Crystal engineering of hand-twisted helical crystals

Article abstract of DOI:10.1021/jacs.6b11835

A strategy is outlined for the design of hand-twisted helical crystals. The starting point in the exercise is the one-dimensional (1D) plastic crystal, 1,4-dibromobenzene, which is then changed to a 1D elastic crystal, exemplified by 4-bromophenyl 4'-chlorobenzoate, by introduction of a molecular synthon -O-CO-in lieu of the supramolecular synthon Br···Br in the precursor. The 1D elastic crystals are next modified to two-dimensional (2D) elastic crystals, of the type 4-bromophenyl 4'-nitrobenzoate where the halogen bonding and C-H· · ·O hydrogen bonding are well-matched. Finally, varying the interaction strengths in these 2D elastic crystals gives plastic crystals with two pairs of bendable faces but without slip planes. Typical examples are 4-chlorophenyl and 4-bromophenyl 4'-nitrobenzoate. This type of 2D plasticity represents a new type of bendable crystals in which plastic behavior is seen with a fair degree of isotropic character in the crystal packing. The presence of two sets of bendable faces, generally orthogonal to each other, allows for the possibility of hand-twisting of the crystals to give grossly helical morphologies. Accordingly, we propose the name hand-twisted helical crystals for these substances.

Full text of DOI:10.1021/jacs.6b11835

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Article
Crystal Engineering of Hand-Twisted Helical Crystals
Subhankar Saha, and Gautam R. Desiraju
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b11835 • Publication Date (Web): 12 Jan 2017
Downloaded from http://pubs.acs.org on January 13, 2017
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Crystal Engineering of Hand-Twisted Helical Crystals
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Subhankar Saha and Gautam R. Desiraju*
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
Keywords: Plastic crystal, elastic crystal, property design, halogen bond, supramolecular synthon
ABSTRACT
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A strategy is outlined for the design of hand-twisted helical crystals. The starting point in the
exercise is the one-directional (1D) plastic crystal, 1,4-dibromobenzene, which is then changed
to a 1D elastic crystal, exemplified by 4-bromophenyl 4’-chlorobenzoate, by introduction of a
molecular synthon −O−CO− in lieu of the supramolecular synthon Br···Br in the precursor. The
1D elastic crystals are next modified to two-directional (2D) elastic crystals, of the type 4-
bromophenyl 4’-nitrobenzoate where the halogen bonding and C−H···O hydrogen bonding are
well matched. Finally, varying the interaction strengths in these 2D elastic crystals gives plastic
crystals with two pairs of bendable faces but without slip planes. Typical examples are 4-
chlorophenyl and 4-bromophenyl 4’-nitrobenzoate. This type of 2D plasticity represents a new
type of bendable crystals in which plastic behavior is seen with a fair degree of isotropic
character in the crystal packing. The presence of two sets of bendable faces, generally orthogonal
to each other, allows for the possibility of hand twisting of the crystals to give grossly helical
morphologies. Accordingly, we propose the name hand-twisted helical crystals for these
substances.
1. INTRODUCTION
Crystal engineering of organic molecular solids1 has been practiced for the last 50 years or so.
Initially, it was recognized that there is a connection between crystal structure and crystal
property; this is exemplified by the work of Schmidt,2 Wegner,3 Thomas4 and others. Solid state
physical and chemical properties were explained on the basis of intermolecular interaction
guided molecular arrangements in the crystals. The term “crystal engineering” was invoked by
Schmidt2 more as a challenge and in terms of setting long term goals. This period also saw
development of various models for crystal structure, notably by Kitaigorodskii,5 Desiraju6 and
Etter,7 to explain observed crystal structures in terms of the packing features and/or the
intermolecular interactions involved. The emphasis next shifted to actual strategies and logic
driven methodologies of crystal design: the concept of the supramolecular synthon was identified
as being crucial to the planning of crystal synthesis.8,9 This phase of the subject continues till
today with the strong modular capability of the synthon being applied in both structural
chemistry10-16 and biology.17-20 But the general emphasis was on structure rather than property
design. In the present work we have tried to take a step in this latter direction and to alter
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properties, even drastically, within a particular structure type. In this sense, the strict
structure‒property correspondence seen in earlier times need not necessarily always be observed.
We suggest that one may now design a desired solid state property starting with a crystal that
does not have it, rather than optimize a property in a crystal that has been obtained through an
initial serendipitous result. There is a subtle shift in emphasis in that the design strategy is
developed heuristically, but of course with structural inputs. Taking up from the design of soft
flexible crystals,31,33 this work is concerned with plastic and elastic crystals.
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The original 1989 definition of crystal engineering6 foresees the development of the subject as:
(a) understanding of intermolecular interactions in the context of crystal packing; (b) design of
new solids where design pertains to structure only and; (c) obtaining desired physical and
chemical properties as described in the present paper. These stages telescope chronologically
and this is natural.
Fig. 1: Development of crystal engineering.
Crystal property design is an attractive and rapidly growing area of organic crystal engineering
today. Mention should be made of work on thermo- and photosalient materials,21-24 colored
crystals,25 semiconductor crystalline materials,26,27 self-healing crystals,28 mechanically
deformable crystals,29-34 pharmaceutical compounds35,63 and others.36,64 Flexible solids, which
include irreversibly bendable plastic and reversibly bendable elastic crystals are of core interest
because of applications to many fields such as optical devices,37,38 organic electronics,39,40
artificial mechanosensors,41 molecular machinery,42 muscle-mimetic biomaterials,43 smart
nanomaterials44 and pharmaceuticals.45-47 In general, one-directional (1D) plasticity with a single
pair of opposite bendable faces results from slip plane assisted structural anisotropy,29 whereas
isotropic interlocked packing gives elasticity.32 These studies and others show the different
structural and interaction requirements for plastic and elastic deformation and bending of
molecular solids.
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Naturally occurring helical or twisted crystals have been known for a few centuries.48 These
twisted architectures are sought out today as functional materials targeting specific optical,
electric or catalytic properties, negative-index metamaterials or invisible materials, and
enantiosensitive plasmonic sensors.49 Recent developments in lithography techniques and
metamaterials have stimulated new interest in older studies on chiroptic materials.50 These
materials are characterized by different absorptions of circularly polarized light which makes for
different refraction. Helical crystals are generally formed through different methods of
crystallization such as slow evaporation of solvent, sublimation, or diffusion limited growth in
different types of gel matrix.49 All these techniques are highly uncertain and hardly predictable to
obtain helical crystal for any given unknown system. In this scenario, a predictable and easy
technique for crystal growth would be useful. Nevertheless, no strategy has been adopted so far
to manually fabricate twisted crystals. Crystal engineering principles are relevant here because
they begin with a bottom-up approach from intermolecular interactions. Twisted crystals
represent changes in the bending direction in a periodic way. This suggests the requirement of
multiple deformation directions in the crystals to make them helical or twisted.
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2. RESULTS AND DISCUSSION
In this paper, we report a number of crystals with multiple sets of plastically bendable faces,
which we then show are twistable. In general, the incorporation of plasticity and multiple
bendable faces within the same crystal would appear to be contradictory29 in terms of the
respective structural requirements. Multiple bendable faces would imply more crystal isotropy,
and this opposes the requirements of plastic bending which requires interaction anisotropy. The
incorporation of various mechanical properties is achieved in a stepwise and gradual manner
(Fig. 2).
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Fig. 2: Flowchart of design steps in the present work.
The initial step starts with a common one-directional (1D) plastic crystal with clear cut
interaction anisotropy. In the next step, this is changed to elastic by varying molecular
orientation so that more isotropic, interlocked packing is favored.32 The 1D elastic crystals are
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next modified to two-directional (2D) elastic through further structural changes.51-53 Finally, the
interaction strengths54,55 in the 2D elastic crystal are changed using known chemical principles,
so that the isotropy is decreased to the extent that plasticity ensues. However, because the basic
structural topology is not changed, there are no slip planes in these two-directional (2D) plastic
bendable crystals. Most of these modular crystal structures are sustained through halogen
bonds,56,57 which are of critical importance in the entire design exercise. The presence of more
than one pair of bendable faces makes these compounds more effective towards deformation.
This last class with 2D plasticity represents a new type of plastically bendable crystals. The
application of hand applied twisting along the lengths of the 2D plastic crystals produces
permanent twisting. Application of the same manual procedure (hand twisting) on common 1D
plastic crystals failed to achieve the same result. In a topological context, we present here the
design of hand-twisted helical crystals of achiral molecules without the use of external chiral
additives.
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We now describe the four steps of the property design.
Step 1: Selection of 1D plastic crystal: 1,4-Dibromobenzene, 1 and strategies towards elasticity
in Step 2
Fig. 3: (a) π···π and Br···Br type-II contacts in the crystal structure of 1 with a slip plane (lilac).
(c) Plastic deformation of a crystal of 1 on (001) face. (b) Br···Br synthons are replaced by
molecular synthons (Z) in Step 2 so that the rings can be oriented along different directions (red
and green arrows) in the model system.
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1, 4-dibromobenzene (1) crystallizes in the P21/n space group (Z’= 0.5). The phenyl rings are
stacked (3.53 Å) along the needle direction [101] (Fig. 3a). These stacked columns are extended
along the [010] direction with Br···Br (3.77 Å, θ1=93.9°, θ2=165.7°) type-II halogen bonds,
constructing slip planes. There are also C–H···Br (d: 3.24 Å) dimer synthons along the same
direction. The third direction [001] also consists of C–H···Br (3.14 Å) hydrogen bonds. A rod
like crystal supported from (001) plane was poked on the opposite face, resulting in permanent
distortion (Fig. 3c), or plastic bending. But it was brittle from the other faces and also towards
external twisting force (see ESI, S12). This results in one-directional plastic bending only. This
is a common compound and 1D plasticity is also common enough a phenomenon now, but this is
the first report of 1D plastic deformation in this compound.
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Design of an elastic bendable crystal from a plastic crystal: The next step involves upgrading 1
to an elastic crystal. In other words, how does one incorporate more isotropic character in the
packing? For this, we considered some molecular modifications. In 1, weak halogen bonded
adjacent stacked columns face along different directions (Fig. 3a: red and green dashed arrows, ~
57°). The breaking of these weak halogen bonds between differently oriented adjacent columns
is expected to lead to permanent molecular movement or plastic deformation. In order to restrict
the breakage, alternate Br···Br supramolecular synthons were replaced with molecular synthons,
namely linker Z (Fig. 3b). Covalent linking of two Ph-rings along the molecular axis prevents
independent migration of columns.32 To increase isotropic packing the –Ar–Z–Ar– compound
selected should be non-planar, and further should be easy to make. A Schiff base (Z is –CH=N–)
linker proved to be reasonable in this regard,32 but suffers due to moisture sensitivity. As an
alternative, the amide linker –NHCO– similar to the imine group with respect to size and shape,
was selected.
Choosing the molecular linker. CSD study: The question, however, is whether or not the amide
linker can consistently maintain the phenyl rings in a non-planar orientation. The compounds P1
and P2 (see ESI S13) were prepared for this. The crystals of amide molecules produced
undesired plasticity, instead of elasticity. We tried to obtain the crystal structures of the amides
many times, but poor internal crystal quality restricted us in so doing. The failure of the model
initiated us to use the Cambridge Structural Database (CSD) to ascertain the behavior of amides
in this context. The CSD was searched for simple amides, Ph−NHCO−Ph, wherein the 2,6
positions on the two phenyl rings are substituted only with H-atoms. 461 hits were obtained. The
histogram (Fig. 4a) shows a preferential orientation for torsion angles in the range 40-80° (202
molecules, 43%) which is useful for our purpose. But there are also hits with angles in the range
0-20° (172 molecules, 37 %) which is not so desirable. Therefore, it was felt that the amide
linker is not completely reliable to achieve a non-planar molecular structure. Perhaps the –
NHCO– fragment allows sufficient conjugation between the two phenyl rings. An isoelectronic
fragment, say an O-atom with less electron donating ability, could lower the tendency towards
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conjugation and with it a planar structure. So we selected phenol esters where the linker Z is –
OCO–. The CSD analysis for these compounds (Fig. 4b) shows a strong preference for inter-ring
torsion angles between 40-90° (379 out of 415, 91 %), showing that the ester group –OCO– is
more suitable than the amide –NHCO– as a linker for non-planarity.
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(a)
(b)
Fig. 4: CSD (November 2014, including updates till May 2015) studies of: (a) amides,
PhNHCOPh and; (b) esters, PhOCOPh to show inter-ring angles
Step 2: One-directional (1D) elastic bendable crystals
(a)
(b)
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(c)
Fig. 5: Crystal packing and representative synthons in form I of 2b: (a) Torsion of phenyl rings
at an angle of ~ 48°; (b) Halogen bonds between π-stacked columns involving Br (type-II) and Cl
(type-I) continue along [001]; (c) Schematic presentation of crystal morphology with marked
faces. Molecules show unidirectional packing along [001].
Considering the ester group as a linker, we prepared compounds 2a-2d. 4-Chlorophenyl 4-
bromobenzoate (2b) has two polymorphs, forms I and II. Both crystallize in the P21/c space
group (Z’=1) with phenyl-phenyl torsion angles 48° and 57° respectively (Fig. 5a). In form I,
molecules are π···π stacked (3.45 Å) along the b-axis. The Br-atoms form type-II Br···Br (3.59
Å, θ1=127.0°, θ2=165.1°) and the Cl-atoms form type-I Cl···Cl (3.63 Å, θ1=θ2=157.8°) contacts
(Fig. 5b). The halogen bonded synthons continue along [001] to form parallel tapes, forming the
one-directional (1D) packing (Fig. 5c) in the ac-plane. These parallel tapes are connected via C–
H···O (2.56 Å) and C–H···Cl (3.2 Å) hydrogen bonds. The comparable energy of interactions
along orthogonal directions makes this crystal structure isotropic. Form II also has an isotropic
structure with type-II (3.86 Å, 82.7°, 148.3°) and type-I (3.9 Å, 142.3°) Br···Br synthons (Fig.
6). But the presence of C–H···π (2.84 Å, 3.05 Å, 3.03 Å) interactions removes the layering of
π···π stacked molecules. 2c and 2d are isostructural to the form I of 2b (see ESI, S5 and S7). The
packing of 2a (ESI, S5-S7) is also very similar with type-II halogen bonds on both sides. There
are also some other compounds (see compound B1 in ESI S14) which do not follow the model
and not discussed further. Form-II of 2b is of this type. Thus while the strategy though works, it
is not 100% efficient. But it is good enough to make further progress.
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Fig. 6: Representative supramolecular synthons in form II of ester 2b.
(a)
(b)
Fig. 7: (a) Bending of a 1D elastic crystal of 2b (form I) from the (001) face (1 through 6). This
operation may be performed repeatedly without crystal breakage. (b) Attempt to bend the same
crystal from the other face parallel to the needle axis broke the crystal immediately.
Mechanism of elastic bending: Crystals of compounds 2a through 2d, including polymorphs,
were tested for bending. About 5 mm long, thin, straight crystals of 2a, 2b (form I), 2c and 2d
were supported from the major face [(100) for 2a and (001) for others] and then poked on the
opposite face, resulting in a bent morphology which was restored to the initial state after removal
of force, i.e. elastic crystals (Fig. 7a) (also see ESI video). The experiment may be repeated.
When these crystals were poked on the other faces along the needle axis, they broke (Fig. 7b)
(see the last part of the video of 2b form I). These crystals can only be bent from one face, hence
1D elastic. The form II of 2b, without π···π stacking, was found to be brittle towards external
mechanical stress (ESI, S10), suggesting the importance of π···π stacking for elasticity.
A possible mechanism for elastic bending is illustrated in Fig. 8. The direction of the halogen
bonds are perpendicular to the bending face and are at equilibrium in the pristine crystal. During
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bending (Fig. 8b) the π···π stacking acts as a spring, permitting regular structural changes at the
bent site where all the halogen bond distances and angles may not be consistent. At the inner
side, the packing is little compressed which may result in shorter halogen bond distances and
angles. There is a concomitant expansion at the outer side causing slightly longer halogen bond
distances and larger angles. The middle portion remains largely unaffected. The polarizable
nature of halogen bonding allows for these deformations. These structural deformations during
bending destabilize the system with respect to the initial thermodynamic equilibrium state and
generate mechanical strain at the site of bending. At this stage, electrostatic type-II halogen
bonds, which because they operate at longer distances, can bring the whole system back to its
initial thermodynamic position, relieving the bent crystal from the stress. This is why the bending
phenomenon is elastic.
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(a)
(b)
Fig. 8: Schematic representation of the bending mechanism, (a) before bending and (b) during
bending.
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Step 3: Design of two-directional (2D) elastic bendable crystals
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Fig. 9: Schematic illustration of (a) parallel halogen bonded (XB) tapes present in 1D elastic
crystals (X= Y= halogen) in this study and (b) design strategy for 2D elastic crystals through C–
H···O (X= halogen, Y= nitro) hydrogen bonds (HBs).
Strategy to make 2D elastic bendable crystals: Access to more than one major bendable direction
(here it is two) requires nearly mutual perpendicular orientations of the molecules (Fig. 9), in
other words a herringbone arrangement (Fig. 9b). The literature shows that orthogonal C–H···O
interactions are potential contacts to maintain such an arrangement.51-53 Compound E3 however
is seen to display only1D elasticity, and there are no side-to-end orthogonal C–H···O interactions
(see ESI S15). Then we used the NO2 derivative and prepared 4-iodophenyl 4-nitrobenzoate (3a)
and 4-iodophenyl 3-methyl-4-nitrobenzoate (3b) (Fig. 2). These crystals showed the desired 2D
pattern, suppressing the problematic issues. The structures are explained hereafter.
Compound 3a crystallizes in the orthorhombic space group P212121 with Z’= 1 and the phenyl-
phenyl torsion angle is 53°. Molecules are π···π stacked (3.45 Å) along the a-axis. The type-II
I···I (3.82 Å, 130.5°, 165.1°) halogen bonds (Fig. 10) connect the stacked columns. These
halogen bonded units with NO2 groups at the end form C–H···O (2.48 Å, 2.58 Å, 2.55 Å)
synthons in a side-to-end fashion along [011] and [011] (Fig. 10b), which eventually orients the
molecules at an angle of ~ 45° besides keeping them parallel, like in the 1D cases discussed
earlier. Such orientation makes the packing identical in both major faces (011) and (011) (Fig.
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10c). The C–H···O (2.40 Å) synthons involving ester and C–H···I (3.36 Å) interactions hold the
adjacent tapes. Compound 3b is isostructural to 3a (see ESI, S5 and S7).
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Long, thin crystals of both 3a and 3b were found to be elastic (see ESI video) when poked on
any of the major faces, (011) or (011). The availability of two bending directions makes the
crystal 2D elastic. The mechanism of deformation is very similar to the 1D case, but here one
end of each molecule is involved in similar halogen bonds whereas the other end has C‒H···O
hydrogen bonds. The system shows reversible bending supported with combined influence from
halogen and hydrogen bonds.
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(a)
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(c)
Fig. 10: Crystal packing and some typical synthons in 3a. (a) Type-II I···I halogen bonds (violet)
exist along [011] and [011], (b) C–H···O (NO2) (red) synthons are present along the same
directions, other interactions like C–H···O (carbonyl) (green), C–H···I (yellow) are also
highlighted. (c) Schematic shape of crystal is shown with labeled faces. C–H···O hydrogen
bonds orient the molecules along two different directions, [011] and [011]. The packing in both
(011) and (011) planes are similar.
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Step 4: Two-directional (2D) plastic bendable crystals
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Methodology: Since halogen bonds play a role for elasticity in compound 3 and other previously
reported cases, lowering their strength vis-à-vis the C–H···O bonds with retention of the 2D
geometry could affect this particular property. It has been shown that varying halogen bond
strength by replacing higher halogens with lower halogens can affect the mechanical property.54
The change in mechanical property (plastic to elastic) by changing the halogen bond strength
within isostructural systems is well documented.55 In this context, the strength of halogen bonds
of the step 3 compounds was varied by substituting the I-atom in 3 with Br and Cl, which are
expected to form weaker halogen bonds (compounds 4a-4e, Fig. 2). For comparison, the
approximate energies of a I···I, Br···Br and Cl···Cl interaction are 10, 7 and 5 kJ/mol,55 and are
comparable with the C–H···O bonds in the structure.
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Fig. 11: Packing in 4a identical to 3a. Type-II Br···Br (lilac) and C–H···O (NO2) (pink) contacts
persist along two directions, [011] and [011].
Compounds, 4a-4e are isostructural to 3a (ESI, S5-S7). They also have C–H···O (NO2) (2.50 Å,
2.61 Å, 2.52 Å) and type-II halogen bond mediated 2D orientation (Fig. 11) as seen previously in
step 3.
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Plastic bending: The bending property of crystals of each compound was tested in a similar
manner. When a crystal of 4a supported from the (011) face was poked on the opposite face, it
took a curved shape (Fig. 12), but did not regain the original shape upon unloading of the
deformation force. Hence the bending is irreversible and thus plastic. All the other compounds in
step 4 exhibit the same behavior.
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Fig. 12: Images of progressive plastic bending in a crystal of 4a on the application of external
mechanical force.
2D bending: To check the 2D plastic bending property, an undistorted crystal of 4c was first bent
(position 1) on either of the major faces along the needle axis (Fig. 13). Thereafter this bent
crystal was further bent on the other face (position 2) along the needle axis. In the Fig. 13a the
two different directions of bending are difficult to visualize. The same crystal is then viewed
from two different angles separately. In Fig. 13b, the bent position 2, parallel to the plane of
paper is clearly visible. But position 1 is directed nearly perpendicular to the plane of the paper
and appears nearly straight, and vice versa for Fig. 13c. These clearly point to the presence of
two different directions for plastic bending in nearly isotropic, interlocked crystals. This type of
behavior deviates from earlier structure-property rationalizations which are based on the idea that
isotropic packing leads to elastic behavior.
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Fig. 13: 2D plastic bending of 4c. (a) A crystal bends at two positions (1 and 2) along two
different directions. In (b) and (c) the same crystal is oriented in such a way so that only one
bending side is visible. The slanted arrow in (c) shows direction of one of the bending faces and
the other one represents the bending direction perpendicular to plane of the paper. The angle
between two bend faces is ~ 45°, which is similar to the crystallographically determined angle.
The replacement of relatively strong I···I interactions by weaker Br···Br or Cl···Cl interactions
means that the restoring capacity of the crystal (which is needed for elasticity) is reduced to the
extent that permanent molecular movement is again seen. That is why step 3 compounds are
elastic, but isotropic (at the molecular level as well as at the crystallographic level) while step 4
compounds are plastic. This explanation raises a question as to why the same atoms (Br, Cl) with
similar type-II geometries exhibit a different bending property, elastic and plastic, for step 2 and
step 4 compounds respectively. In order to answer this question, we calculated the electrostatic
potential (ESP) surface maps for Br in the two model compounds 5 and 6 (Fig. 14). The higher
ESP value for the Br (137 kJ/mol) attached to the electron withdrawing carbonyl aromatic ring,
compared to the Br (118 kJ/mol) connected to electron donating phenolic part suggests the
presence of stronger halogen bonding ability in the former (5), that is in the step 2 examples.
Thus the weaker nature of the type-II Br···Br interactions result in plasticity in the step 4
compounds. Generally, our observations are in accord with the fact that the shear modulus of
these crystals is of the order of 5 GPa (for a measured Young’s modulus = 15 GPa),65 and with a
crystal size of 5×0.2×0.2 mm3, it is expected that one needs around 0.15 J/mm3 in other words, ~
28 kJ/mol to plastically deform the crystal (see ESI for details). This is comparable with the
energies of individual halogen and weak hydrogen bonds involved (~7 kJ/mol). The implication
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is that these types of interactions could be quite useful in the design of similar elastic and plastic
organic crystals.
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Fig. 14: Electrostatic potential (ESP) maps for model systems 5 and 6.
Hand-twisted helical crystals: The crystals of the step 4 compounds were examined and tested
for hand-crafted twisting. Initially we faced many difficulties in performing the twisting
experiment on straight crystals due to their small size. Then a butter (waxy) paper based template
method was applied to do the same. A straight crystal (Fig. 15a) of 4b was placed at the middle
of a rectangular shape butter paper-1 (Fig. 15b1) and covered with another similar size paper-2
(Fig. 15b2). The two ends of the papers were then gently hand held. Then, the papers were
twisted slowly across the plane of the paper (as in a toffee wrapper) to a torsion angle of ~ 150°
(Fig. 15b3). The twisting force was then retracted (Fig. 15b4). The pair of butter papers acts both
as holder and support during the process of hand mediated twisting of the soft crystal. The
resulting crystal was examined under the microscope. The enlarged images in Fig. 15c1-c3 show
the presence of twist in the crystal. The twist is clear at the middle and less clear at the bottom
due to the fracture of the crystal. Other compounds of this series also show similar twisting. This
whole process describes the manual crafting of twisted crystals which we prefer to accordingly
term hand-twisted helical crystals. A similar experiment on the crystal of common 1D plastic
1,4-dibromobenzene (1) results in breakage of the crystal into pieces without any sign of twist
(ESI, S12).
Helical or twisted crystals are crystals wherein there is a bending in different directions in a
grossly periodic way (Fig. 16). The manual twisting can be explained as combination of multiple
steps. First, any position of a crystal starts to bend along anyone of the two bending directions
(arrow-1, coming out of the plane of the paper). At the same time, the adjacent positions begin to
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deform along the other direction (arrow-2 on the plane of the paper). Here the bending along two
different directions is possible because of the existence of two bending faces in the crystal. This
process can continue “periodically” to form a complete helical shaped crystal.
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Fig. 15: (a) An undistorted crystal of 4b before performing the twist experiment. (b1) The same
crystal on the butter paper-1 and (b2) covered with another butter paper-2. (b3) Twisting of the
butter paper encapsulated crystal by hand. (b4) After releasing the twist force. (c1-c3) Enlarged
view of the hand-twisted helical crystal as seen from different angles. The twisting at the middle
is marked with red dotted lines.
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Fig. 16: A schematic twisted crystal with opposite directions of bending creating a torque, shown
as arrow-1 (out of the plane) and arrow-2 (in the plane bending), the simultaneous application of
which generates a helical shape.
3. CONCLUSIONS
We have shown here that pre-identified properties in a molecular crystal, in this case plastic and
elastic mechanical deformation, may be deliberately engineered with a controlled variation of
intermolecular interactions. In the present example, a series of aromatic compounds including
halogenated phenyl benzoates are a convenient template to illustrate these ideas. We started with
a normal plastic crystal namely 1,4-dibromobenzene and have, in succeeding steps, introduced
one- and then two-directional elasticity, and finally by halogen bond manipulations, two-
directional plastic deformation. The fact that a nominally isotropic packing can lead to plastic
deformability is a new paradigm in the mechanical property engineering of molecular crystals.
There is accordingly an evolution in the border line region between elastic crystals and new
generation plastic crystals. When does an isotropic packing lead to elasticity and when does it
lead to two-directional plasticity is an open question that this study throws up. We also show that
it is not really necessary to have a slip plane to induce plastic deformation in a molecular crystal.
As far as helical shaped crystals are concerned, these have been known since long and these
morphologies are reported as occurring under certain conditions of crystal growth and solvent.
The present work shows that one may apply a deliberate crystal design strategy that can be
applied in a predictable way to obtain helical crystals to obtain the first hand-twisted helical
crystals. Crystal engineering has been practiced for roughly fifty years and it is now possible to
discern certain historical trends from the early years. The early days of crystal engineering dealt
with structure-property correlations based on intermolecular interactions. Next, there was a shift
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in emphasis towards synthon based logic-driven design of desired crystal structure with
predictive ability. At the present time, and in the example provided in this paper, it is seen that
both structure and property are modular attributes. Therefore both can be subjected to design
strategies, perhaps independently. Whether or not such exercises would constitute a new third
generation of crystal engineering, is something that can only be known if and when more
examples are revealed.
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There is still another aspect of this study that is of relevance especially when crystal engineering
is taken as a form of supramolecular synthesis.8 It should be clear to the practitioner of synthesis
that any synthetic plan, such as one based on synthon based retrosynthesis, is a general outline
which provides a road map for actions, but it is only the experiments done in the laboratory that
determine the actual route taken—and in the end, these details are everything because they take
the synthesis “through the region of uncertainty”.66 The role of imagination and intuition cannot
be denied in synthetic chemistry. Attempts to disguise the intuitive component in any synthetic
exercise through some kind of thought readjustment, and a repackaging of partial thoughts that
preceded the successful synthetic step in a more causative idiom, falsely allows one to imagine
that the process of discovery arose entirely from intellectual thinking and a purely logic driven
reductionist thought process.67 The steps described above for the design of a 2D plastic crystal
are just that, a menu or a recipe for crystal engineering of such materials. The general strategy
would work in many cases. But the success in carrying out the plan would depend on how one
addresses details that emerge in the actual experiments. In the end, difficult syntheses are not
only difficult to carry out but also difficult to communicate. The transition between reason and
imagination,10 between science and art is not at all new to organic synthesis.68 It is our contention
that this phase has surely arrived now in crystal engineering, and that it will be contained only
by the imagination of its practitioners.
4. EXPERIMENTAL SECTION
4.1 Synthesis and crystallization: Synthesis and crystallization procedures of all step 2 to step 4
compounds are given in supplementary information (S1).
4.2 Single crystal X-ray diffraction (SCXRD): X-ray data of all crystals were collected on a
Rigaku Mercury 375R/M CCD (XtaLAB mini) diffractometer using graphite monochromator
Mo Kα radiation, with an attached Rigaku low temperature gas spray cooling device. The data
were processed with the Rigaku CrystalClear 2.0 software.58 Structure solution and refinements
were executed using SHELX9759,60 implemented in the WinGXsuite.61 All the non-hydrogen
elements were refined anisotropically and hydrogen atoms were refined isotropically. Hydrogen
atom positions were fixed based on riding model. The crystallographic data are given in the
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supplementary information, (S3). The face indexing images of the crystals are given in the ESI,
S4.
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4.3 Powder X-ray diffraction (PXRD): Powder X-ray data (ESI, S8) were collected on a Philips
X’pert Pro X-ray powder diffractometer with attached X’cellerator detector. The sample was
scanned for 2θ = 5°‒40°.
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4.4 Differential Scanning Calorimetry: The DSC (ESI, S2) data were collected on a Mettler
Toledo DSC 823e instrument with a heating rate of 5°C per minute in a liquid nitrogen
atmosphere.
4.5 Computation: Electrostatic potential surface maps were calculated for model bromoaromatics
5 and 6. All the DFT calculations were performed in Gaussian 09 using B3LYP function and 6-
311G (d, p) as basis set.62 Experimentally determined atom coordinates were used as input in
Gaussian for each molecule. Compound 5 was prepared and the crystal structure was determined
in this regard. Coordinates for 6 were taken from the CSD. Electrostatic potential surfaces were
obtained by molecular electron density 0.004 electron/Å. Color coding was used to locate
positively charged (blue) and negatively charged surfaces (red).
ASSOCIATED CONTENT
Supporting Information
Synthesis and crystallization, DSC diagrams, crystallographic information table, face indexing
images, crystal packing of 2a, 2c, 2d, 3b, 4a, 4b, 4c and 4e, representative synthons in 1D elastic
2a and 2D plastic 4a, crystal structure description, PXRD of 4d, images of elastic bending of 3a,
image of brittle crystal of 2b (form II), X-Ray diffraction images before and after bending and
image of a crystal of 1,4-dibromobenzene before and after butter paper supported twisting test,
these can be found in the supporting information which is available free of charge via the internet
at http:// pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*desiraju@sscu.iisc.ernet.in
Notes
The authors declare no competing financial interests.
ACKNOWLEDGEMENTS
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SS thanks the CSIR for a SRF. GRD thanks the DST for the award of a J. C. Bose fellowship.
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Crystal Engineering of Hand-Twisted Helical Crystals
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Subhankar Saha and Gautam R. Desiraju*
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Crystal structures with similar topology of molecules may have quite different mechanical properties because
of small relative differences in intermolecular interactions.
263x191mm (150 x 150 DPI)
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