image: Figure 1 | Structural and optical characterizations of the two manganese-based metal halides. a, b Crystal structure diagrams of C5H11N3(MnCl3·H2O)Br (a) and C5H11N3(MnCl3·H2O)Cl (b), with the free halide ions highlighted in red circles. c Simulated XRD patterns from single crystal diffraction data and experimental ones of powder samples. d, e Calculated density of states. f, g PL, PLE, and absorption spectra. h PL decay curves.
Credit: Yibo Chen et al.
Stimuli-responsive organic-inorganic metal halides are promising for information-related applications due to their adjustable luminescent properties under external stimuli. However, the rational design of such materials is hindered by the unclear structure-property relationship, especially the overlooked role of free halide ions in the crystal lattice.
In a new paper published in Light: Science & Applications, a team of scientists led by Professor Yibo Chen from Guangzhou University, in collaboration with researchers from Tianjin University, has developed a new strategy to unlock the stimuli-responsive photoluminescence performance of manganese-based metal halides via free halide ion substitution. They synthesized two manganese-based metal halide single crystals with identical crystal structure but different free halide ions, C5H11N3(MnCl3·H2O)Cl with free Cl⁻ and C5H11N3(MnCl3·H2O)Br with free Br⁻. The experimental results show that C5H11N3(MnCl3·H2O)Br exhibits reversible photoluminescence color switching between red and green upon heating or water exposure, while C5H11N3(MnCl3·H2O)Cl shows no responsive behavior under the same conditions.
Extensive experimental characterizations and theoretical analyses reveal the intrinsic mechanism of the responsive performance. The substitution of Br⁻ weakens the hydrogen bonding around water molecules in the crystal, facilitating the escape of coordinated water molecules upon heating. The loss of water molecules further triggers structural reorganization and the transformation of the coordination configuration of Mn2+ from six-coordinated octahedron to four-coordinated tetrahedron, which leads to the significant change in photoluminescence color. Moreover, the stimuli-responsive photoluminescence switching of C5H11N3(MnCl3·H2O)Br shows excellent reversibility, with only a slight decrease in luminescence intensity after 15 heating-cooling cycles and stable peak position and full width at half maximum.
To explore the practical application potential of the material, the research team fabricated flexible luminescent films by combining C5H11N3(MnCl3·H2O)Br with polydimethylsiloxane. The films demonstrate outstanding performance in multiple application scenarios. For planar temperature sensing, the film can visually reflect the real-time and spatial temperature distribution of the detected object, providing more detailed thermal information than traditional point temperature sensing. In thermal stamping, the film can realize reversible pattern printing by thermal stimulation, with the printed green pattern recovering to the original red state by absorbing ambient moisture. In the field of information encryption and anti-counterfeiting, the film can be combined with non-responsive C5H11N3(MnCl3·H2O)Cl to fabricate encrypted patterns, which realize information encryption and decryption through thermal stimulation, showing great potential in dynamic anti-counterfeiting.
This study for the first time clarifies the crucial role of free halide ions in regulating the stimuli-responsive performance of metal halides, breaking the traditional cognition of free halide ions as only charge-balancing components in the crystal lattice. The proposed free halide ion substitution strategy provides a simple and efficient method for the design and development of smart stimuli-responsive optical materials. The fabricated manganese-based metal halide materials with reversible photoluminescence switching performance show broad application prospects in smart sensing, information encryption, flexible electronics and other cutting-edge information-related fields.
The scientists of the research team said, "The discovery of the regulatory role of free halide ions opens a new pathway for the development of metal halide-based smart optical materials. In the future, we will further expand the regulation strategy of weak interactions in the crystal and design multi-stimuli responsive materials with higher sensitivity and stronger reversibility to meet the growing demand for smart optical materials in practical applications."
Journal
Light Science & Applications
Article Title
Substitution of free halide ions unlocks responsive photoluminescence switching in manganese-based metal halides