Explain conjugation, energy gaps and longer-wavelength absorption

Conjugation allows several adjacent p orbitals to combine into a larger set of delocalised π molecular orbitals. As the conjugated system becomes more extensive, the frontier π levels generally become more closely spaced.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Explain conjugation, energy gaps and longer-wavelength absorption

Connect conjugation to the frontier-orbital gap

A smaller HOMO–LUMO gap needs a lower-energy photon. Since E = hc/λ, lower energy means absorption at a longer wavelength.

Bathochromic shift and conjugation

A bathochromic shift, commonly called a red shift, is movement of an absorption band to longer wavelength. It does not necessarily mean that the compound itself looks red.

Conjugation requires continuous overlap of adjacent p orbitals. An sp³ centre or a strongly twisted arrangement can interrupt overlap and reduce the expected shift.

A hypsochromic or blue shift is movement to shorter wavelength and higher energy.

Why the gap becomes smaller

Combining more p orbitals produces more π molecular-orbital levels within a broadly comparable energy range. This often brings the highest occupied and lowest unoccupied levels closer together.

The lowest useful π→π* transition therefore needs less energy as conjugation increases. The absorption can move from the far ultraviolet towards the near ultraviolet and, in very extended systems, into the visible region.

The trend is general rather than absolute. Molecular planarity, substituents and solvent can alter orbital overlap and shift the spectrum, so compare structures carefully.

Worked example: ethene and buta-1,3-diene

Ethene has one π bonding level and one π* antibonding level. Buta-1,3-diene has four delocalised π levels, creating a smaller gap between its occupied ψ2 and unoccupied ψ3 orbitals.

The diene therefore absorbs its lowest π→π* transition at longer wavelength than ethene.

Try this

Explain why buta-1,3-diene absorbs at longer wavelength than ethene.

Check your answer

Its four interacting p orbitals produce more closely spaced π levels and a smaller HOMO–LUMO gap, so a lower-energy, longer-wavelength photon is absorbed.

Practise the energy–wavelength link

Write the reasoning in three steps: more effective conjugation → smaller frontier-orbital gap → lower photon energy and longer wavelength.

Do not reverse the final step: wavelength is inversely proportional to photon energy.

Try this

A chromophore's HOMO–LUMO gap decreases. What happens to the wavelength of its corresponding absorption?

Check your answer

The wavelength increases because the required photon energy decreases and E = hc/λ.

Compare interrupted and continuous systems

Buta-1,3-diene is CH₂=CH–CH=CH₂. Penta-1,4-diene is CH₂=CH–CH₂–CH=CH₂.

Check whether p orbitals overlap continuously along each chain before comparing the frontier-orbital gaps.

Try this

Which should absorb at longer wavelength, buta-1,3-diene or penta-1,4-diene? Explain.

Check your answer

Buta-1,3-diene. Its two C=C bonds are joined directly, so p-orbital overlap is continuous and the frontier-orbital gap narrows. In penta-1,4-diene the sp³ CH₂ carbon has no p orbital in the π system, so its two π bonds are not conjugated.

Common mistake: longer wavelength means more energy

A red shift is a move to longer wavelength and lower photon energy, not higher energy. Use E = hc/λ whenever the trend feels uncertain.

More double bonds do not guarantee more conjugation. Their p orbitals must be arranged for continuous overlap.

Try this

Correct: ‘Conjugation raises transition energy, causing a red shift.’

Check your answer

Conjugation generally lowers the relevant transition energy by narrowing the frontier-orbital gap, causing absorption at longer wavelength.

Check your understanding

A strong explanation links p-orbital overlap and π-level spacing to photon energy and wavelength, while noting whether conjugation is genuinely continuous.

Next, use absorbance quantitatively through the Beer–Lambert law.

Try this

Give the complete causal chain from increased conjugation to a red shift.

Check your answer

Increased conjugation gives more closely spaced delocalised π levels, narrows the HOMO–LUMO gap, lowers the required photon energy and increases the absorption wavelength.