Page 353 - Instant notes

I7
PHOTOCHEMISTRY IN THE REAL
WORLD

Key Notes
The majority of chemical reaction sequences in the atmosphere
are initiated by a photodissociation reaction in which absorption
of a solar photon of visible or ultraviolet light promotes an
electronic transition leading to bond dissociation and formation
of reactive radicals. The photodissociation of O 2 leading to the
formation of the ozone layer in the stratosphere is an example.
The energy to drive photosynthesis in plants is provided by the
absorption of photons of visible light by molecules of
chlorophyll. The excited electron produced in the electronic
transition increases the redox potential of a neighboring electron
acceptor molecule. Plants appear green because chlorophyll
molecules strongly absorb the blue and red components of the
visible spectrum whilst green is reflected.
The electronic transition induced in the retinal molecule by
absorption of a photon of visible light causes the molecule to
change configuration between cis and trans isomeric forms. The
trans isomer of retinal is unable to retain its binding to its
associated protein in the eye’s retina and the dissociation triggers
a nerve impulse to the brain.
Chemiluminescence is the emission of light from an
electronically excited state produced by a chemical reaction (e.g.
the blue emission from hydrocarbon radicals in a flame).
Bioluminescence is a chemiluminescent reaction that occurs in a
living organism. The light emitted by the luciferin reaction in a
firefly is a familiar example.

Related topic Electronic spectroscopy (I6)

Atmospheric photochemistry

The chemistry of the atmosphere is initiated by reactions involving the absorption of light
from the sun. The energy of visible and ultraviolet photons is sufficient to cause bond
dissociation and the formation of radical photodissociation products. One of the most
important photochemical processes in the atmosphere leads to the formation of the ozone