Direct Investigation of the Reorientational Dynamics of A-site Cations in 2D Organic-Inorganic Hybrid Perovskite by Solid-State NMR

doi:10.21203/rs.3.rs-183259/v1

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Direct Investigation of the Reorientational Dynamics of A-site Cations in 2D Organic-Inorganic Hybrid Perovskite by Solid-State NMR

https://doi.org/10.21203/rs.3.rs-183259/v1

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published 21 Mar, 2022

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The reorientational dynamics of A-site cations in organic-inorganic hybrid perovskites (OIHPs) play a pivotal role in determining the physical properties of OIHPs. The nuclear magnetic resonance (NMR) relaxation method is a powerful tool for studying molecular dynamics. However, it faces a significant challenge when applied to measure two-dimensional (2D) OIHPs, where the small signals of A-site cations overlap with the much larger signals of the organic spacers. Here, we demonstrate a novel strategy to tackle the spectral overlapping problem while investigating the dynamics of A-site cations by using rotational-echo double resonance (REDOR) NMR combined with the synthesis of 2D OIHPs, consisting of isotopically enriched A-site cations of ¹³C-methyl-¹⁵N-ammonium, [U-¹³C,U-¹⁵N]MA. The direct investigation of the reorientational dynamics of MA allows us to unveil the interplay between the A-site cation dynamics and the structural rigidity of the organic spacers, so providing a molecular-level insight into the design of 2D OIHPs.

Materials Chemistry

Catalysis

Energy Engineering

organic-inorganic hybrid perovskites (OIHPs)

nuclear magnetic resonance (NMR)

Organic inorganic hybrid perovskites (OIHPs) have attracted a significant amount of attention for photovoltaic applications since their power conversion efficiency (PCE) has reached over 25%¹, which is already approaching the performance of the commercial Si-based solar cells. The impressive PCE of OIHP photovoltaics is mainly attributed to their outstanding physical properties, such as a high optical absorption coefficient², low exciton binding energy³, and long and balanced electron − hole diffusion length^4,5. The chemical formula of OIHP is represented as ABX₃ where A is an organic cation, such as methylammonium (MA⁺), B is the divalent metal cation, such as lead(II) ion (Pb²⁺), and X is a halide anion^6,7. Although the electronic structure of OIHP near band edges is mainly determined by the inorganic parts of the Pb and halide elements, the role of A-site organic cations has been reported to play an important role in affecting the device performance and stability, instead of only acting as a passive component for a charge compensation for the [PbI₃]⁻ lattice^8–13. For example, it has been reported that theoretically, the molecular rotations of A-site cations may cause the distortion of the PbI₆ octahedral cage and the consequent dynamical change of the band structure might be at the origin of the slow carrier recombination and the superior conversion efficiency of CH₃NH₃PbI₃¹⁴. In addition, several experimental techniques have been applied to reveal the dynamics of A-site cations, including solid-state nuclear magnetic resonance (ssNMR)^10,15,16, neutron powder diffraction (NPD)¹⁷, and inelastic neutron scattering^18,19. In particular, ssNMR has emerged as a useful tool for studying the cation reorientational dynamics in OIHP. The interplay between the charge carrier lifetimes and the reorientational dynamics of A-site cations in OIHP photovoltaics has been clearly elucidated, providing evidence of the polaronic nature of charge carriers in PV perovskites^10,15,20.

Recently, another new class of two-dimensional organic-inorganic hybrid perovskites (2D OIHPs) have attracted great attention owing to their superior ambient stability, as well as their promising optoelectronic properties^21–24. Ruddlesden-Popper perovskites (RPPs) are typical examples of this class of layered 2D organic-inorganic hybrid perovskites (2D OIHPs), having the generic chemical formula A′₂A_n−1M_nX_3n+1, where A′ represents an organic spacer, such as long chain alkylammonium cation (1-butylammonium, BA⁺) or phenyl alkylammonium cation (2- phenethylammonium, PEA⁺), A is an organic cation, M is a metal, X is a halide, and n is the number of octahedral slabs per unit cell^25–28. The layered 2D OIHP consists of a self-assembled periodic array of inorganic perovskite layers of corner-sharing PbX₆ octahedral slabs, separated by the organic spacers in the lattice framework^29,30. Accordingly, they exhibit a naturally formed “multiple quantum-well (MQW)” structure. The semiconducting inorganic PbX₆ perovskites, with a smaller band gap, act as potential “wells”, while the insulating organic layers, possessing a larger band gap, act as potential “barriers”. The value of n is the number of inorganic octahedral slabs per unit cell that determines the width of the QW^31–33. The layered 2D OIHPs have emerged as a new class of outstanding optoelectronic materials due to their unique tunable physical properties, and structural flexibility, by controlling the number of n values or the thickness of the perovskite slabs^34–36. Because the layered 2D OIHPs consist of soft organic spacers between the flexible inorganic layers, they exhibit a great structural versatility compared to their 3D bulk counterparts³⁷. It has been reported that the manipulations of molecular A-site cations and organic spacers may cause the structural rearrangement, or deformation, of inorganic perovskite cages in 2D OIHPs, which may further influence their electronic band structures near the band edges, and the corresponding optical and electronic behaviours^38–40. Nevertheless, unlike their 3D OIHP counterparts consisting of only A-site cations, the detection of reorientation dynamics of A-site cations in 2D OIHPs with n ≥ 2 by ssNMR becomes a challenging task because 2D OIHPs consist of both molecular A-site cations and organic spacers. The detection of NMR resonance signals of the A-site cations in 2D OIHPs is difficult due to the presence of large fractions of organic spacer cations, where the potential spectral overlapping of the resonance signals of A-site cations and the organic spacers may result in a small signal of A-site cations buried in a larger signal of spacer cations. Subsequently, the conventional ²H and ¹⁴N NMR relaxation methods for characterising the dynamics of cations in 3D OIHP^10,11,15,16 will not be applicable for studying the dynamics of A-site cations in 2D OIHPs. Until now, only the dynamics of the organic spacers at the 2D OIHP crystals of (4NPEA)₂PbI₄ and (PEA)₂PbI₄ with n = 1, which consist of no A-site cations, have been analysed based on the temperature dependence of the ssNMR spectral line shapes⁴⁰. However, this technique is still not applicable for the investigation of the dynamics of A-site cations in 2D OIHP with n ≥ 2 due to the spectral overlapping problem. In this work, we employ a novel strategy to tackle the spectral overlapping problem of 2D OIHPs with n = 2 while investigating the environments and dynamics of the A-site molecular cations by the advanced NMR techniques, combined with the synthesis of 2D OIHPs consisting of isotopically enriched A-site cations of ¹³C-methyl-¹⁵N-ammonium, [U-¹³C,U-¹⁵N]MA. We first applied two-dimensional ¹³C-¹⁵N correlation double cross-polarization magic-angle spinning spectroscopy⁴¹, 2D (¹³C,¹⁵N) DCP MAS, to observe the NMR signal of MA with a substantial sensitivity enhancement and without the interference of the signal of the spacer cations. We further performed REDOR NMR^42,43 to investigate the reorientational dynamics of MA in 2D OIHPs, which provides the motional average to the dipolar coupling between ¹³C and ¹⁵N nuclei of MA. Accordingly, both the environments and the dynamics of A-site molecular cations can be unequivocally revealed by overcoming the spectral overlapping problem. The direct detection of the reorientational dynamics of MA allows us to further examine the interplay between the rigidity of organic spacers and the A-site cations dynamics of 2D OIHPs. Our result of unveiling the reorientational dynamics of the A-site cation in 2D OIHPs by REDOR NMR may provide new insights into the future design of 2D OIHP materials.

The 2D OIHP crystals, including (BA)₂MAPb₂I₇ (n = 2) and 2D (PEA)₂MAPb₂I₇ (n = 2), were synthesised using a slow evaporation at a constant-temperature (SECT) growth method. In order to study the local environments and the dynamics of the A-site cations in 2D OIHP crystals while avoiding the signal of A-site cations overlapping with those of the spacer cations, we adopted an isotopic labelling strategy in preparing the materials. Firstly, [U-¹³C,U-¹⁵N]MA was incorporated as an A-site cation in the synthesis of 2D OIHP crystals. On the one hand, the use of [U-¹³C,U-¹⁵N]MA allowed for the NMR detection of the MA cation with a greatly enhanced sensitivity. On the other hand, by measuring the 2D (¹³C,¹⁵N) DCP MAS spectra of 2D OIHP crystals synthesised with [U-¹³C,U-¹⁵N]MA, we can detect the NMR signal sorely from the dipolar coupled ¹³C -¹⁵N spin pair of [U-¹³C,U-¹⁵N]MA. For comparison, the growth of 2D OIHP crystals consisting of the conventional naturally abundant MA as the A-site cation, were also synthesised. The as-grown crystals consisting of both natural-abundance and [U-¹³C,U-¹⁵N]MA cations exhibited the typical single crystal structure and purity of 2D OIHP, which was confirmed by X-ray diffraction (XRD) analysis as shown in Fig. S1. The corresponding Photoluminescence (PL) spectra of 2D OIHP crystals, consisting of both the natural-abundance MA and the uniformly isotopically labelled MA cations, are almost identical as shown in Fig. S2. Thus, the structural and the physical properties of 2D OIHPs did not alter by using [U-¹³C,U-¹⁵N]MA in the synthesis of 2D OIHP crystals. As shown in Fig. 1(a), the signal of MA of 2D (BA)₂(MA)Pb₂I₇ (n = 2), synthesised with the natural-abundance MA, is poor and largely overlapped with the signal of C2 carbon of BA. As indicated in Fig. 1(b), by recording the 2D (¹³C,¹⁵N) DCP MAS spectra of 2D OIHP crystals synthesised with [U-¹³C,U-¹⁵N]MA, we obtained the cross peaks correlated with the frequency coordinates of the carbon-nitrogen spin pair of [U-¹³C,U-¹⁵N]MA. Namely, we can observe the signal of [U-¹³C,U-¹⁵N]MA alone without worrying about the interference of the signals of the spacer cations. This allowed for the further unambiguous assignment of the resonance peaks of MA in Fig. 1(a). There are two cross peaks which appear in the 2D (¹³C,¹⁵N) DCP MAS spectrum of 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), as shown in Fig. 1(b), indicating the existence of the two different local environments of MA. The major component is associated with the 31.4 ppm peak of the ¹³C direct excitation MAS spectrum overlayed on top of the 2D (¹³C,¹⁵N) DCP MAS spectrum, while the minor component is associated with the 27.2 ppm peak, which may be attributed to be the minor structural defect. The amount of the minor MA component was estimated to be roughly 3% of the total amount of MA based on the ratio of the peak intensities of the ¹³C direct excitation MAS spectrum. In contrast, only one local environment of MA in 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) was observed in Fig. 1(b). Interestingly, the intensity of the cross peak of the major MA component of 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) is less than that of the minor MA component, suggesting that the major MA component is associated with the lower transfer efficiency of double cross polarization. The reason for the lower DCP transfer efficiency is attributed to the presence of the reorientational motion of the C-N bond of MA. In summary, the result of the 2D (¹³C,¹⁵N) DCP MAS spectral analysis of 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) and 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) suggests that the local environments and the dynamics of the A-site cations in 2D OIHP crystals depend on the selection of organic spacers. The information of the MA reorientational dynamics can be further studied by ¹³C{¹⁵N}REDOR NMR in a quantitative manner, which measures the effective dipolar interaction of the ¹³C and ¹⁵N nuclei pair of MAs, averaged by the presence of the reorientational motion of MA.

REDOR NMR has been widely used for measuring the dipolar interaction between a selected heteronuclear spin pair. For a typical solid-state powder sample, the reorientational motion of a dipolar coupled internuclear vector is absent, and the internuclear dipolar interaction is a function of the internuclear distance. REDOR NMR can be used to measure the time evolution of the internuclear dipolar interaction, and the internuclear spin distance at angstrom resolution can be obtained⁴⁴. While REDOR NMR is popular for the accurate measurement of internuclear distances at atomic resolution, it can also be applied to study the molecular motion of a dipolar-coupled nuclear spin pair^42,43. In our case, where the ¹³C and ¹⁵N nuclear spins of [U-¹³C,U-¹⁵N]MA are covalently bonded and the internuclear distance is fixed, the presence of the reorientational motion of the vector between a ¹³C and ¹⁵N spin pair would result in a reduction of the effective dipolar interaction between them^42,43. We first used [U-¹³C,U-¹⁵N]MAI powder, where the reorientational motion of the ¹³C-¹⁵N nuclear spin vector is absent, as an example to show that REDOR NMR can be used to extract the distance information between the ¹³C and ¹⁵N spin of MA. The pulse sequence is shown in Fig. 2(a), where ¹³C is the observed spin and ¹⁵N is the dephasing spin. Two sets of the interleaved experimental REDOR signals were recorded. The full echo signal, denoted as S₀, was recorded without the dephasing pulses and the reduced echo signal, denoted as S, was recorded with the dephasing pulses switched on to recouple the dipolar interaction of the ¹³C-¹⁵N spin pairs. The data was plotted as a REDOR dephasing curve of ΔS/S₀ (N×Tr) versus different dephasing times N×Tr, where Tr is the rotor period and N is 2(n + 1). The distance dependent nature of the REDOR dephasing curve allows for the extraction of the internuclear distance information at angstrom resolution, provided that the reorientational motion of the spin pair is absent. As indicated in Fig. 2(b), the experimental ¹³C{¹⁵N}REDOR results (orange open circles) are consistent with the simulated dephasing curve (orange dashed line) of [U-¹³C,U-¹⁵N]MAI powder using the C-N bond length as 1.51 Å, which was determined by single crystal X-ray crystallography⁴⁵.

We then systematically studied the reorientational dynamics of [U-¹³C,U-¹⁵N]MA incorporated in different perovskite crystals by measuring the effective ¹³C-¹⁵N dipolar interactions of [U-¹³C,U-¹⁵N]MA using ¹³C{¹⁵N}REDOR NMR. The presence of the reorientational motion of the ¹³C-¹⁵N spin vector of MA may result in a reduced REDOR dephasing value (ΔS/S₀) when compared to the ΔS/S₀ value recorded for the spin pair in the absence of motion. The faster the reorientational motion for the spin pair is, the more reduction of the REDOR dephasing value (ΔS/S₀) is obtained. For the [U-¹³C,U-¹⁵N]MAI powder sample, the ¹³C{¹⁵N}REDOR curve reached the maximum value around 1 since the reorientational motion of MA is absent. In contrast, the reduced REDOR ΔS/S₀ values were observed for the other three samples, including 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), and 3D [U-¹³C,U-¹⁵N]MAPbI₃. No organic spacer is present in 3D [U-¹³C,U-¹⁵N]MAPbI₃ such that we included the dynamics of MA in 3D [U-¹³C,U-¹⁵N]MAPbI₃ for comparison, as it is free from the influence of organic spacer cations. As shown in Figs. 2(c)-(f), we chose to compare the ¹³C{¹⁵N}REDOR dephasing values (ΔS/S₀) at 2.4 ms of the dephasing time of the four samples, denoted as (ΔS/S₀)_2.4ms, so that the xy-8 phasing cycling could be applied to the dephasing pulses. Given that 10 kHz of the MAS frequency was used in this series of REDOR experiments, a reduced ¹³C{¹⁵N} REDOR (ΔS/S₀)_2.4ms value indicates that the presence of the reorientational motion of the C-N vector partially averaged out the ¹³C-¹⁵N dipole-dipole interaction over the period of 100 µs. The degree of the reorientational motion of the C-N vectors can be reflected by the reduction of the ¹³C{¹⁵N} REDOR (ΔS/S₀)_2.4ms values. The minor MA peak of 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) was observed to be fully dephased at 2.4 ms of the dephasing time, indicating the absence of the reorientational motion of MA. In contrast, the (ΔS/S₀)_2.4ms values observed for 3D [U-¹³C,U-¹⁵N]MAPbI₃ and the major MA peak in 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) were less than 10%, indicating that MA molecules in these two samples underwent rapid reorientational motion. Interestingly, the (ΔS/S₀)_2.4ms value measured for the 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) sample was greater than the (ΔS/S₀)_2.4ms values for 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) by more than 10%. This result indicates that the MA in 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) underwent a more restricted reorientational motion than the MA in 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), suggesting that the local environments for MA in these two materials were quite different. Taking these results together, it shows that the choice of the incorporated spacer cation in 2D OIHP may affect the molecular motion of the A-site cation, even though there is no bonding between MA and the organic spacers.

To explore the spacer-dependent dynamics of the A-site cation, the REDOR (ΔS/S₀)_2.4ms values of ¹³C{¹⁵N}REDOR experiments were recorded at various temperatures. As plotted in Fig. 3, no significant changes of the REDOR (ΔS/S₀)_2.4ms values were measured for 3D [U-¹³C,U-¹⁵N]MAPbI₃ during the cooling process from 308 to 243 K, indicating that the rapid reorientational motion of the A-site MA did not change significantly in this temperature range. By contrast, there is an obvious change of the REDOR (ΔS/S₀)_2.4ms values of 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) which were found to increase from 0.06 at 308K, to 0.43 at 243K, implying that the rapid reorientational motion of MA was slowed down as the temperature decreased. For 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), the reorientational motion of MA in 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) changed only slightly during cooling, as seen from the slightly increased REDOR (ΔS/S₀)_2.4ms value. The above results suggest that the reorientational motion of the A-site cations due to cooling in 2D OIHP crystals is spacer-dependent. Therefore, we would like to investigate the response of the spacer cations by ssNMR to understand how the A-site cation’s dynamics of 2D OIHPs are affected by organic spacers during cooling.

It is well-known that the structural response or deformation of the octahedral layers, induced by changing the packing geometry of the organic spacers, may strongly affect the optical and electronic properties of 2D OIHPs as they are exposed to external stimuli, such as temperature^32,38,40. The structures of (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) and (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) at temperatures of 300 K and 250 K can be analysed by using their XRD patterns, shown in Figs. 4(a) and 4(b). There is a clear structural change for the (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) with cooling in the temperature range from 300K to 250 K, while there is no change in the XRD patterns for the (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2). Complementary to the structural lattice information obtained from XRD, the ssNMR measurements may further provide the local chemical environment information of the spacer cations of 2D OIHPs during the cooling process. Figures 4(c) and 4(d) exhibit the magnified ¹³C CPMAS NMR spectra, highlighting the resonance peaks of the spacer cations of (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) and 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), respectively. An obvious conformational change of the spacer cation BA in 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) is observed during the cooling process, where the ¹³C resonance peaks of both C3 and C4 of BA are split to two peaks as the temperature decreases in the range from 268 to 243K. The peak splitting shows the existence of two BA conformations in this temperature range. In contrast, neither peak splitting nor peak position changes were found in the ¹³C CPMAS NMR spectrum of 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) during the cooling process, indicating no change in the conformation and the chemical environment of PEA. The full-scaled ¹³C CPMAS NMR spectra which highlight the resonance peaks of the molecular cation MA, are also shown in Fig. S3. For 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), there is no obvious change of the peak position and intensity for the ¹³C resonance peak of MA. In contrast, the major peak of MA in 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) is observed to have a clear shift to up-field, and a significant increase in intensity, suggesting a change in the MA surrounding chemical environment with cooling from 298 to 268K. The result indicates that the organic spacers play an important role in determining the structural and the correlated physical properties. 2D OIHPs consist of long alkyl-chain-based BA cations which result in a more fluctuating structure at ambient conditions compared to the phenyl-group-based PEA ones, consistent with the recent theoretical results³⁷. The CH-π stacking among the aromatic-ring containing spacer of PEA cations may restrict the thermal motions between two inorganic perovskite layers. By contrast, there is no such interaction among the alkyl BA cations, which may result in more flexibility in these chain-like molecules and introduce more structural dynamics in the inorganic perovskite layers. Accordingly, the 2D OIHP crystal, consisting of PEA organic spacers, exhibits a relatively rigid structure with a lower conformational freedom of the crystal structure associated with closer packing, as compared to the BA counterpart. A similar result about the flexibility of the spacer cations BA and PEA in 2D OIHP crystals (n = 1) which consist of no A-site cations, was also observed as shown in Fig S5, indicating that such temperature-induced structural change of 2D OIHPs with cooling is mainly attributed to the influence of the organic spacers. Because 2D OIHPs consisting of BA spacers exhibit less structural rigidity, the structural deformation of the inorganic PbI₆ octahedral layers induced by changing the packing geometry of the organic spacers occurs with cooling, as seen from both the ssNMR and XRD data. In contrast, there is no such structural deformation of the inorganic octahedral layers in the 2D OIHPs consisting of PEA spacers with cooling in this temperature range. Consequently, the reorientational motion of A-site cations in BA-containing 2D OIHPs is more sensitive to the cooling process compared to PEA-containing 2D OIHPs. The interplay between the rigidity of the organic spacers and the A-site cations dynamics of 2D OIHPs can be unambiguously revealed in our studies.

The reorientational motion of the C-N bond of MA modulates the effective dipolar interaction between the ¹³C and ¹⁵N nuclei of [U-¹³C,U-¹⁵N]MA incorporated in 2D OIHP crystals with n ≥ 2. As a result, the information of the reorientational dynamics of MA can be obtained by using REDOR NMR, which measures the effective dipolar interactions between ¹³C and ¹⁵N of [U-¹³C,U-¹⁵N]MA. REDOR NMR analyses were performed on the four different samples, including [U-¹³C,U-¹⁵N]MAI powder, 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2), and 3D [U-¹³C,U-¹⁵N]MAPbI₃, to compare the reorientational dynamics of [U-¹³C,U-¹⁵N]MA incorporated in different materials. The reorientational motion of methylammonium is absent for the [U-¹³C,U-¹⁵N]MAI powder sample, evident from the consistency between the experimental REDOR curve and the simulated REDOR curve using the carbon-nitrogen bond length obtained in an X-ray crystallisation study. The A-site cation in 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) was found to undergo a more restricted reorientational motion at room temperature when compared to 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) and 3D [U-¹³C,U-¹⁵N]MAPbI₃. Moreover, an obvious change of the reorientational dynamics of in 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) occurred when the temperature was cooled down from 298 to 268K, evident from an obvious increase in the ¹³C{¹⁵N} REDOR (ΔS/S₀)_2.4ms value. It is worth mentioning that the significant change in the reorientational dynamics can also be correlated to the significant change of the chemical environment of MA, evident from the clear up-field shift and a significant increase in intensity of the ¹³C CPMAS resonance peak of MA during cooling from 298 to 268K. In contrast, only slight changes of MA reorientational dynamics were observed in 2D (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) and 3D [U-¹³C,U-¹⁵N]MAPbI₃ with cooling. Accordingly, the change of the dynamics of the A-site cations in response to temperature change depend on the choice of the spacer cations of 2D OIHPs.

Since the choice of the spacer cations in 2D OIHPs may affect the reorientational dynamics of the A-site cations, we used temperature dependent ¹³C CPMAS NMR and temperature dependent XRD to investigate the structural changes of the organic spacer cations and of the inorganic frameworks of 2D OIHP crystals with cooling, respectively. 2D OIHP containing BA exhibits less structural rigidity than 2D OIHP containing PEA. The structural deformation of the octahedral perovskite layers induced by the change of the conformation of the alkyl BA spacers in the BA-containing 2D OIHPs occurs with cooling. In contrast, there is no such structural change in both the framework of inorganic perovskites and the conformation of organic spacers in the PEA-containing 2D OIHPs, as evident from the ssNMR and XRD analyses. Consequently, the structural deformation of the inorganic octahedral layers induced by changing the packing geometry of the BA organic spacers occurs on cooling, resulting in the slowing down of the reorientational motion of the A-site cations. When the rigid spacer cation PEA was incorporated in 2D OIHP crystals, there is no such significant change in the conformation of the spacer cation and the inorganic frameworks during cooling. Consequently, the reorientational dynamics of the A-site cation were found to stay relatively unchanged in the PEA-containing 2D OIHPs. In conclusion, the ssNMR study provides information on the reorientational dynamics of the A-site cation in 2D OIHP crystals. The study further unveiled that the dynamics of the A-site cation in 2D OIHP is influenced by the choice of the organic spacers. The interplay between the rigidity of the organic spacers and the A-site cations dynamics of 2D OIHPs is clearly unveiled, even though there is no direct bonding between the A-site cations and the organic spacers. Our results may provide a deep insight in the future design of 2D OIHPs at a molecular level.

Synthesis of 2D organic-inorganic hybrid perovskite. The chemicals, including lead(II) oxide (PbO, ≥ 99.9%), 57% aqueous hydriodic acid (HI) in H₂O, 50% aqueous hypophosphorous acid (H₃PO₂) in H₂O, 99.5% 1-butylamine (BA), 99% 2-phenethylamine (PEA), and ≥ 99% natural abundance methylamine hydroiodide (MAI), were purchased from Sigma-Aldrich (St. Louis). The synthesis of ¹³C-methyl-¹⁵N-ammonium iodide, [U-¹³C,U-¹⁵N]MAI is described in the supporting information SM1. The 2D OIHP crystals were synthesised according to Chen et al.²⁷ Briefly, a mixed solution containing 10 ml of hydriodic acid, 57 wt.% in H₂O, and 1.7 ml of hypophosphorous acid solution, 50 wt.% in H₂O, was prepared to dissolve 10 mmol PbO at 80°C and stirred continuously at 600 rpm to obtain a yellow coloured PbI₂ solution. BAI was obtained by adding 10 mmol BA in 5mL HI solution incubated in an ice bath. The BAI solution was slowly added into the PbI₂ solution at 80°C to obtain orange-colour precipitates, and then the mixed solution was heated up to 100℃ to dissolve the reprecipitates. The solution was cooled down to room temperature to obtain orange flakes of 2D (BA)₂PbI₄ (n = 1) compound. For the synthesis of 2D (BA)₂(MA)Pb₂I₇ (n = 2), a stoichiometric quantity of the 5 mmol MAI was first dissolved into the PbI₂ solution to produce black precipitates of MAPbI₃, and then re-dissolved by heating up to 110°C to obtain a clear yellow solution. BAI solution was obtained by adding 7 mmol BA in 5mL HI solution incubated in an ice bath. The BAI solution was then added dropwise into the clear yellow solution at 110°C. Finally, the solution was cooled down to room temperature to obtain red flakes of 2D (BA)₂MAPb₂I₇ (n = 2) compound. Similar procedures were followed for preparing 2D (PEA)₂(MA)_n−1Pb_nI_3n+1 crystals with n = 1 and n = 2, except that the different molar ratios of the precursors were used. The molar ratios of the precursors, PbO:MAI:PEA, were 1.72:0:3.45 and 6:18:1, for preparing the crystals with n = 1 and n = 2, respectively. ¹³C-methyl-¹⁵N-ammonium iodide, abbreviated as [U-¹³C,U-¹⁵N]MAI, was used to replace the natural-abundance MA for the synthesis of 2D (BA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2) and (PEA)₂([U-¹³C,U-¹⁵N]MA)Pb₂I₇ (n = 2). The synthesis of [U-¹³C,U-¹⁵N]MAI is described in the supporting information.

Single Crystals Growth. A saturated 2D OIHP solution was prepared by dissolving 2D OIHP compounds in a mixed solution containing 10 ml of hydriodic acid, 57 wt.% in H₂O, and 1.7 ml of hypophosphorous acid solution, 50 wt.% in H₂O, under stirring at a constant temperature of 60°C in an oil bath. The well stabilised 2D OIHP saturated solution was allowed to evaporate at a constant temperature of 62°C for several days to obtain the high quality 2D OIHP crystals. The crystals were carefully stored in a glove box to avoid moisture.

Solid-state NMR experiments. REDOR experiments were carried out on a wide-bore 14.1-T Bruker AVIII spectrometer equipped with a 3.2-mm triple-resonance magic-angle-spinning (MAS) probe head. The Larmor frequencies for ¹H, ¹³C and ¹⁵N are 600.21 MHz, 150.92 MHz, and 60.81 MHz, respectively. The sample spinning rate was 10 kHz. The ¹³C polarization was built up by the cross-polarization (CP) scheme with a π/2 pulse of 5 µs, and a CP contact of 1.5 ms. The π pulse durations for ¹³C and ¹⁵N were 10 µs and 12 µs, respectively. During the REDOR sequence, the ¹H CW decoupling scheme with a 100-kHz rf field was adopted, while the ¹H TPPM decoupling with a 70-kHz rf field as used during signal acquisition.

Acknowledgments

We thank the Ministry of Science and Technology (MOST) and iMate Program Academia Sinica.

Author Contributions

T.Y.Y and C.W.C conceived the idea and supervised this work; C.C.L performed the synthesis of 2D OIHP crystals, XRD, PL, and the absorption experiments; C.Y.H and Y.C.W performed the synthesis of 3D MAPbI₃ crystals; T.Y.Y, S.J.H and P.H.W performed NMR experiments; T.P.C performed temperature-dependent XRD and DSC; V.M.G performed the synthesis of ¹³C-methyl-¹⁵N-ammonium iodide; T.Y.Y, C.W.C, P.T.C, R.S and C.C.L discussed and analysed the results; T.Y.Y, C.W.C and C.C.L wrote the manuscript together.

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Direct Investigation of the Reorientational Dynamics of A-site Cations in 2D Organic-Inorganic Hybrid Perovskite by Solid-State NMR

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