Observation of photoinduced ultrafast dynamics in a halogen-bonded supramolecular system

Observation of photoinduced ultrafast dynamics in a halogen-bonded supramolecular system

This illustration shows [Fe(Iqsal)2]+ cation being photoexcited and transferring energy through a halogen bond, inducing the strain effect in [Ni(dmit)2] anion, causing a structural change. Credit: Tadahiko Ishikawa, Nature Communications

Researchers have discovered how halogen bonding can be exploited to drive sequential dynamics in multifunctional crystals, providing crucial insights into the development of ultrafast response times for multilevel optical storage.

Halogen bonds are intermolecular interactions that arise from the attraction between a halogen atom (elements of group 17 in the periodic table) and another lone-pair atom, generally a molecular entity with high electron density. Understanding the distinctive and highly directional nature of halogen bonds is essential for crystal engineering and the study of photo-induced structural deformations, which is essential for the development of innovative photo-functional materials.

However, the influence of halogen bonds on fast photoinduced changes within supramolecular systems remains largely unexplored due to the lack of experimental techniques that can directly observe halogen bonding in action.

To solve this problem, a team of researchers, led by Assistant Professor Tadahiko Ishikawa from the Department of Chemistry in the School of Science at Tokyo Institute of Technology (Tokyo Tech), Associate Professor Kazuyuki Takahashi affiliated with Kobe University, Dr. Yifeng Jiang associated with the European X-ray Free Electron Laser Facility (EuXFEL) and Professor RJ Dwayne Miller associated with the University of Toronto explored the photoinduced dynamics associated with halogen bonds of the prototype multifunctional halogen-bonded system [Fe(Iqsal)2][Ni(dmit)2]· CH3CN·H2O on the ultrafast time scale, caused by the 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, is detailed in the journal. Nature Communications.

Observation of photoinduced ultrafast dynamics in a halogen-bonded supramolecular system

Halogen bonds can be used to tune the functional properties of photoactive supramolecular systems. Credit: Tokyo Institute of Technology

SCO is a phenomenon observed in some transition metal coordination complexes, where a spin transition between low spin (LS) and high spin (HS) states is induced through changes in temperature, pressure or light. SCO accompanies relatively large volume changes and can be controlled by photoinducing different responses in multifunctional crystals. [Fe(Iqsal)2][Ni(dmit)2]· CH3CN·H2O is a typical example of such multifunctional crystals, which exhibits SCO-related phase transitions both thermally and photon. In this system, [Fe(Iqsal)2]+ cations and [Ni(dmit)2] the anions are connected by halogen bonds.

“SCO i [Fe(Iqsal)2]+ cations lead to a phase transition between the low temperature (LT) and high temperature (HT) phases in our target material due to intermolecular interactions,” explains Ishikawa.

“The LT phase displays the LS state of [Fe(Iqsal)2]+ cations and strong dimerization of [Ni(dmit)2] anions, while the HT phase exhibits HS state cations and weak anion dimerization. The question is how the halogen bond drives the electron density and spin changes in the impact functions as part of these phase transitions. Can we control the phase and material properties?”

The researchers investigated the photoinduced ultrafast molecular dynamics involving the SCO of [Fe(Iqsal)2]+ cations and dimerization of [Ni(dmit)2] anions by combining three methods: time-resolved transient visible absorption spectroscopy, time-resolved mid-infrared reflectance spectroscopy, and ultrafast electron diffraction to study the dynamics from different perspectives, covering electronic aspects , vibrational and structural of the system.

This comprehensive approach allowed a thorough investigation of photo-induced changes in states, providing a deeper understanding of the underlying processes and mediators involved. They discovered the existence of a photoinduced transient intermediate state (TIS) distinct 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 reached on the ultrafast time scale, within a few picoseconds, while the final state, similar to the HT phase, is reached through the subsequent slow dynamics over approximately 50 picoseconds.

Furthermore, to elucidate the role of halogen bonds in the aforementioned photoinduced sequence dynamics, the researchers performed quantum chemistry calculations using ultrafast electron diffraction results. Their analysis revealed the stability of the halogen bonds between the cation and the anion that drives the sequential dynamics.

Photoexcitation of [Fe(Iqsal)2]+ cation expands the shell of the SCO ligand, reaching the TIS. This state, being unstable, transfers excess energy [Fe(Iqsal)2]+ cation of [Ni(dmit)2] anions via vibrational energy transfer via halogen bonds.

In addition, the rapid expansion of the SCO ligand shell creates strain in the near part [Ni(dmit)2] anions in the direction of the halogen bond. These two effects result in dimer attenuation [Ni(dmit)2] anions. The researchers developed a short video to illustrate these ultrafast dynamics.






Overall, the present results highlight the importance of halogen bonds in photoinduced dynamics, providing a better understanding of synergistic spin transitions.

“Our study highlights the importance of ultrafast probes in monitoring ultrafast electronic and structural dynamics,” notes Jiang. “Overall, our study highlights the potential for using halogen bonds for tuned functional control in photoactive supermolecular systems, with applications in fast multi-level optical data storage.”

More information:
Yifeng Jiang et al, Direct observation of photoinduced sequential spin transitions in a halogen-bonded hybrid system by complementary ultrafast optical and electronic probes, Nature Communications (2024). DOI: 10.1038/s41467-024-48529-1

Provided by Tokyo Institute of Technology

citation: Observation of ultrafast photoinduced dynamics in a halogen-bonded supramolecular system (2024, June 4) Retrieved June 5, 2024 from https://phys.org/news/2024-06-ultrafast-photoinduced-dynamics-halogen -bonded.html

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