Click on the diagram to display the various bonding and antibonding molecular orbitals and energies.

• Energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in butadiene is smaller than in ethylene(ethene)
• Addition of successive conjugated double bonds progressively narrows the gap between HOMO and LUMO
• Thus longer wavelength (lower energy) radiation required for the transition