data quickly, we require rapid and (near-)complete GS ! ES conversion on illumination, and we would ideally want the conversion to be achievable with low power.
To maintain data integrity, the excited state should be metastable – perhaps
indefinitely – at a specified operating temperature, which would ideally be at or
near room temperature to avoid the need for elaborate cooling systems. A third
property that may be desirable is the presence of a “reporter” that could be used to
read out the majority isomer in a region of the recording surface. This might, for
example, be a discriminatory infrared (IR) absorption, a change in the UV or visible
absorption at a well-defined wavelength or a quantifiable change in a bulk physical
property (e.g. the refractive index).
Though in practice linkage isomer materials are still a considerable way from
being implemented in real devices, several studies have successfully demonstrated
how this could be achieved. In 2010, Woike et al. demonstrated that the change in
the refractive index due to photoisomerisation in three different [Fe(CN) 5 (NO)]
2À
systems could be used to write phase gratings in the crystals, showcasing their
potential use for holographic data storage [50]. In a different approach, Cole et al.
have investigated the use of ruthenium sulphur dioxide linkage isomers as molecular
motors and demonstrated photo-induced nanomechanical transduction in crystals of
[Ru(SO 2 )(NH 3 ) 4 (3-chloropyridine)]Y 2 (Y ¼ p-tosylate or 4-chlorobenzenesulfonate)
[51]. Here, photoactivation leads to conversion to both the MS1 and MS2 isomers,
depending on the irradiation wavelength, and an additional rotation of the benzene ring
in the counter-anion is observed alongside the MS1 conversion as a result of changes
in intermolecular interactions. Incorporation of ruthenium sulphur dioxide materials
into thin films of polyvinyl alcohol (PVA) has also been demonstrated [52], showing
how single crystals might be encapsulated into device media.
3 Steady-State and Pseudo-Steady-State
Photocrystallographic Methodology
In this section we will illustrate how a typical photocrystallography experiment is
conducted in the laboratory. We will also introduce the Johnson-Mehl-AvramiKolmogorov (JMAK) kinetic model for the photoactivated excitation and thermal
decay processes, which will form the basis for discussing faster pump-probe experiments in Sect. 4.
3.1 Experimental Setup
In a typical photocrystallography experiment, a crystal is mounted on the X-ray
diffractometer and irradiated in situ using a light source. In a laboratory setting,
integrating a high-power pulsed laser setup is often impractical, and so lower-power
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
209
To maintain data integrity, the excited state should be metastable – perhaps
indefinitely – at a specified operating temperature, which would ideally be at or
near room temperature to avoid the need for elaborate cooling systems. A third
property that may be desirable is the presence of a “reporter” that could be used to
read out the majority isomer in a region of the recording surface. This might, for
example, be a discriminatory infrared (IR) absorption, a change in the UV or visible
absorption at a well-defined wavelength or a quantifiable change in a bulk physical
property (e.g. the refractive index).
Though in practice linkage isomer materials are still a considerable way from
being implemented in real devices, several studies have successfully demonstrated
how this could be achieved. In 2010, Woike et al. demonstrated that the change in
the refractive index due to photoisomerisation in three different [Fe(CN) 5 (NO)]
2À
systems could be used to write phase gratings in the crystals, showcasing their
potential use for holographic data storage [50]. In a different approach, Cole et al.
have investigated the use of ruthenium sulphur dioxide linkage isomers as molecular
motors and demonstrated photo-induced nanomechanical transduction in crystals of
[Ru(SO 2 )(NH 3 ) 4 (3-chloropyridine)]Y 2 (Y ¼ p-tosylate or 4-chlorobenzenesulfonate)
[51]. Here, photoactivation leads to conversion to both the MS1 and MS2 isomers,
depending on the irradiation wavelength, and an additional rotation of the benzene ring
in the counter-anion is observed alongside the MS1 conversion as a result of changes
in intermolecular interactions. Incorporation of ruthenium sulphur dioxide materials
into thin films of polyvinyl alcohol (PVA) has also been demonstrated [52], showing
how single crystals might be encapsulated into device media.
3 Steady-State and Pseudo-Steady-State
Photocrystallographic Methodology
In this section we will illustrate how a typical photocrystallography experiment is
conducted in the laboratory. We will also introduce the Johnson-Mehl-AvramiKolmogorov (JMAK) kinetic model for the photoactivated excitation and thermal
decay processes, which will form the basis for discussing faster pump-probe experiments in Sect. 4.
3.1 Experimental Setup
In a typical photocrystallography experiment, a crystal is mounted on the X-ray
diffractometer and irradiated in situ using a light source. In a laboratory setting,
integrating a high-power pulsed laser setup is often impractical, and so lower-power
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
209