Reorientational dynamics of methylbenzylammonium cations in the chiral metal halide composite MBAMnCl₃·2H₂O

Abstract

We report results from quasielastic neutron scattering (QENS) measurements of the organic cation dynamics in the manganese-based organic–inorganic chiral metal halide composites (R)-MBAMnCl₃·2H₂O and (S)-MBAMnCl₃·2H₂O, where (R)-/(S)-MBA (methylbenzylammonium) = C₆H₅CH(CH₃)NH₃⁺. Measurements upon heating from 44 to 375 K reveal how the dynamics evolve from (an)harmonic vibrations to localized, reorientational motions of the (R)-/(S)-MBA cations at approximately 270 K, for both materials. Analysis of the elastic incoherent structure factors (EISFs) of the materials suggests that the spatial geometry of the reorientational dynamics changes, and becomes more complex, with increasing temperature. Up to 325 K, the dynamics can be described as three-fold (C₃) jump-diffusion dynamics or continuous rotational diffusion of the methyl (–CH₃) and ammonium (–NH₃) groups of the (R)-/(S)-MBA cations. At higher temperatures, 350–375 K, the analysis points towards the additional presence of continuous rotational diffusion of the –C₆H₅ group of the (R)-/(S)-MBA cation around its long molecular axis, as well as a whole-cation reorientation, here described as two-fold (C₂) jump-diffusion dynamics between two different cation orientations. Such whole-cation dynamics have previously been observed mainly in materials with smaller organic cations. This suggests that the dynamics of the (R)-/(S)-MBA cation are relatively mobile in nature, with relatively weak interactions with the surrounding lattice.

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1 Introduction

Metal halide perovskites (MHPs), of the general three-dimensional (3D) ABX₃ structure, and their low-dimensional variants, such as two-dimensional (2D) A₂BX₄, where A is an organic cation, B is a metal cation, and X is a halide anion, are valued for their unique optoelectronic properties and promise for application in both solar cells^1,2 and light emitting diodes.^3,4 Furthermore, they show promise for application in low-dimensional magnetism⁵ and thermoelectrics.⁶ However, a critical challenge in this field is to understand how the optoelectronic properties are related to the dynamical properties of the materials, especially with regard to the organic cation dynamics.

The current understanding of the organic cation dynamics in MHPs is based primarily on results obtained from computer simulations, nuclear magnetic resonance and QENS experiments. QENS studies, which are of specific concern here, have mostly focused on the prototypical 3D MHPs (MA,FA)PbX₃ (MA = methylammonium; FA = formamidinium; X = I, Br).^7–15 For low-dimensional MHPs, QENS studies of the organic cation dynamics have focused on the 2D MHPs BA₂PbBr₄ (BA = butylammonium), ODAPbBr₄ and GABA₂PbBr₄ (ODA = 1,8-diaminooctammonium, and GABA = 4-aminobutyric acid),¹⁶ BA₂PbI₄ and PEA₂PbI₄ (PEA = phenetylammonium),^17,18 (OA)₂PbI₄ (OA = octylammonium),¹⁹ and, most recently, (1,3-PDA)PbBr₄ (PDA = phenylenediammonium), (1,4-PDA)PbBr₄, and (1,4-XDA)PbBr₄ (XDA = xylylenediammonium), and the zero-dimensional (0D) “perovskitoid” (1,3-XDA)₂PbBr₆.²⁰ QENS studies have, typically, revealed various rotational (reorientational) motions of the organic cations, on the 1–100 ps timescale, and how the timescale and geometry of the dynamics change with temperature, which in some cases have been correlated with the materials’ optoelectronic properties.^16,17,20

Here, in a variable temperature QENS study, we investigate the nature of organic cation dynamics in the novel, manganese (Mn)-based chiral MHPs (R)-/(S)-MBAMnCl₃·2H₂O, also formulated as (R)-/(S)-(HMBA)₂[Mn₂Cl₆(H₂O)₄].^21–23 These materials show promise for use in applications such as circular dichroism and circularly polarized light,²⁴ with potential application in, e.g., quantum computing,²⁵ data storage and encryption,²⁶ bio-molecular sensing,²⁷ imaging,²⁸ and asymmetric photochemical synthesis,²⁹ which may not be achievable with achiral MHPs. Compared to the prototypical chiral MHP (R)-/(S)-MBA₂PbI₄,³⁰ (R)-/(S)-MBAMnCl₃·2H₂O is a lead-free and hence more environmentally benign material. Additionally, the presence of luminescent Mn²⁺ ions gives rise to luminescence in the green and red wavelength regions, based on electronic d–d transitions,³⁰ which, along with blue emission originating from the (R)-/(S)-MBA cations, results in a broadband emission.²² The aim of the study is to determine the nature of the organic cation dynamics, how it evolves with temperature and relates to the nature of organic cation dynamics in other MHPs, and how it correlates with the luminescence properties of the materials.

2 Experimental details

2.1 Sample synthesis and structural analysis

Powder samples of (R)-/(S)-MBAMnCl₃·2H₂O, approximately 0.3 g per composition, were prepared by grinding single crystals of the respective material. Single crystals of (R)-/(S)-MBAMnCl₃·2H₂O were prepared by first dissolving (R)-/(S)-MBA (1 mmol), manganese(II) chloride (1 mmol), and hydrochloric acid (1 mmol) in ethanol (30 mL), which was immediately followed by slow evaporation of ethanol at room temperature for about one week. When the synthesis occurs in a relatively humid environment, crystallization by evaporation at room temperature becomes difficult. It is necessary to heat the mixed solution to 40–60 °C to facilitate crystallization. Finally, the material is dried and then annealed in a furnace at about 60 °C for one hour. The yield of all Mn-based organic–inorganic halide materials is about 60–70%. All chemicals (α-methylbenzylamine (99%), hydrochloric acid (48 wt% in H₂O, ≥99.99%), manganese(II) chloride (MnCl₂, 99.9%), and ethanol (C₂H₅OH, 99.7%)) were commercially purchased from Sigma-Aldrich and used without further purification.²²

Measurements and analyses of single-crystal and powder X-ray diffraction (SCXRD and PXRD) data of the samples (SCXRD used the (R)-MBAMnCl₃·2H₂O sample) confirmed phase purity and that the structures of both materials are in agreement with the structures reported in ref. 21, see S1 in the supporting information (SI). Specifically, (S)-MBAMnCl₃·2H₂O exhibits an orthorhombic structure of space group P2₁, with the lattice parameters a = 6.0631(2) Å, b = 36.3750(10) Å, and c = 6.3840(2) Å.²³ It may be described as a 0D MHP with two edge-shared [MnCl₆] octahedra but with two of the Cl atoms replaced by H₂O molecules. These [Mn₂Cl₈(H₂O)₄]²⁻ dimers are surrounded by (S)-MBA cations with strong interactions between the hydrogens of the (S)-MBA cations and the Cl atoms (Fig. 1).

Fig. 1 Schematic illustration of (left) the crystal structure of (S)-MBAMnCl₃·2H₂O and (right) the (R)-MBA and (S)-MBA cations.²³ The solid lines represent the unit cell.

2.2 Quasielastic neutron scattering

The QENS experiment was performed on the direct geometry time-of-flight spectrometer FOCUS at the Swiss Spallation Neutron Source SINQ at the Paul Scherrer Institute in Switzerland.^31,32 Each sample was finely distributed in an aluminium foil rolled cylinder whose expanded size was 5 cm × 4 cm. The aluminium foil was put in an aluminium can, which was sealed with a lead wire. We first took short (5 min) measurements upon cooling from 340 to 44 K (cooling rate ≈2.5–3.5 K min⁻¹), using 4 Å incident wavelength neutrons. The usage of 4 Å incident wavelength neutrons yielded an energy resolution at the elastic line of around 0.2 meV at full width at half maximum (FWHM, Γ), an accessible energy transfer (E) range from −20 to 5 meV, and an accessible momentum transfer (Q) range from 0.53 to 2.63 Å⁻¹ around the elastic peak. This was immediately followed by long (2 h) measurements at T = 44, 270, 300, 325, 350, and 375 K. After general and instrument-specific data reductions and corrections of the data, that included normalization of the detector efficiency and subtraction of the scattering of the sample cell alone, the measured data were transformed into the dynamical structure factor S(Q,E), which results from the sum of coherent and incoherent scattering contributions. The S(Q,E)s, as presented here, were binned into 15 Q-values.

For (R)-/(S)-MBAMnCl₃·2H₂O, it is important to note that H contributes approximately 92% to the total scattering cross section of the investigated materials and that the scattering from H is predominantly incoherent. Since we do not expect any dynamics of the inorganic sub-lattice within the here probed timescale of picoseconds, the quasielastic scattering is assumed to be related to self-dynamics of the H of the organic cations; however, one should note that a smaller concentration of H is also present as bridging and terminal water molecules of the Mn–Cl complexes (see Fig. 1), which may contribute to quasielastic scattering. However, previous studies show that the H of the bridging and terminal water molecules experience stronger interactions with the neighboring Cl atoms,³³ suggesting that their reorientational dynamics, if they exist, occur on a slower timescale than the timescale of reorientational dynamics of (R)-/(S)-MBA. Therefore, any reorientational dynamics of these water molecules have not been considered here. All data manipulations were done using the software DAVE.³⁴

3 Results and discussion

3.1 Elastic intensity scans and intensity maps

In order to determine the temperature range in which the organic cation dynamics is accessible to the instrument, we first analyze the temperature dependence of the elastic intensity (I_elastic), as here extracted by integration over the energy range of −0.1 < E < 0.1 meV. Fig. 2 shows the elastic intensity, as derived in this way, for both materials. The measurements upon cooling, which were performed first, show a marked increase in elastic intensity between 330 and 300 K, especially for (S)-MBAMnCl₃·2H₂O. Upon further cooling, the increase in elastic intensity occurs at a decreased and similar rate for both materials. The measurements upon heating follow the same trend as the measurements upon cooling, but show that the marked variation in elastic intensity (at 330–300 K) is shifted to higher temperatures by approximately 20 K. The marked change in elastic intensity in the temperature range of 300–350 K indicates the onset (on heating) and freezing out (on cooling) of diffusional dynamics in both materials, whereas the practically linear behavior below ≈300 K reflects the increase (upon temperature increase) and decrease (upon temperature decrease) of (an)harmonic vibrational dynamics in the materials. Due to the generally soft nature of MHPs, it is important to note that the material may exhibit pronounced anharmonicity even at temperatures well below the onset of reorientational dynamics.^35–37

Fig. 2 Elastic intensity, I_elastic, of (R)-/(S)-MBAMnCl₃·2H₂O, as measured upon cooling (first cycle) and heating (second cycle). The data have been normalized to one at 44 K. The dashed lines are guides to the eye and manifest the practically linear behavior of the temperature dependence of the data below 200 K.

The onset of diffusional dynamics of the MBA cations is further reflected in maps of the intensity of the scattered neutrons, as plotted as a function of temperature and energy transfer, [I(T,E), see Fig. 3]. As can be seen, there is a clear quasielastic broadening at temperatures above approximately 250 K for (R)-MBAMnCl₃·2H₂O and 290 K for (S)-MBAMnCl₃·2H₂O, in quite good agreement with Fig. 2. We also observe that the quasielastic signal appears somewhat broader for (S)-MBAMnCl₃·2H₂O, as compared to (R)-MBAMnCl₃·2H₂O, thus indicating faster dynamics in the former.

Fig. 3 I(T,E) maps, as measured upon cooling for (a) (R)-MBAMnCl₃·2H₂O and (b) (S)-MBAMnCl₃·2H₂O. The data have been summed over the whole Q-range, 0.53–2.63 Å⁻¹.

3.2 Dynamical structure factors

For a detailed analysis of the neutron scattering data, S(Q,E) was fitted to the following function:

Here, [script L](E; Γ_i) are Lorentzian functions with FWHM Γ_i, bkg(Q) is a linear background, A₀(Q) and A_i(Q) are the elastic and quasielastic incoherent structure factors, which were normalized to , and R(Q,E) is the instrumental resolution function that is approximated with the data measured at 44 K, and I(Q) is a scaling factor. Depending on sample composition and temperature, one (n = 1) or two (n = 2) Lorentzians were needed to adequately describe the QENS signal.

Fig. 4 shows S(Q,E) at the lowest and highest measured temperatures, 270 and 375 K, for both materials. Data are shown for Q = 1.6 Å⁻¹ and included in the plots are the fits according to eqn (1). Data at all temperatures are shown in Fig. S4 and S5. The S(Q,E) at 270 and 300 K can be adequately fitted to one Lorentzian (L₁), whereas S(Q,E) at T ≥ 325 K requires the use of two Lorentzians (L₁ and L₂), for both materials. This indicates significant changes in the dynamics with increasing temperature, especially the observation of at least one more picosecond-timescale dynamical process at the three highest temperatures, but only small (if any) differences in the observed organic cation dynamics between the two materials. Importantly, the FWHM of the two Lorentzians, Γ₁ (narrower) and Γ₂ (broader), does not show any dependency on Q at any of the measured temperatures or for any of the two materials (Fig. 5). Note, the data points for Q < 1 Å⁻¹ are considered to be less reliable, because of weaker quasielastic scattering in this range and the possible contribution of small angle scattering due to the powder nature of the samples; therefore, these data points are not considered. The practically Q-independent behavior of the data suggests that the probed dynamics are localized in nature. This is exactly as expected, because of the relatively short timescale of the dynamics as probed here (0.3–13 ps). To increase the robustness of our analysis, we refitted the S(Q,E) with Γ₁ and Γ₂ fixed to their Q-independent mean values (shown as dashed lines in Fig. 5). Notably, the refitting of S(Q,E) did not have any visible effect on the quality of the fits to S(Q,E), which supports our approach.

Fig. 4 S(Q,E) of (R)-/(S)-MBAMnCl₃·2H₂O, together with free fits according to eqn (1), at 270 and 375 K, respectively, and Q = 1.6 Å⁻¹.

Fig. 5 FWHM of the Lorentzians, Γ₁ and Γ₂, of (R)-/(S)-MBAMnCl₃·2H₂O, as extracted from the data. Dashed lines are fits to a constant value at each temperature. Data points marked with grey crosses are outliers, not taken into account in the fits.

Fig. 6 shows a plot of the temperature dependence of the correlation time of the dynamics, τ₁ and τ₂, as derived by taking τ_i = 2ℏ/Γ_i (i = 1 and 2).³⁸ As can be seen, the correlation times of the two dynamics processes are (within error) practically temperature independent between 325 and 375 K and take on values of about 5 ps for the slower process and 0.5 ps for the faster. It follows that there is no strong difference in the correlation time of the observed dynamics between the two materials. The fact that the data show no significant temperature dependence in this temperature range suggests that our measurements are not sensitive enough to capture the correspondingly low activation energy of the dynamics. At the lower temperatures, 270 and 300 K, we find that the correlation time for the slower process is somewhat faster for (R)-MBAMnCl₃·2H₂O (τ₁ ≈ 2 ps) than for (S)-MBAMnCl₃·2H₂O (τ₁ ≈ 5 ps). This is in agreement with the observation of the somewhat broader quasielastic scattering at the relatively low-temperature range for (R)-MBAMnCl₃·2H₂O in the I(T,E) maps, as shown in Fig. 3.

Fig. 6 Temperature dependence of the correlation times, τ₁ and τ₂, for (R)-/(S)-MBAMnCl₃·2H₂O, as extracted from the data.

3.3 Elastic incoherent structure factor

Information about the spatial geometry of the localized, reorientational dynamics of the organic cations can be obtained from analysis of the elastic incoherent structure factor (EISF), which is defined as the ratio of the elastic and total scattering intensity, i.e. . Fig. 7 shows the EISFs at 270, 300, 325, 350, and 375 K, as extracted from the data (markers), together with geometrically and physically realistic models (lines) of reorientational dynamics of the H atoms on the MBA cations.

Fig. 7 Experimental EISF (markers) for (a) (R)-MBAMnCl₃·2H₂O and (b) (S)-MBAMnCl₃·2H₂O, together with fits to various jump-diffusion models. The EIFS were extracted from the data.

The experimentally determined EISFs exhibit a decreasing trend with increasing Q and temperature, quite similarly for the two different materials. The systematic decrease with increasing temperature suggests an increase in the number of atoms undergoing the dynamics and hence contributing to the QENS signal within the energy range as probed here. At the three lowest temperatures, 270, 300, and 325 K, the EISF can be best approximated with the model for three-fold (C₃) or continuous (C_cont.) rotational diffusion of the –CH₃ and –NH₃ groups, together with a constant c, which fit equally well to both materials. The constant c accounts for additional elastic scattering, probably mostly due to H that experiences motions too slow to be resolved by the instrument resolution, which corresponds to dynamics characterized at a timescale of ∼0.3–13 ps. c decreases with increasing temperature for both materials. The somewhat larger value of c at 325 K for (S)-MBAMnCl₃·2H₂O (c = 0.18), as compared to (R)-MBAMnCl₃·2H₂O (c = 0), suggests that a larger portion of the –CH₃ and –NH₃ groups are dynamically immobile in (S)-MBAMnCl₃·2H₂O at this temperature. This is in agreement with the slightly higher onset temperature of the dynamics for (S)-MBAMnCl₃·2H₂O, as shown in Fig. 2 and 3.

At even higher temperatures, 350 and 375 K, the EISF shows a more pronounced decrease with increasing Q, which reflects more complex dynamics. This is in agreement with the need for using two Lorentzian functions to fit the QENS spectra. The addition of a dynamical component that takes into account C_cont. rotational diffusion of the –C₆H₅ group leads to a lowering of the EISF, but not enough to fit the experimental data (cf. dashed black curve shown in Fig. 7). In this context, we note that the MBA cation may exhibit two possible orientations in the structure. These refer to the cases when the terminal –NH₃ group points towards any of the two Cl⁻ ions of a neighboring [Mn₂Cl₈(H₂O)₄]²⁻ dimer. Effectively, these two cation orientations differ by a rotation of 48° around the molecule's mass center, which means that the change in orientation of the MBA cation may be described as a C₂ jump-diffusion mechanism between the two orientations. Indeed, a model which contains the C_cont. rotational diffusion of the –CH₃, –NH₃, and –C₆H₅ groups, together with such a C₂ model for a portion of the MBA cations yields an excellent agreement with the experimental data (Fig. 7). A schematic illustration of the dynamics models considered in our analysis is shown in Fig. 8. Mathematical expressions for all models are given in Section S3. A compilation of our tentative assignments of the localized dynamics, as based on our fitting of the EISF data, is shown in Table 1.

Fig. 8 Schematic illustration of localized dynamics of the (R)-MBA cations, as considered in the EISF analysis: (upper) C₃ and C_cont. rotational diffusion of the –CH₃ and –NH₃ groups, C_cont. rotational diffusion of the –C₆H₅ group, and (lower) C₂ rotational diffusion of the whole MBA cation around its mass center.

Table 1 Compilation of dynamics data obtained from the fitting of the experimental EISFs

Materials	T (K)	c	Tentative assignments of localized dynamics
a Refers to the C2 (MBA) part only.
(R)-MBAMnCl3·2H2O	270	0.78	C3 (–CH3 + –NH3)
	270	0.78	Ccont. (–CH3 + –NH3)
	300	0.65	C3 (–CH3 + –NH3)
	300	0.66	Ccont. (–CH3 + –NH3)
	325	0.00	C3 (–CH3 + –NH3)
	325	0.00	Ccont. (–CH3 + –NH3)
	350	0.69^a	Ccont. (–CH3 + –NH3 + –C6H5) ⊗ C2 (MBA)
	375	0.21^a	Ccont. (–CH3 + –NH3 + –C6H5) ⊗ C2 (MBA)
(S)-MBAMnCl3·2H2O	270	0.79	C3 (–CH3 + –NH3)
	270	0.83	Ccont. (–CH3 + –NH3)
	300	0.65	C3 (–CH3 + –NH3)
	300	0.66	Ccont. (–CH3 + –NH3)
	325	0.16	C3 (–CH3 + –NH3)
	325	0.18	Ccont. (–CH3 + –NH3)
	350	0.40^a	Ccont. (–CH3 + –NH3 + –C6H5) ⊗ C2 (MBA)
	375	0.02^a	Ccont. (–CH3 + –NH3 + –C6H5) ⊗ C2 (MBA)

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4 Discussion

By bringing together the results from the QENS analysis, we can now understand several new features pertaining to the organic cation dynamics in (R)-/(S)-MBAMnCl₃·2H₂O.

A general observation is that the localized dynamics of the (R)-/(S)-MBA cations is highly temperature-dependent, evolving from C₃ or C_cont. rotational diffusion of the –CH₃ and –NH₃ groups at temperatures between 270 and 325 K, to the additional presence of C_cont. rotational diffusion of the –C₆H₅ group and C₂ rotational diffusion of the whole (R)-/(S)-MBA cation at 350 and 375 K, for both materials.

The observation of room- to lower-temperature dynamics assigned to C₃ or C_cont. rotational dynamics of the terminal –CH₃ and –NH₃ groups have been found in QENS studies of several other MHPs, including the 3D MHP system MAPbX₃ (MA = CH₃NH₃, X = I,^8–11,39,40 Br,^{7,12,13,41,42} and Cl^39,43), and lower-dimensional systems such as (OA)₂PbI₄,¹⁹ BA₂PbI₄ and PEA₂PbI₄,^17,18 BA₂PbBr₄, ODAPbBr₄ and GABA₂PbBr₄,¹⁶ and (1,3-PDA)PbBr₄, (1,4-PDA)PbBr₄, (1,4-XDA)PbBr₄, and (1,3-XDA)₂PbBr₆,²⁰ which thus appears as a generic feature of MHPs containing organic cations with functional –NH₃ and/or –CH₃ groups, independent of the specific type of organic cation and inorganic sub-lattice. The timescale of the dynamics, however, varies markedly between different materials. As an example, while the timescale of the –CH₃ and –NH₃ group dynamics in (R)-/(S)-MBAMnCl₃·2H₂O (≈ 0.5–5 ps, T = 270–375 K) is similar to that in 3D MHPs,⁹ it is typically faster than for 2D MHPs (usually tens to hundreds of picoseconds).^17,19,44 In this context, we note that the variation in timescale of the dynamics is expected to be primarily affected by the degree of hydrogen/halogen bonding between the –CH₃/NH₃ group to the closest halide ion, which is related to the free volume of the solid. A clear example of this is found in the cubic phase of MAPbX₃ (X = Cl, Br, and I), for which the timescale of –CH₃/NH₃ dynamics decreases systematically with increasing unit cell size.^7,9,39 The generally faster timescale of these dynamics in (R)-/(S)-MBAMnCl₃·2H₂O, when compared to 2D MHPs, thus indicates a larger free volume and weaker interactions between the terminal –CH₃/NH₃ groups and the inorganic sublattice. This is in agreement with the typically longer hydrogen/halogen-bond distances in layered materials.^{7,17,19,44,45} Furthermore, it is probably this relatively large free volume within the 0D framework in (R)-/(S)-MBAMnCl₃·2H₂O that allows for whole-cation reorientation on the relatively short timescale of picoseconds. Whole-cation dynamics have previously only been observed in QENS studies of 3D MHPs with relatively small organic cations, such as for MA and FA dynamics in (MA,FA)Pb(I,Br,Cl)₃,^{7–14,39–43,46–48} with the exception of one recent report on the high-temperature phase of BA₂PbI₄.¹⁷

With regard to the comparison of the dynamics data, the lower onset temperature (cf. Fig. 3) and generally faster dynamics (cf. Fig. 6) of (R)-MBAMnCl₃·2H₂O, when compared to (S)-MBAMnCl₃·2H₂O, may reflect subtle diastereomeric packing effects. Note that these two crystals do not belong to enantiomorphic space groups; thus, strictly speaking, they are not enantiomers. As our XRD data do not reveal any significant differences in their structure, only some minor, statistically insignificant, differences in bond lengths and angles can be observed (see SI). Instead, we speculate that the observed differences may be related to different amounts of hydrogen-containing impurities in the samples, such as ethanol or the new material (R)-MBAMnCl₃·C₂H₅OH that was found to be present in a small quantity based on our SCXRD and PXRD analyses (see SI).

Finally, we note that the picosecond-timescale dynamics of the (R)-/(S)-MBA cations induce transient hydrogen/halogen-bonding interactions between the –CH₃/NH₃ groups and the inorganic sub-lattice and, in effect, give rise to local structural fluctuations (dynamic disorder) that become increasingly pronounced with increasing temperature. As found for other low-dimensional MHPs,²⁰ such temperature-dependent dynamic disorder is likely to promote non-radiative recombination of photoexcited excitons and, hence, to lead to a decrease in the emission intensity upon increasing temperature – an unwanted phenomenon commonly referred to as thermal quenching of luminescence. The relatively high mobility of (R)-/(S)-MBA cations in (R)-/(S)-MBAMnCl₃·2H₂O, due to its unique 0D structure, may thus motivate efforts to tune and restrict the dynamics by structural modification as a route towards higher resistance towards thermal quenching of luminescence.

5 Conclusions

To conclude, our QENS study of (R)-/(S)-MBAMnCl₃·2H₂O unravelled the nature of the organic cation, (R)-/(S)-MBA, dynamics on the picosecond timescale and how it changes with temperature from 44 to 375 K. In the low-temperature regime, up to approximately 270 K, the probed dynamics are limited to (an)harmonic vibrations, whereas at higher temperatures, the (R)-/(S)-MBA cations undergo reorientational dynamics akin to certain high-symmetry motions. At temperatures between 270 and 325 K, the dynamics data can be described as C₃ or C_cont. rotational diffusion of the –CH₃ and –NH₃ groups of the (R)-/(S)-MBA cation. At higher temperatures, the dynamics data point towards the additional presence of both C_cont. rotational diffusion of the –C₆H₅ group of the (R)-/(S)-MBA cation, as well as C₂ jump-diffusion dynamics of the whole (R)-/(S)-MBA cation between two different cation orientations. The observation of whole-cation dynamics on the picosecond timescale for large molecules such as (R)-/(S)-MBA is rare and probably originates from the relatively large free volume with the 0D structure of (R)-/(S)-MBAMnCl₃·2H₂O.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data from this article, including the neutron scattering data as measured at the Paul Scherrer Institute (PSI), are available in ref. 49 and 50.

Supplementary information (SI): X-ray diffraction and structural analyses, additional quasielastic neutron scattering data and fits, and additional information about the EISF analysis (PDF). See DOI: https://doi.org/10.1039/d6tc00535g.

CCDC 2379524 and 2379426 contain the supplementary crystallographic data for this paper.^51a,b

Acknowledgements

This research was funded by the Swedish Research Council (Grant No. 2021-04807 and 2025-04685) and the Swedish Foundation for Strategic Research within the Swedish National Graduate School in Neutron Scattering, SwedNess (Grant No. GSn15-0008). This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland.

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