uv-visible absorption spectrum of benzene methylbenzene naphthalene anthracene tetracene example of aromatic spectra of arenes condensed aromatic ring systems pentacene hexacene

Advanced Level Organic Chemistry: UV and visible light absorption spectroscopy - ARENES

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Doc Brown's Advanced Chemistry: PART 15.5 uv and visible light absorption spectroscopy - ARENES

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11-12 & AP honors chemistry courses: The uv-visible absorption spectroscopy of selected aromatic compounds - arenes [uv-visible spectra page updated Mar 9th 2026 *]

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15.5.1 The origin of colour, the wavelengths of visible light, our perception!

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

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The uv and visible absorption spectrum of some aromatic arene compounds

The uv and visible absorption spectrum of methylbenzene

uv-visible absorption spectrum of methylbenzene C6H5CH3 example of aromatic spectra of arenes

Methylbenzene is colourless because the ∆E values are too high for visible light photon to effect excitation of an electron to a higher energy level.

However, below 380 nm (start of uv region), you do get absorption of uv photons from 275 nm downwards - several broad peaks and note the fine structure peaks from associated vibration levels in the methylbenzene molecule.

There is a λ_max of 190 nm.

uv-visible absorption spectrum of benzene C6H6 example of aromatic spectra of arenes

benzene image adapted from NIST https://www.nist.gov/

Benzene is colourless with the highest λ_max of 177 nm.

Note some evidence of fine structure due to vibrational modes superimposed on the electron transition excitations - the 'ripples' in the 190 to 205 nm region.

We can now compare the effect on uv-visible light absorption for a series of arenes of increasingly larger conjugated ring systems than benzene itself.

uv-visible absorption spectra of naphthalene anthracene tetracene example of aromatic spectrum of arenes condensed aromatic ring systems

Image adapted from https://publiclab.org/notes/warren/8-5-2011/uv-visible-spectral-features-benzine-and-some-pahs

Starting with single aromatic ringed benzene, colourless with the highest λ_max of 177 nm.

naphthalene, colourless, λ_max of 220 nm, no absorption in the visible light region.

anthracene, λ_max of 253, but still no absorption in the visible light region, but close!

Tetracene is pale orange powder with λ_max of 275 nm in the uv, but the absorption spectrum is stretches right over to the visible light region, with appreciable absorption in the violet-blue region.

Pentacene is a purple powder, plenty of absorption in the visible light region, hence a coloured compound

Hexacene is a green-blue powder, therefore plenty of absorption in the yellow-orange-red region of the visible light spectrum.

You can see that as the 'linear con-joined' aromatic conjugated ring systems get larger:

the uv-visible absorption spectra generally moves to longer wavelengths,

resulting in a shift from colourless molecules to coloured molecules as some absorption bands occur with wavelengths of >380 nm in the visible light photon region.

diagram image of the visualisation of the wavelengths of visible light in nanometres nm

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