: in this case we add an auxochrome group (Chlorine) that increases conjugation. Because of this, we observe an increase in the absorption lambda max, as well as a red shift. Again, does the increase in lambda max always refer to the HOMO-LUMO transition or to other orbitals?

Last edited: Jun 1, 2023

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