A Mini-Review on the Structure–Property Relationships of Octahedral Mn²⁺ Chlorido Organic–Inorganic Hybrid Complexes: A Platform for Tunable Luminescence and Multifunctionality

Abstract

Pb-free organic–inorganic hybrid compounds are emerging as a versatile class of light-emitting materials, due to their optoelectronic, magneto-optic, field-sensing properties, low toxicity, and natural abundance. Particularly, ionic Mn²⁺ chlorido organic–inorganic hybrid compounds display high photoluminescent quantum yields, large Stokes shift, and are considered a structurally rich class of materials. A carefully selected wide variety of organic cations can be combined with [MnCl₆]^4– octahedra to obtain low-dimensional organic–inorganic structures such as two-dimensional (2D) sheets, one-dimensional (1D) chains, or zero-dimensional (0D) isolated octahedra, where each structure dimensionality displays unique photoluminescent properties. In this mini-review, the focus is on the structure–property relationships in ionic low-dimensional compounds containing [MnCl₆]^4– octahedra and a variety of organic cations. Specifically, we explore how the choice of organic cations can influence not only the characteristic red-orange emissions but also the emergent coupled functionalities, including ferroelectricity and magnetism. Conceptual connections are established to better understand the structural and property-related phenomena that govern these compounds as next-generation multifunctional materials.

Introduction

Low-dimensional organic–inorganic (O–I) hybrid compounds containing [MnCl₆]^4– octahedra have been known for decades. The photoluminescent (PL) properties for low-dimensional compounds were first reported in 1962, where red or pink luminescence was observed in layered (RNH₃)₂[MnCl₄] (R = CH₃ or C₂H₅) compounds. The start of the 21st century ushered in a renewed interest in these materials, due to their wide band gaps (>1.7 eV), large Stokes shift (>100 nm), and demonstration of PL with adequately high photoluminescent quantum yields (PLQY) for optoelectronic applications. In addition, they have emerged as alternatives to Pb-based materials for magnetic and thermochromic properties and reduced toxicity. Also, they are naturally abundant. Further, they have been studied for potential use in X-ray imaging, anticounterfeiting, and tandem solar cells. ,

For low-dimensional Mn²⁺ chlorido hybrid compounds, the restrictions imposed by Goldschmidt’s tolerance factor for three-dimensional (3D) perovskites are lifted, meaning that the anionic [MnCl₆]^4– octahedra could combine with an array of organic cations. It can be hypothesized that a vast number of permutations exist, rendering this class of materials structurally diverse.

The choice of organic cation can tune the connectivity of [MnCl₆]^4– octahedra, thereby controlling the structural dimensionality and PL properties, as shown in Figure . In two-dimensional (2D) structures, the general formula is A­[MnCl₄] or A₂[MnCl₄] and comprises corner-sharing [MnCl₆]^4– octahedra that extend along two axes to form inorganic sheets, which are separated by 1+ or +2 cations. One-dimensional (1D) structures of formula AMnCl₃ consist of face-sharing [MnCl₆]^4– octahedra that extend in one direction to form linear chains, while edge-sharing variants are of the formula AMnCl₃B (B = neutral or auxiliary ligands). In zero-dimensional (0D) structures, isolated [MnCl₆]^4– octahedra are surrounded by organic cations of formula A₄MnCl₆.

1.

(a–c) Two-dimensional structure of [H₃N­(CH₂) _m NH₃]­MnCl₄ (m  = 2, 3, and 4) compounds. (d) The photoluminescence excitation (PLE) spectra of [H₃N­(CH₂) _m NH₃]­MnCl₄ (m  = 2, 3, and 4) compounds. (e) Emission spectra and (f) TRPL spectra under the excitation of 417 nm (2.97 eV). Figure reprinted (adapted) with permission from ref Copyright 2023 AIP Publishing. (g) Packing diagram structures of (Hmpy)­MnCl₃ (Hmpy = n-methylpyrrolidinium) of the high temperature phase (top) and low temperature phase (bottom) (h) Low temperature phase orange crystals of (Hmpy)­MnCl₃ (Hmpy = n-methylpyrrolidinium) under ambient light (top) and under UV light (bottom). (i) Schematic diagram showing the energy adsorption, migration, and emission processes of (Hmpy)­MnCl₃ (Hmpy = n-methylpyrrolidinium). (j) Absorption and emission spectra of (Hmpy)­MnCl₃ (Hmpy = n-methylpyrrolidinium) at room temperature. Figure reprinted (adapted) with permission from ref . Copyright 2017 American Chemical Society. (k) Photographs of the (C₆H₁₀N₂)₂MnCl₆·2H₂O single crystal under sunlight (top) and UV-254 nm excitation (bottom). (l) Crystal structure of the (C₆H₁₀N₂)₂MnCl₆·2H₂O single crystal along the c-axis. Optical properties of (C₆H₁₀N₂)₂MnCl₆·2H₂O samples annealed at different temperatures. (m) Absorption spectra. The inset shows the fitted band gap. (n) PL curves measured at different excitation wavelengths. (o) PL lifetime of (C₆H₁₀N₂)₂MnCl₆·2H₂O at RT measured using an integrating sphere. Figure reprinted (adapted) with permission from ref . Copyright 2023 American Chemical Society.

In the inorganic layer, several mechanisms can contribute to emission, including radiative recombination of excitons confined within the lattice, recombination involving intrinsic or extrinsic defect states, or relaxation of self-trapped excitons (STE) associated with local lattice distortions. The emission arises from the characteristic ⁴T_1g → ⁶A_1g transition from Mn²⁺ ions. In systems with shorter Mn···Mn interatomic distances, magnetic exchange interactions between neighboring Mn²⁺ ions can lead to energy transfer and broad, red-shifted emission bands, sometimes associated with Mn···Mn coupling.

However, the moisture stability of ionic Mn²⁺ chlorido hybrids remains, with challenge and structure stability depending on lattice energy (Mn–Cl bond strength and packing density), the number of hydrogen-bonded networks, and the hydrophobicity of the organic cations. Proposed methods to increase stability include thermal annealing, exposure to aprotic solvents, stabilizing the structure through cation choices that increase hydrogen bonding interactions, or polymer encapsulation, using epoxy resin. However, structure stability in humid environments is compound specific. For example, the 2D arylamine-based series (C₆H₅C _x H_2x NH₃)₂MnCl₄ (x = 0, 1, 2, 3) exhibits remarkable thermal and structural stability under ambient conditions, maintaining their crystallinity and phase integrity up to approximately 430 to 470 K. The polar structures of the series (C₆H₅C _x H_2x NH₃)₂MnCl₄ (x = 0, 1, 2, 3) (x = 1–3) remains stable up to 400 K, with reversible phase transitions and no mass loss below 400 K, underscoring the stabilizing effect of extensive ⁺N–H···Cl^– hydrogen bonding and rigid π-π packing within the organic bilayers. In contrast, the 0D system (C₆H₁₀N₂)₂MnCl₆·2H₂O exhibits excellent thermal and luminescent reversibility, characterized by a reversible phase transition between 331 and 365 K, accompanied by switchable PL. The material retains approximately 50% of its original PL intensity after 20 heating–cooling cycles (room temperature ↔ 373 K), with no permanent structural degradation, and decomposes only beyond 506 K after dehydration.

In addition to luminescent properties, the Mn²⁺ chlorido hybrids consist of paramagnetic Mn²⁺ ions, and when placed in an applied magnetic field, the spins of the Mn²⁺ ions can either be parallel (↑↑) to the applied magnetic field, for ferromagnetic (FM) coupling, or antiparallel (↑↓) in the case of antiferromagnetic (AFM) coupling. Magnetic interactions in 2D Mn²⁺ chlorido perovskites display AFM interactions in the presence of spin-canting. Similarly, the PL properties are influenced by the degree of d-d orbital overlap of adjacent Mn²⁺ ions. The interplay between magnetism and luminescence arises from the coupling of magnetic spin states within the electronic energy levels responsible for emission processes. This magneto-optic relationship offers unique opportunities for tailoring optical materials through magnetic interactions.

In this mini-review, we highlight recent research on compounds containing octahedral Mn²⁺ chlorido anions and their reported PL properties. As emerging candidates for photovoltaic applications, these materials are being studied to better understand their structure-PL property relationships critical for materials design. First, we will establish the electronic, optical, and structural properties that govern PL behaviors in these materials. Then, we will systematically explore how the organic cation selection dictates dimensionality (2D, 1D, and 0D) and, consequently, the photophysical outcomes. While several examples in this mini-review draw on Mn²⁺ chlorido hybrids from several studies, similar structure–property relationships recur across bromido families. Comparable trends are reported for Mn²⁺ bromido systems, indicating that the design principles summarized here are transferable to bromido analogues. , Focus is given to the chlorido analogues as they exhibit higher luminescence intensity as compared to their bromido counterparts. In contrast, very few studies have been conducted on Mn²⁺ iodo hybrids due to their chemical instability, given that these compounds are poor emitters.

Electronic Structure and Optical Properties

Spectroscopic Characteristics

The Mn²⁺ ion is a d ⁵ metal ion with partially filled 3d orbitals and is a high-spin (S = 5/2) under weak octahedral crystal field effects (CFE), where the ground state is ⁶A_1g. As shown in Figure , Tanabe and Sugano diagrams model the energy levels of Mn²⁺ ions, accounting for both electron–electron interactions and CFE, and illustrate how the energies of excited states vary with crystal field strength (D _q /B), assuming a fixed ratio of C/B of approximately 4.5. In high-spin d ⁵ Mn²⁺ (O_h), the ground state is ⁶A_1g (⁶S) and is largely unaffected by CFE. The observed spin-forbidden transitions between 330 and 450 nm correlate to the ⁶A_1g → ⁴T_1g(G) and ⁶A_1g → ⁴T_2g(G) transitions. The ⁴A_1g(G)/⁴E_g(G) and ⁴T_1g(P) levels lie at shorter wavelengths (<300 nm) and are typically weak or masked by ligand-to-metal charge-transfer (LMCT) absorption.

2.

Tanabe–Sugano diagram of Mn²⁺ (d ⁵) systems. Figure reprinted (adapted) with permission from ref . Copyright 2023 AIP Publishing.

Following photoexcitation, electrons rapidly undergo nonradiative relaxation to the lowest excited ⁴T_1g state, from which radiative decay to the ground ⁶A_1g state gives rise to an emission near 600–620 nm. The energy emitted is dependent on the crystal field strength, D _q. For octahedral Mn²⁺ complexes, D _q is larger due to stronger CF splitting, resulting in a lower energy red-orange emission. For example, (pyrrolidinium)­[MnX₃] (X = Cl^– or Br^–) is a 1D structure, comprising octahedral Mn²⁺ ions and emits red light. Likewise, (NH₃(CH₂)₅NH₃)­[MnCl₆], which is a 2D perovskite compound, emits orange light under UV excitation due to the d-d transition from the octahedral Mn²⁺ ions.

In certain Mn²⁺-based hybrid systems, i.e., dopants, higher Mn²⁺ concentrations corresponding to shorter Mn···Mn separations lead to low-energy emission bands extending into the near-infrared (NIR) region. Historically, these emissions were attributed to AFM superexchange-coupled Mn²⁺ pairs or clusters, in which strong d-d exchange interactions allowing spin- and parity-forbidden transitions to relax radiatively at longer wavelengths. However, recent photophysical and electron paramagnetic resonance (EPR) analyses indicate that such red-shifted or NIR bands can instead originate from trap-mediated recombination processes or mixed-valence centers (Mn²⁺/Mn³⁺) that form under high Mn²⁺ loadings, rather than from purely exchange-coupled dimers. Also, NIR contributions may arise from charge-transfer or intramolecular excitations within conjugated or π-stacked organic cations, which couple weakly to the Mn–Cl framework.

Decay Pathways

The PL behavior of Mn²⁺ chlorido hybrids is governed by a combination of intrinsic and extrinsic decay pathways that are influenced by the coordination geometry, structural dimensionality, and lattice dynamics of the material. Upon UV excitation, an electron–hole pair (exciton) stabilized by Coulombic attraction is created. The energy required to split Coulombic pairs is called the exciton binding energy (E _b). A higher E _b signifies greater exciton stability, effectively suppressing thermal dissociation and thereby promoting radiative recombination pathways, resulting in enhanced luminescence efficiency.

In lattices, where [MnCl₆]^4– octahedra are spatially isolated by bulky organic cations, strong exciton–phonon coupling promotes the formation of self-trapped excitons (STE), which constitute one of the dominant radiative recombination pathways. This localization causes the surrounding lattice to relax, forming an STE that emits light upon recombination.

Halide identity plays a critical role in determining the likelihood and efficiency of STE formation. In layered 2D halide perovskites, structural distortion and metal-halide bonding characteristics influence the trapping and detrapping behavior of excitons, with Cl^– based systems exhibiting deeper self-trapping potentials than Br^– or I^–analogues. This enhanced localization arises from the larger band gap and the deeper valence band of Cl^–, which stabilizes the STE state and suppresses nonradiative decay. In contrast, Br^– and I^– perovskites have shallower potential wells and reduced lattice distortions, making STEs less stable. Comparative studies of (pyrrolidinium)­[MnCl₃] and (pyrrolidinium)­[MnBr₃] show equivalent emission wavelengths of 640 nm, but the Cl^– analogue achieves twice the PLQY of 56%, directly illustrating the impact of halide substitution on photophysical performance. Overall, these findings confirm intrinsic contributions, despite debates on defect-related contributions, temperature- and pressure-dependent PL measurements support intrinsic STEs as the dominant mechanism.

Structural Features of Low-Dimensional Mn²⁺ Chlorido Hybrids

2D Mn²⁺ Chlorido Perovskites

The 2D systems are of the general formula A₂[MnCl₄], when a monovalent cation is used, or A­[MnCl₄] when a divalent cation is used. The inorganic layer consists of corner-sharing [MnCl₆]^4– octahedra extending in two dimensions to form sheets that are separated by a bilayer of monovalent cations, but an organic monolayer is formed when divalent cations are used, as shown in Figure a. The [MnCl₆]^4– octahedra exhibit no Jähn–Teller (J–T) distortions and are connected through alternating Mn–Cl bonds and Mn···Cl semicoordinate interactions. The structure is considered a “soft” structure, meaning that distortions of the Mn–Cl···Mn bond angles, as shown in Figure b, can easily occur due to the steric effects of the cation, giving the inorganic layer a corrugated appearance. Also, the ammonium group from the cations is able to penetrate the inorganic layer, as shown in Figure c, thereby increasing structural corrugation and enhancing PL intensity.

3.

(a) 2D layered perovskite structure containing either an organic monolayer (left) or bilayer (right). (b) Corner sharing of octahedra produces Mn–Cl···Mn bond angles and the formation of cavities. (c) A positive penetration depth of an ammonium head.

1D Mn²⁺ Chlorido Hybrids

In 1D systems, the [MnCl₆]^4– octahedra are linked by corner-, edge-, or face-sharing chlorido ligands. The mode of [MnCl₆]^4– octahedra connectivity directly affects the local symmetry and environment of the Mn²⁺ ions, therefore influencing the optical and electronic properties of the material. The inorganic framework propagates along a single crystallographic axis, surrounded by cations that reside between the voids and are connected by hydrogen bonding interactions to the inorganic component.

0D Mn²⁺ Chlorido Hybrids

The formation of 0D systems usually requires the incorporation of large or bulky cations that isolate [MnCl₆]^4– octahedra through steric effects. Consequently, the Mn···Mn interatomic distance is larger in 0D structures compared to 1D or 2D structures, thereby enhancing quantum confinement effects (QCE).

Structure-PL Relationships of Octahedral Mn²⁺ Chlorido Hybrids

Influence of Cations on PL Properties of 2D Mn²⁺ Chlorido Perovskite Systems

Table lists the 2D Mn²⁺ chlorido perovskites containing either an n-alkyldiammonium, n-alkylammonium, or arylammonium cation. To our knowledge, these are the only ionic 2D Mn²⁺ chlorido perovskites with PL properties reported. In general, the emission band is typically between 581 and 620 nm under UV excitation of approximately 417 nm. The emission originates from the ⁴T_1g → ⁶A_1g transition for octahedral Mn²⁺ ions. 2D Mn²⁺ chlorido perovskites are direct-gap semiconductor materials that exhibit wider band gaps, making them particularly advantageous for applications requiring excitation by high-energy photons. For example, (PEA)₂[MnCl₄] (PEA = phenylethylammonium), with n = 1 exhibiting pronounced quantum confinement and a direct band gap of 4.57 eV.

1. Structural and PL Properties of 2D Perovskite Structures Containing [MnCl₆]^4– Octahedra.

compound	DJ/RP	Mn···Mn interatomic distance (Å)	λex (nm)	λem (nm)	τ av (μs)	PLQY (%)	FWHM (nm)/(eV)	emission color	year of publication	refs
n-Alkyldiammonium Cations
(NH3(CH2)2NH3)[MnCl4]	DJ	5.123	417	620 (2 eV)	24.4	8.2	0.23 eV	red	2023
(NH3(CH2)3NH3)[MnCl4]	DJ	5.038	417	620 (2 eV)	40.4	15.6	0.245 eV	red
(NH3(CH2)4NH3)[MnCl4]	DJ	5.146	417	620 (2 eV)	56.3	23.4	0.245 eV	red
(NH3(CH2)5NH3)[MnCl4]	DJ	5.158	417	581			60	orange	2016
n-Alkylammonium Cations
(CH3NH3)2[MnCl4]	RP	5.12	418	606	109	41	72	red	2024
Arylammonium Cations
(C6H5(CH2)NH3)2[MnCl4]	RP	5.163	417	610	19.1		77.61/0.26	red	2022
(C6H5(CH2)2NH3)2[MnCl4]	RP	5.130	418	610	21.4		75.99/0.25	red
(C6H5(CH2)3NH3)2[MnCl4]	RP	5.168	417	610	27.1		79.76/0.26	red
(C6H5(CH2) 4 NH3)2[MnCl4]	RP	5.137	416	610	24.6		84.01/0.28	red

Alkyl cations play a pivotal role in guiding the self-assembly of 2D layered structures during synthesis. In layered structures, these cations act as insulating barriers, confining charge carriers within the inorganic layer and limiting out-of-plane halide ion migration, thereby improving photostability. They induce QCE, which influences the electronic and optical properties. Also, these cations serve as dielectric moderators by influencing the electrostatic interactions between electrons and holes, stabilizing excitons, and improving luminescence properties.

Several factors are considered when selecting a suitable organic cation as a spacer for 2D Mn²⁺ chlorido perovskite materials. First, the net positive charge of the cation should be +1 or +2, depending on the constituents to maintain overall charge neutrality. For ammonium cations, the number of protonation sites should be considered rather than the number of nitrogen atoms. Strong charge-assisted ⁺N–H···Cl^– hydrogen bonding interactions form between the ammonium group of the cation and the chlorido ligands of the inorganic layer and can be seen as the “glue” that holds the structure together. These ⁺N–H···Cl^– hydrogen bonding interactions can be classical, bifurcated, or trifurcated, as shown in Figure a–c, respectively. An increase in hydrogen bonding interactions produces a more rigid structure, which influences luminescent properties by stabilizing the crystal structure, thereby reducing nonradiative recombination pathways. Bound by hydrogen bonding interactions, the ammonium group resides in the cavities created in the inorganic layer by the Mn²⁺ ions and chlorido ligands. The encroachment of the ammonium group on the inorganic layer results in the buckling of the bridging Mn–Cl···Mn bonds, giving the inorganic layer a corrugated appearance. Recently, Wang et al. reported that an increase in the penetration of the ammonium group of the aniline cation increased the distortion of the inorganic layer. Consequently, the octahedra became more tilted through hydrogen bonding interactions, resulting in a larger optical band gap and stronger PL emissions.

4.

(a) Classical, (b) bifurcated, and (c) trifurcated hydrogen bonding interactions.

Furthermore, the band gap, which is the basic characteristic of light-harvesting materials, is influenced by interlayer interactions. The n-alkylammonium and n-alkyldiammonium cations influence the band gap in a similar manner, with a PL emission band shifting to lower energies as the interlayer distance decreases. The mechanisms of the interlayer interactions influencing the PL properties are not yet fully understood, warranting further investigation to achieve a more comprehensive understanding.

The 2D Mn²⁺ chlorido perovskites with PL properties reported contain shorter cations of four or fewer carbon atoms, leaving scope for further exploration of the PL properties of compounds containing longer chain cations. Promising results have been observed in the (NH₃(CH₂)₉NH₃)₂[MnCl₆] compound containing the (NH₃(CH₂)₉NH₃)⁺ cation, which demonstrated moderate structural rigidity, while maintaining efficient energy transfer, making it a suitable candidate for tunable luminescent applications.

The cation orientation in the crystal lattice has a direct effect on the inorganic distortions. Short n-alkylammonium cations commonly exhibit minimal tilting and are noninterdigitated, thus maintaining a rigid inorganic framework, resulting in enhanced orbital overlap and PL intensity. However, as the chain length increases, the cations tilt relative to the inorganic layers, with N–C–C-C or C–C–C-C gauche bonds present along the cation. This tilting could introduce distortions in the Mn²⁺ coordination environment, thus affecting the ligand field and modifying PL emission characteristics. In contrast, n-alkyldiammonium cations are interdigitated, remaining anchored to the inorganic layer through strong charge-assisted ⁺N–H···Cl^– hydrogen bonds at both ends of the cation. This structural rigidity reduces cation mobility and minimizes fluctuations in the inorganic framework, which may lead to more stable PL emissions.

Additionally, the organic cation can be used to break symmetry elements and thereby enhance luminescent properties. In arylammonium-containing Mn²⁺ chlorido hybrids, inversion symmetry breaking arises primarily from the rotational freedom of the arylammonium cation, resulting in asymmetric hydrogen-bonding interactions of the organic cations, which breaks the inversion center at the Mn²⁺ site and distorts the [MnCl₆]^4– octahedra. This loss of inversion symmetry relaxes the Laporte-forbidden ⁴T_1g → ⁶A_1g transition, enhancing the electric-dipole contribution to emission and thereby strengthening the Mn²⁺ PL, as observed in polar systems such as (pyrrolidinium)­MnCl₃.

The Odd–Even Effect

In Mn²⁺ chlorido hybrids, the odd–even effect is revealed as a structural modulation driven by the odd or even number of CH₂ groups in n-alkylammonium or n-alkyldiammonium cations. A single CH₂ group variation can alter the tilt of the organic spacer, the hydrogen bonding network to the [MnCl₆]^4– octahedra, and the packing arrangement between adjacent inorganic sheets or chains. These geometric shifts directly influence Mn···Mn separations and octahedral distortion parameters, both of which play a decisive role in the strength of Mn–Cl···Mn superexchange interactions and the magnitude of the crystal field strength (CFS).

In addition, 2D Mn²⁺ chlorido perovskites may adopt two key frameworks based on the shifting of adjacent inorganic layers, namely Dion-Jacobson (DJ) and Ruddleson-Popper (RP) structures. While the authors acknowledge that alternating cationic interlayer (ACI) structures exist, to our knowledge, there are no published studies on the PL properties of ACI-type 2D Mn-based perovskites.

DJ and RP Perovskites

The 2D DJ perovskites have a general formula A″A _n–1B _n X_3n+1, where n is the number of inorganic layers and A″ is a large diammonium cation. DJ perovskites accommodate diammonium cations sandwiched between inorganic layers. Adjacent inorganic layers are eclipsed at an offset of (0,0) or (1/2,0) of the Mn···Mn distance and exhibit shorter interlayer distances than RP perovskite structures. For RP perovskites, the general formula adopted is A′₂A _n–1B _n X_3n+1, where n is the number of inorganic layers and A’ is a monoammonium cation. Adjacent inorganic layers are staggered, with an offset of (1/2, 1/2) of the Mn···Mn distance. Typically, two monoammonium cations are incorporated between inorganic sheets and are bonded by van der Waals interactions, providing a versatile framework. The selection and geometry of the organic cation critically influence whether a system adopts DJ or RP packing, with the odd–even effect playing a decisive role. In n-alkylammonium or n-alkyldiammonium cations, an even number of CH₂ units favors more planar cation orientation, promoting eclipsed layer alignment and thus DJ-type structures. Conversely, odd-chain cations introduce tilting in the organic spacer and altered hydrogen-bonding networks, which encourage staggered layers typical of RP structures. These geometric modulations directly impact Mn···Mn interlayer distances, octahedral distortion, and, consequently, CFS, and exciton confinement.

In DJ systems, (H₃N­(CH₂) _m NH₃)­[MnCl₆] (m = 2–4), Panda et al. demonstrated that increasing diammonium chain length expanded the Mn···Mn interlayer spacing from 8.6 to 10.8 Å, prolonged PL lifetimes from 24 to 56 μs, and improved PLQY from 8 to 23%. These results show that controlled interlayer separation mediated by diammonium cations enhances exciton localization and suppresses nonradiative decay. In contrast, He et al. reported that in RP structures (C₆H₅(CH₂) _x NH₃)₂[MnCl₆] (x = 1–4), odd-numbered cations introduced asymmetry in the organic spacers, leading to staggered layers, multimodal emission, and tunable exciton confinement. Importantly, they observed Förster resonance energy transfer (FRET) from the arylamine cation to the [MnCl₆]^4– octahedra, further demonstrating how cation identity directly dictates optical behaviors in these low-dimensional systems.

Influence of Cations on PL Properties of Octahedral 1D and 0D Mn²⁺ Chlorido Hybrids

Tables and list selected 1D and 0D Mn²⁺ chlorido hybrids containing various cations and corresponding PL properties. Among the most cations studied are pyrrolinium and guanidinium cations. When combined with MnCl₂, pyrrolinium cations form ferroelectric materials and guanidinium cations form dielectric materials. For example, a ferroelectric response was observed for thermally induced phase transitions of (C₄H₈N)­[MnCl₃] ((C₄H₁₀N)⁺ = pyrrolinium cation) from a high-temperature centrosymmetric structure to a low-temperature noncentrosymmetric structure. Conversely, (CH₆N₃)­[MnCl₃] ((CH₆N₃)⁺ = guanidinium cation) exhibited 30 K wide hysteresis and orange-red emission wavelength of 645 nm when excited at 365 nm. The odd–even effect also affects ferroic and PL responses. Pyrrolinium-based 1D hybrids with even-chain cations, (C₄H₈N)­[MnCl₃], exhibit ferroelectric phase transitions coupled with strong red emission, whereas odd-chain guanidinium analogues, (CH₆N₃)­[MnCl₃], act as dielectric materials with orange-red emission. This demonstrates that the parity of the cation chain directly impacts structural symmetry, hydrogen bonding patterns, and photophysical performance.

2. Structural and PL Properties of 1D Hybrid Structures Containing [MnCl₆]^4– Octahedra.

compound	cation	Mn···Mn interatomic distance (Å)	λex (nm)	λem (nm)	τ av (μs)	PLQY (%)	FWHM (nm)	emission color	year of publication	refs
(C5H12N)2[MnCl3]	n-methylpyrrolidinium	3.23	525	632	54.5	80		red	2017
(C4H12N)[MnCl3]	tetramethylammonium	3.25	443	645	98.6	99		red	2024
(C6H14N)[MnCl3]	3-methylpiperidine	3.29	373	644	450	60	95	red	2024
(C3H11N2)[MnCl3]	trimethylhydrazinium	3.19	365	653		6.4		red	2022
(C4H8N)[MnCl3]	3-pyrrolinium	3.23	375	635	28.2		333.6	orange–red	2015
(C4H10N)[MnCl3]	pyrrolinium	3.24	451	645	56.0		9.7 × 10–4	red	2019
(C5H12N)[MnCl4]	piperidinium	3.28	372	650	372.4	28		red	2023
(C6H10N2)4[Mn3Cl13]	4-(aminomethylpyridinium)	5.13	420	628	1660	4.9		red	2021

3. Structural and PL Properties of 0D Hybrid Structures Containing [MnCl₆]^4– Octahedra.

compound	cation	Mn···Mn interatomic distance (Å)	λex (nm)	λem (nm)	PLQY (%)	τav (μs)	FWHM (nm)	emission color	year of publication	refs
(C6H10N2)2[MnCl6]	2-ammoniomethylpyridinium	7.56	274	618	15.85	6210		red	2023
(C6H18N3)2[MnCl6]	2-azaniumylethylpiperzine	9.87	273	625	3.34	61.81		red	2023
(C3H8N6)2[MnCl6]	2,4,6-triaminopyrimidine	5.40	354, 423, 515	612	21.9	2960		red	2021
(C8H14N2)2[MnCl6]	m-xylylenediamine	7.52	365, 420, 518	618	31.1	1100		red

Finally, dual-emission behaviors underscore the interplay of organic and inorganic components. [(N-AEP)₂MnCl₆]·2Cl^–, ((N-AEP) = C₆H₁₈N₃ ³⁺) displays excitation-dependent emission, with high intensity red-orange emission at 625 nm from Mn²⁺ d-d transitions and secondary blue emission at 460 nm attributed to the organic cation. Tunable chromaticity between warm and cold white light confirms that cation selection, chain parity, and layer stacking collectively govern the electronic structure and emissive properties. These examples illustrate how the choice of organic cation not only influences ferroic responses but also modulates emission properties.

In addition, a 1D Mn²⁺ chlorido hybrid was synthesized by Su et al., crystallizing in a hexagonal lattice, with space group, P ₆3/m. A near-unity PLQY of 98.6% was achieved for the (TMA)­[MnCl₃] (TMA⁺ = tetramethylammonium) compound. A strong broad-band red emission peak at 645 nm originating from the ⁴T_1g → ⁶A_1g transition of Mn²⁺ ions with a FWHM of 99 nm, indicating its suitability for broadband applications. The structural rigidity imparted by the TMA⁺ cation contributes to its exceptional thermal stability, with the PL intensity remaining unchanged at 300 °C.

The organic cations can act as the structure and electronic modulator that influences the PL properties of the material. To achieve efficient PL, the band alignment of the organic and inorganic components should favor radiative recombination and the bandgap of the cation should be large enough to accommodate the energy levels of the Mn²⁺ ions. In general, a Type-I band alignment, where the highest occupied molecular orbital (HOMO) of the organic cation lies below the Mn²⁺ 3d occupied states and the lowest unoccupied molecular orbital (LUMO) lies above the Mn²⁺ 3d unoccupied states, is favorable for PL. This alignment confines both electrons and holes within the [MnCl₆]^4– octahedra, facilitating efficient radiative recombination. Type-II alignment, where the organic cation’s LUMO lies below the Mn²⁺ 3d states, can promote charge separation and result in nonradiative losses.

In 1D and 0D systems, the Mn···Mn interatomic distances influence the PLQY more than 2D systems. Where the Mn···Mn interatomic distances are smaller, i.e., a high concentration of Mn²⁺ ions, the PL is quenched via nonradiative energy transfer due to multiphonon interaction, where energy is dissipated as heat rather than photons, as shown in Figure . Consequently, larger cations are commonly used to increase Mn···Mn interatomic distances and improve the PLQY.

5.

Correlation between shortest Mn···Mn distances and PLQY. Red circles represent the compounds synthesized by Mao et al. blue squares are taken from the literature. Figure is reprinted (adapted) with permission from ref . Copyright 2020, American Chemical Society.

In principle, 1D and 0D low-dimensional structures are more conducive to a high PLQY with a long lifetime, due to the strong QCE and the hydrogen bond network formed by protonated nitrogen atoms. The emission wavelength in Mn²⁺ materials is primarily dictated by the ligand field around the [MnCl₆]^4– octahedra. The cation affects this field by modulating octahedral distortion. In 0D systems, closely packed cations can compress the octahedra, leading to blue-shifted emissions due to increased crystal field strength. Conversely, bulky or flexible cations may introduce relaxed packing and promote octahedral expansion, resulting in red-shifted PL. Aromatic cations often induce a red-shifted emission and high energy absorption bands (240–340 nm) correlate to n–σ* and σ–σ* transitions.

Applications of Octahedral Mn²⁺ Chlorido Compounds

In this section, we highlight recent advancements in octahedral Mn²⁺ chlorido hybrid materials, with a focus on their ferroelectric and dielectric properties relevant to emerging photovoltaics, as well as other optoelectronic applications.

Ferroelectric and Dielectric Photovoltaics

Mn²⁺ chlorido hybrid materials exhibit intriguing multifunctional properties arising from their ability to respond to external electric fields, mechanical stress, and temperature. These behaviors can be broadly classified into ferroelectric and dielectric responses, both of which are intimately tied to the symmetry and dynamics of the crystal lattice and the ordering of organic cations.

Ferroelectric photovoltaics are observed in noncentrosymmetric crystals, where spontaneous electric polarization can be reversed under an applied electric field. In Mn²⁺ chlorido hybrids, ferroelectric phase transitions involve symmetry breaking from a high-temperature paraelectric phase to a low-temperature ferroelectric phase below the Curie temperature (T _C). Such transitions typically occur within one of the ten polar point groups (e.g., triclinic 1 (C ₁), orthorhombic mm2 (C _2v ), monoclinic 2 (C ₂), m (C _s), trigonal 3 (C ₃), 3m (C _3v ), tetragonal 4 (C ₄), 4mm (C _4v ), hexagonal 6 (C ₆), and 6mm (C _6v )), which dictate the orientation and switchability of ferroelectric domains. These domains are crucial in defining dielectric, piezoelectric, and pyroelectric responses. For example, You et al. demonstrated that 1D ferroelectric (Me₃NCH₂Cl)­[MnCl₃] exhibits a high T _C of 406 K and a piezoelectric coefficient d ₃₃ of 185 pC/N, comparable to classical piezoelectric ceramics such as BaTiO₃. Under external stress, the polarization vector can switch between multiple domain orientations, resulting in a large polarization change and enhanced piezoreponsivity.

Dielectric photovoltaics, on the other hand, are associated with centrosymmetric crystals that lack intrinsic polarization but can be polarized by an external field. The dielectric response is quantified through the dielectric constant (ε′) and dielectric loss (ε″), which reflect the material’s ability to store and dissipate electrostatic energy. In Mn²⁺ chlorido systems, dielectric switching is often coupled with temperature-dependent structural phase transitions that reorient the organic cations within the inorganic framework. For instance, Lv et al. reported a phase transition in the 2D perovskite (NH₃(CH₂)₅NH₃)­[MnCl₄] at 298 K, where the organic cations reorient along different crystallographic axes, leading to significant changes in cavity shape and dielectric properties. The real part of the dielectric constant (ε′) increases sharply from ∼5.5 in the low-temperature phase to 10.25 at the phase transition, marking a high dielectric state approximately 1.86 times greater than the low dielectric phase. This switching behavior is largely frequency independent, as ε′ values remain nearly unchanged across varying frequencies. Notably, dielectric anomalies near 298 K are prominent along the a- and c-axes, while minimal changes are observed along the b-axis. At the phase transition temperature, ε′ increases by approximately 4.50-, 1.14-, and 2.86-fold along the a-, b-, and c-axes, respectively.

White Light and Warm White Light Emission in 2D Mn²⁺ Chlorido Perovskites

2D Mn²⁺ chlorido perovskites have emerged as promising candidates for white light-emitting diodes (WLEDs) due to their strong absorption in the UV and blue regions and large Stokes shifts (>100 nm). Their optical performance is closely tied to the inorganic octahedral [MnCl₆]^4– units, which exhibit characteristic d-d transitions responsible for red to orange-red emission, and the organic cations, which influence crystal packing, structural rigidity, and energy transfer efficiency.

For instance, (CH₃NH₂)₂[MnCl₄] single crystals exhibit strong red emission under blue light excitation, with a primary peak at 608 nm, a full width at half-maximum (FWHM) of 72 nm, and a luminescence lifetime in the microsecond range. Wang et al. proposed that integrating a blue-light-excited red emitter such as (CH₃NH₃)₂[MnCl₄] into YAG:Ce³⁺-based WLEDs could improve the color rendering index (CRI) and reduce correlated color temperature (CCT), enhancing overall photoelectric performance. Notably, substituting Cl^– with I^– in (CH₃NH₃)₂[MnI₄] extended the absorption edge into the visible spectrum, producing photoresponses at 300, 460, and 530 nm, demonstrating that halide engineering directly tunes optical and device performance.

Beyond general white light emission, warm white light LEDs exploit the broad-band PL of Mn²⁺ combined with blue-emitting components. Alkyl- and guanidinium cation-containing 2D Mn²⁺ perovskites allow for tunable emission, as demonstrated by Wang et al., who synthesized a blue-light-excited red emitter, (CH₆N₃)₂[MnCl₄]. When integrated with YAG:Ce³⁺, this material exhibited red emission at 650 nm, a PLQY of 55.9%, and excellent thermal stability. Alloying with 8 mol % Zn²⁺ further enhanced the luminescence efficiency, achieving a CCT of 3984 K, high luminous efficiency (90.41 lm W^–1), and a CRI of 93.7, illustrating the potential of cation and metal ion substitution to optimize both optical and thermal performance.

The combination of high absorption in the blue region, efficient energy transfer from organic cations to Mn²⁺ centers, and tunable emission wavelengths underscores the versatility of 2D Mn²⁺ chlorido perovskites for next-generation optoelectronic devices. Mechanistically, the ability to manipulate emission color, quantum efficiency, and stability via cation engineering, halide substitution, and alloying strategies highlights the critical role of structure–property relationships in device performance. Future work could explore alternative UV-excited red emitters such as (NH₃(CH₂)₃NH₃)­[MnCl₄], which emit at 620 nm, to further enhance warm-white light output and device efficiency, bridging the gap between fundamental photophysics and practical LED applications.

Magneto-Optics

The coupling of physical properties such as magnetization and optical behavior gives rise to multifunctional materials. This combination of properties has led to magneto-optical applications that aid in the design of new dual-purpose materials. For example, compounds such as (CH₃(CH₂) _n NH₃)₂[MnCl₄] demonstrate the duality of magnetic sensing and optical response, leveraging their sensitivity to magnetic field variations for magneto-optic device applications.

In 2D and 1D Mn²⁺ chlorido hybrids, magnetic coupling occurs between Mn²⁺ ions that are linked through a nonmagnetic chlorido ligand, often referred to as the single halide, Mn–Cl···Mn superexchange pathway. The closer the bridging Mn–Cl···Mn angle is to 180°, the stronger the AFM coupling between Mn²⁺ ions. Mn²⁺ is a d ⁵, S = 5/2 ion, with unpaired electrons residing in the d-orbitals of the Mn²⁺ ions, with weak spin–orbit coupling, and small deviations from perfect antiparallel alignment may yield spin-canting in 2D Mn²⁺ chlorido hybrid systems. To our knowledge, the only compound that did not exhibit spin-canting behavior was (NH₃(CH₂)₂NH₃)­[MnCl₄], and this was attributed to its DJ structure. The authors proposed that the perfect alignment of neighboring inorganic layers in DJ structures prevents spin canting, unlike the RP structures, which generally exhibit some degree of staggering between the layers. These observations suggested that staggered inorganic layers play a role in enabling spin canting, which introduces slight deviations from magnetic alignment within the Mn²⁺ sublattice, leading to changes in the selection rules for electronic transitions. Therefore, spin-canting is speculated to weaken PL properties in RP perovskites through the misalignment of interlayer chlorido orbitals.

Additionally, Zhang et al. reported a long-range AFM ordering in (CH₃NH₃)₂[MnCl₄] perovskites, with a weakly canted FM below the ordering temperature, T _N of 47 K. It was observed that the AFM interactions weaken the electronic overlap, and exciton recombination becomes less efficient, leading to quenching of emission intensity. FM ordering, however, enhances exciton binding energy by aligning spins and improves emission intensity. These dynamics underscore the dual role of magnetic ordering in stabilizing excitons and determining the radiative decay pathways that dominate optical output.

Conclusions and Future Outlook

Octahedral Mn²⁺ chlorido hybrids represent a promising and tunable class of low-dimensional materials with unique photophysical and magnetic properties driven by structural flexibility. The choice and design of organic cations play a central role in structural dimensionality, modulating interlayer spacing, hydrogen bonding. The extent of octahedral distortions directly impacts self-trapping, emissions, and magnetic ordering. Strategies such as tuning hydrophobicity through cation substitution and enhancing hydrogen bonding networks provide methods to improve both stability and optical performance. The role of the organic layer extends beyond structural templating. Polarizable and conjugated organic cations can modulate the dielectric environment, influence exciton binding energies, and even improve interlayer conductivity.

In summary, five design principles could be applied to these materials:

Design Principle 1: Electronic Structure Dictates Emission Color and Efficiency

The weak, spin-forbidden d-d transitions of Mn²⁺ yield long emissions that are sensitive to the ligand field environment. Thus, tuning crystal field strength, through halide choice, dimensionality, or cation distortion, provides a design handle for controlling emission color (orange vs red) and lifetime.

Design Principle 2: Organic Cation Engineering Governs Exciton Binding Energy, Quantum Confinement, and Lattice Distortion

The nature of hydrogen bonding and steric effects from the organic cation controls interlayer spacing, Mn···Mn separation, and octahedral tilting. These, in turn, regulate quantum confinement, exciton binding energy, and nonradiative losses. Thus, the organic cation choice is therefore one of the primary strategies for tuning both luminescence efficiency and multifunctional responses.

Design Principle 3: Dimensionality Controls Quantum Confinement and Excitonic Dynamics

Moving from 2D to 1D to 0D structures systematically increases confinement, stabilizes self-trapped excitons, and alters radiative vs nonradiative pathways. Thus, dimensionality is a predictable design for tailoring PL color and lifetime.

Design Principle 4: Subtle Molecular Packing Effects (Odd–Even Chain Variations) Induce Photophysical Shifts

Even single CH₂ differences alter symmetry, hydrogen bonding topology, and interlayer order, leading to measurable shifts in band gaps and emission behavior. This principle underlines how small, chemically accessible modifications can yield substantial tuning of optical properties.

Design Principle 5: Coupling Optical and Magnetic Degrees of Freedom Enables Multifunctionality

Because Mn²⁺ provides both spin (magnetic) and d-d optical transitions, structural tuning can simultaneously affect luminescence and magnetic ordering. This dual tunability makes Mn²⁺ chlorido hybrids a model system for designing multifunctional Pb-free hybrids that bridge optoelectronics and spintronics.

These principles also offer guidance for application-driven material selection. For WLEDs and display technologies, Principles 1–3 are key. Emission color and efficiency are optimized by adjusting CFS and confinement through halide identity, dimensionality, and cation distortion. Highly emissive 0D and 1D frameworks, with strong confinement and large Stokes shifts, are ideal for narrow band phosphors, whereas 2D analogues combine efficient transport with moderate confinement, suitable for light-harvesting and photovoltaic devices. For ferroelectric and dielectric applications, Principle 4 provides a structural design route. Odd-chain diammonium spacers often yield polar, switchable noncentrosymmetric phases, while even-chain analogues stabilize centrosymmetric dielectric structures. For magneto-optic and spintronic systems, Principle 5 becomes central. Mn–Cl···Mn connectivity and layer stacking tune magnetic superexchange and spin-phonon coupling, enabling emission that responds to magnetic ordering and temperature.

As synthetic control and structural design strategies continue to advance, octahedral Mn²⁺ chlorido hybrids are poised to play a central role in next-generation optoelectronic, ferroelectric, and magneto-optical technologies. Considerable progress has already been achieved in engineering broadband PL with large Stokes shifts and temperature-dependent emission, yet challenges remain in achieving uniformly high PLQY across all dimensional classes, ensuring long-term stability, and realizing scalable synthesis routes. A deeper understanding of excited-state dynamics and defect-mediated processes is essential to unlock the full potential of these materials. At the same time, the largely unexplored ferroelectric and magnetoelectric Mn²⁺ chlorido systems offer additional functional dimensions, promising the convergence of optical, magnetic, and electronic properties within a single multifunctional platform. Finally, it should be acknowledged that several of the mechanistic insights and design frameworks discussed herein are primarily derived from a few leading research groups. Although these contributions have been instrumental in shaping current understanding, independent replication across diverse experimental settings will be essential to confirm the robustness and generalizability of these conclusions.

Biographies

Shalene Natalia Bothma received her PhD in Chemistry from the University of Pretoria, South Africa, in 2024. She is currently a postdoctoral researcher at Sol Plaatje University, South Africa, where she specializes in the structure–property relationships of organic–inorganic hybrid materials and nanomaterials for tunable photoluminescence and multifunctional applications.

Nicola Luigi Bragazzi, MD, PhD, is a physician-scientist, data scientist, and statistician with expertise in public health, epidemiology, and biophysics. His work centres on research methodology, artificial intelligence, and advanced statistical modeling applied to precision medicine and global health. He has published extensively across disciplines, bridging biomedical sciences with data-driven approaches. Dr. Bragazzi has been recognized among the world’s top 2% most-cited scientists. He is currently the Director of the Centre for Applied Data Sciences at Sol Plaatje University, South Africa.

Roy Peter Forbes is a Senior Researcher in the School of Chemistry at Wits who earned his PhD from the University of the Witwatersrand in 2011. He specializes in the characterization of functional materials using both lab-based and synchrotron X-ray techniques, with research interests that include battery materials, solid-state electrolytes, heterogeneous catalysts, pharmaceuticals, ceramics, and metals. His work emphasizes understanding structure–property relationships across a diverse set of materials, contributing to advancements in energy and functional material research.

Odireleng Martin Ntwaeaborwa is Professor of Physics and Dean of the Faculty of Natural and Applied Sciences at Sol Plaatje University, South Africa. His research focuses on surface science, as well as powders and thin films of optoelectronic nanomaterials with applications in solid-state lighting, solar energy conversion, photodynamic therapy, and gas sensing. He has authored over 282 peer-reviewed journal articles, 7 book chapters, 1 edited book, and more than 120 conference proceedings. He is often invited to deliver keynote and plenary talks at national and international conferences.

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.