Question

Many organic molecules that absorb light in the visible range possess molecular structures with extensive conjugation. With reference to molecular electronic orbitals and their relative energies, briefly summarize how the degree of conjugation within a molecule affects the UV-vis absorption spectrum.

Answer 1

You must know that radiation with higher energy, or shorter wavelength in the UV (200-400 nm) and visible (400-700 nm) range of the electromagnetic spectrum causes many organic molecules to undergo various electronic transitions. It means that when a certain amount of energy from UV or visible light is absorbed by a molecule, one of its electrons jumps from a lower energy to a higher energy molecular orbital. UV-visible spectroscopy (=200-800 nm) studies the changes in the electronic energy levels within molecules which arise due to due to transfer of pi electrons or electrons from non-bonding orbitals. It generally gives us information about -electron system, conjugated unsaturation, aromatic compounds and conjugated non-bonding electron system.

Let us first take an example of a non-conjugated molecule, i.e. hydrogen H₂. The molecular orbital diagram of H₂ is shown below:

If the hydrogen molecule is exposed to light of a wavelength with energy equal to ΔE, this wavelength will be absorbed and the energy is used to excite one of the electrons from the HOMO to the LUMO (σ to the σ* orbital). This is called as a σ - σ* transition. ΔE for this electronic transition is 258 kcal/mol, corresponding to light with a wavelength of 111 nm. This does not belong to UV-visible region. Henc, no spectra would be obtained for hydrogen molecule in UV-vis spectroscopy.

Now, let us take an example of a simple conjugated molecule ethene, i.e. CH₂=CH₂. Since, energy gap is shorter than σ - σ* gap, ethene absorbs light at 165 nm (a longer wavelength than molecular hydrogen). Thus, ethene molecule undergoes transition.The molecular diagram showing this transition is given below:

The electronic transitions of both molecular hydrogen and ethene are too energetic to be accurately recorded by standard UV spectrophotometers, which generally have a range of 220 – 800 nm. UV-vis spectroscopy is in the study of molecules with conjugated systems. In these groups, the energy gap for π -π* transitions is smaller than for isolated double bonds, and thus the wavelength absorbed is longer. Molecules or parts of molecules that absorb light strongly in the UV-vis region are called chromophores.

Now, take an example of a simplest conjugated molecule 1,3-Butadiene i.e. CH₂-CH=CH-CH₂. The molecular orbital diagram showing four pi- MO's is:

In this molecule, the energy gap between and i.e. HOMO and LUMO is 132 kcal/mol, which is indeed smaller than that of our isolated pi-bond system ethene. Thus, here the wavelength of the transition is 217 nm.

Similarly, for a higher conjugated pi-bond system example 1,3,5-Hexatriene, the energy gap between HOMO and LUMO is 111 kcal/mol and the transition occurs at around 258 nm.

Thus, as the conjugated system becomes stronger, the energy required for the transition decreases (as the HOMO-LUMO energy gap becomes more and more narrow) and hence the molecule absorbs at a higher wavelength. Infact, in molecules with extended pi systems, the HOMO-LUMO energy gap becomes so small that absorption occurs in the visible region rather than the UV region of the electromagnetic spectrum. For example, Beta-carotene, which is a system of 11 conjugated bonds absorbs light with wavelength in the blue region of the visible spectrum and transmits wavelength belonging to the red-yellow region. That is why, beta-carotene appears orange in colour. Mostly, all molecules with extended pi-bond system are coloured due to this reason only.