Advanced Chemistry: PART
Visible light absorption spectroscopy of chlorophyll a
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB
KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry
of analysis and molecular structure determination
SPECTROSCOPY INDEXES *
All Advanced Organic
Chemistry Notes *
The origin of colour, the wavelengths of
visible light, our perception!
theory, spectrometer, examples of absorption & reflectance spectra
spectra - index of examples: uses, applications, more on the
chemistry of colour
brown * Use
mobile phone/ipad etc. in 'landscape' mode *
This is a BIG website, take time to explore it
The uv and visible absorption spectrum of
Spectra mage adapted from
Image of the parent structure of porphyrin
adapted from https://en.wikipedia.org/wiki/Porphyrin
Image of chlorophyll molecule adapted from
absorbs strongly in the blue and red regions with λmax of
λmax of 660 nm in organic solvents, and 435 nm and 670-680 nm in cells of all photosynthesising
This means there is high percentage transmission/reflected of green light.,
which is why plants look green.
Photosynthetic plants contain three types of pigments:
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
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.
transition metals Appendix 2. for an
Introduction to complexes
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
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.
the colour of leaves and the seasons
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
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 λmax of
.~430 and 670 nm.
Anthocyanins do not absorb in the red region of the visible
spectrum, and has as single λmax of
~530 nm, centred around the green region of the visible light
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.
Key words & phrases: interpreting the uv-visible absorption
spectrum of chlorophyll a, identifying the maximum absorption peaks in the uv-visible
absorption spectrum of chlorophyll a, explaining the uv-visible absorption spectrum of
chlorophyll a, how to use the visible absorption spectra of chlorophyll a to explain the colour of
chlorophyll a, applications of the uv-visible absorption spectrum of chlorophyll
a interpreting the uv-visible absorption spectra of chlorophyll a,
identifying the maximum absorption peaks in the uv-visible absorption spectra of
chlorophyll a, explaining the uv-visible absorption spectra of chlorophyll a, how to use the visible
absorption spectra of chlorophyll a to explain the different colours of
chlorophyll a, applications of the uv-visible absorption spectra of chlorophyll
a the molecular structure of chlorophyll a the role of magnesium ion in
chlorophyll a in photosynthesis
UV and visible spectroscopy index
All Advanced Organic
Use My Google search site box
Website map buttons below
|Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications