Arrange in increasing order of ${\lambda }_{max}$ in UV-visible spectra: (i) ethene, (ii) buta-1,3-diene, (iii) hexa-2,4-diene ($\mathrm{CH}{}_3\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CHCH}{}_3$), (iv) hexa-1,3,5-triene.

Increasing conjugation lowers the energy of the pi to pi* transition and shifts ${\lambda }_{max}$ to longer wavelength. Alkyl substituents on the diene give a further small red shift, so hexa-2,4-diene absorbs above buta-1,3-diene. The triene has the longest conjugated system and absorbs at the longest wavelength.