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.