biological processes that occur in crystals can be monitored in “real time”, and threedimensional structural information on species that exist in the solid state for only
fractions of a second may be obtained [16–18]. Regarding the external media for
inducing reactions in crystals, the use of “light” has proved the most popular and is
arguably the least likely to cause crystal degradation. Photochemically induced
chemical reactions are relatively easy to control. The size, shape and intensity of
the incident beam can be controlled using a wide variety of optical equipment and,
when using a tuneable source, the wavelength can be varied to match the process
involved. This has led to the development of “photocrystallography” [19], a term
attributed to Philip Coppens [20], one of the pioneers in the area, although the
crystallography of photoactivated species is also included in the description of
“photosalient” processes [21] or, more generally, as “time-resolved” crystallography. With all the technological developments, the speed of a full single-crystal X-ray
data collection has been reduced to minutes particularly when a high-intensity
synchrotron source is used. This has provided the opportunity to undertake a
whole range of new crystallographic experiments that can follow the dynamics of
a chemical process and obtain a full three-dimensional picture of photoactivated
species. This chapter will highlight some of the technological and experimental
results that have been obtained in the first 20 years of the twenty-first century.
Of all the technological advances that have facilitated the growth of
photocrystallography, it is the more general availability of synchrotrons as research
tools that has proved to be most important. They have been used to apply a range of
analytical techniques to study chemical and biological samples by providing highintensity electromagnetic radiation as the probe. A synchrotron is a storage ring in
which electrons are accelerated around the ring at speeds approaching the speed of
light and, as a consequence, are subject to relativistic effects. This causes the
electrons to circulate around the ring in discrete bunches. The resultant radiation is
used in different experimental stations which are at the end of beamlines radiating
out tangentially from the storage ring. Some of these beamlines are dedicated to
crystallographic experiments and use X-rays with wavelengths in the range
0.4–2.5 Å [22]. A schematic representation of a synchrotron ring is illustrated in
Fig. 1 which shows how the electrons are initially accelerated from their source
through a linear accelerator, through a booster synchrotron into the main storage ring
where their circular orbits are controlled by magnetic fields and their energies are
controlled by insertion devices which guide the X-ray beam to the experimental
hutch where the X-ray diffractometer sits.
2 Photocrystallographic Methodology
The type of photocrystallographic experiment that can be carried out is dependent on
the lifetime of the photoactivated species that is being studied. The lifetimes of the
various processes can range from hours to femtoseconds as are summarised in Fig. 2
so that the type of experiment that can be carried out and the results that can be
242
P. R. Raithby
fractions of a second may be obtained [16–18]. Regarding the external media for
inducing reactions in crystals, the use of “light” has proved the most popular and is
arguably the least likely to cause crystal degradation. Photochemically induced
chemical reactions are relatively easy to control. The size, shape and intensity of
the incident beam can be controlled using a wide variety of optical equipment and,
when using a tuneable source, the wavelength can be varied to match the process
involved. This has led to the development of “photocrystallography” [19], a term
attributed to Philip Coppens [20], one of the pioneers in the area, although the
crystallography of photoactivated species is also included in the description of
“photosalient” processes [21] or, more generally, as “time-resolved” crystallography. With all the technological developments, the speed of a full single-crystal X-ray
data collection has been reduced to minutes particularly when a high-intensity
synchrotron source is used. This has provided the opportunity to undertake a
whole range of new crystallographic experiments that can follow the dynamics of
a chemical process and obtain a full three-dimensional picture of photoactivated
species. This chapter will highlight some of the technological and experimental
results that have been obtained in the first 20 years of the twenty-first century.
Of all the technological advances that have facilitated the growth of
photocrystallography, it is the more general availability of synchrotrons as research
tools that has proved to be most important. They have been used to apply a range of
analytical techniques to study chemical and biological samples by providing highintensity electromagnetic radiation as the probe. A synchrotron is a storage ring in
which electrons are accelerated around the ring at speeds approaching the speed of
light and, as a consequence, are subject to relativistic effects. This causes the
electrons to circulate around the ring in discrete bunches. The resultant radiation is
used in different experimental stations which are at the end of beamlines radiating
out tangentially from the storage ring. Some of these beamlines are dedicated to
crystallographic experiments and use X-rays with wavelengths in the range
0.4–2.5 Å [22]. A schematic representation of a synchrotron ring is illustrated in
Fig. 1 which shows how the electrons are initially accelerated from their source
through a linear accelerator, through a booster synchrotron into the main storage ring
where their circular orbits are controlled by magnetic fields and their energies are
controlled by insertion devices which guide the X-ray beam to the experimental
hutch where the X-ray diffractometer sits.
2 Photocrystallographic Methodology
The type of photocrystallographic experiment that can be carried out is dependent on
the lifetime of the photoactivated species that is being studied. The lifetimes of the
various processes can range from hours to femtoseconds as are summarised in Fig. 2
so that the type of experiment that can be carried out and the results that can be
242
P. R. Raithby