Chemistry Reference
In-Depth Information
a sequence of photophysical processes or photo-
chemical reactions, just as if it had been excited
directly by photon absorption. One of the main
values of photosensitisation is that it can give access
to the excited states of molecules that absorb only
weakly or in inconvenient regions of the spectrum.
The generation of singlet oxygen from triplet
ground-state O
2
(see Fig. 18.3) provides a good
example. Singlet oxygen is an interesting reagent
with unique properties, but it cannot be generated
in useful quantities by irradiating the ground-state
molecule in its extremely weak (spin-forbidden)
absorption band. Fortunately, it can be generated
readily by means of photosensitising dyes, such as
Rose Bengal, Methylene Blue or Eosin Y.
In determining the economic viability of a
prospective photochemical process, however, the
overall energy efficiency of the system is of more
practical significance than the quantum yield of
product formation. We can define the overall energy
efficiency as the amount of desired product derived
from a given input of electrical energy. This takes
into account inefficiencies in converting electrical
energy into light, and light wasted by scattering or
absorption by materials other than the reactant, as
well as the energy wastage due to photophysical
processes or the generation of by-products following
excitation of the reactant. Estimation of the overall
energy efficiency of a process normally is straight-
forward and provides information that is directly
relevant to the reactor actually employed for the
process.
Quantum yields
From the preceding discussion, it is clear that an
excited-state molecule, once it is generated by
photon absorption or sensitisation, can undergo any
one of a number of photophysical or photochemical
processes. In general, there will be competition
between these pathways. The number of moles of
reactant undergoing a particular process
i
per ein-
stein of light absorbed is termed the
quantum yield
of
that process j
i
. In chain processes, such as frequently
occur in radical reactions, where many product mol-
ecules can result from a single photon absorption,
quantum yields can exceed unity. In all other cases,
the sum of the quantum yields of all competing path-
ways will be equal to unity:
Acknowledgements
The author wishes to thank the EPSRC and the Royal
Academy of Engineering for the award of a Clean
Technology Fellowship, which provided him with
the opportunity to begin research into the exploita-
tion of photochemistry for cleaner manufacturing
processes, as well as other aspects of clean technol-
ogy. As a consequence of this original award, the
author has been able to develop industrial collabo-
rations with A. H. Marks and Co. Ltd and Air Prod-
ucts, Inc., linked to academic collaborations with
chemical engineers and chemists, especially with
C. M. Gordon, D. C. Sherrington, S. J. Shilton, R. L.
Skelton and R. Withnall. The author thanks all these
collaborators and the members of his research group
for continuing stimulation.
Â
f
i
=
1
i
From the standpoint of photochemical synthesis,
the photophysical processes illustrated in Fig. 18.14
are unproductive and thus are referred to as
energy-
wasting
processes. Fluorescence, for example, may
be very useful in analytical applications but it is
unwanted if it competes efficiently with an intended
photochemical reaction. The quantum yield of for-
mation of the desired product therefore is an impor-
tant quantity in assessing the potential utility of any
photoreaction. Unfortunately, the measurement of
quantum yields usually is inconvenient, especially
if quantum yields for a particular design of reactor
are needed. For this reason, the measurement of
quantum yields often is not carried out in photo-
chemical studies.
References
1.
(a) Anastas, P. T., & Williamson, T. C.
Green Chemistry:
Frontiers in Benign Chemical Syntheses and Processes.
Oxford University Press, Oxford, 1998; (b) Clark, J. H.
Chemistry and Waste Minimization.
Blackie, London,
1995; (c) Kirkwood, R. C., & Longley, A. J.
Clean Tech-
nology and the Environment.
Blackie, London, 1995.
2.
Boule, P.
Environmental Photochemistry.
Springer,
Berlin, 1999.
3.
Schiavello, M.
Heterogeneous Photocatalysis.
John Wiley,
Chichester, 1997.
4.
Weissermel, K., & Arpe, H.-J.
Industrial Organic Chem-
istry
(Lindley, C. R., trans.), 3rd edn. VCH, Weinheim,
1997.
