The system at the heart of the study is the thiaporphyrin, a porphyrin variant in which one pyrrole ring of the macrocycle is replaced by a thiophene. Porphyrins are among the most versatile ligands in coordination chemistry, and the thiaporphyrin framework brings an additional twist: as a monoanionic ligand, it only partially compensates the charge of a divalent metal ion. When a divalent metal such as copper is complexed, the resulting π-electronic complex remains a cation, and it therefore must pair with an anion in any isolable salt. This charge imbalance, usually treated as a chemical bookkeeping detail, is precisely what the Ritsumeikan team exploited. By choosing different counteranions, they could rewrite the electrostatic landscape of the crystal without touching the copper center that carries the spin.

Incorporating copper(II) is what makes the strategy magnetically meaningful. The Cu(II) ion contributes a single unpaired electron, so each complex cation is simultaneously a charged building block and a paramagnetic unit. Whether two such units interact magnetically depends on how closely their spin-bearing cores approach and how those cores are oriented relative to one another. Intermolecular spin–spin interactions in Cu(II) complexes of π-electronic macrocycles had previously been reported in only a limited number of cases, which left open a fundamental question: could the assembly mode of these ion pairs be deliberately steered, and would the steering translate into measurable changes in magnetic behavior?

To answer that question, the researchers synthesized two thiaporphyrin Cu(II) complex cations and initially isolated them as chloride salts. They then exchanged the chloride counterions for a chemically diverse panel of anions: tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), tetrakis(pentafluorophenyl)borate (B(C6F5)4−, abbreviated FABA−), and pentacyanocyclopentadienide (PCCp−). This set spans a wide range of shapes and electronic characters, from compact, roughly spherical inorganic anions to a large, weakly coordinating borate and a flat, π-electronic cyclopentadienide derivative decorated with five cyano groups. Each ion pair was then interrogated with a combination of single-crystal X-ray analysis, solid-state electron spin resonance (ESR) spectroscopy, magnetic susceptibility measurements, UV/visible spectroscopy, and theoretical calculations, allowing the team to connect crystallographic structure directly to magnetic response.

The structural contrast that emerged was striking. With the π-electronic PCCp− counteranion, one of the Cu(II) complexes formed a charge-by-charge assembly, in which cations and anions alternate in π-stacked ion pairs, each positive unit sandwiched against a negative partner. A second PCCp− ion pair adopted a different motif, featuring axial Cu–N coordination involving the anion. Critically, in the charge-by-charge arrangement the spin density remained largely localized on the CuN3S core of the cation, with negligible delocalization onto the PCCp− anion. Because the paramagnetic centers were effectively insulated from one another by the interleaved anions, no significant intermolecular spin–spin interactions were detected. The alternating stack, elegant as a supramolecular architecture, is magnetically quiet.

The nonplanar counteranions told a very different story. Ion pairs containing BF4−, PF6−, or FABA− did not interleave with the cations in the same way. Instead, the complex cations stacked face-to-face with one another, forming π-stacked cation dimers that further assembled in a two-by-two packing mode. Here the anions occupy the space around the dimers rather than between the stacked cations, and the two spin-bearing CuN3S cores are brought into close, well-defined proximity. ESR and magnetic susceptibility measurements on these dimer-based structures indicated antiferromagnetic interactions, meaning the neighboring spins couple in an opposing alignment that suppresses the net magnetic moment. Theoretical spin-density calculations supported the experimental observations, showing opposite spins localized on the respective stacked Cu(II)-containing cations, exactly the picture expected for an antiferromagnetically coupled dimer.

Perhaps most instructively, the study showed that even among the dimer-forming salts, the strength of the antiferromagnetic interaction was not uniform. Differences in local S/N contacts between stacked cations and differences in how the dimers packed against one another were associated with differences in the magnitude of the coupling. In other words, the counteranion influenced not only whether dimerization occurred at all, but also the fine geometry of the dimers and their higher-order arrangement in the crystal. The distance and orientation of the CuN3S units proved to be the crucial factors governing spin–spin coupling, while chalcogen-bonding and dipole–dipole interactions helped stabilize the stacked dimers. Magnetism, in this system, is a readout of packing geometry at multiple hierarchical levels.

From a materials-design standpoint, the implications reach beyond porphyrin chemistry. Spintronics, the exploitation of electron spin alongside charge in devices, has long sought molecular platforms in which magnetic interactions can be tuned rationally. Conventional approaches modify the ligand or the metal to adjust magnetic coupling, but the Ritsumeikan strategy operates one level higher: it leaves the paramagnetic unit untouched and rewrites the assembly script through ion pairing. Because counteranion exchange is a comparatively mild post-synthetic operation, the same complex cation could in principle be delivered into crystals with different magnetic characters simply by changing the salt it is crystallized from. As Professor Maeda noted, the Cu(II) complexation of thiaporphyrins affords paramagnetic π-electronic cations that modulate ion-pairing assembly modes in combination with coexisting anions, and the design of π-electronic systems with charge and spin would provide fascinating strategies for the construction of supramolecular spintronic materials.

The study also refines the conceptual vocabulary of charged π-electronic systems. Oppositely charged species tend to form charge-by-charge assemblies driven by electrostatic attraction, whereas like-charged π-electronic units can, under favorable intermolecular interactions, overcome electrostatic repulsion and form stacked dimers. Which of these competing tendencies wins depends on a delicate balance of electrostatics, dispersion forces, and directional secondary interactions such as chalcogen bonding. By mapping how four distinct anions tip that balance in a single family of complexes, the researchers have turned a qualitative intuition into a set of experimentally grounded structural criteria: keep the spin cores apart and the material is magnetically silent; bring them into stacked proximity with the right contacts and antiferromagnetic coupling appears.

What remains to be explored is how far this ion-pairing lever can be pushed. The findings demonstrate that counteranions can direct the assembly of paramagnetic molecular cations and, in turn, modulate their collective magnetic properties, but extending the approach to ferromagnetic coupling, to switchable or responsive materials, and to processable thin films will require further work. Even so, the message of the study is clear and broadly applicable: in charged π-electronic systems, the counterion is not passive baggage but an architectural agent. By using ion pairing to control dimerization and spin arrangements, chemists now have a molecular design strategy for constructing supramolecular spintronic materials in which magnetic behavior can be tuned through assembly, one carefully chosen counteranion at a time.

Subject of Research: Counteranion-controlled ion-pairing assembly and magnetic properties of thiaporphyrin Cu(II) complex cations

Article Title: Counteranions reshape molecular packing to tune magnetism

Article References: Counteranions reshape molecular packing to tune magnetism. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: thiaporphyrin, copper(II) complexes, counteranions, ion pairing, antiferromagnetism, π-stacking, supramolecular chemistry, spintronics, molecular magnetism, chalcogen bonding, crystal packing, Chemical Science

Writing in Nature Materials, the team describes how a single chemical modification—a chlorine atom placed at the para position of a phenethylammonium cation—transforms the stability of two-dimensional tin iodide perovskites. The resulting material, based on the 4-chloro-phenethylammonium (4ClPEA) cation, forms ultrastable two-dimensional and quasi-2D tin halide perovskites that retain bright photoluminescence for several months when simply left out in ambient air. When incorporated into 2D/3D perovskite solar cells, the chlorinated cation delivers devices with a power conversion efficiency of 16.2 percent, alongside operational stability exceeding 1,000 hours at 55 degrees Celsius in air—a combination of efficiency and durability that few tin-based devices have ever approached.

The key insight behind the work lies in how the organic spacer cations that separate the inorganic tin iodide sheets influence what happens between those sheets. In the layered Ruddlesden–Popper architecture of two-dimensional perovskites, bulky organic cations cap the corners of the octahedral framework and stack on top of one another between the inorganic slabs. The researchers systematically compared a family of halogen-substituted phenethylammonium cations—4XPEA, where X is hydrogen, fluorine, chlorine, or bromine—in the model compound (4XPEA)2SnI4. Single-crystal X-ray diffraction revealed that the chlorinated variant packs the organic layers more tightly than any of its siblings, driven by stronger π-stacking interactions between the aromatic rings.

That tighter packing is not merely a crystallographic curiosity. Using computational modeling and diffusion analysis, the team showed that the densely interlocked organic layers in (4ClPEA)2SnI4 substantially impede the diffusion of oxygen and water molecules into the vulnerable inorganic sheets. In effect, the chlorinated aromatic rings act as a molecular raincoat, sealing the gaps through which air would otherwise infiltrate and oxidize the tin(II) centers. The fluorinated and brominated analogues, by contrast, leave the interlayer region more permeable, and their films degrade noticeably faster under identical conditions. The result establishes a direct structure–property relationship: the tighter the interlayer packing, the greater the resistance to oxidative degradation.

Stability alone, however, would be of limited value if the material could not be turned into a high-quality solar absorber. The second half of the study addresses the notoriously uncontrolled crystallization of tin halide perovskites. Tin-based films tend to crystallize too quickly and too chaotically during deposition, producing pinholes, poor orientation, and defective grain boundaries that accelerate degradation and squander charge carriers. Here again, the 4ClPEA cation proved decisive. When added to three-dimensional tin iodide perovskite precursor solutions, it steers film growth toward markedly improved crystallinity and preferred crystallographic orientation, yielding dense, well-ordered 2D/3D heterostructured films.

Grazing-incidence wide-angle X-ray scattering measurements confirmed the enhanced orientation and phase purity of the treated films, while time-of-flight secondary ion mass spectrometry mapped the distribution of the organic cation through the film thickness. Density functional theory calculations provided a mechanistic underpinning, indicating favorable interactions between the chlorinated aromatic ring and the iodide species at the perovskite surface—an anion–π interaction that helps template orderly growth. The combination of a well-oriented 3D absorber with a protective 2D capping layer is precisely the architecture that the field has been pursuing for lead-based perovskites, and the study demonstrates that it can be realized in the tin system with the right molecular tool.

The device results are striking in context. Lead-free tin perovskite solar cells have historically lagged far behind their lead-containing counterparts, with certified efficiencies only recently crossing the 14 percent threshold and most devices failing within hours or days of operation. The 16.2 percent efficiency achieved with the 4ClPEA-based 2D/3D films places this work among the best-performing tin perovskite photovoltaics reported to date. More importantly, the devices did not merely perform well on the bench immediately after fabrication; they endured more than 1,000 hours of continuous operation at an elevated temperature of 55 degrees Celsius in ambient air, conditions that combine thermal stress, oxygen exposure, and moisture—the three horsemen of perovskite apocalypse.

The broader significance of the work extends beyond a single efficiency number. Tin halide perovskites are considered essential building blocks for the next generation of all-perovskite tandem solar cells, where a narrow-bandgap tin or tin–lead bottom cell would be paired with a wide-bandgap lead top cell to push efficiencies beyond what silicon can deliver. Lead toxicity, however, remains a persistent regulatory and public-acceptance obstacle, and studies quantifying the biological impact of lead leakage from perovskite modules have underscored the risk of assuming any safe threshold. A durable, efficient, entirely lead-free absorber would remove that obstacle while simplifying encapsulation requirements, and the chlorinated-cation strategy offers a generalizable design principle: choose spacer cations whose substituents promote tight interlayer packing and strong surface interactions.

The researchers also emphasize that the approach is rooted in fundamental chemistry rather than device engineering tricks. By comparing the full series of halogenated phenethylammonium cations, they isolated the effect of the substituent on π-stacking, interlayer spacing, and barrier properties, showing that chlorine occupies a sweet spot among hydrogen, fluorine, and bromine. The team has filed a patent application on the technology, and the design motif—electron-withdrawing substituents on aromatic spacer cations to tighten packing and passivate surfaces—could plausibly be extended to germanium-based perovskites, low-dimensional emitters, and other tin-containing optoelectronic devices where air sensitivity has been the limiting factor.

Challenges remain before tin perovskite solar cells can compete commercially. The efficiency gap with lead-based devices, which now exceed 27 percent, is still substantial, and scaling the chemistry from laboratory cells to modules will require further optimization of deposition and encapsulation. But the demonstration that a single atom of chlorine, placed with chemical intent on an organic cation, can buy months of ambient stability and a thousand hours of hot operation represents a genuine inflection point. For a field that has watched its most promising lead-free materials crumble within days, the message is clear: the path to practical tin photovoltaics may run through the careful engineering of the molecules that sit between the crystals, not just the crystals themselves.

Subject of Research: Stable 2D/3D tin halide perovskite solar cells enabled by a chlorinated organic spacer cation

Article Title: A chlorinated organic cation enables stable 2D/3D tin iodide perovskite photovoltaics

Article References: Triggs, C. T., Chen, L., Xie, J., Weadock, N. J., Zhang, Z., Ye, J. Y., Kerner, R. A., Taddei, M., Yang, F., Addison, B., Wang, X., Liu, T., Harvey, S. P., Toney, M. F., Beard, M. C., Yan, Y., Zhu, K., & Jin, S. (2026). A chlorinated organic cation enables stable 2D/3D tin iodide perovskite photovoltaics. Nature Materials. https://doi.org/10.1038/s41563-026-02726-z

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02726-z

Keywords: tin halide perovskite, perovskite solar cells, lead-free photovoltaics, 4-chloro-phenethylammonium, 2D/3D heterostructure, air stability, π-stacking, crystallization control, power conversion efficiency, Ruddlesden–Popper perovskite, tin oxidation, Nature Materials