Chemistry Reference
In-Depth Information
The most important asset of the technique is definitely its capability to provide
direct insight into atomic-scale changes that accompany the excitation, either
during the time-span of the excited state or after deexcitation and related reactions.
This first-hand information on the geometry of the reaction coordinate can be
conveniently coupled with that on the energy diagram from other experimental
techniques to arrive at complete description of the light-induced phenomena.
Having said that, at the present stage, the method has certain limitations, mainly
related to the requirements that it puts to the system of interest and to its technical
realization. The low efficiency of the excitation represents one of the main burdens
in obtaining a sufficiently strong signal of the photoconverted species. Moreover,
the optimization of the experimental conditions which are necessary to obtain
sufficiently high yields is still a time-consuming process; the result often depends
on the particular system and is not universally applicable. In that respect, the
utilization of crystal engineering [
86
,
87
] to design crystal motifs which will
comply with these requirements (for instance, by retardation of time-decay by
modulation of the intermolecular interactions or trapping of the intermediates)
might be one alternative approach. With several examples, the group of Coppens
has recently demonstrated [
88
,
89
] the usefulness of supramolecular chemistry in
the course of “dilution” of the reactive species, although such an approach has its
drawbacks, for example, such that are related to the (electronic) interaction of the
medium and the photoactive species.
As some of the further developments in the field, development of techniques to
probe ultrafast events in
irreversible
processes [
90
], the development of the experi-
mental setups which will allow opportunities to study a larger number of “slow”
processes (in the millisecond and microsecond range), and study of the ultrafast
(sub-picosecond) processes are definitely some of the important directions for new
developments in the future. Moreover, now, when the basic principles of the
method have been developed, it seems natural that the applications should be
focused on systems with actual practical interest, for example, systems that are
important for alternative energy sources or application in electronics. X-ray free
electron lasers that are nowadays becoming available are expected to shorten the
timescales, an important direction for further development, both for physics as well
as for structural biology [
91
]. These future applications will also require fast-
determining detectors, which are currently a potential bottleneck in the process of
further squeezing the timescale. While the importance of X-ray photodiffraction is
being increasingly recognized, other X-ray-based methods to study processes in
time-domain science, in solution as well as solid state, have simultaneously
emerged, so that they now provide a platform to combine information with the
results obtained with X-ray photodiffraction. As some of the recent representative
examples, solution-state dynamics of small photoexcited molecules can be
observed with X-ray scattering at picoseconds scale [
92
] and solvent reorganization
around photoexcited species can be studied by X-ray absorption spectroscopy [
93
,
94
]. The group of Lin Chen has greatly advanced time-resolved X-ray absorption
techniques to study metal-to-ligand charge transfer excited states in copper(I)
complexes. [
95
,
96
] Time-resolved diffraction using neutrons or electrons for
