Melting of an Anchored Bilayer: Phase Transitions in the Organic

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Melting of an Anchored Bilayer: Phase Transitions in the Organic-Inorganic Hybrid Pervoskite (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) S. Barman† and S. Vasudevan*,†,‡ Solid State and Structural Chemistry Unit, and Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-566012, India ReceiVed: April 27, 2009; ReVised Manuscript ReceiVed: June 19, 2009

The conformation, organization, and phase transitions of alkyl chains in organic-inorganic hybrids based on the double pervoskite-slab lead iodides, (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) have been investigated by X-ray diffraction, calorimetry, and infrared vibrational spectroscopy. In these hybrid solids, double pervoskite (CH3NH3)Pb2I7 slabs are interleaved with alkyl ammonium chains with the anchored alkyl chains arranged as tilted bilayers and adopting a planar all-trans conformation at room temperature. The (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 compounds exhibit a single reversible phase transition above room temperature with the associated enthalpy change varying linearly with alkyl chain length. This transition corresponds to the melting in two-dimensions of the alkyl chains of the anchored bilayer and is characterized by increased conformational disorder of the methylene units of the chain and loss of tilt angle coherence leading to an increase in the interslab spacing. By monitoring features in the infrared spectra that are characteristic of the global conformation of the alkyl chains, a quantitative relation between conformational disorder and melting of the anchored bilayer is established. It is found that, irrespective of the alkyl chain length, melting occurs when at least 60% of the chains in the anchored bilayer of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 have one or more gauche defects. This concentration is determined by the underlying lattice to which the alkyl chains are anchored. Introduction Layered organic-inorganic compounds formed by interleaving long-chain alkyl groups between inorganic sheets and held in place by either ionic or hydrogen bonding have been widely studied.1 Interest in these compounds may be attributed to the fact that they can in principle combine properties of both the inorganic and organic parts in a single system. In particular, molecular hybrids based on Pb(II) or Sn(II) iodide pervoskite layers have attracted widespread attention.2-6 These materials have been regarded as natural semiconductor/insulator quantum well structures exhibiting strong excitonic absorption and emission at room temperature. The Pb(II) and Sn(II) hybrids have the advantage that, depending on the preparative conditions, they can crystallize in different phases of the RuddelsonPopper series,7,8 ((CH3NH3)m-1(CH3(CH2)nNH3)2MmI3m+1: M ) Pb, Sn; m ) 1, 2, ..., ∝.These structures consist of pervoskite slabs of varying thickness interleaved with alkyl ammonium bilayers. The two extreme members m ) 1 and m ) ∝ are the K2NiF4 and cubic pervoskite structures, respectively. The fact that physical and structural properties may be tuned either by changing the alkyl chain length or by modulating the thickness of the inorganic slabs makes these an exciting class of compounds. Alkyl chains in long chain alkyl ammonium layered perovskites are usually arranged as bilayers9-11 and hence share similarity with phospholipid bilayers that are known to undergo structural phase transitions with temperature. However, unlike lipid bilayers where individual molecules can undergo lateral * To whom correspondence should be addressed. E-mail: svipc@ ipc.iisc.ernet.in. † Solid State and Structural Chemistry Unit. ‡ Department of Inorganic and Physical Chemistry.

diffusion and also flip-flop between layers, the alkyl chain bilayers in these compounds are anchored firmly to the inorganic sheet and therefore are characterized by the total absence of translational mobility. The degrees of freedom of the alkyl chains of the anchored bilayer in these hybrid materials are restricted to changes in conformation. A closely related and extensively studied system is the self-assembled monolayers on even surfaces.12-16 Long chain alkyl ammonium layered pervoskites are known to exhibit one or more solid-solid phase transitions with temperature.17-23 The main transition is associated with an increase in conformational disorder of the alkyl chains and in analogy with similar behavior in lipid bilayers may be classified as melting of the anchored bilayer in twodimensions. The inorganic moiety however shows no change in structure in this temperature range, and the alkyl chains remain tethered to the sheets even above the transition. The single layer CH3(CH2)nNH3)2PbI4, for example, exhibits two phase transitions above room temperature. The first is associated with the dynamic rotational disordering of the ammonium headgroup while the second with the melting of the alkyl chains.11 In this system, as in lipid bilayers and similar alkyl chain assemblies, it is the accumulation of gauche defects with temperature that drives the melting transition. Quantitative information on the relation between the extent of conformational disorder and melting in these systems are difficult to establish. This in part is because the lipid bilayers are essentially fluidlike systems making experimental investigations more demanding. On the other hand, the anchored bilayer in these organic-inorganic hybrids is in principle more amenable to study by solidstate spectroscopic techniques that would allow a more quantitative relation between the extent of disorder and the melting phenomena to be established.

10.1021/jp903840j CCC: $40.75  2009 American Chemical Society Published on Web 08/10/2009

Phase Transitions in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 In this study, we report a detailed investigation of the thermal behavior of the alkyl chains in the anchored bilayer in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17). These compounds exhibit a single phase transition above room temperature. It is shown that this transition is associated with a two-dimensional melting of the alkyl chains of the bilayer. X-ray diffraction and calorimetric measurements have been used to understand the effect of alkyl chain length on the transition, and infrared spectroscopy has been used to monitor changes in conformation. We make use of the progression bands in the infrared spectrum that arise from a coupling of vibrational modes of methylene units in trans registry to monitor the conformation of the alkyl chains.10 These bands are characteristic of the global conformation of the hydrocarbon chain and can provide a quantitative measure of the concentration of all-trans planar chains in the ensemble and its variation as the temperature approaches that of the melt.24 We show that there is a critical concentration of conformational disorder at which melting occurs that is the same irrespective of the alkyl chain length in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7. These results highlight the universal nature of melting in this class of materials.

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Figure 1. (a) Powder X-ray diffraction patterns of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 for different values of n and (b) interlayer spacing as a function of the alkyl chain length, n.

Experimental Section The double-layer pervoskites (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) were prepared by evaporation of a solution containing CH3NH3I and CH3(CH2)nNH3I with PbI2 in tetrahydrofuran. Red colored crystallites of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 were formed during evaporation. The compounds were washed with distilled water until the filtrate showed complete absence of acid. The overall reaction may be written as

2PbI2 + CH3NH3I + 2CH3(CH2)nNH3I f (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 The thermal behavior of these compounds was investigated using a Perkin-Elmer DSC 2C differential scanning calorimetry operated at a scanning rate of 5 K min-1 under N2 atmosphere. The temperature scale and enthalpy were calibrated using an indium standard (Tm ) 429 K and ∆H ) 28.5 J g-1). Powder X-ray diffraction patterns as a function of temperature were recorded using Cu KR radiation on a Shimadzu XD-D1 diffractometer operated in the θ-2θ Bragg-Brentano geometry. Samples were mounted on an aluminum block that could be heated in a controlled manner. Variable temperature infrared spectra were recorded on a Perkin-Elmer spectrum 2000 FT-IR spectrometer in the diffuse reflectance mode using a DRIFT (P/N 19900 series) accessory with a cooled MCT detector. Results and Discussion X-ray Diffraction. The X-ray diffraction patterns of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 9, 11, 13, 15, 17) are shown in Figure 1a. The compounds show a well-developed progression of intense 00l Bragg reflections that may be indexed to a unique interlayer spacing (see also the Supporting Information). The 00l reflections may be counted as corresponding to l ) 1, 2, 3, ... or l ) 2, 4, 6, .... We have chosen the latter based on the structure of the single layer (CH3(CH2)nNH3)2PbI4 pervoskites.25 The X-ray patterns indicate the layer nature of these compounds; only 00l reflections are observed indicating an extremely high degree of preferred orientation arising as a consequence of their layered morphology. The plot of interlayer spacing versus

Figure 2. Schematic structure of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 where Pb2I73- layers are separated by an all-trans alkyl ammonium chain bilayer. The CH3NH3 sits in the octahedral void in the Pb2I7 sheet. Either of the arrangements shown above is consistent with the observed X-ray lattice spacing. The unit cell is indicated.

number of methylene carbons in the alkyl chain shows a linear increase with slope 1.2 (Figure 1b). On the basis of the reported structures of the corresponding single-layer pervoskites,9,25,26 (CH3(CH2)nNH3)2PbI4 (n ) 9, 11, 13, 15, 17), it is reasonable to conclude that the double-layer (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 compounds too have a layer structure with the (CH3NH3)Pb2I7 sheets separated by alkyl chain bilayers. For an all-trans alkyl chain, the addition of a methylene unit to the chain increments its length by 1.25 Å. If the alkyl chains in these layered pervoskites are in an all-trans conformation, then the slope of Figure 1b indicates that the chains are arranged as tilted bilayers with a tilt angle, the angle between the interlayer normal and the molecular axis of the alkyl chains, of 63° as shown schematically in Figure 2. In principle, the observed lattice spacing of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 may also be accounted for by an interdigitated bilayer, but such

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Figure 3. (a) DSC traces of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 for different values of n and (b) the enthalpy change ∆H vs n.

TABLE 1: Summary of the Transition Enthalpies and Entropies Accompanying the Melting of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 n

Tm (K)

∆H (kJ mol-1)

∆ S (J K-1mol-1)/CH2

11 13 15 17

356 362 369 377

41.4 50.9 59.8 75.5

10.5 10.8 10.8 11.7

an arrangement may be ruled out, since it would lead to an unphysically large density of alkyl chains in the interslab region. The observed tilt angle is also in agreement with the value calculated from the ratio of the cross-sectional area of an alltrans alkyl chain, Ac, to the available surface area per anchored surfactant chain, As. The tilt angle so determined, cos-1(Ac/ As), has a value of 65°. This value of the tilt angle would satisfy the criteria for formation of a tilted bilayer arrived at from theoretical considerations and would exhibit a reversible phase transition.27 Phase Transitions. Differential Scanning Calorimetry Measurements. The differential scanning calorimetric traces of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) are shown in Figure 3a. All four compounds exhibit a single first-order endothermic transition on heating while on cooling the corresponding exotherm is observed but at a lower temperature. The DSC data are summarized in Table 1. In analogy with phase transitions in lipids28 and transitions in the single-layer perovskites,11,21 this transition is assigned to a melting of the anchored bilayer. The endothermic transition temperature increases with increasing carbon chain length and so does the enthalpy change, ∆H, which shows a linear variation with n (Figure 3b). The ∆H values for the melting transition are comparable with the values reported for similar organic-inorganic hybrid layered systems.11,25,26 The entropy change per CH2 unit in the chain, as calculated from the DSC data, is nearly constant for the four compounds and has a value of ∼11 J K-1 mol-1 (Table 1). X-ray Diffraction. The thermal variation of the interlayer spacing of (CH3(CH2)nNH3)2Pb2I7 compounds was monitored by variable temperature powder X-ray diffraction measurements. The powder X-ray diffraction patterns for a typical compound,

(CH3NH3)(CH3(CH2)nNH3)2Pb2I7, n ) 15 at temperatures between 300 and 390 K in intervals of 10 K, are shown in Figure 4a. It may be seen that, at the temperature corresponding to the endotherm in the DSC (Figure 3), the diffraction peaks are shifted to lower Bragg angles corresponding to an expansion of the lattice along the c-axis, increased interslab spacing. The change in the crystallographic C parameter (taken as twice the inter-inorganic slab spacing) as a function of temperature is displayed for the four compounds (both heating and cooling runs) in the four panels of Figure 4b. Vibrational Spectroscopy. Vibrational spectroscopy has been extensively used for probing conformation of alkyl chain assemblies.29 A typical room temperature spectrum of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 is shown in Figure 5 (see also the Supporting Information). The intense bands that appear between 2800 and 3000 cm-1 are the C-H stretching modes of the methyl and methylene groups of the alkyl chains. The bands at 2848 and 2916 cm-1 are assigned to the symmetric (νs(CH2), d+) and antisymmetric (νas(CH2), d-) stretching modes of the methylene groups while the peak at 2954 cm-1 is assigned to the antisymmetric (νas(CH3)) stretching vibration of the terminal methyl group. The methyl symmetric stretching mode expected at 2871 cm-1 is not seen. It is well-known that the methylene stretching mode frequencies are sensitive to the conformation of the alkyl chain shifting to higher frequencies with increased conformational disorder.30,31 For an all-trans alkyl chain, as in the case of crystalline n-alkanes, the symmetric and antisymmetric stretching modes of the methylene (CH2)n groups appear in the range 2846-2849 cm-1 and 2916-2918 cm-1, respectively. With an increasing number of gauche conformers, as in the high temperature disordered liquid phases of n-alkanes, the position of these peaks shifts to higher wavenumbers, typically 2856-2858 cm-1 for the symmetric stretching mode and 2924-2928 cm-1 for the antisymmetric stretching mode. The observed peak positions of the symmetric (2848 cm-1) and antisymmetric (2916 cm-1) C-H stretching modes of the methylene groups in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) indicate that, at room temperature, the alkyl chains are essentially in an all-trans conformation. The peak at 720 cm-1 is assigned to the CH2 rocking vibration, and the fact that

Phase Transitions in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7


Figure 4. (a) X-ray diffraction patterns of (CH3NH3)(CH3(CH2)15 NH3)2Pb2I7 at different temperatures between 300 and 390 K at intervals of 10 K. The XRD patterns above the transition are shown in red. (b) The interlayer spacing of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 at different temperatures for n ) 11, 13, 15, 17.

Figure 5. Infrared spectrum of (CH3NH3)(CH3(CH2)15NH3)2Pb2I7.

this band as well as the CH2 scissoring mode at 1467 cm-1 appear as singlets implies hexagonal subcell packing.32 These bands are known to split into two distinct components as a result of crystal or factor-group splitting in orthorhombic or monoclinic packing.33 Increase in temperature induces changes in alkyl chain conformation. The methylene stretching modes that are sensitive to chain conformation shift gradually with temperature to higher frequencies. The infrared spectra of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 n ) 15 at different temperatures are shown in Figure 6a. There is an abrupt shift of the methylene C-H stretching frequencies, both νs and νas, to higher wavenumbers at the temperature corresponding to the endotherm in the DSC. The

νs and νas frequencies of all four compounds have been plotted as a function of the reduced temperature, T/Tm, in Figure 6b (Tm is the temperature of the melting endotherm in the DSC). It may be seen that the transition (T/Tm ) 1) is accompanied by a jump in the C-H stretching frequencies. The values of the symmetric and antisymmetric methylene stretching modes frequencies after the transition, 2854 and 2925 cm-1, respectively, are typical of alkyl chains having a large concentration of gauche disorder like in the alkane melts.30,31 The methylene stretching modes in the infrared spectra of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 indicate that the first-order transition is characterized by an increased conformational disorder of the alkyl chains. This behavior is similar to that observed in a number of bilayer assemblies on melting and suggests that the endothermic transition in these compounds corresponds to a melting of the anchored alkyl chain bilayer. It is possible to identify specific conformational sequences of the alkyl chain that contain gauche bonds from their characteristic signature in the methylene wagging region of the infrared spectrum.34 Thus, a peak at 1341 cm-1 indicates a penultimate bond oriented such that the methylene group is in gauche conformation relative to the methylene group three carbons away. A peak at 1354 cm-1 is characteristic of two adjacent gauche bonds (double gauche), and a bond at 1364 cm-1 arises from a kink (successive gauche-trans-gauche conformers in the chain). These are localized modes, and hence, the area under the peak is proportional to the concentration of the specific bond sequences. The methyl umbrella deformation mode that appear at 1375 cm-1 is taken as an internal standard for normalization because its position is unaffected by the conformation of the rest of the chain. For the n ) 15 compound, this region of the normalized infrared spectrum at different temperatures is shown in Figure 7. At 300 K, the defect peak intensities are insignificant, but as the temperature is raised, peaks at 1341 cm-1 that correspond to end-gauche defects and at 1364 cm-1 corresponding to kink defects appear. The intensity of these features grows with increasing temperature. Double

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Figure 6. (a) Infrared spectra of (CH3NH3)(CH3(CH2)15NH3)2Pb2I7 in the C-H stretching region at different temperatures between 290 and 380 K in an interval of 5 K. Spectra recorded above the Tm are shown in red. (b) The methylene C-H symmetric and antisymmetric stretching frequencies plotted against reduced temperature (T/Tm).

Infrared Progression Bands. Vibrational spectra of long alkyl chain molecules have been interpreted on the basis of vibrational modes of an infinite polymethylene chain.35,36 Conformational order causes a coupling of the vibrational modes of methylene units in trans registry giving rise to a series of progression bands in the infrared. These modes are delocalized over the length of the all-trans segment, and their spacing and position depend on the number of coupled trans-CH2 units and hence can provide a quantitative measure of chain conformation. Vibrational modes in an all-trans methylene chain are described through a coupled oscillator model for which the eigenvalues are given by37

4π2ν2 ) H0 + 2

∑ Hm(cos mφk)

where H0 and Hm are the matrix elements of the secular determinant. The φ’s are the phase difference between adjacent oscillators as given by Figure 7. Infrared spectrum in the localized wagging defect mode region of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 n ) 15 at different temperatures between 290 and 380 K at 5 K intervals. Spectra above the Tm are shown in red.

gauche defects, which would normally manifest at 1354 cm-1, are however not seen. The nature of the observed defects (kinks and end-gauche) are typical for a system of densely packed alkyl chains, the dense packing preventing those defects that require large volumes such as double gauche from making an appearance. Although the methylene stretching modes are sensitive to chain conformation, it is not possible to obtain quantitative information on the extent of conformational disorder from the position of these modes. This information may however be obtained from an analysis of the progression bands in the infrared spectrum described in the following subsequent section.

φk ) kπ/(N + 1), (k ) 1, 2, ..., N) where N is the number of coupled oscillators or as in the present, coupled trans-CH2 units. For an infinite polymethylene chain, only vibrational modes at φ ) 0 or π are infrared and/or Raman allowed. For a finite chain, however, in addition to the φ ) 0 or π modes, a series of bands, namely progression bands, appear in the vibrational spectrum. The progression bands appearing in the spectrum are analyzed by assigning a k value after identifying the particular mode to which it belongs. When correctly assigned, a smooth curve results from a plot of νk versus φk. The progression bands arising from the coupling of CH2 wagging (ν3), twisting-rocking (ν7), rocking-twisting (ν8), and C-C skeletal stretch (ν4) modes have been studied in detail for n-alkanes,38 n-alcohols,39 fatty acids,40 soaps,24 and surfactants intercalated in layered solids.41,42



Figure 9. Dispersion of the methylene wagging and rocking-twisting band progressions. The symbols are experimental band positions, and the solid curve is the calculated dispersion curve for an infinite polymethylene chain.35,36 Figure 8. Infrared spectra of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 for n ) 11, 13, 15, 17 showing methylene (a) wagging (Wk) and twisting (Tk) and (b) rocking-twisting (Rk) progression bands.

The progression bands in the infrared spectrum of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) are shown in Figure 8a,b. The series of bands that appear in the 1400-1150 cm-1 region are the wagging (ν3,Wk) progression modes (Figure 8a). The twisting-rocking (ν7,Tk) progression modes appear as weak shoulders to the wagging modes and are not clearly seen for all compounds. The CH2 rocking (ν8, Rk) modes appear between 1000 and 700 cm-1 (Figure 8b). It may be seen from Figure 8 a,b that, as the alkyl chain length increases, the number of progression bands increases with a corresponding decrease in the interband separation. For all four compounds k values could be assigned to the bands in the progression series assuming that

all n-methylene units in the alkyl chains of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 are in trans registry. The positions and assignments of the progression bands are provided in Table 2 and also indicated in Figure 8. The observed band positions have been plotted as a function of the phase difference, φk, in Figure 9. It may be seen that, for a particular mode, all points lie on an identical curve irrespective of the chain length. In fact, the experimental dispersion lies on the calculated dispersion curve for an infinite polymethylene chain.35,36 The fact that all experimental points lie on a smooth dispersion curve validates the assignment of k values in Figure 8 that, in turn, justifies the assumption that, in the (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 pervoskites, all methylene units irrespective of chain length are in trans registry. These results unequivocally establish that, in the (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 compounds, the alkyl chains adopt an all-trans planar conformation at room temperature and

TABLE 2: Observed Peak Positions of Methylene Wagging and Rocking-Twisting Band Progressions in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 n ) 17

n ) 15

n ) 13

n ) 11

ν (cm-1)

assign

ν (cm-1)

assign

ν (cm-1)

assign

ν (cm-1)

assign

1326 1313 1305 1288 1284 1272 1251 1235 1214 1192 945 881 850 820 791 768 742 720

W9 W8 W7 W6 T7 W5 W4 W3 W2 W1 R4 R6 R7 R8 R9 R10 R11 R12-R17

1325 1315 1303 1283 1263 1248 1244 1226 1220 1195 927 895 858 823 793 767 720

W8 W7 W6 W5 W4 T4 W3 T3 W2 W1 R4 R5 R6 R7 R8 R9 R10-R15

1324 1314 1297 1278 1260 1251 1234 1227 1210 1200 889 867 826 791 764 720

W7 W6 W5 W4 T4 W3 T3 W2 T2 W1 R4 R5 R6 R7 R8 R9-R13

1326 1313 1293 1277 1266 1248 1238 1206 1185 924 877 835 793 764 741 720

W6 W5 W4 T4 W3 T3 W2 W1 T1 R3 R4 R5 R6 R7 R8 R10, R11

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Figure 10. Infrared spectra in the (a) wagging and (b) rocking progression band regions for (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 n ) 15 recorded at different temperatures between 300 and 375 K in 5 K intervals. Spectra above the Tm are shown in red.

that the progression bands in Figure 8 provide a characteristic signature of alkyl chains having all n-CH2 units in trans registry. Increase in temperature induces conformational disorder in the methylene chains. Chains having one or more gauche defects will no longer contribute to the intensity of the progression bands that are associated with the all-trans chains. The intensities of these bands are therefore directly proportional to the concentration of all-trans chains. If gauche defects appear at different locations in different chains, the length of the all-trans segments in each chain would be different. The intensities of the progression bands associated with these all-trans segments would be too small to be observed, and no additional progression series would be seen with increase in temperature. The infrared spectra at different temperatures in the wagging (1400-1100 cm-1) and rocking (1050-750 cm-1) band progression series of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 n ) 15 are shown in Figure 10. It may be seen that, with increase in temperature, there is decrease in the intensity of the progressions but with no additional feature appearing. The endothermic melting transition is characterized by the abrupt disappearance of progression band intensity. The melt is therefore characterized by the total absence of all-trans chains. The progression band intensities of Figure 10 can provide a quantitative measure of methylene chain conformation, since only all-trans alkyl methylene chains contribute to the progression band intensities. The intensity of the progression bands is therfore directly proportional to the concentration of all-trans chains present in the ensemble, and the ratio of the integrated intensities of the progression bands at any two temperatures would be a direct measure of the concentration of all-trans chains at the two temperatures.24 In (CH3NH3)(CH3(CH2)nNH3)2Pb2I7, the intensity of the gauche defect bands in the infrared spectrum at 300 K are negligible (Figure 7) implying that all alkyl chains are ordered in an all-trans conformation.


Figure 11. Temperature variation of the fraction of all-trans chains in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 for n ) 13, 15, 17. Tm is the melting temperature.

The ratio of the intensities of the progression bands at any temperature with respect to the intensity at 300 K would be therefore directly proportional to the fraction of alkyl chains in (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 that have an all-trans conformation at that temperature. This fraction has been plotted as a function of the reduced temperature T/Tm in Figure 11. The intensity at different temperatures was obtained by summing up the progression band intensities for both wagging and rocking modes for all values of k associated with that series. It may be seen that, for all values of n, the temperature variation of the integrated intensities is identical and in fact lie on the same curve. It may also be seen that, in all compounds, the melting transition occurs when ∼60% of the chains are disordered (have at least one gauche defect). The fact that, in the (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 compounds, the thermal evolution of conformational disorder is identical irrespective of the alkyl chain lengths and that melting occurs for the same concentration of conformationally disordered chains clearly highlights the universal nature of melting in these compounds. This is not surprising; the alkyl chains irrespective of their length are anchored to the same underlying (CH3NH3)Pb2I7 lattice (Figure 2) so that the distance between the ammonium “head” groups of the alkyl chains is identical and fixed with no lateral expansion possible. A conformationally disordered chain occupies a larger lateral area as compared to an all-trans chain. There is a critical concentration of disordered alkyl chains that the structure can sustain that is independent of chain length. However, increased space or “free” volume per chain is created if the tilt coherence of the chains is lost. Loss of tilt coherence leading to a splaying of the chains of the bilayer would allow for the increased “tail-to-tail” distance requirement of the conformationally disordered chains whose “head-to-head” distance is fixed because they remain tethered to the inorganic (CH3NH3)Pb2I7 sheet. Loss of tilt coherence in turn would lead to an increased interlayer spacing as shown in Figure 12. This



Figure 12. Schematic depiction of the melting transition in the (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 compounds. The structure on the left is the room temperature structure (300 K) in which all alkyl chains are in all-trans registry. The figure in the middle panel is the structure just before the melting transition. The structure on the right is the structure after melting where more than 60% of the chains are conformationally disordered and the tilt angle of the individual chains are different.

increase in interlaying spacing is indeed observed in the X- ray diffraction of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 above the melting transition (Figure 4). The melting of the anchored bilayer corresponds to a loss of tilt coherence of the alkyl chain when the concentration of the disordered chains (chains having one or more gauche bond) exceeds a critical concentration (60%) that is determined by the underlying lattice to which the chains are anchored. Conclusions The layered organic-inorganic hybrid pervoskite (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 is composed of alkyl ammonium chains anchored to double pervoskite (CH3NH3)Pb2I7 slabs. X-ray diffraction and spectroscopic measurements show that at 300K the alkyl ammonium chains are arranged as tilted bilayers with all alkyl chains adopting a planar all-trans conformation. The latter was established from an analysis of the progression band series in the infrared that arise from the coupling of vibrational modes of methylene units in trans registry. The progression bands in the spectra of (CH3NH3)(CH3(CH2)nNH3)2Pb2I7 (n ) 11, 13, 15, 17) indicate that, irrespective of chain length, all n CH2 units are in trans registry at room temperature. The progression bands in these compounds provide a characteristic signature for alkyl chains that adopt an all-trans conformation. With increase in temperature, the (CH3NH3) (CH3(CH2)nNH3)2Pb2I7 compounds exhibit a first-order phase transition with the associated enthalpy change varying linearly with chain length and accompanied by an increase in interslab spacing. This transition is associated with the melting in twodimensions of the alkyl chains of the bilayer and is characterized by an increased conformational disorder of the methylene units of the chain and loss of tilt angle coherence leading to an increase in the interslab spacing. By monitoring the intensities of the progression bands in the infrared spectra as a function of temperature, a quantitative relation between conformational disorder and melting of the anchored bilayer is established. It

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JP903840J

Melting of an Anchored Bilayer: Phase Transitions in the Organic

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