Chemistry Reference
In-Depth Information
improved performance. The advent of efficient
excimer lamps, offering essentially monochroma-
tic light at a variety of wavelengths, has provided an
additional technological advance to aid reactor
design. Photochemistry also seems well adapted to
process intensification through the development of
small-scale continuous reactors.
Photochemical reactions can become competi-
tive when at least one of the following conditions
prevails:
5.1 Light and energy
Light is an everyday phenomenon, but trying to
understand it in terms of quantum and relativity
theories takes us into territory still beset with many
mysteries. Fortunately, for most photochemical pur-
poses, a pragmatic mix of particle and wave models
serves well enough as a conceptual basis.
In the
wave model
, light is simply an oscillating
electrical field, with an associated oscillating mag-
netic field, propagating as a wave through space. A
light wave has both a frequency and a wavelength.
For visible and ultraviolet (UV) light the frequencies
are very high (in the region of 10
15
Hz) and the wave-
lengths are short (100-700 nm). The UV-visible light
lies in the middle of a continuum of
electromagnetic
radiation
, in which infrared light, microwaves and
radio waves have longer wavelengths and lower
frequencies, whereas x-rays and gamma rays have
shorter wavelengths and higher frequencies.
The oscillating electric fields of light waves can
interact with the electrically charged particles in
matter (electrons and nuclei), inducing motion in
the particles and increasing the energy of the matter.
It has been known for a long time, however, that
such transfer of energy from light to matter can
occur only in discrete packets, termed
quanta
or
photons
. The energy of each photon
E
p
is related to
the frequency n of the light wave:
(1)
There is no economical conventional reaction as
an alternative.
(2)
High selectivity in the production of a high-
value fine chemical or pharmaceutical negates
the extra costs involved in photochemical
processing.
(3)
A photochemical reaction accomplishes in one
stage what would require a sequence of several
thermal reactions.
(4)
A photochemical reaction avoids the genera-
tion of a by-product, the presence of which
would result in costly purification or disposal
procedures.
(5)
A chain reaction results in high quantum
yields.
How many such processes actually will emerge and
be commercialised will depend on the direction
and emphasis of future research and development
in clean technology. In an age when whatever
is natural or 'organic' is so highly esteemed, we
should reflect that nature's synthetic methodology is
photosynthetic.
E
p
=
h
n
where
h
is Planck's constant (6.6260755 ¥ 10
-34
J·s).
Thus, for light of a given frequency, energy can be
transferred to matter only in integral multiples of the
photon energy
E
p
.
When we consider this aspect of energy transfer,
light seems to behave more like a stream of particles
than a wave. Energy can be abstracted from the light
beam by removing individual photons in any inte-
gral number, but energy corresponding to a fraction
of a photon cannot be abstracted. Nevertheless, in
this
particle model
, photons differ significantly from
more familiar particles such as billiard balls. The
latter can transfer part of their energy in collisions,
losing some of their speed and generally changing
direction as a result. Photons, on the other hand,
transfer energy only by being absorbed. A photon
therefore must lose all its energy or none. (A pos-
sible exception is found in the inelastic scattering of
light, e.g. in vibrational Raman scattering. Here the
5 The Basics of Photochemistry
There are many excellent texts on photochemistry
[32]. The reader is referred to these for detailed treat-
ments of the theory of light, excited states of atoms
and molecules, photophysical processes, mecha-
nisms of photochemical reactions and synthetic
applications. There are, however, a few important
principles that should be kept in mind when consid-
ering the technological exploitation of photoche-
mistry. This section has been included, really as an
appendix, to serve as a reminder of these principles
for some or a brief introduction for others, but it may
be neglected by any reader who is reasonably famil-
iar with the subject.
