Inorganic-organic hybrid molecular materials are widely viewed as a synthetic route to single-phase magnetoelectric (ME) materials that work at room temperature under low magnetic fields, but their discovery remains example-driven: no validated, transferable set of chemical and structural design rules exists that predicts whether a candidate molecular crystal will show ME coupling, or with what magnetoelectric tensor magnitude. Recent demonstrations (a paramagnetic Yb(III) ferroelectric in 2020, an electronic-ferroelectric film in 2021, (TMCM)[FeCl4_4] in 2024 with α31≈89\alpha_{31}\approx 89 mV Oe−1^{-1} cm−1^{-1}, and a {Co3IIIDyIII}\{\mathrm{Co}^{III}_{3}\mathrm{Dy}^{III}\} complex in 2026 reaching about 250 mV cm−1^{-1} Oe−1^{-1}) each propose a family-specific heuristic, most commonly magnetostriction-mediated coupling in paramagnetic ferroelectrics, while the strict case of a molecular material that is genuinely multiferroic at room temperature (long-range magnetic order coexisting with ferroelectricity, with usable coupling) remains unrealized. The problem is to establish predictive design rules for room-temperature, low-field ME coupling in single-phase molecular materials and to validate them by synthesizing new compounds whose measured ME response matches a priori predictions.

Background

A multiferroic is a material in which two or more ferroic orders (most often ferroelectricity, a switchable electric polarization, and magnetic order) coexist in a single phase. A magnetoelectric (ME) material couples them: an applied magnetic field changes the electric polarization PP, and an applied electric field changes the magnetization. The strength of the linear coupling is measured by the ME tensor αij=∂Pi/∂Hj\alpha_{ij}=\partial P_i/\partial H_j (conventionally reported, for example, in mV Oe−1^{-1} cm−1^{-1}), while related observable effects include the magnetodielectric effect (a magnetic-field-induced change of the dielectric permittivity). Single-phase ME materials are scarce because ferroelectricity and magnetism compete at the levels of symmetry (ferroelectricity requires a polar, non-centrosymmetric structure; magnetic order does not supply it) and electronic structure (usual ferroelectric mechanisms favor empty dd shells, magnetism requires partially filled ones). In inorganic solids only a handful of systems, such as BiFeO3_3 and some hexaferrites, show undisputed room-temperature ME behavior. Inorganic-organic hybrid molecular materials — coordination complexes, hybrid organic-ammonium metal-halide perovskites, and metal-organic frameworks — offer an alternative: they combine the designability and synthetic flexibility of molecular chemistry with the magnetism of transition-metal or lanthanide ions, and their soft lattices can translate magnetic-field-induced strain into large changes of dipole arrangement. Molecular multiferroics are known (for example the metal-formate frameworks, in which magnetic ordering sets in only at a few kelvin), but their coupling is confined to low temperature, weak tensor magnitudes, or high fields, which is the limitation this problem targets. Three results define the current frontier. First, Long et al. (Science 367, 671 (2020)) demonstrated strong ME coupling at room temperature and low field in a paramagnetic ferroelectric ytterbium(III) coordination complex [Zn(OAc)(L)Yb(NO3_3)2_2] (OAc = acetate; L = a chiral salen-type Schiff-base ligand): an applied magnetic field strains the crystal (magnetostriction, the field-induced change of shape or volume), and that strain changes the polarization — establishing that long-range magnetic order is not required for usable coupling. Second, Liu et al. (JACS 143, 5779 (2021)) achieved room-temperature ME coupling in a thin film of [(n-C3_3H7_7)4_4N][FeIII^{III}FeII^{II}(dto)3_3] (dto = C2_2O2_2S2_2, the dithiooxalate dianion), an electronic ferroelectric in which the switchable polarization arises from order-disorder/electronic charge arrangement rather than ionic displacement. Third, Hu, Song et al. (Nat. Commun. 15, 4702 (2024)) showed that the molecular ferroelectric (TMCM)[FeCl4_4] (TMCM = trimethylchloromethylammonium) exhibits room-temperature ME coupling under fields of about 1 kOe and below, with α31≈89\alpha_{31}\approx 89 mV Oe−1^{-1} cm−1^{-1} and a polarization change of roughly 12% at 40 kOe, driven by field-induced stretching of Cl···Cl contacts and shrinking of the [FeCl4_4]−^- anions; most recently a homochiral {Co3IIIDyIII}\{\mathrm{Co}^{III}_{3}\mathrm{Dy}^{III}\} molecular crystal (JACS 2026) reached about 250 mV cm−1^{-1} Oe−1^{-1} at room temperature via magnetostriction-piezoelectric coupling, and a chiral Ni2+^{2+}/Yb3+^{3+} complex showed room-temperature magnetic-field-driven 180° polarization reversal (Materials Today 2026). What is still missing, and is explicitly identified as the goal in the 2021 Advanced Materials review of the field and the 2025 Dalton Transactions perspective on room-temperature, low-field-induced ME coupling in molecular complexes, is the layer above these examples: a predictive understanding connecting chemical and structural descriptors (ion identity and anisotropy, coordination geometry, lattice softness, halogen and hydrogen-bond contacts, polar space group) to ME tensor magnitudes, so that materials can be designed rather than found. A further, distinct open problem in the same field is genuine room-temperature molecular multiferroicity: no single-phase molecular material is known in which long-range magnetic order and ferroelectricity coexist above 300 K with usable coupling — every room-temperature molecular ME example to date is a paramagnetic ferroelectric in which the coupling is strain- or electronic-structure-mediated.

Problem Statement

Establish a validated, transferable set of chemical and structural design rules that predicts, for inorganic-organic hybrid molecular materials, whether a candidate single-phase crystal will exhibit magnetoelectric (ME) coupling at room temperature under low magnetic fields, and with what magnitude of ME response — the linear ME tensor αij=∂Pi/∂Hj\alpha_{ij}=\partial P_i/\partial H_j or an equivalent measured ME observable — and demonstrate the rules by synthesizing new molecular materials whose measured room-temperature, low-field ME response matches predictions made before synthesis. “Low field” is used in the qualitative sense of the cited literature (fields of order kOe or below); the benchmark magnitudes reported above for individual compounds describe the field's present capability rather than thresholds an answer must hit. The rules must connect chemical and structural features of the candidate crystal — for example features such as magnetic-ion identity and anisotropy, coordination geometry, polar space group and switching mechanism, lattice compliance, or non-covalent contacts such as halogen···halogen interactions — to the ME response; the descriptor basis is not prescribed, and a different set of chemical or structural descriptors that yields validated a priori predictions serves the same goal. The rules must be predictive rather than retrospective: they must forecast the response of compounds not yet made, not rationalize known examples after the fact.

References

  1. Xiao-Lin Liu, Dong Li, Hai-Xia Zhao, Xin-Wei Dong, La-Sheng Long, Lan-Sun Zheng, “Inorganic–Organic Hybrid Molecular Materials: From Multiferroic to Magnetoelectric”, Advanced Materials 33, 2004542 (2021). DOI: 10.1002/adma.202004542, https://doi.org/10.1002/adma.202004542
  2. Yi-Fan Zhang, Ling-Ao Gui, Yan Peng, Zhao-Bo Hu, You Song, “Perspective on room temperature and low-field-induced magnetoelectric coupling in molecular complexes”, Dalton Transactions 54, 881–888 (2025; online 28 November 2024). DOI: 10.1039/d4dt02876g, https://doi.org/10.1039/d4dt02876g
  3. Jérôme Long, Maxim S. Ivanov, Vladimir A. Khomchenko, et al., “Room temperature magnetoelectric coupling in a molecular ferroelectric ytterbium(III) complex”, Science 367, 671–676 (2020). DOI: 10.1126/science.aaz2795, https://doi.org/10.1126/science.aaz2795
  4. Xiaolin Liu, Bin Wang, Xiaofeng Huang, Xinwei Dong, Yanping Ren, Haixia Zhao, Lasheng Long, Lansun Zheng, “Room-Temperature Magnetoelectric Coupling in Electronic Ferroelectric Film based on [(n-C3_3H7_7)4_4N][FeIII^{III}FeII^{II}(dto)3_3] (dto = C2_2O2_2S2_2)”, Journal of the American Chemical Society 143, 5779–5785 (2021). DOI: 10.1021/jacs.1c00601, https://doi.org/10.1021/jacs.1c00601
  5. Zhao-Bo Hu, Xinyu Yang, Jinlei Zhang, Ling-Ao Gui, Yi-Fan Zhang, Xiao-Dong Liu, Zi-Han Zhou, Yucheng Jiang, Yi Zhang, Shuai Dong, You Song, “Molecular ferroelectric with low-magnetic-field magnetoelectricity at room temperature”, Nature Communications 15, 4702 (2024). DOI: 10.1038/s41467-024-49053-y, https://doi.org/10.1038/s41467-024-49053-y
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