Anion effects and spontaneous resolution of fluorescent tripyridinyl Co(II) spin crossover complexes
image: Schematic representation of the anion and guest effects. The achiral nitrate complex (left) crystallizes from CH2Cl2 with co-planar pyrene rings and exhibits spin crossover with hysteresis. The tetrafluoroborate/perchlorate complexes (right) crystallize from DMF with a large twist between pyrene rings, leading to chiral structures and spontaneous resolution.
Credit: Nano Research, Tsinghua University Press
Molecular materials exhibiting spin crossover (SCO) are promising candidates for applications in high-density data storage, sensors, and molecular switches due to their ability to reversibly change magnetic states under external stimuli. A key challenge lies in integrating this magnetic bistability with other functionalities, such as luminescence and chirality within these systems.
A team of chemists led by Prof. Hui-Zhong Kou from Tsinghua University has made significant progress on this front. Their research demonstrates that in a series of fluorescent terpyridine-based cobalt(II) complexes, the choice of counteranions and solvents plays a critical role in determining both the structure and properties of the resulting materials.
The team published their findings in Nano Research. The complexes were constructed from a terpyridine ligand functionalized with a pyrenyl fluorophore, but their properties are significantly different based on the accompanying anion (NO₃⁻, BF₄⁻, or ClO₄⁻).
"The counteranions and the solvents significantly influence both the magnetic behavior and chiral properties of these complexes," explained Prof. Kou. "This provides a clear design strategy for tuning multifunctional molecular materials." The nitrate-containing complex exhibited an incomplete SCO transition with a thermal hysteresis loop of about 20 Kelvin near room temperature—a rare "memory" effect for cobalt(II) systems. In contrast, the tetrafluoroborate and perchlorate complexes crystallized into chiral structures from a DMF solution. Here, the two pyrene planes within each molecule are twisted by approximately 73°, endowing the complex with inherent handedness. During crystallization, these chiral molecules spontaneously separated into enantiopure crystals, a process known as spontaneous resolution.
"Spontaneous resolution of chiral coordination complexes from achiral ligands is a challenging task," noted Prof. Kou. "In this case, it was incidentally induced by the combination of bulky tetrahedral anions and a polar solvent, which favored a twisted molecular conformation." Although variable-temperature fluorescence measurements indicated that the spin-state switching and luminescence properties in the nitrate complex operated independently, this points to the ongoing challenge and future direction of designing coupled multifunctional systems.
The work underscores anion and solvent engineering as powerful, often overlooked tools for controlling molecular magnetism and supramolecular assembly. The demonstrated ability to induce chirality without chiral synthons is particularly valuable for developing new materials for asymmetric catalysis or chiral devices. These findings offer a strategic blueprint for the rational design of next-generation smart materials where multiple physical properties can be intergrated at the molecular level.
Other contributors include Xuan Wang, Lin Miao, Zhiwei Wen and Yan Ju from the Engineering Research Center of Advanced Rare Earth Materials (Ministry of Education), Department of Chemistry, Tsinghua University in Beijing, China; and Juan Yuan from the School of Pharmacy, Henan University of Chinese Medicine in Zhengzho, China.
This research was supported by the National Natural Science Foundation of China (Project Nos. 22271171 and 22101076).
DOI Link:
https://doi.org/10.26599/NR.2026.94908408
About Nano Research
Nano Research is a peer-reviewed, open access, international and interdisciplinary research journal, sponsored by Tsinghua University and the Chinese Chemical Society, published by Tsinghua University Press on the platform SciOpen. It publishes original high-quality research and significant review articles on all aspects of nanoscience and nanotechnology, ranging from basic aspects of the science of nanoscale materials to practical applications of such materials. After 18 years of development, it has become one of the most influential academic journals in the nano field. Nano Research has published more than 1,000 papers every year from 2022, with its cumulative count surpassing 8,000 articles. In 2025 InCites Journal Citation Reports, its 2025 IF is 9.4 (8.3, 5 years), and it continues to be the Q1 area among the four subject classifications. Nano Research Award, established by Nano Research together with TUP and Springer Nature in 2013, and Nano Research Young Innovators (NR45) Awards, established by Nano Research in 2018, have become international academic awards with global influence.