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, we 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.

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. The low-temperature (LT) phase exhibits LS state of the [Fe(Iqsal)₂]⁺ cations and strong dimerization of the [Ni(dmit)₂]^– anions, while the high-temperature (HT) phase exhibits HS state cations and weak dimerization of anions, which has been considered as the result of the halogen bonds. The question is how does the halogen bond direct electron density and spin changes to impact functions as part of undergoing these phase transitions. Another question is whether the phase and material properties can be controlled.

We 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. We 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, we conducted quantum chemistry calculations using the ultrafast electron diffraction results. Our 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. Please watch a short video, see below, that illustrates 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. Our study indicates the potential for utilizing halogen bonds for fine-tuned functional control in photo-active supramolecular systems, with applications in fast multilevel optical data storage.