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

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination

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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

(c) doc b

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 λmax of 190 nm.

 

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

(c) doc b benzene image adapted from NIST https://www.nist.gov/ 

Benzene is colourless with the highest λmax 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 λmax of 177 nm.

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

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

Tetracene is pale orange powder with λmax 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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