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TLDR, what is the effect of electron donating and withdrawing groups on the UV-Vis absorption spectra of pi-conjugated system?

For a conjugated system, my understanding is that as the extent of conjugation increases, the difference in energy between the highest unoccupied molecular orbital and the lowest unoccupied molecular orbital decreases. The delocalization of electrons in a highly conjugated molecule provides the molecule with a greater number of molecular orbitals, decreasing their spacing. Conjugated compounds which absorb visible light do so when the difference in energy of the homo and lumo correspond to a frequency in the visible spectrum.

My question is in regards to the affects of electron donating and withdrawing groups and the absorption of light by these compounds in the visible spectrum.

My intuition is that electron donating groups will cause greater delocalization of electrons within the compound, creating more relevant MO's, decreasing spacing and reducing the HOMO- LUMO energy gap, causing the compound to absorb lower energy (longer wavelength light). Is my intuition correct?

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  • $\begingroup$ You are talking about electron withdrawing groups (so they have an inductive effect) but it is nothing compared to mesomeric effect, which is known to influence color. Also a withdrawing group or a giving group should have an opposite effect which your intuition does not account for. $\endgroup$ – SteffX Mar 14 at 15:31
  • $\begingroup$ @SteffX Would you mind articulating this opposite effect, in addition to explaining the relationship between mesomerism and the reflection of light by a compound? $\endgroup$ – D W Mar 14 at 15:42
  • $\begingroup$ As itt stands the question is very complicated. I disagree with others as for your wording does not limit the discussion to different electronegativity in an elementary book sense. Of course electrons donating or withdrawing substituents / groups influence colour. But it would be better to focus on absorption of a single molecule. Your link to reflection does indeed complicate the issue. Your intuition is correct but the question reflects misconception or at least is not clearly formulated. But in general yes, there are effects on colour. Mesomeric effects basically extend the conjug. system. $\endgroup$ – Alchimista Mar 14 at 16:08
  • $\begingroup$ @Alchimista I suppose I should narrow the context then, suppose the molecule in question is an azo dye, let's say methyl orange, which reflects the 520nm and 460nm in acidic and basic environments respectively. In an acidic environment, if one were to substitute the benzene ring with an electron donating group, such as OMe, would the subsituted molecule emit light at a lower wavelength? $\endgroup$ – D W Mar 14 at 16:31
  • $\begingroup$ Is the reflection that doesn't fit in here. Forget colour for a while and just look at the molecule Establish what happens to its electronic levels, and specifically to the energy gap. The phenyl ring in your ex Is part of the system, removing it will increase the gap almost no matter of what group is going to replace it. You get a totally different molecule. should better place -OMe on that phenyl. Most likely whatever mesomeric effect on a conjugated system is bathichromic, and enhance absorbivity, like a little extension of the pi-system. You should play with a carotene or so for an idea $\endgroup$ – Alchimista Mar 14 at 19:20
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Yes, your intuition is correct.

Among others properties, systems with pi-electrons delocalisation are actually chromophore whose UV-Vis absorption spectrum is mainly dictated by the HOMO-LUMO transition.

An extension of the delocalised system results in a narrowing of the gap and thus shifts the spectrum to red. The effect is dramatic when extending, e.g., the length of a polyene chain.

However, although with different impact, the same is true if a parent system is substituted with group leading to additional resonance, both because of electron donating or withdrawing effects.

This is fundamentally no different from considering a C=C double bond, which is itself a chromophore, and functionalize it with the various auxochromes that are commonly listed when introducing UV-Vis electronic absorption spectroscopy.

An auxochrome as listed won't necessarily delocalise over the conjugated system in complicated structure, or it can marginally do so. Thus, don't assume that if there is a methoxy - just to mention an auxochrome - somewhere in the molecule or name, the spectrum of the latter is necessarily red shifted as compared to the parent molecule lacking that auxochrome.

If the auxochrome can indeed partecipate in delocalisation, then its effect is bathochromic (red shift) and generally hyperchromic (higher absorbivity).

Rules for predicting the wavelength in the absorption spectrum of conjugated systems do exist, though they fail quite early with increasing the conjugation length.

Consider that the conjugation length of a system is better described by an effective conjugation length, as for the system is susceptible of various effects, first of all departure from planarity due to hindrance. The same is true for the effect of the substituent. If you think of a very long polyene, an end substitution doesn't necessarily extend its effect all along the formally conjugated system.

Similarly the concomitant presence of electron withdrawing and donating groups can have negligible or no effect, as their influences can somehow cancel each others.

Again, is very important to consider that the effective delocalisation is not only influenced by the size of the system, but by an ensemble of parameters, such as the already mentioned steryc hindrance, structural motifs that might pin electrons (think of a phenylene in the middle of an otherwise polyene chain), solvent, etc.

With a bit of caution, you can certainly take that substitution of a conjugated system with groups having mesomeric effects results in absorption maximum at longer wavelength as well as in a higher absorptivity.

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