largely intact. The crystal engineering methodologies applied to the design of monomers with suitable separations and orientations have included templating methods
using both metal ions and hydrogen bonds and co-crystallisations and host-guest
chemistry. Because the solid-state [2 + 2] cycloaddition process is irreversible,
diffraction data can be collected at various stages throughout the reaction simply
by pausing the irradiation at convenient intervals. Solid-state kinetic data has been
obtained in this way on cycloaddition reactions by following the photoreaction as a
function of irradiation time using single-crystal X-ray diffraction methods [74–76].
Irreversible solid-state photoreactions permit the full three-dimensional structures
of the starting material and of the product to be determined using conventional
single-crystal X-ray crystallographic methods as long as crystal integrity is
maintained throughout the process. However, for reversible dynamic processes in
the solid state, effectively snapshots of the structures in their excited states must be
obtained, taking into account the lifetime of the activated species. Over the last four
decades, the importance of fully reversible photoactivated processes has been
realised because of their application in real-world technologies [77], including
sensors, read-write data storage media, non-linear optics [78], molecular switches,
amphidynamic materials [77] and molecular actuators [79–81]. Examples of reversible photochemical processes include metal-metal bond-length changes [82, 83],
linkage isomerisation processes [8, 23, 84, 85] and light-induced spin-state trapping
behaviour [13]. The photocrystallographic studies have established that the structural changes can be promoted and controlled in the solid state, with the concomitant
control over physical properties such as colour [86, 87], luminescence [8, 17, 24, 86,
87] and refractive index [88–90].
Steady-state and pseudo-steady-state photocrystallographic techniques have been
used to identify metastable species with much of the research focussing on the
identification of metastable linkage isomers and on the products of light-induced
excited spin-state trapping (LIESST) experiments. This work is covered in previous
chapter of this book by Skelton et al. and will not be discussed in detail here other
than to give a brief outline of the overall findings that can be related to the faster
time-resolved photocrystallographic experiments that will be described in the next
section.
The majority of photocrystallographic studies of transition metal complexes that
undergo linkage isomerism under photoactivation have focussed on nitrosyl [91–
94], sulphur dioxide [17, 85, 95] and nitro complexes [8]. Some general conclusions
as to the processes involved can be drawn from these studies. In all cases the
percentage of conversion obtained and the isomer formed are highly dependent on
the wavelength of light used and the temperature at which the experiment is carried
out. For a single-crystal-to-single-crystal process to occur, there is no change in the
crystal system and the unit cell parameters do not change by more than 2%. There is
a temperature at which the metastable limit is reached above which the excited state
has a finite lifetime before returning to the ground state. The interconversion is also
dependent on the flexibility of the crystal lattice and on the steric and electronic
environment of the ambidentate ligand and the metal centre, as evidenced by only
small changes in cell volume being observed. There needs to be sufficient space in
Time-Resolved Single-Crystal X-Ray Crystallography
253
using both metal ions and hydrogen bonds and co-crystallisations and host-guest
chemistry. Because the solid-state [2 + 2] cycloaddition process is irreversible,
diffraction data can be collected at various stages throughout the reaction simply
by pausing the irradiation at convenient intervals. Solid-state kinetic data has been
obtained in this way on cycloaddition reactions by following the photoreaction as a
function of irradiation time using single-crystal X-ray diffraction methods [74–76].
Irreversible solid-state photoreactions permit the full three-dimensional structures
of the starting material and of the product to be determined using conventional
single-crystal X-ray crystallographic methods as long as crystal integrity is
maintained throughout the process. However, for reversible dynamic processes in
the solid state, effectively snapshots of the structures in their excited states must be
obtained, taking into account the lifetime of the activated species. Over the last four
decades, the importance of fully reversible photoactivated processes has been
realised because of their application in real-world technologies [77], including
sensors, read-write data storage media, non-linear optics [78], molecular switches,
amphidynamic materials [77] and molecular actuators [79–81]. Examples of reversible photochemical processes include metal-metal bond-length changes [82, 83],
linkage isomerisation processes [8, 23, 84, 85] and light-induced spin-state trapping
behaviour [13]. The photocrystallographic studies have established that the structural changes can be promoted and controlled in the solid state, with the concomitant
control over physical properties such as colour [86, 87], luminescence [8, 17, 24, 86,
87] and refractive index [88–90].
Steady-state and pseudo-steady-state photocrystallographic techniques have been
used to identify metastable species with much of the research focussing on the
identification of metastable linkage isomers and on the products of light-induced
excited spin-state trapping (LIESST) experiments. This work is covered in previous
chapter of this book by Skelton et al. and will not be discussed in detail here other
than to give a brief outline of the overall findings that can be related to the faster
time-resolved photocrystallographic experiments that will be described in the next
section.
The majority of photocrystallographic studies of transition metal complexes that
undergo linkage isomerism under photoactivation have focussed on nitrosyl [91–
94], sulphur dioxide [17, 85, 95] and nitro complexes [8]. Some general conclusions
as to the processes involved can be drawn from these studies. In all cases the
percentage of conversion obtained and the isomer formed are highly dependent on
the wavelength of light used and the temperature at which the experiment is carried
out. For a single-crystal-to-single-crystal process to occur, there is no change in the
crystal system and the unit cell parameters do not change by more than 2%. There is
a temperature at which the metastable limit is reached above which the excited state
has a finite lifetime before returning to the ground state. The interconversion is also
dependent on the flexibility of the crystal lattice and on the steric and electronic
environment of the ambidentate ligand and the metal centre, as evidenced by only
small changes in cell volume being observed. There needs to be sufficient space in
Time-Resolved Single-Crystal X-Ray Crystallography
253