Intro to UV Spectroscopy

In UV spectroscopy molecules or materials are irradiated with ultraviolet radiation.  The UV radiation is absorbed by the molecules and in doing so an electron is bumped (excited) from a low lying HOMO to a higher energy LUMO.

Ground State                               Excited State (Higher E)     

The difference in energy between the excited and ground state is referred to as the HOMO-LUMO gap.  The HOMO-LUMO gap is effected by the functional groups attached to the double bond and to the degree of conjugation.

In general

• The more conjugation the lower the HOMO-LUMO gap (larger λ_max).
• The more alkyl substituents the lower the HOMO-LUMO gap (larger λ_max).

Interpreting UV spectra

A UV spectra has absorbance on the y-axis and wavelength of UV radiation on the x-axis.  Recall that the amount of light absorbed by a collection of molecule is governed by Beer's law.  There is normally a maximum in wavelength observed in a UV spectra which is designated λ_max.  For example below is the UV spectra for Lycopene and its  λ_max=471 nm (nanometers).

Lycopene is highly conjugated having 11 double bonds in conjugation and therefore has a small HOMO-LUMO gap.  In general the smaller the HOMO-LUMO gap energy, the lower the energy of the photon absorbed.  Recall that the energy of a photon E = hν = hc/λ.  So the smaller the HOMO-LUMO gap the larger the λ_max.

Lycopene's λ_max=471, which is in the blue end of the spectrum.  Since Lycopene absorbs all the blue light, all other light namely the red light is reflected and as a result appears red in ripe tomatoes.

Lycopene

Below are the wavelengths of visible light.  Notice how the λ_max=471 is in the blue region.

Structure and  λ_max

As discussed above the;

• The more conjugation the lower the HOMO-LUMO gap (larger λ_max).
• The more alkyl substituents the lower the HOMO-LUMO gap (larger λ_max).

Thus 1,3-pentadiene would have a smaller HOMO-LUMO gap and therefore a larger  λ_max than 1,4-pentadiene.  Note that 1,3-pentadiene is conjugated, while 1,4-pentadiene is not conjugated.

Adding an alkyl group such as a methyl group increases the λ_max.  For example 4-methyl-1,3-pentadiene has a greater λ_max than 1,3-pentadiene.