This question has 3 parts, each related to the title.

In biochem, we were shown the chemical structure of chlorophyll. The light-absorbing head has many carbon-rings with alternating single and double bonds (conjugation). We were taught that this arrangement of bonds is what allows the molecule to absorb light energy.

Question 1: I understand that pi-bonds are generally better at absorbing photons than single bonds, but why do conjugated bonds increase this ability?

Question 2: Also, this molecule does not seem to be have any resonance structures needed for delocalized bonds and thus does not have the light absorbing capabilities like that graphene/graphite or benzene rings. (If we could engineer an artificial chlorophyll molecule would this increase its efficiency?)

The head of the molecule also contains a Mg ions that is coordinate bonded to four surrounding nitrogen atoms.

Question 3: If the alternating double and single bonds are responsible for the absorption of light, what role does this metal complex serve in the molecule? Other pigments like carotenoid don't have this metal complex and they're still able to absorb light.


but why do conjugated bonds increase this ability

Adsorption of light results in excitation of an electron, moving from low-level (usually bonding or non-bonding) to a higher (usually antibonding) orbital. (for understanding of what bonding, non-bonding and antiboding orbitals are, see here)

Generally, when two orbitals of same energy partially overlap, two more are formed: one with slightly higher energy and one with slightly lower. When orbitals of conjugated bonds interact, energy of one bonding orbital goes up and of one antibonding orbital goes does, narrowing the gap. When the gap is withing or closely near ~1.6-3.2 eV it can participate in visible light adsorption.

this molecule does not seem to be have any resonance structures

It does, see carefully.

what role does this metal complex serve in the molecule

The relevant metal-free molecule do exist in nature. It has, however, different color. $Mg$ ion seems to adjust the gap mentioned above. For a more clear understanding of the matter you might look for data on phtalocianin pigments, that may be easily obtained with different central atom in a very similar structure.

  • $\begingroup$ this MO thoery seems to be really useful as it keeps popping up wherever I go. Do you know any sources that would allow me to learn this concept? Thanks ! $\endgroup$ – Aniekan Umoren Jul 17 '17 at 0:06
  • $\begingroup$ @AniekanUmoren Modern university-level inorganic-chemistry textbooks usually provide a qualitative introduction. Quantum-chemistry books that are not focused on DFT provide a more detailed introduction. $\endgroup$ – permeakra Jul 17 '17 at 6:10
  • $\begingroup$ @ AniekanUmoren Chlorophylls play a major role in light harvesting and in electron transfer in photosynthetic organisms, plants, algae and bacteria. The Mg seems to adjusts the redox potential wrt. electron transfer in the protein of photosynthetic organisms. It may also play a role in binding to the protein. (The metal free chlorophylls are called pheophytins and have slightly different spectra. Generally chlorophylls are derivatives of chlorins and are closely related to porphyrins) $\endgroup$ – porphyrin Jul 17 '17 at 11:34

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