Successive solid transformations induced electron transfer and switchable functions
Electron-transfer is a common phenomenon in nature and plays important roles in biology, energy, materials, catalysis and other fields. Intermetallic electron-transfer not only changes the valence states and electron configurations of the participant metal ions but also switches the coupling interactions between them. Therefore, the control of electron-transfer provides an efficient way to tune the magnetic, electric and optical properties of materials. Typical examples showing external stimuli tuned intermetallic electron-transfer behaviors are cyanide-bridged complexes. The modulation of electron-transfer behavior is realized mainly based on solution reaction. It is a direct method to regulate the redox potential and electron-transfer behavior of metal centers via chemical modification, such as ligand substitution, anion and solvent exchange in solution reaction. However, it is a formidable challenge in the solid state to reversibly control different electron-transfer behaviors and realize the coupling and synergy between the multi-functional properties under external stimuli.
Tao Liu's group from the Dalian University of Technology has been committed to the construction and multi-functional research of light-induced electron-transfer compounds over a long term. Recently, they achieved the coupling and synergy between magnetic, polar, fluorescence and thermal expansion properties via electron rearrangement induced variation in energy transfer, charge distribution and bond distance, providing a new method to read magnetic state through light, electric and mechanical signal. Some related works were published in Angew. Chem. Int. Ed. 2017, 56, 7663-7668; Angew. Chem. Int. Ed. 2017, 56, 13052-13055; Chem. Sci. 2018, 9, 617-622; Chem. Sci. 2018, 9, 2892-2897.
Recently, they adopted an ancillary ligand with an extended p-conjugation system to prepare a tetranuclear {Fe2Co2} compound. Three crystalline phases with different electron-transfer behaviors were obtained via successive solid transformations from solvated 1* 2CH3OH* 4H2O, to desolvated 1, and its polymorph 1a through subsequent desolvation and vapor induction. 1* 2CH3OH* 4H2O possesses the {FeIILS(μ-CN)CoIIILS} (LS = low spin) state at room temperature, which undergoes a thermally-induced reversible electron-transfer in mother liquor (see Movie 1). Furthermore, 1* 2CH3OH* 4H2O undergoes a desolvated process upon heating in air and then generates 1 (see Movie 2). 1 showes the {FeIIILS(μ-CN)CoIIHS} (HS = high spin) state at room temprature. No thermally-induced electron-transfer behavior was observed in 1. Polymorph 1a possesses the {FeIILS(μ-CN)CoIIILS} state at 298 K. It showed reversible intermetallic electron-transfer between {FeIILSCoIIILS} and {FeIIILSCoIIHS} states upon thermal treatment or alternative irradiation with 808 and 532 nm lasers at cryogenic temperatures, which realize the rapidly reversible control of electron-transfer and rearrangement. Structure-magnetic property relationship indicates that the enhancement of intermolecular π* * * π interactions between ligands around cobalt centers induces the changes of coordination spheres and strength of ligand field, which can tune the redox potential of cobalt centers and finally result in different electron-transfer behaviors. Moreover, the distances of π-π interactions can be modulated in a continuous range of 3.3-3.8 Å. The enhancement of intermolecular π* * * π interactions in the process of desolvation and structural rearrangement provides an important driving force for the successive solid transformations. This work indicates that the introduction of π* * * π interactions not only provides a strategy to manipulate successive solid transformations with different electronic states but also offers an access to construct switchable multifunctional materials displaying stimuli-induced dynamic changes functions in the future.
More information:
Cheng-Qi Jiao et al, Manipulation of successive crystalline transformations to control electron transfer and switchable functions, National Science Review (2018). DOI: 10.1093/nsr/nwy033
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