dependence of the lattice parameters was measured using partial data collected for
each delay time with 60 frames with 10 s of exposure for every 1
step of the
diffractometer ϕ axis. Typical excitation densities were 150 μm
À2 , with a laser
diameter of ca. 500 μm (FWHM). After data processing of the full data sets for the
monoclinic polymorph, photodifference maps were calculated for the 500 ps and
50 μs time delays. The maps show significant changes in electron density upon
excitation (Fig. 11). The response to the excitation at 500 ps is shown by the
sideways shift of electron density from the Fe1 atom towards O2 (Fig. 11a). This
shift becomes even more pronounced at the 50 μs time point (Fig. 11b). There is also
an increase in the Fe–N distances by ca. 0.05 Å. The estimated population of the
excited state at 500 ps is in the range 1.5–2%, and this increases to ca. 10.5% after
50 μs. Overall, the results show significant shifts of the Fe atom and the two O atoms
upon excitation, and the results are consistent with the complementary spectroscopy
that was undertaken.
In a subsequent study of the Fe(III) spin-crossover material, [Fe
(3-MeO-SalEen) 2 ][PF 6 ], the switch from LS to HS only occurs at the molecular
level as clearly shown by the linear dependence of the fraction of photoswitched
molecules with the excitation density as well as with the initial fraction of low spin
molecules. The inter-system crossing from the photoexcited LS (S ¼ 1/2) to HS
(S ¼ 5/2) occurs within approximately 200 fs and is accompanied by coherent
non-equilibrium vibrational relaxation in the photoinduced HS state. These results
reveal similar dynamical features to those already reported for LIESST in related Fe
(II) systems [137]. The activation of coherent molecular vibrations is an essential
requirement for reaching the HS potential on the timescale of molecular motions,
whereas their fast damping allows an efficient trapping in the HS potential
[138]. The observed coherent oscillations are attributed to photoinduced molecules
in the HS states, and the results are supported by Raman spectroscopy at thermal
equilibrium and DFT analyses of molecular vibrations and TD-DFT calculations of
optical absorption.
4 Conclusions
Time-resolved crystallography has developed extensively over the last three
decades, and the results presented in this chapter show that under favourable
conditions it is possible to obtain full three-dimensional structural data on chemical
species that have lifetimes of microseconds or less [139]. These advances, coupled
with similar advances in instrumentation and computer power, open up possibilities
for monitoring chemical processes in the solid state in a way that has not previously
been possible. At present the restrictions of maintaining crystal integrity remain, and
the reactions and processes are mostly limited to the interaction of the solid to
external media such as light, as discussed in this chapter, but also to changes in
temperature and pressure and the influence of magnetic and electric fields (although
crystal integrity is retained in some solid-gas reactions). The potential is enormous,
Time-Resolved Single-Crystal X-Ray Crystallography
263
each delay time with 60 frames with 10 s of exposure for every 1
step of the
diffractometer ϕ axis. Typical excitation densities were 150 μm
À2 , with a laser
diameter of ca. 500 μm (FWHM). After data processing of the full data sets for the
monoclinic polymorph, photodifference maps were calculated for the 500 ps and
50 μs time delays. The maps show significant changes in electron density upon
excitation (Fig. 11). The response to the excitation at 500 ps is shown by the
sideways shift of electron density from the Fe1 atom towards O2 (Fig. 11a). This
shift becomes even more pronounced at the 50 μs time point (Fig. 11b). There is also
an increase in the Fe–N distances by ca. 0.05 Å. The estimated population of the
excited state at 500 ps is in the range 1.5–2%, and this increases to ca. 10.5% after
50 μs. Overall, the results show significant shifts of the Fe atom and the two O atoms
upon excitation, and the results are consistent with the complementary spectroscopy
that was undertaken.
In a subsequent study of the Fe(III) spin-crossover material, [Fe
(3-MeO-SalEen) 2 ][PF 6 ], the switch from LS to HS only occurs at the molecular
level as clearly shown by the linear dependence of the fraction of photoswitched
molecules with the excitation density as well as with the initial fraction of low spin
molecules. The inter-system crossing from the photoexcited LS (S ¼ 1/2) to HS
(S ¼ 5/2) occurs within approximately 200 fs and is accompanied by coherent
non-equilibrium vibrational relaxation in the photoinduced HS state. These results
reveal similar dynamical features to those already reported for LIESST in related Fe
(II) systems [137]. The activation of coherent molecular vibrations is an essential
requirement for reaching the HS potential on the timescale of molecular motions,
whereas their fast damping allows an efficient trapping in the HS potential
[138]. The observed coherent oscillations are attributed to photoinduced molecules
in the HS states, and the results are supported by Raman spectroscopy at thermal
equilibrium and DFT analyses of molecular vibrations and TD-DFT calculations of
optical absorption.
4 Conclusions
Time-resolved crystallography has developed extensively over the last three
decades, and the results presented in this chapter show that under favourable
conditions it is possible to obtain full three-dimensional structural data on chemical
species that have lifetimes of microseconds or less [139]. These advances, coupled
with similar advances in instrumentation and computer power, open up possibilities
for monitoring chemical processes in the solid state in a way that has not previously
been possible. At present the restrictions of maintaining crystal integrity remain, and
the reactions and processes are mostly limited to the interaction of the solid to
external media such as light, as discussed in this chapter, but also to changes in
temperature and pressure and the influence of magnetic and electric fields (although
crystal integrity is retained in some solid-gas reactions). The potential is enormous,
Time-Resolved Single-Crystal X-Ray Crystallography
263