Question

Provide brief definitions for the following: a) conjugation; b) polymer, c) HOMO and LUMO. 2) Why do conjugated compounds absorb light in the Ultraviolet region? Why do more conjugated compounds absorb at higher wavelengths/lower energies? What is required for organic compounds to absorb in the Visible light region?

Answer 1

a) Conjugation: A molecule is said to conjugated when it has alternating single and multiple bonds. Molecules with alternating double bonds and lone pairs or carbanion or carbocation are also said to be conjugation.

b) Polymer: Polymer is a substance which is made up of a number of repeating units called monomers.

c) HOMO: HOMO is the molecular orbital which is the highest (in terms of energy) occupied (by electrons) molecular orbital.

d) LUMO: LUMO is the molecular orbital which is the lowest (in terms of energy) unoccupied molecular orbital.

2) The electrons in the conjugated system will absorb the photon of light of ultraviolet region wavelength, hence conjugated systems will absorb ultraviolet light.

With the increase in conjugation in a compound, the energy gap between HOMO and LUMO will decrease and hence the compound will absorb at higher wavelength light.

For organic compounds to absorb in the visible light region they should have long conjugated system in their structure.