(AGENPARL) – mar 04 giugno 2024 *Source*: Tokyo Institute of Technology
*Immediate release:* June 4, 2024
Headline: Observing Ultrafast Photoinduced Dynamics in a Halogen-Bonded
Supramolecular System
*Researchers uncover how the halogen bond can be exploited to direct
sequential dynamics in the multi-functional crystals, offering crucial
insights for developing ultrafast-response times for multilevel optical
storage.*
Halogen bonds are intermolecular interactions that arise from the
attraction between a halogen atom (group 17 elements in the periodic table)
and another atom with lone pairs, more generally a molecular entity with
high electron density. Understanding the distinctive and highly directional
nature of halogen bonds is crucial for crystal engineering and studying
photoinduced structural deformations, which is key for the development of
innovative photo-functional materials.
However, the influence of halogen bonds on the rapid photoinduced changes
within supramolecular systems remains largely unexplored due to a lack of
experimental techniques that can directly observe the halogen bond in
action.
To solve this problem, a team of researchers, led by Assistant Professor
Tadahiko Ishikawa from the Department of Chemistry at the School of Science
at Tokyo Institute of Technology (Tokyo Tech), Associate Professor Kazuyuki
Takahashi affiliated with Kobe University, Dr. Yifeng Jiang affiliated with
the European X-Ray Free-Electron Laser Facility (EuXFEL), and Professor R.
J. Dwayne Miller affiliated with University of Toronto, explored the
photoinduced dynamics associated with the halogen bonds of the prototypical
halogen-bonded multifunctional system [Fe(Iqsal)2][Ni(dmit)2]·CH3CN·H2O on
the ultrafast timescale, triggered by change in electron spin or spin
crossover (SCO) mechanics. The study, which is a collaborative research
project involving Tokyo Tech, EuXFEL, University of Potsdam, University of
Toronto, University of Tsukuba, and Kobe University, has been detailed in
the journal *Nature Communications*.
SCO is a phenomenon observed in some transition-metal coordination
complexes, wherein a spin transition between low-spin (LS) and high-spin
(HS) states is triggered through changes in temperature, pressure, or
light. SCO accompanies relatively large volume changes and can be
controlled by photoinducing different responses in the multifunctional
crystals. [Fe(Iqsal)2][Ni(dmit)2]·CH3CN·H2O is a typical example of such
multifunctional crystals, which exhibits both thermally- and photo-induced
SCO-related phase transitions. In this system, [Fe(Iqsal)2]+ cations and
[Ni(dmit)2]– anions are bound by halogen bonds.
“SCO of the [Fe(Iqsal)2]+ cations leads to a phase transition between
low-temperature (LT) and high-temperature (HT) phases in our target
material due to intermolecular interactions,” explains Ishikawa. “The LT
phase exhibits LS state of the [Fe(Iqsal)2]+ cations and strong
dimerization of the [Ni(dmit)2]– anions, while the HT phase exhibits HS
state cations and weak dimerization of anions. The question is how does the
halogen bond direct electron density and spin changes to impact functions
as part of undergoing these phase transitions. Can we control the phase and
material properties?”
The researchers investigated the ultrafast photoinduced molecular dynamics
involving SCO of the [Fe(Iqsal)2]+ cations and dimerization of the [Ni(dmit)
2]– anions by combining three methods: time-resolved transient visible
absorption spectroscopy, time-resolved mid-infrared reflectivity
spectroscopy, and ultrafast electron diffraction to study the dynamics from
different viewpoints, covering electronic, vibrational, and structural
aspects of the system. This comprehensive approach allowed for a thorough
investigation of the photoinduced change of the states, providing a deeper
understanding of the underlying processes and intermediates involved. They
discovered the existence of a photoinduced transient intermediate state
(TIS) different from the LT and HT phases, characterized by the HS state of
[Fe(Iqsal)2]+ cations with strong dimerization of [Ni(dmit)2]– anions. This
TIS state is achieved in the ultrafast timescale, within a few picoseconds,
while the final state, similar to the HT phase, is achieved through
sequential slow dynamics over approximately 50 picoseconds.
Furthermore, to elucidate the role of the halogen bonds in the
above-mentioned photoinduced sequential dynamics, the researchers conducted
quantum chemistry calculations using the ultrafast electron diffraction
results. Their analysis revealed the persistence of halogen bonds between
the cation and the anion guiding the sequential dynamics. Photoexcitation
of the [Fe(Iqsal)2]+ cation expands the SCO ligand shell, reaching TIS.
This state, being unstable, transfers the excess energy of the [Fe(Iqsal)2]+
cation to the [Ni(dmit)2]– anions through vibrational energy transfer via
halogen bonds. In addition, the rapid expansion of the SCO ligand shell
builds strain on the nearest [Ni(dmit)2]– anions in the halogen bond
direction. These two effects result in dimer softening of the [Ni(dmit)2]–
anions. The researchers developed a short video
to illustrate these ultrafast
dynamics.
Overall, the present results underscore the importance of halogen bonds in
the photoinduced dynamics, offering a better understanding of the
synergistic spin transition. “Our study highlights the importance of
ultrafast investigations in monitoring ultrafast electronic and structural
dynamics,” remarks Jiang. “Overall, our study highlights the potential for
utilizing halogen bonds for fine-tuned functional control in photo-active
supramolecular systems, with applications in fast multilevel optical data
storage.”
*Reference*
Authors:
Yifeng Jiang1,*, Stuart Hayes2, Simon Bittmann3, Antoine Sarracini2, Lai
Chung Liu4, Henrike M. Müller-Werkmeister5, Atsuhiro Miyawaki6, Masaki Hada7,
Shinnosuke Nakano8, Ryoya Takahashi8, Samiran Banu8, Shin-ya Koshihara8,
Kazuyuki Takahashi6,*, Tadahiko Ishikawa8,*, and R. J. Dwayne Miller 2,*
Title:
Direct observation of photoinduced sequential spin transition in a
halogen-bonded hybrid system by complementary ultrafast optical and
electron probes
Journal:
*Nature Communications*
DOI:
10.1038/s41467-024-48529-1

Affiliations:
1 European XFEL, Germany
2 Departments of Chemistry and Physics, University of Toronto, Canada
3 Max Planck Institute for the Structure and Dynamics of Matter, Germany
4 Uncharted Software, Canada
5 Institute of Chemistry, University of Potsdam, Germany
6 Department of Chemistry, Graduate School of Science, Kobe University,
Japan
7 Tsukuba Research Center for Energy Materials Science, University of
Tsukuba, Japan
8 Department of Chemistry, School of Science, Tokyo Institute of
Technology, Japan
Figure 1 Title: Schematic image of the intermolecular interaction induced
by spin crossover ligand expansion
Figure 1 Caption: This illustration shows the [Fe(Iqsal)2]+ cation being
photoexcited and transferring energy through a halogen bond, inducing
strain effect on the [Ni(dmit)2]? anion, causing a structural change.
credit: Tadahiko Ishikawa
Figure 2: https://tokyotech.box.com/s/fpz66pz7habqz4o2w1qtavsrw03cxpap
credit: Tokyo Institute of Technology
*Contact: *Emiko Kawaguchi, Public Relations Department, Tokyo Institute
*About Tokyo Institute of Technology*
Tokyo Tech stands at the forefront of research and higher education as the
leading university for science and technology in Japan. Tokyo Tech
researchers excel in fields ranging from materials science to biology,
computer science, and physics. Founded in 1881, Tokyo Tech hosts over
10,000 undergraduate and graduate students per year, who develop into
scientific leaders and some of the most sought-after engineers in industry.
Embodying the Japanese philosophy of “monotsukuri,” meaning “technical
ingenuity and innovation,” the Tokyo Tech community strives to contribute
to society through high-impact research.
https://www.titech.ac.jp/english/