The potentially complex energy redistribution processes that occur in
inorganic transition metal complexes make the interpretation of their relaxation
dynamics a significant challenge. Unlike in organic molecules, where rates for
processes such as intersystem crossing can take up to and in excess of many
nanoseconds, the timescales involved in inorganic materials are of picosecond /
femtosecond timescales and therefore require a more advanced experimental setup
to study them. The dynamic processes that are of interest are the evolution of
the electron population after excitation through external stimuli, via the various
charge transfer and decay processes. These processes can also induce changes in
the population of the valence orbitals on the metal ion centres causing a change
to the overall spin state. This work aims to develop techniques to monitor the
redistribution of the electron population along with the magnetisation of the
materials of interest.
The dynamic processes are much too fast to be captured using standard spectrometers
and magnetic techniques, so a technique which can operate on these
very fast timescales is required. Ultrafast laser spectroscopy allows study of
the electronic dynamics using a technique called transient transmission which
involves studying changes in the transmission spectrum as a function of time. Two
laser pulses are employed, one to perturb the sample and another to interrogate
the sample. By varying the time delay between the two pulses a picture of
how the spectrum changes over time can be constructed. This process allows a
picture to be built, of how the electronic population redistributes and decays after
excitation. In order to study the magnetisation dynamics of such materials the
samples are required to exhibit a magnetic signal. To this end, a magnetically
ordered family of inorganic compounds known as Prussian Blues were chosen
as the system of interest. These materials consist of transition metal ions linked
through cyanide bridging ligands in a rock-salt type structure. Laser spectroscopy
was used to monitor the changes in the magnetisation of the sample through a
technique called magneto-optical Faraday rotation. This involves monitoring the
polarisation of the laser pulses after interaction with the sample, again, at various
time delays to measure how the magnetisation of the sample changes over time.
These techniques were applied to three chromium based Prussian Blues, vanadium-chromium
(VCr), iron-chromium (FeCr) and the chromium-chromium (CrCr)
analogues. Through the systematic substitution of the metal ion adjacent to the
Cr sites, it was found that the rate of intersystem crossing could be influenced by
the nature of the metal center. In the case of both VCr and FeCr analogues, the
transfer of population to the final excited state occurred incredibly fast, within the
temporal profile of the pump pulse. However, in the case of the CrCr analogue,
this population transfer was slowed down suffciently that a growth of the final
excited state was observed over the course of the initial 0.5 ps after excitation.
The synthesis of these materials was carried out to optimise the morphology of
the thin films for use in the laser measurements. During this work it was found
that some of these materials exhibit electrochromic activity which was explored
in isolated films and as part of multilayered heterostructures. This work was also
incredibly helpful with understanding and predicting the spectral signatures of
redox processes involved after photo-excitation.
This line of research offers the potential to gain a deeper understanding of the
dynamic processes occurring in these functional materials which will serve as
model systems. The information gained can then be applied to a broader range
of complexes. Functional materials which possess much higher magnetic ordering
temperatures or larger magnetisation would have a greater potential to be used
in future practical applications
