4.7 Sub-second Linkage Isomer Studies
Solid-state linkage isomer systems have traditionally been studied for their ability to
access long-lived metastable states at low temperatures, and as such comparatively
little work has been done to explore the potential for fast single-crystal switching at
higher temperatures. However, the solid-state kinetics discussed in Sect. 3 highlight
the strong temperature dependence of the excited-state lifetime, and spectroscopic
studies conducted at or near ambient temperature indicate that faster pump-probe
diffraction studies should be possible if the correct experiment temperature is
selected [25].
In 2017, Casaretto et al. reported the first laboratory-based pump-probe diffraction study following the photoisomerisation in crystals of the archetypal iron-nitrosyl
system sodium nitroprusside [82]. The laboratory pump-probe setup incorporated an
electronically gated HPC detector, a microfocus X-ray source and a complementary
transient absorption spectroscopy setup. The study followed changes in the relative
intensity of Bragg reflections known to change significantly through the
isomerisation, which were used to infer a conversion of c.a. 1% to a short-lived
photo-induced isomer with 6 ms time resolution at 150 K. The results of this study
are encouraging and pave the way towards further pump-probe diffraction studies on
linkage isomer crystals, aiming to obtain full 3D crystal structures in line with the
fast time-resolved experiments on other photoactive materials conducted at synchrotron and XFEL facilities.
5 Conclusions
This chapter has outlined some of the key developments and challenges in the study
of solid-state linkage isomerism as a model for solid-state photochemical reactions
and in particular has highlighted the wealth of structural and kinetic information that
can be obtained by in situ single-crystal photocrystallography experiments. While
long-lived metastable isomers are easily characterised using traditional X-ray
photocrystallography methods in the laboratory, identifying shorter-lived excitedstate species requires time-resolved experiments at synchrotron sources. With the
increasing availability of state-of-the-art X-ray sources and detectors, it is conceivable that time-resolved photocrystallography measurements at millisecond and possibly even microsecond timescales may soon be routinely achievable in the
laboratory. This will greatly support the development of faster time-resolved diffraction methods at synchrotron and XFEL facilities, helping to transform what is
currently a specialist research area into a routine and widely available technique for
real-time visualisation of solid-state photochemical reactions. This is an exciting
prospect with the potential to revolutionise our understanding of photoactivated
processes in the solid state and to drive the development of new and improved
functional materials for applications including catalysis and molecular data storage.
234
L. E. Hatcher et al.
Solid-state linkage isomer systems have traditionally been studied for their ability to
access long-lived metastable states at low temperatures, and as such comparatively
little work has been done to explore the potential for fast single-crystal switching at
higher temperatures. However, the solid-state kinetics discussed in Sect. 3 highlight
the strong temperature dependence of the excited-state lifetime, and spectroscopic
studies conducted at or near ambient temperature indicate that faster pump-probe
diffraction studies should be possible if the correct experiment temperature is
selected [25].
In 2017, Casaretto et al. reported the first laboratory-based pump-probe diffraction study following the photoisomerisation in crystals of the archetypal iron-nitrosyl
system sodium nitroprusside [82]. The laboratory pump-probe setup incorporated an
electronically gated HPC detector, a microfocus X-ray source and a complementary
transient absorption spectroscopy setup. The study followed changes in the relative
intensity of Bragg reflections known to change significantly through the
isomerisation, which were used to infer a conversion of c.a. 1% to a short-lived
photo-induced isomer with 6 ms time resolution at 150 K. The results of this study
are encouraging and pave the way towards further pump-probe diffraction studies on
linkage isomer crystals, aiming to obtain full 3D crystal structures in line with the
fast time-resolved experiments on other photoactive materials conducted at synchrotron and XFEL facilities.
5 Conclusions
This chapter has outlined some of the key developments and challenges in the study
of solid-state linkage isomerism as a model for solid-state photochemical reactions
and in particular has highlighted the wealth of structural and kinetic information that
can be obtained by in situ single-crystal photocrystallography experiments. While
long-lived metastable isomers are easily characterised using traditional X-ray
photocrystallography methods in the laboratory, identifying shorter-lived excitedstate species requires time-resolved experiments at synchrotron sources. With the
increasing availability of state-of-the-art X-ray sources and detectors, it is conceivable that time-resolved photocrystallography measurements at millisecond and possibly even microsecond timescales may soon be routinely achievable in the
laboratory. This will greatly support the development of faster time-resolved diffraction methods at synchrotron and XFEL facilities, helping to transform what is
currently a specialist research area into a routine and widely available technique for
real-time visualisation of solid-state photochemical reactions. This is an exciting
prospect with the potential to revolutionise our understanding of photoactivated
processes in the solid state and to drive the development of new and improved
functional materials for applications including catalysis and molecular data storage.
234
L. E. Hatcher et al.