Cation dynamics by 1H and 13C MAS NMR in hybrid organic–inorganic (CH3CH2NH3)2CuCl4

To understand the dynamics of the cation in layered perovskite-type (CH3CH2NH3)2CuCl4, the temperature-dependent chemical shifts and spin–lattice relaxation times T1ρ in the rotating frame have been measured using 1H magic angle spinning nuclear magnetic resonance (MAS NMR) and 13C cross-polarization (CP)/MAS NMR techniques. Each proton and carbon in the (CH3CH2NH3)+ cation is distinguished in MAS NMR spectra. The Bloembergen–Purcell–Pound (BPP) curves for 1H T1ρ in CH3CH2 and NH3, and for the 13C T1ρ in CH3 and CH2 are revealed to have minima at low temperatures. This implies that the curves represent the CH3 and NH3+ rotational motions. The amplitude of the cationic motion is enhanced at the C-end, that is, the N-end of the organic cation is fixed to the inorganic layer through N–H⋯Cl hydrogen bonds, and T1ρ becomes short with larger-amplitude molecular motions.


I. Introduction
Metal-organic hybrids, which consist of organic and inorganic components, have recently attracted much attention because these materials have many possibilities for the tailoring of their functionalities and physical properties including optical, electrical and magnetic properties by adjusting the organic and/or metal building blocks. Hybrid metal-organic compounds based on the perovskite structures are of increasing interest due to their potential use for solar cells. 1,2 However, toxicity and chemical instability issues of halide perovskites still remain as the main drawbacks for use in solar cells. The crystalline structure of compounds of the type (C n H 2n+1 NH 3 ) 2 MCl 4 , where n ¼ 1, 2, 3 . and M represents divalent metals (M ¼ Cu, Cd, .), may be described as a sequence of alternating organic-inorganic layers. [3][4][5][6] Many compounds in this family have been extensively investigated and have demonstrated successive phase transitions. This family of materials crystallizes in the layered perovskite structure, which consists of innite, staggered layers of corner-sharing MCl 6 octahedra interleaved by alkylammonium cations. 7 Because of the layered character of their structure, these crystals become appropriate substances for investigations of two-dimensional electronic systems. The cavities between the octahedra are occupied by the ammonium heads of the organic cations, which, importantly, form strong N-H/Cl hydrogen bonds to any of the eight chloride ions. 8 Ethylammonium copper chloride (CH 3 CH 2 NH 3 ) 2 CuCl 4 is a layered perovskite-type compound that undergoes a complicated sequence of phase transitions. Differential scanning calorimetry (DSC) data indicates several phase transitions, at 236 K (¼T C4 ), 330 K (¼T C3 ), 357 K (¼T C2 ), and 371 K (¼T C1 ), as temperature increases. [9][10][11][12][13][14] The peaks at 236 K, 330 K, and 371 K are very weak and can perhaps correspond to second-order transformations. 13 The phase transitions in this crystal are mostly connected with changes in the arrangement of the alkylammonium chains. Fig. 1 shows the room-temperature orthorhombic crystal structure of (CH 3 CH 2 NH 3 ) 2 CuCl 4 . 8,15 The hybrids have the orthorhombic crystal structure with the space group Pbca, and the lattice constants are a ¼ 7.47Å, b ¼ 7.35Å, and c ¼ 21.18Å at room temperature. 16 The CuCl 6 octahedra are strongly distorted with elongated Cu-Cl bonds orthogonal to each other on adjacent octahedra. The CuCl 6 sheets are sandwiched between two layers of alkylammonium. The structure of the organic component consists of a double layer of alkylammonium ions with their charged ends, the nitrogen atoms, oriented to the nearest CuCl 6 plane. 4 The complete structure is constituted by corner-sharing CuCl 6 octahedra, forming the inorganic layers, and bilayers of organic cations attached to the octahedra by their NH 3 heads. 17,18 The structural geometry and molecular motions of the organic molecules within the layered hybrid structure is important for determining the inuence of temperature on the evolution of the structural phase transitions in the perovskite structure. Physical properties in particular depend on the characteristics of metallic anion and the organic cation.
In the present study, the crystal structure and thermal stability for (CH 3 CH 2 NH 3 ) 2 CuCl 4 was observed by means of conventional Xray, thermogravimetric analysis (TGA), and optical polarizing microscopy. In order to clarify the structural geometry and dynamics of the cation in the organic-inorganic (CH 3 CH 2 NH 3 ) 2 -CuCl 4 , we investigated the chemical shis and the spin-lattice relaxation time T 1r in the rotating frame using 1 H magic angle spinning nuclear magnetic resonance (MAS NMR) and 13 C crosspolarization (CP)/MAS NMR. The CH 3 CH 2 and NH 4 groups of the CH 3 CH 2 NH 3 cation are distinguishable in 1 H MAS NMR spectra, and the CH 3 and CH 2 groups are distinguished by 13 C CP/ MAS NMR spectra. We investigated the 1 H and 13 C dynamics in the (CH 3 CH 2 NH 3 ) + cation near the phase-transition temperatures.

II. Experimental method
Crystals of (CH 3 CH 2 NH 3 ) 2 CuCl 4 were obtained by slow evaporation at 25 C from an aqueous solution of C 2 H 5 NH 2 $HCl and CuCl 2 $2H 2 O in the stoichiometric 2 : 1 proportion. The obtained crystals were yellow square plates, typically 5 mm Â 5 mm in area and 0.5 mm in thickness.
The structure of the (CH 3 CH 2 NH 3 ) 2 CuCl 4 crystals was determined at room temperature with an X-ray diffraction system (PANalytical, X'pert pro MPD) with a Cu-Ka (l ¼ 1.5418) radiation source. Measurements were taken in a q-2q geometry from 10 to 60 at 45 kV and with a tube power of 40 mA. And, the TGA curve at a heating rate of 10 C min À1 was measured under N 2 atmosphere, and the mass of the powdered sample used in the TGA experiment was 11.41 mg.
The chemical shis and the T 1r values for (CH 3 CH 2 NH 3 ) 2 -CuCl 4 were obtained by 1 H MAS NMR and 13 C CP/MAS NMR at Larmor frequencies of u 0 /2p ¼ 400.13 and 100.61 MHz, respectively, using Bruker 400 MHz NMR spectrometers at the Korea Basic Science Institute, Western Seoul Center. Crystalline powdered samples were placed within a 4 mm CP/MAS probe, and the MAS rate for 1 H and 13 C measurements, to minimize spinning sideband overlap, was set to 10 kHz. The 1 H T 1r values were determined using a p/2Àt sequence by varying the duration of spin-locking pulses. 13 C T 1r values were measured by varying the duration of the spin-locking pulse applied aer the CP preparation period. The width of the p/2 pulse used for measuring T 1r for 1 H and 13 C was 3.7 ms, with the spin-locking eld at 67.56 kHz. The chemical shis and T 1r were measured over a temperature range of 180-430 K.

III. Experimental results
The measured structure at room temperature exhibited orthorhombic symmetry with cell parameters of a ¼ 7.
This result is consistent with the results reported by Steadman and Willett. 16 The TGA curve of (CH 3 CH 2 NH 3 ) 2 CuCl 4 is shown in Fig. 2 for measuring thermal stability. The rst occurrence of mass loss begins at approximately 430 K (T d ), which is the onset of partial thermal decomposition. The second weight loss of 25.1% near 530 K is due to the removal of the CH 3 CH 2 NH 3 Cl from the compound, leaving intermediate CH 3 CH 2 NH 3 CuCl 3 that belongs to another known class of compounds ABX 3 . Near 560 K, CuCl 2 remains as the residue and when it reaches 580 K, the total weight loss becomes 65.55%. The color of the crystal is dark yellow at room temperature although it has slightly inhomogeneous hue due to surface roughness. As the temperature increases, the color of the crystal varies from dark yellow (300 K, 350 K), brown (400 K), to dark brown (450 K, 500 K), and then they start melting at 530 K as shown in the inset in Fig. 2. The TGA and optical polarizing microscopy results show that the crystal above 430 K allows CH 3 to partially escape by the breaking the weak C-N bond.
The 1 H NMR spectra at a frequency of 400.13 MHz were obtained by MAS NMR. The 1 H spectrum recorded at room  This journal is © The Royal Society of Chemistry 2018 temperature is shown in the inset in Fig. 3; the spectrum shows two peaks at chemical shis of d ¼ 0.23 and 12.12 ppm, which are assigned to the protons of the CH 3 CH 2 and NH 3 groups, respectively. The spinning sidebands for CH 3 CH 2 are marked with asterisks and those for NH 3 are marked with open circles. However, the different 1 H signals from CH 3 and CH 2 cannot be resolved, and therefore the combined CH 3 CH 2 peak is very broad and has a larger intensity due to the overlap of the CH 3 and CH 2 peaks. The peak with the lower chemical shi is attributed to the protons in CH 3 CH 2 , and that of the higher chemical shi is attributed to the protons in NH 3 . The 1 H chemical shis for the alkyl and ammonium groups slowly and monotonously vary with temperature, indicating that the surrounding environments of the protons in the alkyl and ammonium groups change continuously, as shown in Fig. 3; here, the chemical shis for protons in CH 3 CH 2 and NH 3 near T C1 , T C2 , and T C3 are nearly constant with temperature, whereas those for protons in CH 3 CH 2 and NH 3 below T C4 change more abruptly.
The T 1r values for the CH 3 CH 2 and NH 3 protons in (CH 3 -CH 2 NH 3 ) 2 CuCl 4 were obtained as a function of temperature. The magnetization traces of both the alkyl and ammonium protons may be described by a single exponential function [19][20][21] where S(t) is the magnetization as a function of the spin-locking pulse duration t, and S 0 is the total nuclear magnetization of the proton at thermal equilibrium. 19 The recovery curves for several delay times were measured, and the T 1r values were obtained from the slopes by the delay time vs. intensity, at several different temperatures. This analysis method was used to obtain the T 1r values for each proton in CH 3 CH 2 and NH 3 which are plotted as a function of inverse temperature in Fig. 4 Here, g H and g C are the gyromagnetic ratios for the 1 H and 13 C nuclei, respectively; N is the number of directly bound protons; r H-C is the H-C internuclear distance; ħ is the reduced Planck constant; u H and u C are the Larmor frequencies of 1 H and 13 C, respectively; and u 1 is the frequency of the spin-locking eld. We analyzed our data assuming that T 1r would show a minimum when u 1 s C ¼ 1, and that the BPP relation between T 1r and the characteristic frequency u 1 could be applied. We sensitively controlled the minima in the T 1r temperature variations and the slopes around the minima. From these results, the value of (g H g C ħ/r H-C 3 ) 2 for the proton in eqn (2) was obtained. We then calculated the temperature dependences of the s C values for protons by using the obtained values of (g H g C ħ/r H-C 3 ) 2 . The temperature dependence of s C follows a simple Arrhenius equation:  where s 0 is a pre-exponential factor, T is the temperature, R is the gas constant, and E a is the activation energy. Thus, the slope of the linear portion of a semi-log plot should yield E a . The E a value for the rotational motion can be obtained from the log s C vs. 1000/T curve shown in Fig. 5; we obtained E a ¼ 12.19 AE 1.30 kJ mol À1 and E a ¼ 8.33 AE 0.50 kJ mol À1 for CH 3 CH 2 and NH 3 , respectively. The rotational motion for alkyl groups is activated, whereas the rotational motion for ammonium groups at the end of the organic cation is less strongly activated. The structural analysis of the carbons in (CH 3 CH 2 NH 3 ) 2 CuCl 4 was performed by 13 C CP/MAS NMR, and the corresponding spectrum is shown in Fig. 6, as a function of temperature; the 13 C CP/MAS NMR spectrum at room temperature shows two signals at chemical shis of d ¼ 50.77 ppm and d ¼ 113.50 ppm with respect to tetramethysilane (TMS), which can be assigned to CH 3 and CH 2 , respectively. The 13 C chemical shi of CH 2 abruptly shis with temperature, whereas that of CH 3 changes only much less with temperature. The full width at half maximum (FWHM) linewidths for the 13 C of CH 3 and CH 2 in Fig. 7 showed a monotonic decrease with increasing temperature, with no particular anomalies attributable to the phase transitions. The linewidth of the 13 C signal assigned to CH 3 is broad compared to that of CH 2 , and the linewidth narrows signicantly with increasing temperature. This narrowing of the 13 C linewidths is attributed to internal motions that the line widths follow the same temperature dependence as some internal motions, hence the motions are responsible for the line widths.
To obtain the 13 C T 1r values, the nuclear magnetization was also measured at several temperatures as a function of delay time. The signal intensity of the nuclear magnetization recovery curves for 13 C is described by a single exponential function as in eqn (1) at all temperatures. The 13 C T 1r values for CH 3 and CH 2 in (CH 3 -CH 2 NH 3 ) 2 CuCl 4 are plotted as a function of inverse temperature in Fig. 8. The temperature dependences of the 13 C MAS NMR T 1r values seem to be similar. The T 1r values for CH 3     increase with temperature in the same manner; whereas, the 13 C T 1r values near the phase-transition temperatures are approximately continuous. The T 1r values for CH 3 and CH 2 at room temperature are 33.85 ms and 109.40 ms, respectively. The amplitude of the cationic motion is enhanced at its CH 3 end, and the central CH 2 moiety is xed to the NH 3 group in the organic cation. The T 1r curve below T C4 can be reproduced by BPP theory. The BPP curves for CH 3 and CH 2 , showing minima at low temperatures, is almost the same as those of the CH 3 CH 2 and NH 3 shis of the 1 H MAS NMR measurements. E a for the rotational motion of CH 3 and CH 2 can be obtained from the log s C vs. 1000/T curve shown in Fig. 5

IV. Conclusion
We discuss the molecular motions for cation of Cu-based hybrid materials, where we replace Pb with nontoxic Cu metal for leadfree perovskite solar cells, and investigate their potential toward solar cell applications based on ionic dynamics of the cation in hybrid organic-inorganic (CH 3 CH 2 NH 3 ) 2 CuCl 4 by NMR studies. The cation dynamics and interionic interactions through hydrogen bonds are expected to be closely related with the physical properties due to the potential applications. The cation dynamics in a layered perovskite-type (CH 3 CH 2 NH 3 ) 2 CuCl 4 were investigated as a function of temperature by 1 H MAS NMR and 13 C CP/MAS NMR experiments. The CH 3 CH 2 and NH 4 units in the CH 3 CH 2 NH 3 cation were distinguished by the 1 H MAS NMR spectra, and the CH 3 and CH 2 units in the CH 3 CH 2 NH 3 cation were also clearly distinguished in the 13 C CP/MAS NMR spectra.
To obtain detailed information about the cation dynamics of this crystal, the spin-lattice relaxation time T 1r in the rotating frame for both 1 H and 13 C were measured, revealing that these atoms undergo rotational motions at low temperatures. The BPP curves for the 1 H T 1r in CH 3 CH 2 and NH 3 , and for the 13 C T 1r in CH 3 and CH 2 , were shown to have a minimum at low temperatures; the T 1r of 1 H and 13 C showed a minimum and is governed by the tumbling motion of the CH 3 CH 2 and NH 3 groups, indicating that the 1 H and 13 C atoms in the CH 3 CH 2 -NH 3 + groups exhibit high mobility at low temperatures. The molecular motions for 1 H and 13 C in the CH 3 CH 2 NH 3 + cation were very free at low temperatures. T 1r provides insight into the changes in the cation reorientation rates at low temperature. The 13 C T 1r values in CH 3 increased with temperature, a trend that has been observed in alkyl chains attached to the (CH 3 -CH 2 NH 3 ) cation due to its greater mobility toward its free end. The CH 3 CH 2 NH 3 cationic motion is enhanced at the opposing end of the cation to the NH 4 + group probably because this group is bound to the inorganic layer through the N-H/Cl hydrogen bonds. The 13 C T 1r is usually dominated by the uctuation of the anisotropic chemical shi, and it becomes shorter with largeramplitude molecular motions. This implies that the amplitude of the cationic motion is enhanced at the C-end, that is, the Nend of the organic cation is xed at the inorganic layer through N-H/Cl hydrogen bonds. The cationic motion, being associated with the uctuation of the molecular axis, is expected to be gradually excited with increasing temperature.

Conflicts of interest
There are no conicts to declare.