Conjugation: Conjugation brings about a bathochromic shift in the absorption bands. The higher the extent of conjugation, the more is the bathochromic shift. Such shift in absorption spectra can easily be explained using molecular orbital theory. Figure 5.3 shows the molecular orbitals drawn for ethylene; 1,3-butadiene; and 1,3,5-hexatriene on a qualitatively same energy scale for comparing their energies.

As is clear from the figure, the energy differences between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases as the conjugation increases. This provides an explanation as to why an electronic transition is possible at lower energy (higher wavelength) as the conjugation increases.

Figure 5.3 Molecular orbitals of ethylene; 1,3-butadiene; and 1,3,5-hexatriene. Notice the decrease in the energy gap of HOMO and LUMO as the conjugation increases.