Chemistry Reference
In-Depth Information
solid-solid phase transition of VO
2
at the femtosecond scale by Cavalleri and
collaborators [
77
].
The method has also been applied to organic crystals, although the number of
available examples is still rather limited. A remarkable example of a detailed study
is the picosecond-scale observation of paraelectric-to-ferroelectric phase transition
in the crystals of an organic charge transfer compound, the molecular complex of
tetrathiafulvalene and chloranil, reported by Koshihara, Collet and their collabora-
tors [
78
,
79
]. Excitation by femtosecond laser pulses above the threshold triggers
partial charge transfer between the alternating donor and acceptor molecules and
results in reversible photoinduced phase transition. The phase change appears as
lowering of the crystal symmetry caused by the change of intermolecular distances
within the stacked columns. Techert and collaborators [
80
] have employed time-
resolved X-ray diffraction to study the mechanism of bond cleavage and formation
during [2 + 2] photocycloaddition reaction on
-styrylpyrylium trifluoromethane-
sulfonate. An extensive study of this system with picosecond time-resolution,
supported by spectroscopic assessment, has provided a detailed insight into the
mechanism of this important reaction. A sequence of processes, involving bond
breaking and formation, phenyl group rearrangement, and anion rotation have been
unraveled, and the kinetics of these processes was also deduced [
80
].
Ultrafast X-ray diffraction has been also employed recently to study the atomic
structure and decay profile of excited molecules with long-lived excited states.
These studies provide valuable information on the primary processes following
photoexcitation. A time-resolved, stroboscopic experiment where the delay between
pump and probe is systematically varied, provided direct atomic-scale evidence and
the structure of the photoexcited state of the paddlewheel ion [Pt
2
(pop)
4
]
4
[
81
].
Shortening of the central Pt-Pt bond and slight lengthening of the Pt-P bonds by
excitation to the lowest excited triplet state were substantiated by density functional
calculations and topological analysis of the charge density [
82
]. Intermolecular
phenomena in electronically coupled systems, such as formation of excited-state
complexes (exciplexes), were also observed by time-resolved X-ray photodiffrac-
tion, for example, in the case of a trimeric pyrazolate copper complex [
83
]. Techert
and Zacchariase [
84
,
85
] have employed picosecond-scale time-resolved powder
X-ray photodiffraction to analyze intermolecular charge transfer in the excited state
of solid 4-(diisopropylamino)benzonitrile. The result [
85
] showed “flattening” of the
molecule by decrease of the torsional angle between the diisopropylamino group and
the phenyl ring of 4
.
a
3 Summary and Future Outlook
Although technically still being at the development stages, X-ray photodiffraction
is already being established as a very promising method for structure analysis of
important photoinduced phenomena related to very fundamental physical and
chemical processes that accompany the interaction of the matter with light.
