Understanding Structural Variations in Elastic Organic Crystals by in

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Understanding Structural Variations in Elastic Organic Crystals by in situ High pressure FTIR Spectroscopy and Nanoindentation Study Somnath Ganguly, Ragaverthini Chinnasamy, Shyamal Parikh, Mangalampalli S R N Kiran, U. Ramamurty, Himal Bhatt, M.N. Deo, Soumyajit Ghosh, and Pallavi Ghalsasi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01684 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Crystal Growth & Design

Understanding Structural Variations in Elastic Organic Crystals by in situ High pressure FTIR Spectroscopy and Nanoindentation Study

Somnath Ganguly,a Ragaverthini Chinnasamy,b Shyamal Parikh,c Mangalampalli S R N Kiran,d U. Ramamurty,e Himal Bhatt,f M. N. Deo,f Soumyajit Ghosh*b and Pallavi Ghalsasi*c

a

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560 012, India

b c

Department of Chemistry, SRM Institute of Science and Technology, Chennai 603 203, India

School of Engineering and Technology, Navrachana University, Vadodara- 391 410, Gujarat, India

d

Nanomechanics Laboratory, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Chennai 603 203, India

e

School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore.

f High pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India b

Department of Chemistry, SRM Institute of Science and Technology, Chennai 603 203, India, E-mail: [email protected] c

School of Engineering and Technology, Navrachana University, Vadodara- 391 410, Gujarat, India, Email: [email protected]

Abstract Organic crystals possessing elasticity are gaining wide attention due to their potential applications in technology. From a design perspective, it is of utmost importance to understand the mechanical behaviour of these crystals and their ability to handle stress. In this paper, we present in situ high pressure Fourier Transform Infrared (HP-FTIR) study on 2,5-dichloro-N-benzylidene-4-chloroaniline (DPA) and 2,6 dichloro-N-benzylidene-4-fluoro3-nitro aniline (DFA) crystals at pressures ranging from ambient pressure to 21.5 GPa and 14.5 GPa respectively along with nanoindentation studies, at room temperature. The infrared stretching wavenumber of the aromatic and aliphatic C–H, H–C=N and C–Cl bands on compression showed blueshifts and increased widths, which reflect structure perturbation caused by changes in intermolecular interactions in the crystals. It was noted that both the crystals DPA and DFA behave in different fashion under high pressure prompting the derivation of different models based on structural changes in lattice. Further, Nanoindentation studies corroborated pressure-induced molecular movement in both the crystals.

Introduction

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Organic crystals that exhibit a high degree of elasticity have many potential applications such as flexible organic electronics,1-4 bioinspired actuators,5-7 biomimetics,8 flexible optical waveguides etc.4,9 The origin of such a behaviour has been attributed to the presence of multiple weak and dispersive interactions within the frameworks that are relatively isotropic.10-15 When these crystals are loaded, stress-induced changes in intermolecular interactions can result, which, in turn, may cause short range structural variations.16 It is important to understand pressure-induced effects on the structure and properties of the molecular crystals, as they may have important implications in industries such as in pharma and photonics etc.4,17-21 High-pressure Fourier Transform Infra-red (HP-FTIR) spectroscopy is a well suited tool to monitor subtle pressure induced variations in the lattice in real time. In an earlier work on energetic 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) crystals, the authors showed small structural rearrangements at high pressure within the lattice.22 Recently, we conducted a study of an elastic crystal 2,3-dichlorobenzylidine-4-bromoaniline (DBA) using HP-FTIR spectroscopy that clearly showed structural rearrangements within the crystals at high pressures.16 In this work, spectral variations of two crystals 2,5 dichloro-N-benzylidene-4chloroaniline (hereafter DPA) and 2,6 dichloro-N-benzylidene-4-fluoro-3-nitroaniline (hereafter DFA) with pressure are examined. Both DPA and DFA exhibit high degrees of elasticity, yet have distinctly different packing features. In situ HP-FTIR spectra on these crystals under quasi-hydrostatic pressure environment were obtained after subjecting them to compression and decompression cycles ranging from ambient to 21.5 GPa and 14.5 GPa respectively. Complementary nanoindentation studies were conducted on both the crystals and the results were compared with spectral data. On the basis of these experiments, we propose a model for pressure-assisted intermolecular-changes that occur during the deformation of the examined materials. Both DPA (CCDC No. 1025338) and DFA (CCDC No. 1050261) are halogen substituted N-benzylideneanilines and are already reported.11,23 Molecular units of these are illustrated in scheme 1. Acicular long needle type crystals were grown from methanol solvent using slow evaporation method.11,23 In both crystals under applied pressure, absence of slip planes along with criss-cross corrugated packing in the lattice favours rupturing of weak and dispersive interactions that allow molecules to be stretched up to short range in bent state and once the pressure is retracted the molecules come back to their original thermodynamic lattice positions thereby rendering elasticity.10-12,23


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Scheme 1: Molecular structures of crystals DPA and DFA. Crystallographic details of DPA and DFA were already discussed in previous reports.11,23 The DPA crystals (Figure 1) grow in monoclinic space group Cc with two molecules in an asymmetric unit. Various C–H···Cl (3.60 Å, 2.94 Å, 127.52° ; 3.72 Å, 2.92 Å, 143.21°; 3.77 Å, 2.88 Å, 157.70°) hydrogen bonded interactions are involved to form corrugated tapes along b axis (Figure 1a). Further the tapes are sustained by π···π stacks along a axis. A notable feature is that hydrogen bonded tapes are π···π stacked in an orthogonal direction of the crystal long axis. Face indexing confirmed the crystal facets as (001)/(001) and (101)/(101).11 Criss-cross arrangement with corrugation angle close to ~120° (Figure 1b) and the formation of π···π stacks along the a axis can be noted on the (001) face (Figure 1c).

(a)

(b)

(c)



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Figure 1. (a) Cl···Cl (green) and C–H···Cl (red) interactions lead to tape formation along the b axis in crystal DPA. (b) π···π stacks along a axis in crystal DPA and ~120° between the molecules in consecutive stacks. (c) Packing extends via π···π stacks along a axis. In the DFA crystals (Figure 2), molecules crystallize in space group P2 1 /c with one molecule per asymmetric unit. Unlike in DPA, different types of weak interactions with varying degrees of energy are present in this case. For example, multiple halogen interactions such as Cl···Cl, F···Cl and C–H···O hydrogen bonds are present in DFA essentially giving rise to “spring like” elastic response (Figure 2a).23 Molecules are linked via π···π stacks along the a axis and corrugated packing features are evident from bendable faces (Figure 2b and S5, ESI). Interestingly, π···π stacks are oriented along crystal long axis. In both DPA and DFA crystals, absence of slip planes coupled with criss-cross packing facilitates easy rupturing of the weak interactions during bending allowing short range movement of molecules. Upon unloading, molecules can easily revert back to their original minimum energy configurations.11,23

(a)

(b)

Figure 2. (a) Multiple halogen interactions such as Cl···Cl (green), F···Cl (yellow) and also C–H···O (red) hydrogen bonds facilitate formation of tape along b axis. (b) π···π stacks are visible along crystal long axis a. Experimental In situ HP-FTIR spectra under quasi-hydrostatic pressure environment were recorded on single crystals using Bruker’s VERTEX80V spectrometer attached with a HYPERION 2000 infrared microscope with 15x objectives equipped with a midinfrared globar source and HgCdTe detector. These measurements were carried out using a short symmetric DAC (diamond anvil cell), equipped with type IIA diamonds and KBr as a pressure transmitting medium. The reference spectra were recorded using pure KBr inside DAC. The sample was filled inside a 150-micron size hole in 50micron thick pre-indented gasket. Pressure was measured by a ruby calibration system.24 The nanoindentation experiments were performed on the major faces of the crystals using a Hysitron Ti950 Triboindenter (Minneapolis, USA) that continuously monitors and records load and displacement with resolutions of 1 nN and 0.2 nm, respectively. A three-sided Berkovich diamond indenter with a tip radius of ~75 nm was used to indent the crystals and image the residual indents in scanning probe microscopy mode (ESI, S6). The peak load, P max of 6 mN was achieved in 10 s, held the P max for 10 s


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and finally unloaded to zero load in 10 s. A minimum of 9 valid indentations were performed on each crystallographic face. The experiments were conducted in a quasistatic load-controlled mode. The methodology employed in quantification of mechanical properties from the load-displacement curves can be found elsewhere.25-27 Results and Discussion DPA: Figure 3a shows the FTIR spectra of DPA between 3600-2400, 1800-1500, and 1000-600 cm1 regions as a function of applied pressure while the spectra obtained in the 3600-2400 cm-1 region is magnified in Figure 3b. While these figures show a number of absorptions of medium and weak intensity, only absorptions corresponding to the C–H aromatic and aliphatic stretches as well as the CH=N and C–Cl stretches will be discussed. The above bands were assigned from a previous work.28,29 It can be clearly seen that notable spectral changes take place as pressure is increased. The CH=N stretching band is sharp at at 1614 cm-1 at ambient pressure. It begins to blueshift and also increases in breadth at pressures above 1 GPa. The C–H aliphatic stretching band at ~2910 cm-1 is a sharp, split band of medium intensity which also blueshifts and increases in breadth first at 1 GPa and then broadens significantly at about 2.6 GPa (spectrum in Fig 3b). At 12 GPa the band is very broad and shallow at 2965 cm-1 and above this pressure it is barely seen. In both the cases, while the blue shift is due to compression of the bond, the observed broadening is related to the changes in local molecular bonding into possibly a disordered state.30

(a)



(b)

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(c)

Figure 3. (a) FTIR spectra of DPA in different spectral regions as function of pressure (b) and (c) FTIR spectra of DPA in the 3600-2400 cm-1 region as a function of increasing and decreasing pressure respectively. The weakness of the C–H aliphatic stretch (~2910 cm-1) at high pressures is due to the random orientation of aliphatic C–H at the kink site of molecule with the intensity h gradually reducing with pressure. Here, under high pressure aliphatic C–H can be accommodated randomly in the available space. Therefore, it is reflected in loss of intensity (reduced value of derivative of dipole moment). The C–H aromatic stretch is a weak split band observed at ~3076 cm-1 at ambient pressure which moves to ~ 3098 cm-1 at 3.9 GPa . A large shift of the band starts to take place at about 5.8 GPa with significant increase in breadth. The band shows an increase in intensity above 5.8 GPa with further blue shift. The blueshift is again due to compression of the bond and the intensity increase is due to closer stacking of C–H aromatic rings and their alignment on compression. The medium intensity sharp C–Cl stretching band appears at ~ 815 cm-1 at ambient pressure. At pressure of about 19 GPa the band is located at about 835 cm-1 and appears very broad and shallow. The striking changes are due to changes involving the C–Cl interactions with pressure. On decompression the CH=N stretch, C–H aromatic and C–H aliphatic stretch and C–Cl stretching bands show reversal to the actual position and width indicating short range changes in molecular order and is indicative of possible elastic nature of the crystal. DFA: Figure 4a and 4b show an extremely weak C–H aliphatic stretch at 2924 cm-1 which weakens further and disappears at pressures above 6.8 GPa. The C–H aromatic stretch appears at 3086 cm-1 as a split band at ambient pressure. This band blue shifts and increases in breadth and intensity and finally lies at 3143 cm-1 at 11.0 GPa . The band is at 3183 cm-1 at 14.6 GPa . At this pressure it appears broad but with increased intensity. As discussed, the blueshift is due to compression of the bond and increased intensity could arise from closer stacked rings. The rings are likely to come closer and undergo increased overlap with pressure. The rings can reorient in a particular direction under a compressed state thereby enhancing intensity. From Fig 4a it can be seen that the C–Cl stretch is seen split at 821 and 832cm-1 at ambient pressure and the bands moves higher to 830 and 848 cm-1 at 6.0 GPa appearing broader. The band


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gradually moves to 837 and 853 cm-1 at 11 GPa appearing as broad and shallow and finally moves to 842 and 860 cm-1 at 14.5 GPa with large increase in breadth. The large breadth of the band is due to changes in molecular bonding of C–Cl at high pressure and possible collapse of local order. The CH=N stretch is split at 1632 cm-1 at ambient pressure moves to 1643 cm-1at 6.8 GPa becoming a broad single band and then moves to 1653 cm-1at 11 GPa and finally to 1657 cm-1 at 14.5 GPa where it appears weak and broad. The blueshift of the band is due to compression of the bond arising out of flattening of corrugation along the a axis. All bands reverse in position, and width on decompression indicating reversal of molecular ordering and interactions to their original thermodynamic position upon pressure retraction which is a clear indication of elasticity. As seen in figures 4(b) and 5(b), the first overtone of the C–H in plane wagging mode at 1305 cm-1 appears at around 2600 cm-1 and becomes more prominent above 5 GPa indicating change in environment surrounding C–H bond.

(a)

(b)

(c)



Figure 4 .(a) Pressure variations of the IR absorptions of (DFA) (b) and (c) FTIR spectra of DFA in the 3600-2400 cm-1 region as a function of increasing and decreasing pressure respectively Wavenumber pressure plots are given in Figure 5 for DPA. The C–H stretches, the CH=N stretch and the C–Cl stretches show sudden increase in slope at 1 GPa and flatter slopes at 6, and 13GPa with another increase in slope at 13GPa indicating changes in non covalent weak interactions occurring at these pressures (DPA). The hollow points in each plot are wavenumbers for decompression and indicate the elastic nature of the crystal. The error bars are barely visible and are smaller than the size of the symbols used.

Figure 5. Variation of the wavenumbers of the C–H, C–Cl and CH=N stretches of DPA.

For DFA (Figure 6) slope-changes can be seen at 2, 8, 12 GPa, again indicating changes in molecular bonding and compressibility at these pressures. There are possibilities of phase changes (short range transient) and each slope signifies new compressibility due to rearrangement of intermolecular interactions. Again, the hollow points in each plot are wavenumbers for decompression and indicative of the elastic nature of the crystal. The error bars are barely visible and are smaller than the size of the symbols used.

Figure 6. Variation of the wavenumbers of the C–H, C–Cl and CH=N stretches of 2,6 dichlorobenzilydene4fluoro-3-nitro aniline (DFA).


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The slopes of wavenumber with pressure dω/dP for DPA and DFA are given in Tables 1 and 2 respectively. The wavenumber–pressure plots are fitted to straight lines and the corresponding R2 values are mentioned in the brackets. No values of R2 are mentioned if the number of data points were limited. Out of two crystals, the C–H aliphatic stretches are very different and reflects overall surrounding environment. The C–H aliphatic stretch is of medium strength, structured and slowly blueshifts and becomes weak and broad with pressure in DPA. In DFA the band appears very weak at ambient pressure and is not seen above 6.8 GPa. The reasons for the higher intensity of C–H aliphatic in DPA could be related to a function of the derivative of C–H stretch dipole moment. During 2-5 GPa for DPA and 2.3-8.4 GPa for DFA, for this band indicates a higher slope for the DFA crystal in these ranges indicating its higher flexibility (Table 1). This is due to multiple interactions like C– H∙∙∙X, Cl∙∙∙Cl in DFA that force the structure to revert back to its original position on pressure reduction (Split C–Cl in DFA,).16,31-33 The C–H aromatic stretch is very weak and split in DPA at ambient pressure. The band blueshifts and broadens gaining some intensity. The small gain in intensity could be due to small increase in overlap of the stacked rings with pressure. The aromatic stretch in DFA is of medium intensity and gains significant intensity with increase in pressure (Figure 7). The lower intensity of the band in DPA at ambient pressure could be due to stacks comprising of smaller number of molecules. The crystal structures indicate orthogonal alignment of the aromatic rings in DPA and a parallel alignment in DFA with respect to crystal axis. The large intensity increase of the aromatic stretch of DFA contrasts with a smaller intensity increase for the band in the case of DPA and shows that pressure increase causes accumulative effects in stacks of DFA with respect to DPA.

DPA Mode

0-1 GPa Wavenumber (cm-1)

C–Cl

1-5 GPa

5-13 GPa

13-21 GPa

dω/dP (cm-1/GPa)

815 2.91(0.94) 2.46(0.93) 0.15(-0.17)

0.36(0.78)

829

1.79(0.8) 2.68(0.89)

0.2(0.93)

0.95(0.99)

N=C

1614 1.82(0.88) 1.67(0.86)

2.66(0.79)

2.49(0.72)

Aliphatic C–H

2921 5.99(0.83) 6.77(0.74)

7.17(0.82)

1.39

Aromatic C–H

3076 6.27(0.88) 8.31(0.84)

4.12(0.98)

2.61(0.63)

Table 1. Pressure Derivatives of Vibrational Wavenumbers of DPA. Values in the bracket are meant for R2 values obtained from linear fits.



0-2.3 GPa

Mode

C–Cl N=O N=C Aliphatic C–H Aromatic C–H

Wavenumber (cm-1) 821 832 1527 1632 2924 3086

DFA 2.3-8.4 GPa

0.11 0.27 0.22 -0.2

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8.4-11.6 GPa

dω/dP (cm-1/GPa) 2.68(1) 0.7(0.56) 3.99(0.89) 0.55(0.22) 2.65(0.9) 0.43(0.85) 2.7(0.92) 0.28(0.41)

11.6-14.6 GPa

1.23 1.27 -1.32 1.22

1 10.23(0.95) 1.61

8(0.99)

2.72(-0.23)

5

Table 2. Pressure Derivatives of Vibrational Wavenumbers of DFA. Values in the bracket are meant for R2 values obtained from linear fits.

(a)

(b)

Figure 7. Intensity increase in aromatic C–H peak as a function of pressure for (a) DPA and (b) DFA. The solid (hollow) symbols are for compression (decompression) and the error bars are smaller than the size of the symbols at many pressures. It was observed that different bands respond to pressures differently since bands are intimately bonded in different local environments. Most bands lose fine structure with pressure increase. The loss of fine structure can also be attributed to symmetry loss at higher pressures. Figures 7 (a) and (b) show comparison of intensity of C–H aromatic vibration with pressure. The solid (hollow) symbols represent increasing (decreasing) pressures. The pronounced increase in the intensity of C–H aromatic vibrational mode of DFA as compared to DPA, is related to the increase in overlap of π···π stacking of rings.


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Nanoindentation The representative load, P and displacement, h responses obtained via nanoindentation on the major faces of DPA and DFA crystals are displayed in Fig. 8. At any given P, h is much higher in DFA as compared to that in DPA, indicating that the former is much softer.17,34 Interestingly; the loading part of DPA was smooth compared to the DFA, which exhibits a few displacement bursts (pop-ins) at higher loads probably due to the shearing of corrugated columnar molecular layers along the indentation direction.11,12 The elastic bending behaviour of both the crystals can be compared on the basis of the unloading part of the P-h curves, which occurs primarily due to elastic recovery. It is evident from Fig. 8 that, while the DFA recovers 50% of h max after the complete unloading, DPA recovers to only 20%. This result clearly indicated that DFA crystals are significantly more elastic in nature. This is possibly the reason for the significantly lower elastic modulus of DFA crystals (1.42 ± 0.242 GPa) as compared to that estimated for DPA crystals (9.5 ± 0.92 GPa), as shown in Table 3. The fact that DFA is elastically more compliant with respect to DPA can be attributed to a multiple number of variable interactions present in them. Though both these crystals share some common packing features, it is the type and nature of weak interactions that dictate the differences in elastic response. The orientation of π···π stacks in the isotropic framework of the lattice plays an important role in elasticity apart from weak and dispersive interactions. In DFA, π···π stacking is along the crystal axis and the criss-cross packing network is perceptible from bendable side faces whereas in DPA π···π stacking is orthogonal to crystal axis. Moreover, in DFA, the presence of multiple halogen interactions such as Cl···Cl, Cl···F as well as C–H···O interactions act as “strain buffer” playing a role in the crystal regaining its original shape when pressure is withdrawn. In fact, these so called “strain buffer” interactions are parallel to the indentation direction and therefore whenever the indenter penetrates, the weak and dispersive interactions squeeze them easily and owing to their more compressible nature it gives rise to low E value in case of DFA. Also, the hardness of DPA (0.289 ± 0.038 GPa) was high compared to DFA (0.131 ± 0.04 GPa) because of the higher resistance offered by the corrugated π···π stacks in DPA that are parallel to the indenter direction. In DPA, π···π stack direction is parallel to indentation direction thereby the tip experience more hindrance in penetrating the crystal owing to relatively stronger π···π interaction. However, in case of DFA, the π···π stacking layers are orthogonal to indenter direction which allows the indenter to penetrate through the crystal easily due to more compressible nature of Cl···Cl, Cl···F and C–H···O interactions. Therefore, the lower hardness value was observed in DFA.

Table 3. Elastic Moduli and Hardness for DPA and DFA Crystal DPA

Crystal DFA

E

9.5 ± 0.92

GPa

1.42 ± 0.242 GPa

H

0.289 ± 0.038 GPa

0.131 ± 0.04 GPa



Figure 8. Representative P-h curves of DFA and DPA crystals, obtained using nanoindentation with a Berkovich tip. Arrows indicate pop-in or displacement bursts in DFA crystals.

A model for pressure induced structural changes Based on high pressure and regular nanoindentation studies we propose a model for structural changes that may occur in the crystals of DPA (Figure 9) and DFA (Figure 10) under applied pressure. Though both are elastically bendable, the differences in elasticity can be related to the presence of a wide variety of weak and dispersive interactions along with relative orientations of π···π stacks along the crystal axis. The compressible nature of Cl···Cl, Cl···F and C–H···O interactions is reflected in the high-pressure changes of important vibrational modes. As mentioned, from changes in these vibrational modes such as the C–H aromatic stretch important conclusions could be drawn regarding the nature of corrugational changes that occur on pressure application. From the structures of the DPA and DFA crystals described above, it is quite likely that in case of DPA, π···π stacking distance shrinks along with flattening and overlap of rings increase as pressure increases (Figure 9). Such flattening is further supported by the fact that the C–H aromatic stretch intensity increases with pressure indicating increasing overlap of rings (Figure 7a). The intensity increases significantly in the case of DFA. It may be noted that because of the orthogonal nature of π···π stacks in DPA this crystal is likely to exhibit lower compressibility compared to the DFA crystal.


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Figure 9. Proposed model of high pressure study of crystals DPA Since the C–H aromatic stretch shows a higher intensity increase in DFA than DPA, as shown in Figures 7(a) and (b), it is likely that the corrugation change in DFA results in greater overlap compared to DPA. The difference in corrugational changes could be further explained. Thus under applied pressure, the relative orientation of molecules may change resulting in changes in corrugation. It is possible that the corrugation angle becomes more acute in the DFA crystal with pressure causing an increase in aromatic C–H intensity (Figure 10). Thus the significant increase in the intensity of C–H aromatic peak in DFA may be associated with increased overlap of rings of neighbouring molecules of same π stacked column rather than rings from adjacent stacks (Figure 10). On the other hand, in DPA, since the pressure is transmitted over the stacks, flattening is more feasible with intra-stack distance reduction leading to increased aromatic C–H peak intensity. We propose that the higher intensity increase in C–H aromatic stretching mode in DFA is caused mostly due to intrastack overlapping of rings in contrast to ring flattening in DPA. Split of C–Cl and split CH=N in DFA indicate multiple interactions involving Cl and C–H···N in DFA that cause higher compliance in the crystal as observed in the nanoindentation study. The higher wavenumber of C–H aliphatic in DFA (2946 cm-1) compared to the C–H aliphatic wavenumber in DPA (2910 cm-1) could be an indication of C–H…O interaction in DFA.

Figure 10. Proposed model of high pressure study of crystals DFA.



Conclusions In conclusion, in situ high pressure studies were conducted on two elastic organic crystals namely, 2,5-dichloro-N-benzylidene-4-chloroaniline (DPA) and 2,6 dichloro-N-benzylidene4-fluoro-3-nitro aniline (DFA) at pressures ranging from ambient pressure to 21 GPa at room temperature. Most of the prominent bands including aromatic and aliphatic C–H and C–Cl bands showed significant blueshifts, increasing widths owing to changes in intermolecular interactions in the crystals. This can be attributed to the compressed state of crystals under the influence of high pressure. Decompression studies showed clear reversibility reflecting the fact that changes are essentially short-range perturbations which retract to original state upon withdrawal of pressure. However, different responses of two crystals are related to two different packing configurations of lattice. It has been noted that relative orientation of π stacks with respect to crystal axis coupled with presence of multiple weak and dispersive interactions play a pivotal role in overall elasticity and are reflected in the FTIR spectral changes with gradual increments of pressure. Therefore, two models were proposed for two crystals showing the effects of pressure on them which further depicts a high energy state of the lattice under the influence of high pressure. Acknowledgments S. G. thanks the Science and Engineering Research Board, Department of Science and Technology, Govt. of India for the Early Career Research Award Grant (No. ECR/2017/000060). M.S.R.N.K. thanks the Science and Engineering Research Board, Department of Science and Technology, Govt. of India for an Early Career Research Award (File No: ECR/2016/000827). R. C. thanks SRM Institute of Science and Technology for PhD fellowship. Pallavi Ghalsasi would like to acknowledge financial support from DAEBRNS (2012/37P/15/BRNS/806) and DST-SERB (EMR/2016/003974). We thank Prof. Gautam R. Desiraju from the Indian Institute of Science, Bangalore, for his constant inspiration. References (1) Chen, W.; Qi, D. C. ; Huang, H.; Gao, X.; Wee, A. T. S. Organic–organic heterojunction interfaces: Effect of molecular orientation. Adv. Funct. Mater. 2011, 21, 410-424. (2) Schmidt-Mende, L. Self-organized discotic liquid crystals for high-efficiency organic photovoltaics. Science 2001, 293, 1119-1122. (3) Bronstein, H. Thieno [3, 2-b] thiophene−diketopyrrolopyrrole-containing polymers for high-performance organic field-effect transistors and organic photovoltaic devices . J. Am. Chem. Soc. 2011, 133, 3272-3275. (4) Liu, H.; Lu, Z.; Zhang, Z.; Wang, Y.; Zhang, H. Highly Elastic Organic Crystals for Flexible Optical Waveguides. Angew. Chem. Int. Ed. 2018, 57, 8448-8452. (5) Fratzl, P.; Barth, F. G. Biomaterial systems for mechanosensing and actuation. Nature 2009, 462, 442-448. (6) Garcia-Garibay, M. A. Molecular crystals on the move: from single‐crystal‐to‐single‐crystal photoreactions to molecular machinery. Angew. Chem. Int. Ed. 2007, 46, 8945-8947.


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For Table of Contents Use Only

Understanding Structural Variations in Elastic Organic Crystals by in situ High pressure FTIR Spectroscopy and Nanoindentation Study

Somnath Ganguly,a Ragaverthini Chinnasamy,b Shyamal Parikh,c Mangalampalli S R N Kiran,d U. Ramamurty,e Himal Bhatt,f M. N. Deo,f Soumyajit Ghosh*b and Pallavi Ghalsasi*c

Elastic Organic Crystals 2,5-dichloro-N-benzylidene-4-chloroaniline (DPA) and 2,6 dichloro-N-benzylidene-4-fluoro-3-nitro aniline (DFA) studied by in situ high pressure Fourier Transform Infrared (HP-FTIR) study and nanoindentation studies at room temperature.



Supporting information is available in the file named ESI_23Feb2019. The list of the contents in the SI is as follows.

S1. Experimental details S2. Single crystal X-ray diffraction S3. Crystallographic Information Table S4. Hirshfeld Surface Analysis S5. Packing images of elastic crystals S6. Residual Indent Images on DPA and DFA after Nanoindentation Experiments


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Understanding Structural Variations in Elastic Organic Crystals by in

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