# What is the origin of the colour of azo dyes?

Is it the same reason as to why transition metal complexes have colour?

• What do you mean for same reason? – G M Mar 26 '14 at 8:36

Yes, it is all about the absorption of light at specific wavelength.

Azobenzene, the parent compound has an absorption maximum around $\lambda$= 430 nm in the visible spectrum.

The interesting part is: The absorption can be tuned by substitution of the arenes. This is done before the azo coupling.

Some examples are Allura Red (1), Chrysoine Resorcinol (2), Janus Green B (3) and Direct Blue 1 (4).

The colours of the molecules are chosen to resemble the colour in solution.

In order to make the dyes soluble in water, either sulfonate groups are attached or the dye molecule is just one fat cation.

As already mentioned by GM in another answer, +M substituents elongate the $\pi$ system (auxochromic effect).

• Basically it is an application of the particle in a box model. – Martin - マーチン Mar 26 '14 at 6:01
• @GM I disagree. The fundamental point isn't about d-orbitals, but absorption in the VIS range, resulting in the complementary colour observable. – Klaus-Dieter Warzecha Mar 26 '14 at 8:03
• @KlausWarzecha this is of course true, but from this point of view most of the substances has their color for the same reason... And you differentiate it only from color due to interference phenomena... I've interpret the answer at a deeper level: understanding if the color is due to a similar change in electronic distribution... I will ask to the user... Thanks for the clarification! – G M Mar 26 '14 at 8:22
• @Klaus Warzecha I understand that the azo-dye absorbs a specific frequency of light and we see the complementary colour. However, I am confused about WHAT absorbs the light. For e.g. in transition metals, there is a d-d transition of an electron. I asked my teacher, she told me that it has something to do with the electron cloud of the ring that absorbs a frequency from visible light and goes to a higher energy level- that didn't make sense to me :/ – Eliza Mar 26 '14 at 16:59
• @Eliza Your teacher is right, no d-d transition here. But $\pi$ systems. Aromatic systems (remember your question about the resonance stabilization) and double bonds. More double bonds in conjugation: better. Ethene absorbs at $\lambda$= 160 nm, 1,3-butadiene at 285 nm, trans-stilbene has a maximum at 295 nm. We have transitions from a bonding to an "antibonding" (not occupied) $\pi$ orbital here :) That's the "electron cloud" your teacher mentioned. – Klaus-Dieter Warzecha Mar 26 '14 at 17:21

# What is the origin of colours?

Most of the colours that we perceive are originate by the selective absorption of some spectral bands and the reflection of the others wavelength, some times with the contribution of fluorescence from the absorption at a higher wavelength. If we exclude colour due to interference (e.g. some butterfly wings!) all the other phenomena are linked with the absorption of the photons. However, this absorption could be due to different causes and so we can differentiate between different absorption mechanisms.

# Absorption of electromagnetic waves through bonding and anti-bonding $$\pi$$ and $$\sigma$$ orbitals

Note: I suppose you are familiar with MO theory

One very spread mechanism of absorption of electromagnetic waves is through electrons in pi and sigma orbitals. The absorption occurs at different wavelengths depending on the compound but however, most of the transitions occur normally in the UV region (400nm is the limit of VIS region). I have made this picture with TikZ from this site you can see that only $$n \rightarrow \pi^{*}$$ is in the visible region.

# How can azo dyes absorb light?

In the case of azo dyes, the Chromophore is $$\ce{-N=N}-$$. Is this group that permits the absorption of light. However, Azobenzene has its absorption peak in UV region (it only absorbs a little bit of blue).

So how can azo dyes absorb light (of course for light we means 400-700 nm radiation) and so be colourful? One way to do that is to form a bigger conjugated system to lower the energy, in this case delocalizing the electrons across the molecule. For azobenzene you can also add two auxochromes the hydroxyl group ($$-OH$$) and the amino group ($$-NH_{2}$$) these groups permit the formation of a charge transfer complex CT Complex that should be something like this:

## Is it the same reason as to why transition metal complexes have colour?

I'm not sure that is right to say that is the same reason whereby transition metal complexes have colour. However this is a tricky question because we have seen that there are many factors that cause the absorption so of course, a transition metal can be considered a conjugated system as said Dan S and so is completely different from this point of view, however, CT complex is a common mechanism used to explain colour in transition metal complex.

• I disagree on azobenzene being colourless. Are you certain that you had a look at the UV-VIS spectrum of azobenzene and not silbene, which indeed is colourless? – Klaus-Dieter Warzecha Mar 26 '14 at 8:07
• @KlausWarzecha You are right my professor told me that was colorless but in fact absorbs some blue... Thanks for the comment.. – G M Mar 26 '14 at 8:32
• We're talking about "azobenzene" here the whole time, but actually we are having two: trans (the more stable one) and cis. We could even isomerize trans -> cis upon irradiation in the 300 nm region, and cis back to trans thermally or by irradiation around 440 nm. I would have to look up the reference though. – Klaus-Dieter Warzecha Mar 26 '14 at 8:44
• Found it: DOI. Sandra Monti and Pietro Bortolus from Bologna, where Ciamician (!) worked. – Klaus-Dieter Warzecha Mar 26 '14 at 8:51
• @G M: Just one more thing, my teacher told me that the more the number of phenyl rings in an azo dye, the more intense is the colour. If $\ce{-N=N}-$ is what absorbs the frequency of light and we see the complementary colour... how does the number of phenyl rings affect the colour we see? – Eliza Mar 26 '14 at 17:11

Is it the same as transition metal colors? If you define this as "an electron is promoted to a higher energy state" and that photon absorption defines the color then the answer is yes. If you define it as an absorption by d orbitals then the answer is no. As others have indicated these dyes can absorb in the visible wavelengths due to the aromatic delocalized pi electrons. These are electrons that are shared over many atoms. In a transition metal you are simply promoting electrons in the d shell from one state to another. The electrons aren't "wandering" over a bunch of other atoms.