Diversity in Mechanical Response in DonorâAcceptor Coupled

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Diversity in Mechanical Response in Donor Acceptor Coupled Cocrystal Stoichiomorphs Based on Pyrene and 1, 8-Dinitroanthraquinone Systems Deepak Chopra, and MANJEET Singh Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Crystal Growth & Design

Diversity in Mechanical Response in Donor Acceptor Coupled Cocrystal Stoichiomorphs Based on Pyrene and 1, 8-Dinitroanthraquinone Systems Manjeet Singha and Deepak Chopra*a a

Crystallography and Crystal Chemistry Laboratory, Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal By-Pass Rode Bhauri, Bhopal-462066, Madhya Pradesh, India.

ABSTRACT:Cocrystals are an emerging class of molecular materials which can achieve desirable properties by selecting suitable constituent molecules and can also exhibit polymorphism, but different stoichiometry of the constituent molecular pairs can also give different structures having different properties is of interest. With an aim of understanding the influence of stoichiometric ratio of the π-conjugated donor and acceptor in altering the crystal packing and their mechanical properties, we have synthesized two stoichiomorphs, namely, DQP1 (1A:1D) and DQP2 (1A:0.5D), alongwith their polymorphs, DQP3 (1A:2*0.5D) and DQP4 (2A:1D) respectively,involving pyrene as the donor (D) and 1, 8dinitroanthraquinone as an acceptor (A). Both stoichiomorphs have a layered structure with different packing patterns.DQP1 and DQP3 have ‒ADADAD‒ type whereas DQP2 and DQP4 have ‒ADAADA‒type of layered packing. The DQP4 shows thermosalient effect, and on heating converts into the polymorph DQP2 via SCSC phase transition (PT) while the crystals of DQP3 shows elastic bending. We discuss this diversity of mechanical properties on the basis of the molecular packing of donor and acceptor molecules. Here, we also demonstrated that the structure-stacking modes and mechanical property of cocrystals can be tuned through the variation of stoichiometric ratio of constituents.

coupled cocrystals) is important to overcome the problem.

INTRODUCTION Organic molecular crystals/cocrystals that are sensitive to external stimuli such as heat, light, mechanical force, etc. can induce mechanical responses such as bending (plastic,1-3 elastic4-8 and more recently superelastic9-11),12−14 shape memory,15 bursting,16−19 jumping,20−24 twisting,25-28 or curling29-31 have attracted increased research interest because they are invaluable for the fabrication of mechanically tunable actuation or energy harvesting devices,32-34 phototransistors,35-36 light-emitting diodes,37 solar cells,38 photonics,39 muscle-mimetic biomaterials,40 bioelectronics,41 flexible electronics,42 smart nanomaterials,43 pharmaceuticals44-46 etc. Apart from that these organic molecular crystals/cocrystals especially π-conjugated are also highly important in materials science due to their ability to form a densely packed three-dimensional structure exhibiting packing anisotropy.47-49 However, organic single crystals are generally not flexible due to their densely packed structure. Onthe other hand, polymers in the solid state such as films and fibers are typical flexible materials50-51 but polymer materials are amorphous, and increasing the crystalline state in these matrix decreases flexibility.52The trade-off between flexibility and crystallinity of materials is often a fatal problem in solid-state materials science. The study on the fabrication of flexible crystals with optical and/or electrochemical properties (donor-acceptor

The π-conjugated donor-acceptor based cocrystals have offered creative stands to realize unique physical properties such as superconductivity,53 metallicity,54 photoconductivity,55 ferroelectricity,56 etc. The bicomponent system based on π-conjugated donor-acceptor molecules have been also theoretically predicted to exhibit good charge-transport behaviors.57-58Polymorphism in donoracceptor based cocrystals exists and has been found in several cocrystals59. In addition, stoichiomorphism (“When studying cocrystals, different structures may arise just as a consequence of different stoichiometry of the two compounds” 60) in cocrystals are relatively less reported. 6162

In the present study, pyrene (PY), 1, 5dinitroanthraquinone (5DNAQ) and 1, 8dinitroanthraquinone (8DNAQ) were selected for cocrystal synthesis based on the possibility of interaction between donor and the acceptor. We have performed single crystal growth of stoichiomorphs from different solvents, via slow evaporation technique at room temperature (24.0 o C) as well as at low temperature (4.0 oC) and found an interesting observation that different stoichiomorphs of same constituents have different mechanical properties.

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Figure 1.Optical images of DQP1, DQP2, DQP3 and DQP4 cocrystals obtained from crystallization.

Scheme 1. Schematic representation of all the cocrystal stoichiomorphs and their polymorphsobtained from the combination of PY, 8DNQD and 5DNQD. We discuss this diversity in mechanical properties on the basis of stoichiometry and the molecular packing of the donor and acceptor molecules. The present study also demonstrated that the structure-stacking modes and mechanical property of cocrystals can be tuned through the variation of stoichiometric ratio of the parent constituents.

RESULTS AND DISCUSSION Preliminary screening of cocrystals of PY with 5DNAQ and 8DNAQ yielded cocrystals only with 8DNAQ (scheme 1). The concept that not all the compounds readily cocrystallize even if they possess necessary hydrogen bonding groups is not new.63 In order for a cocrystal to be stable at certain conditions, the interactions in its structure should be compatible with those in the starting compounds. Thus, the absence of cocrystal formation with 5DNAQ may be due to their failure to provide a hydrogen bond network competitive enough to the structures of parent compounds. The single crystal of DQP1 and DQP2 [Figure 1(a) and (b)] were obtained upon crystallizing 5 mg of the ground material from ~ 3 mL of dichloromethane (DCM) and acetonitrile (ACN) respectively, via slow evaporation techniqueat room temperature. The single crystal of DQP3 [needle-shaped, Figure 1(b)] was concomitantly obtained during the cocrystallization of DQP2 in the ACN solvent and it has 2:1 stoichiometric molar ratio of PY and 8DNAQ respectively. The single crystal of DQP4 [Figure 1(c)] was obtained upon crystallizing 5 mg of the ground material from ~ 5 mL of tetrahydrofuran (THF) also via slow evaporation technique at room temperature. The formation of all cocrystals was confirmed by DSC, powder and single crystal X-ray diffraction.

other: the first peak has been observed at 288.0 °C (ΔHt = −25.73 J/g) while the second at 320.0 °C (ΔHt = −13.92 J/g) with the same scanning rate (Figure 2, blue color) and the DSC thermograms of DQP2 displays single endothermic peak observed at 291.0 °C (ΔHt = −33.96 J/g) with the same scanning rate (Figure 2, red color). Further, the DSC thermograms of DQP4 (blockshaped crystal) has three endothermic peaks: the first peak is observed at 103.0 °C (ΔHt = −1.69 J/g), the second is at 287.0 °C (ΔHt = −52.72 J/g) while the third peak at 299.0 °C (ΔHt = −3.75 J/g) with the scanning rate of 5.0 °C/min (Figure 3, blue color). One interesting observation in the DSC thermograms of DQP4 is that when we reverse (new experiment) the heating cycle from 160.0 °C up to 30. 0 °C (Figure 3, black color) and again start heating (second cycle of the same material, red color, Figure 3) up to 315.0 °C then only one endothermic peak is observed at 292.0 °C (ΔHt = −39.41 J/g). This study suggest that there is a phase transition at around 110.0 °C because the

Figure 2.DSC traces recorded at 5 °C/min for crystals of DQP1, DQP2 cocrystal stoichiomorphs along with their staring materials.

Differential Scanning Calorimetry (DSC) and Hot Stage Microscopy (HSM) The DSC thermograms of the starting materials, 8DNAQ and PY display single endothermic peak (Figure 2, light blue and black color) observed at 320.0 °C (ΔHt = −85.27 J/g) and 150.0 °C (ΔHt = −126.31 J/g) respectively, with the scanning rate of 5.0 °C/min. The DSC thermogram of DQP1 displays two endothermic peaks adjacent to one

Figure 3.DSC traces accompanying the phase transition recorded at 5 °C/min for crystals of DQP4 cocrystal polymorph.


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endothermic peak observed at 103.0 °C (ΔHt = −1.69 J/g) in first heating cycle of DQP4 (Figure 3, black color) disappeared in the second heating cycle as well as the third peak which was observed at 299.0 °C (ΔHt = −3.75 J/g) in the first cycle (Figure 3, blue color) also disappeared in second heating cycle (Figure 3, red color) and the peak which was seen in second heating cycle (Figure 3, red color) has a higher onset value (292.0 °C) compared to second peak onset value (287.0 °C) during first experiment (Figure 3, blue color). In order to monitor the occurrence of the phase transition, Hot Stage Microscopy (HSM) experiments were performed on a stereomicroscope equipped with a hot stage apparatus and the photographs and video were taken with a Leica polarizing microscope. The single crystal was placed on a glass slide and the images of the experiments were recorded. HSM experiments show (Figure 4) an interesting phenomenon, the single crystal to single crystal (SCSC) phase transition occurs which is accompanied by shattering of one crystal (block shape) into many small crystals (rhombus shape, yellow circle Figure 4 and 5). HSM experiments have been performed by heating an acicular crystal that lies on its major face (1-1-1) (Figure S1) from 25.0 °C to 135.0 °C at the rate of 5.0 °C/min. We observed that the conversion process starts at 100.0°C (Figure 5, Movie SM1) accompanied by simultaneous elongation of (1-1-1) face and finally the whole crystal shatters into small crystals having major phase (100), between 130.0 °C to 135.0 °C (Figure 5, Figure S1), which can be physically separated. DQP3 crystals were obtained concomitantly during solution evaporative crystallization of DQP2 in ACN solvent. Since it has long needle shape [red circle, Figure 1(b)] therefore, an acicular crystal of the DQP3 was subject to bending forces manually in two-point geometry, using needle under optical microscope. Surprisingly, DQP3 crystal is flexible and bends under mechanical stress. The reversible bending−relaxation phenomenon of the crystal can be cycled many times and the crystal bending angle exceeded almost 180° (Figure 6).

Figure 4.Images of the DQP4 cocrystal polymorph crystal before and after SCSC phase transition.

polymorph after phase transition(second heating cycle, red color, Figure 3) also indicates that the SCSC phase transition of DQP4 to DQP2 occurred because DSC trace of second cycle exactly matches with DQP2 (both have only one endothermic peak with almost same heat of fusion as well as peak value, see red color in Figure 2 and 3). These results (VTPXRD and DSC) suggest that DQP4 is converted in to DQP2 on heating via SCSC phase transition. Here, we can say that DQP4 and DQP2 are monotropic because DQP4 is irreversibly converted into DQP2 before their melting.65 One more interesting observation observed during VTPXRD experiment was that the XRD pattern of DQP4 at 65.0 °C is different from both phases (DQP4 and DQP2, see Figure 7) which indicates that there is one more phase in between both phases but in the DSC experiment no signature of that phase transition was observed. To confirm that, we havecollected SCXRD data of DQP4 at 65°C (DQP4_1, Table1) and found that it is the packing polymorph of DQP4 (Figure S4). The experimental PXRD pattern of DQP4 (collected at 65.0 °C during VTPXRD experiment) completely matched with the simulated pattern of DQP4_1 (Figure S5).Profile fitting for the PXRD pattern was done to rationalize the conversion of phase DQP4 to DQP_1, (Figure S6) fittedusing the lattice

Variable Temperature Powder X-Ray Diffraction (VTPXRD) To confirm the SCSC phase transition in DQP4, the VTPXRD study was carried out for DQP4 from 25.0 °C to 115.0 °C at a heating rate of 5.0 °C/min. The XRD pattern was recorded at 45, 65, 85, 100, 105, 110 and 115 °C (Figure S2) respectively. It can be visualized that the peaks shift towards lower 2θ values with increasing temperature, and this is on account of the expansion of the crystallographic axes.5 The PXRD patterns of DQP4 at 115 oC is different from the room temperature (25.0 oC, Figure 7) pattern and it was found to completely match with the experimental PXRD pattern of DQP2 (recorded at 25.0 oC, Figure 8). Profile fitting refinement64 for the obtained PXRD pattern was done to rationalize the presence of phase DQP2(Figure S3) obtained after PT of DQP4 at 115 oC. It is interesting to mention here that the DSC trace of DQP4

Figure 5.SCSC phase transition depicting elongation along the (1-1-1) plane of DQP4 cocrystal polymorph on heating at 5 °C/min followed by shattering of one crystal into many small crystals with the (100) plane (yellow circles).

Figure 6. Bending and relaxation process of DQP3 cocrystal polymorph, this operation can be performed repeatedly without crystal breakage.



Figure 7. Experimental PXRD patterns of DQP4 cocrystal o polymorph recorded at 25, 65 and 115 C. b) Overlay diagram of PXRD showing change in their peak position along with splitting of peaks.

parameters obtained after crystal structure determination of DQP_1 at 65 oC. DQP4_1 phase is unstable because below ~65 oC it converts to DQP4 and above that temperature it is converted to DQP2 therefore we can say it is the metastable form with respect to both the phases.66 To understand the observed macroscopic phenomena at molecular level, a detailed investigation of the key structural features of all the phases is of extreme necessity.

Packing Analysis of All Cocrystal Stoichiomorphs All the obtained cocrystal stoichiomorphs and their polymorphshave a layered structure but there are significant differences in the intermolecular interactions, molecular stacking sequences, and molecular orientations. The details of the crystallographic data of all the stoichiometric variants ofcocrystal polymorphs are tabulated in Table S1. The lists of all relevant intermolecular interactions for all the cocrystal stoichiomorphs and their polymorphsare tabulated in Table S2-S6. DQP1: DQP1 crystallized in the orthorhombic system with Pca21 space group and includes one molecule of 8DNAQ and one molecule of PY in the asymmetric unit [Figure S7(a)]. In this stoichiomorph, both the nitro groups in 8DNAQ molecule are not coplanar with thering. These are twisted on account of the steric hindrance between the neighboring carbonyl group with the nitro groups. The torsion angleof boththe nitro groupswith their associated rings is same (71o)[Figure S8(a)].It has a layered structure as ‒A‒D‒A‒D‒, where A represents the 8DNAQ (acceptor) and D represents the pyrene (donor) having molecular stacking [mixed stack with molecular centroid distance, in the range of 3.819 to 3.884 Å, Figure 9(a)]. These molecular layers are stacked alterna-

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tively along the stacking direction (b axis), forming a molecular column. The total interaction energy (IE) for ‒A‒D‒ molecular pairs are varies alternatively as ‒61.9 kJ mol-1 and ‒60.6 kJ mol-1 in a molecular column [Figure S9(a)] and each stacking is comprised of both the electrostatic and dispersive components [Figure S9(c) and (d)].These molecular columns are connected to their adjacent columns via C−H•••O hydrogen bonding interactions, having IE between -4.4 to -8.3 kJ mol-1 [see tube size in Figure 9(b), Figure S9(a)] and these inter-column interactions are most probably due to the twisting of the nitro groups with respect to their associated ring.In the 8DNAQ molecular layer [ac plane, Figure 9(c)], 8DNAQ molecules are also connected via C−H•••O (in the range of 2.24 to 2.78 Å) hydrogen bonds interactions (Table S2). The molecules within a layer are more weakly bounded as compared to the molecular stacking, the pairwise IEs are in the range of -8.3 to -31.2 kJ mol-1 for 8DNAQ molecules [Figure S9(b)]. Within the molecular layers (PY as well as 8DNAQ molecular), the molecules are not in a plane with respect to each other. They are arranged in two alternating stacks with a tilt angle of 31.63° along the stacking direction [Figure 9(a)]. DQP2 and DQP3 (Concomitant cocrystal stoichiomorphs): DQP2 crystallized in the monoclinic system with C2/c space group and includes one molecule of 8DNAQ and a half molecule of PY in the asymmetric unit [Figure S7(b)]. In DQP2, both nitro groups in 8DNAQ molecule are also twisted with respect to the ring, but the torsion angles for both the groups are different (75o and 83o)[Figure S8(b)].It also has layered structure but in different arrangement (‒ADA‒ADA‒) as compared to DQP1. Here, we can consider three layers (‒ADA‒) as one layer [blue shadow, Figure 9(d)] which stacked along the crystallographic a axis. The molecular centroid distance between PY and 8DNAQ is 3.80 Å with IE of -59.8 kJ mol-1 and between 8DNAQ and 8DNAQ, the molecular centroid distance is 5.92 Å with IE of -38.8 kJ mol-1 [Figure 9(d), Figure S10(a)]. There exists destabilization to the extent of 7.4 kJ mol-1 between the 8DNAQ molecules [yellow circle in Figure 9(e)]. In this stoichiomorph, PY layer are also connected with 8DNAQ layers (displaced as compared to DQP1) via C−H•••O (in the range of 2.54 to2.81 Å) hydrogen bonds interactions (Table S3), having

Figure 8.Comparison of the experimental PXRD patterns of o DQP4 after heating at 115 C with DQP2 cocrystal polymorph o recorded at room temperature (25 C).


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Figure 9. a) Supramolecular packing of DQP1 having tilt angle of 31.63° along stacking direction. b) Energy framework of DQP1. c) C−H•••O interactions between 8DNAQ molecules in DQP1. d) Supramolecular packing of DQP2. e) Energy framework of DQP2 along the c axis show destabilizing interactions (in yellow circle) between two 8DNAQ layers. f) C−H•••N (pink shades) and C−H•••Ointeractions between 8DNAQ molecules in DQP2. g) Supramolecular packing and h) having tilt angle of 24.12° along stacking direction in DQP3. i) C−H•••O interactions between 8DNAQ molecules in DQP3. j) Interlocking pattern of the column (pink shadow) along the staking directionin DQP3. k) Energy framework of DQP3. l) Supramolecular packing of DQP4 having tilt angle of 11.88° along stacking direction. m) Supramolecular layer of molecules integrated via stacking interactions in DQP4. n) Energy framework of DQP4. o) C−H•••O interactions between 8DNAQ molecules in DQP4.



IE in range of -9.4 to -14.6 kJ mol-1[Figure S10(b)].In the 8DNAQ molecular layer, 8DNAQ molecules are connected via C−H•••N [2.64 Å, pink shades, Figure 9(f)] and C−H•••O (in the range of 2.19 to 2.70 Å) hydrogen bonds interactions [Table S3, Figure 9(f)] and the pairwise energies of these molecular layers are in the range of -10.7 to 30.0 kJ mol-1[Figure S10(c)]. All the molecules within a layer (both PY as well as 8DNAQ) are almost in a plane with respect to each other in 2D [Figure 9(d) and (e)]. DQP3 polymorph crystals were obtained concomitantly during solution evaporative crystallization of DQP2 in ACN solvent. The long needle shape crystal [red circle, Figure 1(b)]crystallized in the monoclinic system with P21/c space group andincludes one molecule of 8DNAQ and two half molecules of PY in the asymmetric unit [Figure S7(c)]. It has long needle shape which is flexible and bends under mechanical stress with bending angle exceeded almost 180° (Figure 6). In this polymorph, both nitro groups of 8DNAQ molecule are also twisted with respect to the ring, havingdifferent torsion angles for both groups (71o and 83o)[Figure S8(c)].It has a similar stacking pattern [‒A‒D‒A‒D‒, Figure 9(g)] as DQP1 stoichiomorph, but has different centroid to centroid distance and orientation. The molecular centroid distance between PY and 8DNAQ are in the range of 3.79 to 3.82 Å with IE in the range of -58.1 to 60.2 kJ mol-1 along the b axis [Figure 9(h) and S11(a)].The molecules within the PY layers as well as 8DNAQ layers are also not in a plane but are tilted at an angle of 24.12o [Figure 9(h)]. The PY molecule is integrated with 8DNAQ molecules via C−H•••O (in the range of 2.50 to 2.78 Å) hydrogen bonds interactions [Figure S11(b),Table S4]. In the 8DNAQ molecular layer, 8DNAQ molecules are integrated via C−H•••N (2.81 Å) and C−H•••O (in the range of 2.41 to 2.71 Å) hydrogen bonds interactions [Figure 9(i), Table S4] and the pairwise interaction energies of these molecular layers are in the range of -11.7 to -24.6 kJ mol-1[Figure S11(c)].In the PY molecular layerPY molecules are connected via weak H•••H (2.37 Å, the sum of van der Waal radii is 2.4 Å) contacts Figure S11(b).

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count of the strong twisting of the nitro groups, the crystal packing, is further stabilized via intermolecular interactions between the out-of-plane nitro groups and the carbon atoms, one nitro group interacts with the C=O group and the carbon atoms of the 8DNAQ ring and the other nitro group interacts only with the carbon atoms of the 8DNAQ ring. The distances lie in the range of 2.9433.031Å[Figure S12]. It has a similar layer pattern [‒ADA‒ADA‒, Figure 9(m)] as DQP2, having molecular centroid distance between PY and 8DNAQ in the range of 3.76 Å to 3.86 Å with IE in the range of -58.7 to -63.2 kJ mol-1, and the molecular centroid distance between 8DNAQ and 8DNAQ is 6.80 Å with IE of -9.5 kJ mol1 [Figure S13(a) and S13(b)].In this polymorph the PY molecule is also connected with 8DNAQ molecules via C−H•••O (in the range of 2.48 to 2.77 Å) hydrogen bonding interactions [Figure S13(c),Table S5] with the IE in the range of -8.3 to -15.6 kJ mol-1. In the 8DNAQ molecular layer, 8DNAQ molecules are also connected via C−H•••O hydrogen bonding interaction [Figure 9(o), Table S5] with the pairwise IE in the range of -12.0 to -30.5 kJ mol-1

The molecular stacking formed by PY and 8DNAQ layers pack into inter-locked columns parallel to the stacking direction along the a axis [pink color shadow, Figure 9(j)], leading to the absence of slip planes between the columns.6 The IE (-8.4 to -10.4 kJ mol-1) of the interlinked columns is very less when compared to the IE (57.8 to -61.3 kJ mol-1) of the layers of stacked molecules of PY and 8DNAQ [see the tube size in Figure 9(k), Figure S11(a)]. The phenomena of the mechanical bending can be attributed to the absence of slip planes along the stacking direction, since it prevents the permanent sliding (plastic bending) of molecular layers.6 DQP4: The DQP4 polymorph crystallized in the monoclinic system with P21/c space group and includes two molecules of 8DNAQ and one molecule of PY in the asymmetric unit [Figure S7(d)]. In this polymorph, both the nitro groups in 8DNAQ molecule are also twisted with respect to the ring, having different torsion angles for the nitro groups (70o and 79o) [Figure S8(d)]. On ac-

Figure 10. a) Structural transformation from DQP4 (left) to DQP2 (right) a) sliding of the layers. b) Reorientation of molecules along with molecular layers. c) Movements of the acceptor molecular layer (pink shadow) and d) Change in the energy frameworks during the phase transition.


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[Figure S13(d)]. The molecules within the PY layers as well as 8DNAQ layers are tilted at an angle of 11.88o with respect to each other [Figure 9(l)]. The DSC traces of DQP4 shows endothermic first-order phase transition to another polymorph (DQP2) between 95 to 115 oC on heating with a characteristic “sawtooth” profile of the thermal effect67that is characteristic for a thermosalient transition (Figure 4 and 5). The transition is irreversible, and there is no exothermic peak on cooling. The thermal analysis alone indicates that the phase formed after phase transition (i.e. DQP2) is more stable than initial form (i.e. DQP4). Inspection of the phase transition by HSM revealed that on heating in the range of 95 to 115 oC crystals of DQP4 shattered [perpendicular to the major face (1-1-1) of the DQP4 crystal] into small rhombus shaped crystals which retains its original crystallinity as they transform to DQP2 (Figure 4 and 5, Movie SM1). This SCSC phase transition is a demonstration of the thermosalient effect, a process where crystals acquire stress and strain due to sudden release of the elastic energy accumulated during a first-order phase transition.13,15,17,18,20 Such type of thermosalient effects where crystals move in response to a structural phase transition have been recently consider to be similar to martensitic transformation.20 Normally, the SCSC phase transition is accompanied by relatively small changes in the dimension of the crystal to retain the integrity of a single crystalline phase,6870 However some exceptionalcases are also reported having extraordinarily large change in initial and final lattices with preserved crystallinity.67In our case, there are also exists a substantial change in initial and final dimensions of the crystal along with retention of their crystallinity after the SCSC phase transition. To understand the origin of this change during SCSC phase transition, we investigated the change in packing patterns on the same crystal before and after the phase transition. The crystal symmetry during the phase transition changes from monoclinic, P21/c to C2/c with a small change of the unit cell volume.

Description of the mechanism of the SCSC phase transition In DQP4 polymorph crystal (with dimensions 0.296 × 0.453 × 1.569 mm) each PY molecule is integrated with four 8DNAQ molecules via C‒H•••O [Figure S13(c)]hydrogen bonding interactions along with other two 8DNAQ molecules via molecular stacking. It has offset layered molecular stacking along c axis with a repetition of three layers [‒ADA‒‒ADA‒, Figure 10(a)] with molecular centroid distance, between PY and 8DNAQ molecules lying in the range of 3.76 to 3.86 Å [IE in the range of -57.8 to -63.2 kJ mol-1, Figure S13(a)]and between8DNAQ molecules is 6.80 Å [IE -9.5 kJ mol-1Figure S13(b)].These intermolecular stacking interactions propagate through the structure along c axis. After the SCSC phase transition (with new dimensions 0.122 × 0.376 × 0.465 mm), the major face (1-1-1) of the DQP4 crystal

transforms to a new face (100), which is the major face of DQP2. The new phase (DQP2) still has the layered structure but there are significant differences in the intermolecular interactions, molecular stacking sequences, and molecular orientations. There are remarkable changes in molecular centroid distance within the repetition units (3.76/3.80 Å, IE -58.7/-59.8 kJ mol-1; 3.86/3.80 Å, IE 63.2 /-59.8 kJ mol-1) and between the repetition units [6.80/5.92 Å, -9.5/-38.8 kJ mol-1; Figure 9(d) and S13(a)].In the DQP4 polymorph, molecular packing are energetically more anisotropic because the IE within a repeating unit (‒A‒D‒A‒) is stronger than between the repeating unit (‒A‒A‒), which can be visualized from energy frameworks [left Figure 10(d)]. Upon application of external stimuli (heat) loosely bound layers moved along the c direction with rupture and reformation of stacking interactions [upper Figure 10(a)]. The movement of layers can be visualized in Figure 10(c) (pink rectangular shadow).Before phase transition in the (8DNAQ) molecular column consisting of acceptor molecules, the molecules are arranged in the face to edge pattern alternatively and after phase transition these molecules are rearranged in the face to face pattern. The distance between the molecular planes is also changed. Before the phase transition, the distance between the molecular planes consisting of donor molecules are 9.345 and 9.697 Å alternatively, and after the phase transition the distance between the same is increased to 10.387 Å [Figure 10(c)]. More importantly, there was a change in the molecular orientation. Before the phasetransition the donor molecules are tilted with respect to the adjacent molecule in the same layer with 11.38oand after phase transition this changed to 6.87o[Figure 10(b)]. This process of phase transition can be understood more clearly by considering only consecutive layer of donor molecules. Both polymorphs (before and after phase transition) differ in the relative positions of the donor molecules in the layers, which brings about different distance between the molecular centroids of PY, the values being 10.859/10.858 Å and 13.346 Å in DQP4 and DQP2, respectively [Figure S14(c)].In DQP4 the donor layers are oriented along c axis, and the alternative molecules of the same layer are tilted opposite to each other [blue arrow, Figure S14(b)];in DQP2 the donor layers are oriented along b axis, and all the molecules of the same layer are almost in a plane [Figure S14(b)].Furthermore, the lattice



Figure 11. Schematic representations of the mechanism of SCSC phase transition of DQP4 cocrystal polymorph on heato o ing from 25 C to 115 C.

Figure 12. 2D supramolecular constructs obtained from XPac analysis for DQP4 and DQP4_1.

parameters, which accompany the structural transformation, reflect the large contraction of c axes (22.66 %, Table 1) and expansion along the crystallographic a and b axes (55.22 % and 1.06 % respectively, Table 1) resulting in accumulated strain in the crystal interior with macroscopic modifications that are related to the SCSC phase transition in the crystal. The overall molecular movement during the process of SCSC (DQP4 to DQP2) phase transition is shown in Figure 11. The VTPXRD analysis suggest that there is another phase also present in between both phases (DQP4 and DQP2), to confirm that we have collected SCXRD data of DQP4 at 338 Kand these SCXRD analysisrevealed that, first of all the DQP4 polymorph convert into a 2D isostructural packing polymorph having almost same cell parameters (see DQP4 and DQP4_1, Table 2) in between 335 K to 340 K, and then finally into DQP2 in between 375 K to 388 K (Figure S4). The 2D isostructurality of the intermediate phase was confirmed by XPac program (version 2.0.2),71−73introduced by Gelbrich and co-workers. All the atomic coordinates (in crystal geometry) except the hydrogen atoms, were considered for the XPac analysis. The crystal structures of DQP4 and DQP4_1 are isomorphous in monoclinic P21/c space group with Z = 4. In both cases, three molecules (connected via π•••π stacking interactions) together act as a single unit (sandwich), and these units are arranged in a layer along the crystallographic c axis. A comparison between the crystal structures DQP4 and DQP4_1 reveals the presence of a 2D supramolecular construct (Figure 12) with a dissimilarity index value of 2.6.

Conclusions We have reported the formation and design of four novel cocrystals(two stoichiomorphs and their polymorphs)based on PY and 8DNAQ compounds. Interestingly, we have found that the cocrystals differ in stoichiometry (stoichiomorph) of the same constituents have different mechanical properties. Both cocrystal stoichio-

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morphs and their polymorphs have layered structure but there are significant differences in the intermolecular interactions, molecular stacking sequences, and molecular orientations. TheDQP2 stoichiomorph and their polymorph DQP4 have ‒ADAADA‒ type layered stacking and the other stoichiomorph DQP1 and their cocrystal polymorph DQP3 has‒ADADAD‒ type of layered stacking. The DQP4 shows thermosalient effect, on heating it converts into DQP2 polymorph (martensitic transformation) through shattering(perpendicular to the major face) of the crystal into many small crystals (rhombus shape) retaining their crystallinity via SCSC phase transition. Structure analysis of both the cocrystal polymorphs, before and after the phase transition indicates a plausible mechanism by which the differences in the layer packing sequences and molecular orientations in two polymorphs are responsible for this oriented phase transition. The DQP3 polymorph shows elastic bending due to interlock stacking of molecular columns and absence of slip planes. Herein, we have demonstrated that the structure-stacking modes and the observed mechanical property in cocrystals can be tuned through the variation in the stoichiometric ratio of the parent constituents with remarkable changes in crystal dimensions. This study not only provides insightsinto the observed differences in mechanical behavior ofdonor-acceptor coupled cocrystal stoichiomorphs based on PY and 8DNAQ, but also hints at the possibility of using such stoichiomorphs in the future as molecular self-actuating devices.

EXPERIMENTAL SECTION Materials All starting compounds pyrene (PY), 1, 8dinitroanthraquinone (8DNAQ) and 1,5dinitroanthraquinone (5DNAQ) with 99 % purity were purchased from Sigma-Aldrich Company and have been used without further purification.

Preparation of Stoichiometry Variant Cocrystal Polymorphs DQP1 and DQP2 were synthesized by cogrinding of PY and 8DNAQ in 1:1 and DQP4 in 1:2 molar ratio respectively, in a mortar-pestle for 45 min, 15 min neat grinding and other 30 by adding a catalytic amount (two or three drops) of MeOH solvent.74 The single crystal of DQP1 and DQP2 [red color crystal, Figure 1(a) and (b)] were obtained upon crystallizing approximately 5 mg of the ground material from ~3 mL of dichloromethane (DCM) and acetonitrile (ACN) respectively, via slow evaporation technique at room temperature. The single crystal of DQP3 [red color needle-shaped, Figure 1(b)] was concomitantly obtained during the cocrystallization of DQP2 in the ACN solvent. DQP4 was synthesized by cogrinding of PY and 8DNAQ in 2:1 molar ratio and their single crystal [red color crystal, Figure 1(c)] was obtained upon crystallizing approximately 5 mg of the ground material from ~ 5 mL of tetrahydrofuran (THF) also via slow evaporation technique at room temperature.

Single Crystal X-ray Diffraction (SCXRD)


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The single crystal X-ray diffraction measurements for all stoichiometry variant cocrystal polymorphs were carried out on a Bruker APEX II Kappa CCD single crystal diffractometer equipped with a graphite monochromator using MoKα radiation (λ = 0.71073 Å) at 100(2) K except DQP4_1 (SCXRD data was collected at 338 K). Unit cell measurement, data collection, integration, scaling and absorption corrections was performed using Bruker APEX II software.75 Multiscan absorption corrections were applied using SADABS.76 The structures were solved by direct methods using SHELXS-9777 and refined with Full-matrix least squares method using SHELXL-201478 present in the program suite WinGX.79 All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms bound to carbon were placed in the calculated positions. The packing diagrams of all cocrystals were generated using Mercury 3.6 software.80

Differential Scanning Calorimetry (DSC) and Hot Stage Microscopy (HSM) The DSC traces of all stoichiometry variant cocrystal polymorphs were recorded using a Perkin-Elmer DSC 6000 instrument under nitrogen gas atmosphere. The samples were precisely weighed between 1.0 to 2.0 mg and were placed in non-hermetic sealed aluminium pan in vacuum. The samples were scanned at a rate of 5.0 °C/min in the range of 30.0 °C –350.0 °C under a dry nitrogen atmosphere at a flow rate of 20 ml/min. Hot Stage Microscopy (HSM) experiments were performed on a stereomicroscope equipped with a hot stage apparatus (operating at a heating rate of 5°C/min) and the photographs and video were taken with a Leica polarizing microscope. The single crystal was placed on a glass slide and the images of the experiments were recorded.

Variable Temperature Powder X-ray Diffraction (VTPXRD) The variable temperature powder X-ray diffraction patterns for cocrystal stoichiomorphs were recorded on a PANalytical Empyrean X-ray diffractometer with CuKα radiation (1.5418 Å). The crystals were placed on a silica sample holder and measured by a continuous scan between 5 to 50° in 2θ with a step size of 0.013103°. PXRD profile fitting refinement was performed with JANA 200639 for the DQP4, DQP2 and DQP4_1stoichiomorphs with the lattice parameters obtained from SCXRD.

Energy framework calculations The energy framework analysis has been performed using CrystalExplorer1781 to visualize the intermolecular interaction topology in allstoichiometry variant cocrystal polymorphs. The “energy framework” was constructed based on the crystal symmetry and the energies are estimated from B3LYP/6-31G(d,p) molecular wave functions calculated at the crystal geometry, summing up the electrostatic, polarization, dispersion and exchange-repulsion terms based on a scaling factor of 1.057, 0.740, 0.871, and 0.618, respectively.82 The interactions energies below a certain energy threshold (4 kJ/mol) are omitted for clarity, and

cylinder thickness (80) is proportional to the intermolecular interaction energies.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Accession Codes CCDC 1847613, 1847615−1847618 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting TheCambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +91-0755-6692392.

ORCID Deepak Chopra: 0000-0002-0018-6007 Manjeet Singh:0000-0002-9081-2242

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Dr. M. S. thanks SERB for the National Post-Doc Fellowship (NPDF, Project No. PDF/2016/001385). We are also thankful to IISER Bhopal for the research facilities and infrastructure.

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Table of Contents

Synopsis: To understand the influence of stoichiometric ratio, in a combination of π-conjugated donor molecule pyrene and an acceptor molecule, 1,8-dinitroanthraquinone, in altering the crystal packing and their associated mechanical properties, two stoichiomorphs, have been synthesized. One stoichiomorph shows thermosalient effect, and on heating converts via SCSC phase transition into another polymorph while the other shows polymorph exhibits elastic bending.


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Diversity in Mechanical Response in DonorâAcceptor Coupled

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Diversity in Mechanical Response in DonorâAcceptor Coupled