The uv and visible absorption spectrum of chlorophyll a

Image of the parent structure of porphyrin adapted from https://en.wikipedia.org/wiki/Porphyrin

Image of chlorophyll molecule adapted from https://www.life.illinois.edu/govindjee/photosynBook/Chapter9.pdf

Chlorophyll absorbs strongly in the blue and red regions with λ_max of 420 nm and λ_max of 660 nm in organic solvents, and 435 nm and 670-680 nm in cells of all photosynthesising plants.

This means there is high percentage transmission/reflected of green light., which is why plants look green.

Photosynthetic plants contain three types of pigments: chlorophylls, carotenoids (see alkene absorption spectra) and phycobilins - here I'm just dealing with chlorophyll a.

As well as plants, chlorophyll is also present in brown, red and blue-green algae

Like haemoglobin, chlorophyll is porphyrin pigment molecule.

In chlorophyll, the two hydrogen atoms of the porphyrin ring (shown in the diagram below and the top of the right diagram of the whole chlorophyll a molecule) are replaced by the Mg²⁺ ion - this is a square planar arrangement of the four bonds and amounts to a complex ion situation.

The chlorophyll molecule is effectively acting as a polydentate ligand in forming the magnesium complex.

See transition metals Appendix 2. for an Introduction to complexes & ligands

This why plants need magnesium - the chlorophyll molecule is essential for photosynthesis and essential for a healthy plant - and don't forget the chlorophyll molecules is as much the basis of most food chains as any other molecule in the chemistry of life!

 

The crucial part of the chlorophyll molecule and where it fits into the whole structure

The structure of the chlorophyll molecule is shown on the right-above and you can see the porphyrin ring at the 'top end', which effectively forms a complex with the magnesium ion Mg²⁺.

Various other organic groups are attached to the porphyrin ring and these can change to give several different forms of chlorophyll molecules.

The porphyrin ring as 26 pi electrons and forms a large conjugated system, together with the magnesium ion, in which the energies required for electron excitation include the energies of visible light photons e.g. in the blue and red regions.

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Note on the colour of leaves and the seasons

Reminder: The chlorophyll molecule strongly absorbs red and blue light in the visible region - the energy absorbed for photosynthesis. The green wavelengths are not absorbed and are reflected giving leaves their characteristic green colour.

In the autumn, when photosynthesis stops, the leaves turn many colours e.g. browns - yellows - reds etc.. This is because the chlorophyll breaks down when photosynthesis stops and the green colour disappears. The leaves no longer absorb in the yellow-red region, so the yellow to orange colours become visible giving the leaves some of their autumn colour, but what happens to the blue and red no longer absorbed?

At the same time other chemical changes may occur, which emphasize orange-red colours through the development of red anthocyanin pigments - they absorb blue and green! So you are still getting blue absorption, but not by chlorophyll. The overall effect is to give us a wonderful spectrum of autumn leaf colours changes from green ==> yellow ==> orange ==> dark red-brown - all captured in one photograph. The photograph above almost displays the full range of autumn colours.

 

Anthocyanins have an extended conjugated ring system that allows electronic transitions by visible light photons.

Chlorophyll absorbs in the blue and red regions of the visible spectrum, with two λ_max of .~430 and 670 nm.

Anthocyanins do not absorb in the red region of the visible spectrum, and has as single λ_max of ~530 nm, centred around the green region of the visible light spectrum.

As you can see from the anthocyanin visible light absorption spectrum (above), it absorbs strongly in the blue to green region of the visible spectrum, but, unlike chlorophyll, it only absorbs very weakly in the red region, so leaves turn from green to yellow to red-brown as the chlorophyll decreases and the anthocyanin pigment concentration increases.

Image from https://harvardforest.fas.harvard.edu/leaves/pigment

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