DATE2025.06.06 #Press Releases

Ultrafast spectroscopy reveals the mechanism of the bright red-blue colour-changing phase transition!

-- Revealed the long-standing mystery of the mechanism of the light-induced charge-transfer spin transition phenomenon --

Presentation points

• In the charge-transfer spin-transition material that change colour between bright blue and red crystals, which occurs first by light irradiation, charge transfer or spin transition? After a quarter of a century of long-standing debate on the question, the mechanism has now been clarified.
• In this bright colour change between blue and red, it has been found by ultrafast spectroscopy, that light induces a charge-transfer phase transition, followed by a spin transition. Light irradiation generates a photoexcited phase by a charge-transfer between metal ions within tens of femtoseconds, followed by a spin transition in 130 femtoseconds.
• The present finding is expected to be a fundamental technology for the design of next-generation materials, such as high-speed photoswitching materials and magneto-optical memory devices, which manipulate colour changes, charge state changes and spin state changes at ultra-high speed.

Overview

A joint research team led by Professor Shin-ichi Ohkoshi of the School of Science, the University of Tokyo and Professor Eric Collet of the University of Rennes, France, has discovered by femtosecond laser ultrafast spectroscopy that in a photoinduced phase transition material where charge transfer and spin transition coexist, charge transfer occurs first, followed by spin transition upon light irradiation. In the charge-transfer spin-transition material, which shows drastic colour-change between bright blue and red crystals, it has long been debated for a quarter of a century whether the charge transfer or the spin transition leads the phase transition, but this time the phenomenon has been revealed that the charge transfer occurs first and then the spin transition. When the blue crystal phase of the cyanido-bridged cobalt-tungsten self-assembly compound, trivalent cobalt low-spin tetravalent tungsten state (Co^III_low-spin-W^IV), is irradiated with pulsed light, photoexcitation originating from the absorption from the charge transfer between cobalt and tungsten occurs within about several tens of femtoseconds, and the photoexcited phase, divalent cobalt  low-spin– pentavalent tungsten state (Co^II_low-spin-W^V) is generated (hereafter called the photoexcited phase), which undergoes a spin transition within 130 femtoseconds to form a red crystal phase, divalent cobalt  high-spin–pentavalent tungsten state (Co^II_high-spin-W^V), i.e., the Co^II low-spin state (S= 1/2) transits to the Co^II high-spin state (S= 3/2). This achievement is expected to serve as a fundamental technology for the design of next-generation materials such as high-speed photoswitching materials and magneto-optical memory devices that manipulate light-induced colour change, charge state change and spin state change at ultrafast speed.

Announcements

Phase transition materials, whose physical properties change dramatically upon photo-stimulation, are attracting attention as functional materials. In particular, light-induced phase transitions, in which the electronic and magnetic states are instantly changed by light irradiation, are expected to have potential for new device applications such as optical memory and optical devices due to their high speed and controllability. In many of these materials, charge transfer (CT) ^{（Note 1）} and spin transition (ST)^{（Note 2）} are intertwined, but the order of their occurrence has been a matter of debate for a quarter of a century and has not been fully understood.

The members of CNRS IRL DYNACOM ^{（Note 3）} , Professor Shin-ichi Ohkoshi, Dr. Kazuki Nakamura and Assistant Professor Koji Nakabayashi of the School of Science at the University of Tokyo, and Professor Eric Collet and Associate Professor Laurent Guerin of the University of Rennes, France, have focused on a cyanido-bridged cobalt (Co)-tungsten (W) self-assembly compound. The material is known to exhibit a phase transition with a temperature hysteresis loop originating from the phase transition between the blue crystal phase, Co^III_low-spin-W^IV state in the low temperature phase, and the red crystal phase, Co^II_higt-spin-W^V state in the high temperature phase, and this material also shows a photoinduced magnetisation phenomenon^{（Note 4）} . In the present study, Cs⁺_0.1(hydroxonium ion)_0.9[Co(4-bromopyridine)_2.3{W(CN)₈}] (referred to as CsCoW) was investigated (Fig. 1a), which shows a bistable temperature phase transition between the high and low temperature phases. When the low-temperature phase of CsCoW is irradiated by light, phase transition occurs and a light-induced phase develops, accompanying colour changes and changes in magnetic properties (Fig. 1b,c).

Figure 1: Blue-red colour-change phase transition material based on cyanido-bridged cobalt-tungsten self-assembly Cs⁺_0.1(hydroxonium ion)_0.9[Co(4-bromopyridine)_2.3{W(CN)₈}] (CsCoW).

(a) Crystal structure of cyanido-bridged cobalt-tungsten-accumulated compounds. (b) Thermal hysteresis^{（Note 5）} observed in the temperature dependence of the product of molar magnetic susceptibility and temperature (_XMT, proportional to the number of spins). (c) UV-Vis-NIR absorption spectra. Before light irradiation (black), after light irradiation (wavelength of irradiated light; 785 nm, intensity: 160 mW cm^-2) and after annealing at 100 K (dotted line), measured at 4 K. The inset shows photographs of the samples before (bottom, corresponding to the low temperature phase) and after (top, corresponding to the light-induced phase) light irradiation.

Using femtosecond time-resolved optical spectroscopy, the research group investigated the ultrafast dynamics of the photo-induced phase transition by observing the optical density change (ΔOD(t))^{（Note 6）} . The results show that photoexcitation first transfers charge from the tungsten tetravalent (W^IV) to the cobalt trivalent low-spin state (Co^III_low-spin), resulting in the formation from the blue crystal phase Co^III_low-spin-W^IV to the Co^II_low-spin-W^V state. This charge transfer was completed in about tens of femtoseconds after photoexcitation. Furthermore, within about 130 femtoseconds after this charge transfer, an ultrafast spin transition was captured, in which the spin state of cobalt transitions from a low-spin state to a high-spin state. This spin transition process revealed the formation of a short-lived photo-excited phase (Co^II_low-spin-W^V), which transitions to the final red crystal phase, the high-spin state (Co^II_high-spin-W^V) (photo-induced phase).

A periodic change in optical density was also observed after the transient peak. It was suggested that the activation of specific crystal lattice vibrations (phonon modes) promotes this spin transition. In particular, the all-symmetric vibrational modes^{（Note 7）} , which are associated with local structural changes in the cyanido-bridging network, are the driving force of the spin transition, and these phonon modes may play a role in accelerating the spin transition process.

Figure 2:  Ultrafast dynamics of light-induced phase transitions observed by subpicosecond time-resolved optical spectroscopy.
(a) Optical density change measured at room temperature. Vertical axis shows time, horizontal axis shows wavelength and colours show intensity. (b) Optical density changes at 600 nm (red circle) and 700 nm (blue circle); the fitting at 700 nm (blue circle) (solid line) represents exponential dynamics towards the photoinduced phase; the fitting at 600 nm (red circle) includes contributions from the photoinduced phase (orange line) and photoexcited phase (green line). The black line in the inset shows the vibrational fitting function. (c) Schematic of the ultrafast dynamics of the light-induced phase transition. When the low-temperature phase is irradiated with light, a photo-excited phase is generated, which is converted to a photo-induced phase within 130 fs by a spin transition with activation of the breathing mode.

In order to observe slower, continuous dynamics, optical measurements were carried out on picosecond timescales. The results show that, first, localised photoswitching occurs within 130 fs and the volume change due to the spin transition generates an internal pressure, which stabilises the larger volume photoinduced phase. Secondly, the activation of lattice vibrations by laser heating is considered to facilitate the transition from the low-temperature phase with small volume to the photo-induced phase with large volume (Fig. 3).

Figure 3: Schematic of Charge-Transfer-induced Spin Transition (CTIST^{（Note 8）}) in cyanido-bridged cobalt-tungsten-accumulated compounds.
When the low-temperature phase (blue) is irradiated with light, a local charge transfer occurs first, which induces a spin transition within 130 fs, resulting in the local formation of a light-induced phase (red). Under high light intensity and high temperature conditions, the light propagates throughout the crystal by thermoelastic transformation.

The results of this research clearly elucidate the relationship between charge transfer and spin transition in a material exhibiting bright colour change and provide new insights into the understanding of the mechanism of the ultrafast light-induced processes ^{（Note 9）} . Based on these results, further optimisation of material design and synthesis methods in the future will enable the practical application of magneto-optical devices with high-speed response and controllability that surpass those of conventional electronic devices. This is expected to have a major ripple effect not only in materials science but also in the fields of electronics and information and communication technology.

Information on Research Group Members

Department of Chemistry, School of Science, The University of Tokyo    
Shin-ichi Ohkoshi, Professor
Dr Kazuki Nakamura (former graduate student) 
Koji Nakabayashi, Assistant Professor

Institute of Physics, University of Rennes, France       
Eric Collet, Professor
Laurent Guérin, Associate Professor
Gaël Privault, postdoctoral researcher
Marius Hervé, postdoctoral researcher

Publications

Journal name	Nature Communications
Title of paper	Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets
Author(s)	Kazuki Nakamura, Laurent Guérin, Gaël Privault, Koji Nakabayashi*, Marius Hervé,  Eric Collet*, and Shin-ichi Ohkoshi* (*Corresponding authors)
DOI Number	10.1038/s41467-025-60401-4

Research Grant

This study was conducted in support of
IRL DYNACOM, CNRS International Research Laboratory, France,
CNRS - University of Tokyo Joint Research Program "Excellence Science",
Cryogenic Research Center, The University of Tokyo

Terminology

Note 1 Charge Transfer (CT)
Tphenomenon whereby electrons are transferred from one atom to another in a molecule or crystal. ↑up

Note 2 Spin Transition (ST)
APhenomenon in which the spin state of a molecule or metal ion is changed by an external stimulus. In the case of transition metal ions, transitions between low (S = 0) and high (S = 2) spin states of iron divalent ions and between low (S = 1/2) and high (S = 3/2) spin states of cobalt divalent ions are known. ↑up

Note 3 CNRS IRL DYNACOM (CNRS International Joint Research Institute)

The CNRS IRL DYNACOM is an international institute for research on the fast time dynamics of optical phase transitions started in 2022 by the University of Tokyo and the University of Rennes under the umbrella of the French Centre National de la Recherche Scientifique (CNRS). It is an international institute for research on the fast time dynamics of optical phase transitions, and is run by Director Shin-ichi Ohkoshi and Deputy Director Eric Collet, among others. Dynamics) developed into DYNACOM. The research is being developed in collaboration with the European Synchrotron Radiation Facility and the Swiss X-ray Free Electron Laser Facility. ↑up

Note 4 Cyanido-bridged cobalt-tungsten-accumulated compounds
The CoW(CN)₈ complex was found by Ohkoshi et al. in 2003 as a photoinduced charge-transfer type phase transition complex. ↑up

Note 5 Thermal Hysteresis
When a phase transition occurs by changing the temperature, the temperature at which the phase transition occurs may differ between the cooling and heating processes. The difference in temperature at this time is referred to as the thermal hysteresis. ↑up

Note 6 Optical density change (ΔOD(t))
Quantitative expression of the change in light intensity passing through a sample, used as an indicator for observing changes in the electronic and spin states of molecules. ΔOD(t) is expressed on a 1/1000 scale. ↑up

Note 7 Totally symmetric vibrational modes
Vibrational modes in which molecules and crystal lattices expand and contract periodically.↑up

Note 8 Charge-Transfer-induced Spin Transition (CTIST)
This term was proposed by Prof Shin-ichi Ohkoshi et al. in 2002.↑up

Note 9
Prof. Eric Collet et. al. reported a charge-transfer phase transition initiated by spin transition for a cobalt-iron complex in 2021. ↑up