For typical UV–Vis questions of this style, the molecule with the greater extent of conjugation (more continuously overlapping double bonds) absorbs at the longer wavelength.

Key idea

  • UV–Vis absorption corresponds to promoting an electron from a lower-energy orbital (often the HOMO) to a higher-energy orbital (often the LUMO).
  • A smaller energy gap between these orbitals means lower-energy, longer-wavelength light is absorbed, because energy and wavelength are inversely related through E=hc/λE=hc/\lambda E=hc/λ.

What makes a molecule absorb at longer wavelength?

The following structural features generally shift absorption to longer wavelengths (a “red shift”):

  • More conjugation :
    Conjugated systems (alternating single and double bonds) have delocalized π\pi π electrons, which lowers the HOMO–LUMO gap and increases the absorption wavelength.
* Example trend in alkenes:
  * Ethene: λmax⁡≈171\lambda_{\max}\approx 171λmax​≈171 nm
  * Buta-1,3-diene (two conjugated double bonds): λmax⁡≈217\lambda_{\max}\approx 217λmax​≈217 nm
  * Hexa-1,3,5-triene (three conjugated double bonds): λmax⁡≈258\lambda_{\max}\approx 258λmax​≈258 nm

Each added conjugated double bond pushes λmax⁡\lambda_{\max}λmax​ to longer wavelength.

  • Better overlap / continuous conjugation :
    A molecule where double bonds are directly conjugated (e.g., 1,3-hexadiene) absorbs at a longer wavelength than one where the double bonds are isolated (e.g., 1,4-hexadiene), because only the conjugated system benefits from delocalization.
  • Electron-donating and -withdrawing substituents :
    Groups that push or pull electron density into a conjugated system can further reduce the energy gap and increase the absorption wavelength (this is how many organic dyes get their color).

Quick practical rule

If you are choosing which molecule absorbs at the longer wavelength among similar structures:

  1. Count the number of conjugated double bonds (and conjugated lone pairs).
  2. Check that they are in a continuous chain or ring (no sp3^33 carbon “breaks” the conjugation).
  3. The molecule with:
    • more conjugated double bonds, and
    • more extended delocalization
      will almost always absorb at the longer wavelength.

If you share the specific pair of molecules you are comparing, a more concrete, step-by-step prediction can be given.