Question

The naphthalene molecule has a structure that corresponds to two benzene molecules fused together:

Thep-electrons in this molecule are delocalized over the entire molecule. The wavelength of maximum absorption in the UV-visible part of the spectrum in benzene is 255 nm. Is the corresponding wavelength shorter or longer than 255 nm for naphthalene?

Short answer

The corresponding wavelength of maximum absorption in the UV-visible part of the spectrum is longer than 255nm for naphthalene.

Step-by-step solution

Effect of incident light

When sample molecules are exposed to light (photons) having similar energy to that of a possible electronic transition within the molecule, light energy will be absorbed and the electron is promoted to a higher energy orbital.

Promotion to the excited state

Energetically favored electron promotion is from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and the species is called an excited state.

The following diagram illustrates this excitation for an isolated double bond (only two pi-orbitals).

Examples of $\text{π}\to \text{π*}$excitation

Effect of conjugation on absorption spectrum

Increased conjugation brings the HOMO and LUMO orbitals closer together. The resultant energy$\left(\text{ΔE}\right)$ required to for delocalization is therefore less, and the wavelength of the photon with this energy is increased accordingly.

The relation is:

$\lambda =\frac{hc}{\Delta E}$

The more conjugated structure in naphthalene causes ‘bathochromic’ (longer wavelength) shifts of the absorption bands in the spectrum and therefore the corresponding wavelength is greater than 255 nm.