Image adapted from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/uv-vis/spectrum.htm

The uv absorption spectrum of buta-1,3-diene H₂C=CH-CH=CH₂, a molecule with a relatively small conjugated system i.e. the minimum alkene conjugated structure with two C=C double bonds separated by a C-C single bond.

For comparison with alkenes below, think of this molecule as R(CH=CH)_nR, where n = 2 (R = H).

A conjugated system is where an alternating single and double bond system produces some overlap of the pi electrons with the sigma bonds. You can consider that the C-C bond has some double bond character and the double bond more sigma bond character - a sort of blurring of the C-C=C-C=C system - note that you must have at least two C=C double bonds separated by a C-C bond to get conjugation in alkenes. (see appendix for more details on conjugated systems)

With alkenes, uv-visible light absorption is due to pi electron excitation, ∆E_elec for π ==> π*

λ_max 217 nm is in the ultraviolet region, so buta-1,3-diene does not absorb visible light, so appears colourless to us.

The conjugation lowers the energy needed to excite the molecule and absorb a uv photon in the process.

For comparison, non-conjugated 'monoenes' have higher ∆E_elec values, needing shorter wavelength photons for excitation e.g. for the uv absorption spectra of other alkenes we have:

ethene H₂C=CH₂ λ_max = 162 nm, propene CH₃CH=CH₂ λ_max = 170 nm,

but-2-ene CH₃-CH=CH-CH₃ λ_max 179 nm (λ_max for buta-1,3-diene was higher at 217 nm).

but, like buta-1,3-diene, the conjugated cyclohexa-1,3-diene has an even larger λ_max of 259 nm.

(data from https://sites.science.oregonstate.edu/~gablek/CH335/Chapter10/bare_UV_vis.htm )

 

(b) Alkenes with more than two C=C double bonds (polyenes)

Having looked at the simplest conjugated alkene system compared to a single alkene bond molecule, you should predict that an even longer conjugated alkenes system should have greater λ_max values and, will they eventually have a ∆E_elec value low enough for visible light photons to be absorbed, hence become coloured?

The answer is yes and look at a carrot in the larder!!!

We now look at series of molecules that have longer conjugated systems of alternating C=C and C-C bonds.

Image adapted from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/uv-vis/spectrum.htm

The series R(CH=CH)_nR, where n = 2 to 5.  If R = H, the molecules would correspond to:

H₂C=CH₂ ethene for n = 1 (see the first spectra data above), λ_max = 162 nm (no conjugation possible)

H₂C=CH-CH=CH₂ buta-1,3-diene for n = 2 (see the first spectra above), λ_max = 217 nm

H₂C=CH-CH=CH-CH=CH₂ hexa-1,3,5-triene for n = 3, λ_max = 262 nm

H₂C=CH-CH=CH-CH=CH-CH=CH₂ octa-1,3,5,7-tetraene for n = 4,  λ_max = 289 nm

H₂C=CH-CH=CH-CH=CH-CH=CH-CH=CH₂ deca-1,3,5,7,9-pentaene for n = 5, λ_max =  340 nm

Non of the absorption peaks are in the visible region, so all of these are colourless compounds, but, when n = 5, the right-hand absorption band is almost in the beginning of the visible region (> 380 nm).

What you observe is a steady trend of increasing λ_max/nm, of decreasing energy (∆E_elec), as the linear conjugated system increases in length by one extra C-C=C 'unit' at a time.

Dodeca-1,3,5,7,9,11-hexaene has a λ_max of 362 nm, continuing the trend from above, maybe very pale yellow?

Looking at another series of polyenes, for the series CH₃(CH=CH)_nCH₃, the highest absorbance peaks are, when :

n = 7, λ_max peaks at 355, 375 and 398 nm

n = 8 λ_max peaks at 375, 395, 420 nm

n = 9 λ_max peaks at 393, 416 and 443 nm

n = 10 λ_max peaks at 406, 431 and 460 nm

These will all be coloured compounds because there is some weakish absorbance in the visible light range of >380 nm because the conjugated polyene structure is now low enough in energy.

Note that the polyene structure, (-CH=CH)_n, is the chromophore, the structural feature responsible for the molecule being coloured from electronic excitation by visible light photons.

Data from https://www.researchgate.net/figure/Synthesis-and-electronic-absorption-characteristics-of-MeCHCH-Me_tbl1_240939369

 

(c) Why are carrots orange coloured?

The beta-carotene molecule is an orange organic pigment found in carrots

Note that the 'monomer' Z-2-methylbuta-1,3-diene (isoprene), with a much shorter delocalised electron system, from which beta-carotene is biosynthesised, is colourless.

The extended conjugated alternate C-C single and C=C double bond system becomes the chromophore, sufficiently low in energy to allow pi electron excitation by visible light photons.

When dissolved in hexane solvent you observe three absorbance peaks 425, 450 and 480 nm.

Therefore the red, orange and yellow light is transmitted, giving carotene (hence carrots) an orange colour.

The visible absorption spectrum of beta carotene, the orange pigment in carrots (the molecular structure is the full E-isomer, all >C=C< in the E 'trans' configuration). The spectrum is obtained using a hexane solution - hexane does not absorb light in the visible region so it doesn't interfere with the spectrum.

The visible absorption spectrum of carotene has a λ_max of ~450 nm with strong absorption in the blue region of the visible spectrum - note the λ_max has now shifted well into the visible region - not surprising with a conjugated system consisting of 11 alternating C=C double bonds !!!

Compare carotene with the colourless polyenes described in section (b) with their shorter conjugated systems.

Extra notes on carotenoids (a few points re-edited from Wikipedia)

There are several molecular forms of carotenes (examples of carotenoids) which are important photosynthetic pigments for photosynthesis.

As already mentioned, they absorb ultraviolet, violet, and blue light and scatter yellow, orange and red light

Carotenes contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll.

They also protect plant tissues by helping to absorb the energy from very reactive singlet oxygen, an excited form of the oxygen molecule O₂ which is formed during photosynthesis.

Carotenes are also responsible for the orange colours of many other fruits, vegetables and fungi (for example, sweet potatoes, chanterelle and orange cantaloupe melon). Carotenes are also responsible for the orange (but not all of the yellow) colours in dry foliage. They also (in lower concentrations) impart the yellow coloration to milk-fat and butter.

 

(d) The colour of astaxanthin

Image adapted from https://www.researchgate.net/figure/Chemical-structure-of-b-carotene-and-of-the-xanthophylls-astaxanthin-and-lutein-main_fig1_6526730

Astaxanthin is a keto-carotenoid used as a dietary supplement and food dye.

As well as alkene groups it has two other organic functional groups, namely a secondary alcohol group and a ketone group, at both ends of the molecule.

The conjugated system (the chromophore) in astaxanthin, compared to carotene, is extended by the ketone group and the λ_max is shifted to a longer wavelength and the colour is a stronger deeper red than 'paler' orange carotene - because the chromophore group is extended compared to carotene, giving a deeper colour.

It is a more reddish colour than beta carotene, because it shows stronger overall absorption in the blue-green regions of the visible spectrum.

Image adapted from https://www.researchgate.net/figure/Absorbance-spectra-of-pure-astaxanthin-concentrations-of-2-and-4-mg-L-respectively-in_fig1_238383838

The λ_max for astaxanthin in trichloromethane solvent occurs at ~492 nm.

 

The spectra of some organic compounds can be significantly different in different solvents e.g. for astaxanthin: