Photoinduced macroscopic polarization change with long-lived metastable state

2021 Fiscal Year Research-status Report

• Project/Area Number: 21K05085
• Research Institution: Kyushu University
• Principal Investigator: SU SHENGQUN 九州大学, 先導物質化学研究所, 特任助教 (90817496)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: polarization change / spin transition / photoexcitation

Outline of Annual Research Achievements

The proposed research aims at developing crystals of spin crossover complexes with Light-Induced Excited Spin-State Trapping (LIESST) effect that exhibit photoinduced polarization change from ground state to excited state with long-lived metastable state. After we tried a series of 2,6-bis(pyrazol-1-yl)pyridine and bis[((2-methylimidazol-4-yl)methylidene)-3-aminopropyl]ethylenediamine derivatives as ligands to develop the Fe(II) spin crossover complexes, and we have obtained several polar Fe(II) spin crossover complex exhibited LIESST effect. However, the single crystals of the most of them are easy to cleavage after the spin transition, which does not facilitate the measuring of macroscopic polarization change of the compounds. Only one compound's single crystal remained intact after undergoing an incomplete spin transition. In this compound, 28% of Fe(II) changes from the low spin state of the ground state to the high spin state of the excited state after photoexcitation. The light-induced excited state with a relaxation time of 4609 s at 10 K will facilitate the test is sufficient to detect electric current in a crystal when it returns to the ground state. When the excited state returned to the ground state, a polarization change of ca. 0.45 μC cm－2 was detected. This study indicates that the combination of the LIESST effect and macroscopic polarization is a feasible way to realize photoinduced persistent polarization change.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

For a long time, our group mainly focused on developing molecular switches based on coordination complexes, especially iron spin-crossover compounds. A series of spin-crossover complexes responsive to light have been synthesized, characterized, and well-studied. We can make full use of those methods of synthesis. In addition, we are familiar with the measurement of the photoinduced magnetic property and polarization change. These should be the main reasons for the smooth progress of this research. In addition, convenient test conditions, such as H-NMR, single-crystal x-ray diffraction and measurement instruments for magnetic property and polarization change are also important.

Strategy for Future Research Activity

Although we have obtained a spin crossover complex with photoinduced polarization change as expected, the structure of the excited state is still unknown because of the low conversion rate and the fact that the excited state can only be stabilized at low temperature, which makes it difficult for us to explore the mechanism of the polarization change. We will continue to design and synthesize new spin-crossover compounds with the high photoinduced conversion rate and the more stable excited state or optimize the already obtained compounds to achieve photoinduced polarization change and grasp the mechanism of polarization change.