Mechanically Flexible Organic Crystals Achieved by Introducing Weak Interactions in Structure: Supramolecular Shape Synthons

Article abstract of DOI:10.1021/jacs.6b05118

Controlling mechanical properties of ordered organic materials remains a formidable challenge, despite their great potential for high performance mechanical actuators, transistors, solar cells, photonics, and bioelectronics. Here we demonstrate a crystal

Full text of DOI:10.1021/jacs.6b05118

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Article
Mechanically Flexible Organic Crystals Achieved by Introducing
Weak Interactions in Structure: Supramolecular Shape Synthons
Gamidi Rama Krishna, RAMESH DEVARAPALLI, GARIMA LAL, and C. Malla Reddy
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Mechanically Flexible Organic Crystals Achieved by Introduc-
ing Weak Interactions in Structure: Supramolecular Shape
Synthons
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Gamidi Rama Krishna,‡ Ramesh Devarapalli,‡ Garima Lal, and C. Malla Reddy*
Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, Mo-
hanpur 741 252, India
Key words: Crystal Engineering, Supramolecular Synthon, Mechanical Properties, Structure-Property, Weak Inter-
actions, Flexible Crystals
ABSTRACT: Controlling mechanical properties of ordered organic materials remains a formidable challenge, despite their
great potential for high performance mechanical actuators, transistors, solar cells, photonics and bioelectronics. Here we
demonstrate a crystal engineering approach to design mechanically reconfigurable, plastically flexible single crystals (of
about 10) of three unrelated types of compounds by introducing active slip planes in structures via different non-
interfering supramolecular weak interactions, namely van der Waals (vdW), π-stacking and hydrogen bonding groups.
Spherical hydrophobic groups, which assemble via shape complementarity (shape synthons), reliably form low energy slip
planes, thus facilitate an impressive mechanical flexibility, which allowed moulding the crystals into alphabets, “o r g a n i
c c r y s t a l”. The study, which reports the preparation of a series of exotic plastic crystals by design for the first time,
demonstrates the potential of soft interactions for tuning mechanical behaviour of ordered molecular materials, including
those from π-conjugated systems.
1. INTRODUCTION
Utilizing collective molecular functions from highly dense
and grain boundary-free, ordered structures of organic
materials is currently attractive owing to their potential
applications in mechanical actuators,1,2 phototransistors,3,4
light-emitting diodes (LEDs),5 solar cells,6 photonics,7
integrated optical waveguides,8 bioelectronics,9 flexible
electronics10 etc. Although the intrinsic structure-function
tunability, light weight and low cost advantages of the
single crystalline organics over inorganic counterparts
make them highly attractive, the poor mechanical per-
formance at single particle level remains a great limitation
for their practical usage on flexible substrates.11 Ordered
single crystals with high mechanical flexibility compara-
ble to that of thin films, elastomers and polymers, can be
highly advantageous for future smart materials. Although
there have been increasing reports on various dynamic
single crystals that for example show plastic or elastic
flexibility,12-14 photomechanical effect,15-19 thermosalient
effect,20 twisting,21 most of these exotic single crystals are
Figure 1. Building blocks utilized for achieving flexible
from serendipitous discoveries.
single crystals. Derivatives that successfully resulted flexible
crystals are shown in black while those failed are in grey.
High plasticity, which is aimed in this study, can offer
some unique and diverse advantages in functional molec-
ular crystals, for instance, excellent mechanochromic lu-
minescence (change of emission color upon mechanical
action) in solid-state fluorophores,22,23 good tabletability
in API solid forms24,25 and lower detonation sensitivity in
explosives.26
These recent studies have revealed excellent structure-
mechanical behavior correlation, but to the best of our
knowledge not a single example was achieved by design. This
goal suffers from the requirement of control over local mo-
lecular movements, which is critical in ensuring effective
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dissipation of stress arising from structural functions. Stress
bonding interactions with themselves. Here, when these
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dissipation in such dynamic crystals occurs typically via the
exchange or modulation of weakest intermolecular interac-
tions, such as van der Waals (vdW), C─H∙∙∙O, lighter halo-
gens, π∙∙∙π or combination of these multiple forces in the
structures, which is vital for preserving the monolithic nature
of crystals during such macroscopic manifestations.27
hydrogen bonding groups engage themselves by forming
zero dimensional (0D) homosynthons, say carboxylic acid
dimers, the other two weaker type interactions shall guide
the rest of the self-assembly process independently and
self-sort to pack with themselves. This in turn shall lead
to the formation of slip planes by vdW groups between
the π-stacked columns of molecular units; a favourable
condition for plastic flexibility. Notably, the vdW and π-
stacking groups, both of which are weak and largely dis-
persive, due to the slight difference in the interaction na-
ture and shape similarity of identical groups are expected
to promote self-sorting and limit the possibility of
synthon cross over, thus shall follow the proposed pack-
ing model.
Although the importance of the weak dispersive inter-
actions is long acknowledged in crystal engineering28 and
materials chemistry,29 they have been perceived as poorly
dependable, hence remained largely unexpended for ac-
complishing predetermined structural architectures that
perform particular tasks. To overcome such burden, here
we targeted the least interfering supramolecular shape
synthons ― synthons formed via hydrophobic groups by
shape complimentary isotropic interactions ― to ration-
ally design mechanically flexible plastic single crystals.
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To test this crystal engineering hypothesis and its gen-
eral applicability, we chose three unrelated classes of
molecules, i) N-substituted naphthalene diimides (NDIs),
ii) substituted benzenes and iii) Schiff bases. Each class is
unique and serves a different purpose: the NDI building
blocks consisting of rigid covalently linked core group,
represent the π-conjugated semiconducting molecular
materials; the substituted benzene carboxylic acids and
amides act as supramolecular building blocks where the
core is uniquely constructed by hydrogen bonded acid
dimer (0D) or amide (1D or 2D) synthons, hence act as
prototype models for synthon based crystal engineering;
Schiff bases, consisting of non-rigid unsymmetrical build-
ing blocks with a greater conformational freedom, belong
to the interesting class of compounds with solid-state
properties such as photochromism33 and thermosalient
nature.34
2. RESULTS AND DISCUSSION
2.1 On Plastic Bending Model: Earlier hypothetical mod-
el12,30 and recent mechanistic studies27 on plastically bend-
able crystals suggest that bending proceeds by segrega-
tion of bent section into flexible layers that slide on top of
each other via reorganization of weaker intermolecular
interactions (vdW and stacking), thereby generating do-
mains with slightly different lattice orientations.
Nanoindentation studies by Desiraju and Ramamurty
suggest that shear sliding of the crystallographic planes
dominates the mechanism of plasticity in molecular crys-
tals rather than other mechanisms such as dislocation
movement.31,32 Hence, we targeted for a structural setup
with a combination of weak interactions, namely vdW
(for shear sliding) and π-stacking (for splaying of mole-
cules to accommodate the bend angle) in perpendicular
direction to achieve plastically bendable crystals.12,27,30
2.3 Covalent π-conjugated Building Blocks: Alkyl Shape
Synthons
In this series, we synthesized35 several naphthalene
diimides (NDIs), with alkyl vdW substituents such as me-
thyl (1a), n-propyl (1b), p-tolyl (1c) and tert-butyl (1d) on
either ends of the central NDI core, which act as covalent
building blocks (Figure 1). The π-conjugated NDIs have
attracted much current interest in materials science and
supramolecular chemistry owing to their striking elec-
tronic (n-type semiconductors) and spectroscopic proper-
ties, and ease of preparation. The crystallization typically
resulted in long needles, with lengths of up to 1 cm, and
were suitable for X-ray structure determination (crystal-
lographic information is given in SI, Table S1). Although
we synthesized some halogen substituted NDI analogues,
their poor solubility prevented further progress.
2.2 Design Strategy for Achieving Plastically Flexible Sin-
gle Crystals
To move from serendipity to design, we considered the
following crystal engineering strategy. A variety of nonin-
terfering groups, namely spherical vdW groups and aro-
matic groups with or without hydrogen bonding func-
tionalities (Figure 1) are strategically targeted. The near
spherical vdW groups can potentially close pack with
themselves to form smooth slip planes in the structure
while the central aromatic core groups tend to close pack
in perpendicular direction, typically via π-stacking (Figure
2). In some cases, a third set of dominant hydrogen bond-
ing groups were employed, as they also do not interfere
with the above two, but prefer to form strong hydrogen
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Figure 2. Illustration of the supramolecular shape synthon based, slip plane model for achieving plastic flexibility in
single crystals. (a) Building block, constructed via covalent or hydrogen bonded linkages, possessing central core (in red) and
peripheral vdW (in blue) groups. (b) Self-assembly of building blocks into target structural model with slip planes (broken blue
lines), driven by shape synthon forming vdW groups. (c) Structural model showing the sliding of flexible molecular sheets along
slip planes upon crystal bending. (d) Prototype naphthalene diimide (1a) based covalent building block and (e) its crystal pack-
ing, which resemble the target slip plane model in (b). (f) Micrographs of a flexible single crystal of 1a, before (left) and after
(right) mechanical bending. (g) Analogie is drawn to compare the bending of sticky yellow pad and plastic crystal via slippage of
individual sheets and slippage of molecular layers, respectively (in c and f). Notice the similarity of the top ends of the bent crys-
tal (image on right, in f) and sticky note pad in (g). (h) Scanning electron microscopy image of the mechanically bent crystals of
1a.
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elongated n-propyl groups and (d) arrangement of the next
layer, where the n-propyl groups pointing in opposite direc-
tion to that of the first, hence interlocked to prevent slip-
page. (e) The brittle mechanical fracture of 1b crystals.
Analysis of crystal structures of 1a (monoclinic P21/c),
1b (orthorhombic Pbca), 1c (monoclinic P21/c) and 1d (tri-
clinic P-1) by visual inspection revealed that in all the cas-
es the vdW groups close pack with themselves to form the
shape synthons (isotropic interactions) as anticipated
while the aromatic groups form π-stacked columns in all
the solids of this series. Notably in case of 1a, 1c and 1d
the shape synthons align along a crystallographic plane,
thus result in slip planes (except in 1b), showing an excel-
lent agreement with our target weak interaction based
self-assemble model. For instance, in 1a the central NDI
aromatic core group forms π-stacked columns while the
Figure 3. Crystal packing and mechanical behavior of 1b
spherical methyl groups close pack with themselves via
and 1d. (a) Crystal packing in 1d with smooth slip planes
vdW shape synthons and form the slip planes parallel to
formed by shape synthons involving spherical tert-butyl
(0 0 2) (Figure 2e). Similarly, the compounds 1c (SI, Figure
groups. (b) Plastic and highly flexible 1d crystals mould into
S1), 1b and 1d (Figure 3a) also formed the π-stacked col-
letters “I I R S”, where first “I” corresponds to an undisturbed
umns separated by slip planes. Notably, in case of 1b the
elongated vdW groups (n-propyl) from adjacent stacks
face in opposite direction.
reference crystal and the second “I” a crystal bent-and-
reconfigured back to original morphology. (c) A layer of 1c
molecules in bc-plane showing zipper type arrangement of
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Mechanical behavior of the crystals was examined by a
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simple qualitative method, i.e. applying mechanical stress
using a pair of forceps and a metal needle while viewing
them under a stereomicroscope.30 To our delight, the sin-
gle crystals of 1a, 1c, and 1d showed excellent plastic flexi-
bility, while notably 1b underwent brittle fracture. The
crystals of 1a, 1c and 1d could be bent to 360° (Figure 2h;
videos S1, S2 and S3 corresponding to 1a, 1c and 1d respec-
tively). To demonstrate this impressive plastic flexibility,
we mould the crystals of 1a (Figure 2h) and 1d (Figure 3b)
into a variety of shapes, such as letters “S E M” and “I I R
S”, respectively. Such plastic flexibility is unprecedented
in π-conjugated organic crystals. The bent crystals remain
monolithic without much damage to the morphological
integrity, even at the extreme bend angles such as in “S”
and “M”. All the crystals bend only on one pair of faces
that is parallel to the slip plane, while break on the per-
pendicular pair of faces under similar conditions (see vid-
eo S4).
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In case of 1b (brittle) the elongated ─CH2─CH2─CH3
vdW groups from stacked columns face in opposite direc-
tion, unlike in the case of spherical ─Me, ─t-Bu groups.
Although the shape synthons are formed in this structure
as well, the uneven surface probably prevents smooth
sliding of molecular slides and make these crystals brittle
(Figure 3c, 3d; video S4).36 This suggests that the nearly
spherical vdW groups, such as –Me and –tBu, that with
smooth potential energy surfaces, can potentially be ex-
ploited to form active slip planes for plastic bending.
Figure 4. Halogen shape synthons on supramolecular
building blocks: Crystal letters: Crystal packing in (a)
2,3-dichlorobenzoic acid, 2a, and (b) 3,4-dichlorobenzoic
acid, 2b, to show the formation of shape synthons by Cl-
groups (in green) and 0D acid dimers (in red). Flexible
crystals of (c) 2a, 2b, 2c and 2d modulated into letters “o r
g a n i c c r y s t a l”.
π-stacking interactions. As anticipated, the Cl groups in
both 2a and 2b not only form type I Cl∙∙∙Cl (in 2a: 3.486Å,
159.79°, 124.83°; in 2b: 3.492Å, 172.34°, 116.70°) shape
synthons, but also aligned to form slip planes parallel to
(0 0 1) in both cases. This is very significant as despite the
different geometrical positioning of Cl groups in 2a (2,3-)
and 2b (3,4-), they form the slip planes in a similar man-
ner, demonstrating the driving potential of Cl groups for
forming the shape synthons. Not only this, the crystals of
both 2a and 2b showed high plastic bending flexibility
upon mechanical action, demonstrating the striking
agreement with the proposed model.
2.4 Hydrogen Bonded Building Block: Halogen Shape
Synthons
As crystal engineering strategies that work very well in
one particular series may fail in another, we have extend-
ed the study to other molecules, including those with
hydrogen bonding groups. The molecular units employed
in this series possess a carboxylic acid or amide group.
The supramolecular dimer formed by these groups acts as
a central core, in place of the rigid covalent moiety in the
NDI series (Figure 1). The vdW groups here are mostly
halogen groups (Cl, Br), except in 2e (-tBu), which are
substituted on aromatic ring. The lighter halogens are
expected to form soft halogen∙∙∙halogen (X∙∙∙X) interac-
tions.37-39 Halogen interactions are generally classified into
two categories: type I (geometrical) and type II (chemical,
polarization based). The type I X∙∙∙X interactions, which
are more favored by lighter halogens Cl and Br, are non-
polar and typically interact based on shape complementa-
rity, hence qualify as shape synthons (softer type) desired
for our model. On contrary, heaviest halogen group, io-
dine prefers the polarization based stronger type II X∙∙∙X
interactions (harder type),37 hence is not desirable.
The monohalo-compounds, 2c (2-chlorobenzoic acid)
and 2d (2-bromobenzoic acid), also form dimers that pack
into columns in the structure. The slip planes are also
formed in them, but are not entirely by the halogen
groups. The single halogen group (less surface area, com-
pared to the dihalo-compounds) and the edge-to-edge
aromatic interactions (which are also vdW type) (SI, Fig-
ure S2) contribute to the slip plane. These two crystals
also show plastic flexibility, which is demonstrated by
shaping them into letters “o r g a n i c c r y s t a l” (Figure
4c). While the simple benzoic acid fails to show plastic
flexibility, perhaps due to the absence of any soft vdW
groups and presence of more specific C−H∙∙∙O interac-
tions between slip stacked molecules in the structure (SI,
Figure S3).
The dichloro compounds, 2a (C2/c) and 2b (P21/c),
form the carboxylic acid dimers (0D) via cyclic O─H∙∙∙O
hydrogen bonds (Table S2, Hydrogen bond table). These
dimers further pack into columns along b-axis with only
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Figure 5. Schematic representation of brittle fracture in amides. (a) Formation of building block and (b) packing of mole-
cules into hydrogen bonded sheets, separated by slip planes. The strong extended H-bonds make the sheets rigid. (c) Tight pack-
ing of molecules within the rigid sheet prevents splaying of molecules, hence fracture ensues. (d) Amide based molecular build-
ing block. (e) Formation of 1D N-H∙∙∙O hydrogen bonds and supramolecular shape synthons via Cl∙∙∙Cl interactions. (f) Illustra-
tion of the molecular packing with 1D hydrogen bonded sheets separated by slip planes in the crystal. Sliding of layers requires
the movement of entire sheet at a time due to its rigidity; splaying of molecules is also restricted by extended hydrogen bonding,
thus the crystals break instead of plastic bending. (g) Fracture of crystals of 3a upon application of a mechanical stress.
To test if the presence of slip planes in crystals alone is
sufficient or the absence of extended hydrogen bonds is
also needed for plastic flexibility, some primary amide
analogues were also studied. Because, in contrast to the
0D acid dimers, amides typically form extended 1D chains
or 2D sheets due to the presence of additional N─H
group. We succeeded to synthesize and obtain suitable
single crystals of amides, 3a (3,4-dichlorobenzamide) and
3b (p-tertiarybutyl benzamide, see Figure 1). Remarkably,
in both 3a and 3b the vdW groups again close pack with
themselves across a plane in the crystals to form the de-
sired slip planes (Figure 5). However, both the crystals
were found to be brittle and showed no sign of plastic
flexibility.
2.5 Unsymmetrical Building Blocks with Multiple Types
of vdW Groups
Contrary to the symmetrical building blocks in the two
earlier categories, in this series there are some unsymmet-
rical Schiff bases of a pharmaceutically significant o-
vanillin compound, with greater conformational flexibility
and multiple types of vdW groups. All the building blocks
in this series, which are easy to prepare, have a common
─OMe group (from vanillin) on one end and ─OMe, ─Cl,
─Br, ─Me, or ─t-Bu on the opposite end (from substitut-
ed anilines), while the central part consists of ─C═N─
(Figure 1). Hence, this increased complexity (unsymmet-
rical molecular geometry with different functional groups
on two ends and greater conformational flexibility) pro-
vides a good test for assessing the reliability and generali-
ty of our proposed design model.
This suggests that the presence of strong N─H∙∙∙O in-
teractions in 1D and 2D networks may make the structure
rigid, hence can prevents local molecular movements in
crystals (Figure 5). Moving entire sheet or layer at a time
for mechanical shearing is energetically expensive, i.e. the
shear stress becomes greater than fracture stress. Hence
the hydrogen bonded sheets may act as rigid sheets rather
than flexible sheets. This suggests that in addition to the
“softness” of the X∙∙∙X or R∙∙∙R interactions, the absence of
rigid extended interactions in the structure is critical in
providing such malleability to crystals. Stronger, but also
“harder” interactions such as hydrogen bonding as seen in
3a and 3b would accumulate larger stress at short dis-
tances, and contribute to enhanced stiffness to cause frac-
ture of the crystal.
Analysis of crystal structures revealed that majority of
the structures are indeed consistent with the proposed
model (Figure 6(a) & 6(b)). For instance, in 4a the slip
planes are formed by the two ─OMe groups on either
ends (SI, Figure S6) while the aromatic groups form the π-
stacking interactions in the orthogonal direction, which
result in a corrugated packing. The compounds 4b and
4c, which are isostructural (P212121), also form slip planes
by close packing of ─OMe, ─Cl/─Br and aromatic C─H
groups (Figure 6(a) & 6(b)). On the other hand, com-
pounds, 4d and 4e, with purely alkyl substituents (─Me,
─t-Bu groups, respectively, on the aniline side) failed to
form slip planes, unlike in the successful cases of 1a and
1d with the same groups, respectively. The qualitative
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mechanical tests revealed that the crystals of 4a to 4c are The fair success of the model in three above classes of
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highly flexible and bend plastically, while 4d and 4e did
not show plastic flexibility under similar conditions. In
the latter two crystals, the crystal structures adopt the π-
stacked layer arrangement with no slip planes by the vdW
groups.12 The failure of vdW groups to form slip planes in
this complex system can be attributed to the interference
from electronegative groups (formation of sp3C-H…O be-
tween –OMe groups) on these molecules (SI, Figure S7,
S8). Nevertheless, the model has facilitated a fair success
even in this complex series.
molecules demonstrates the potential of weaker interac-
tions for reliably achieving desired structures, which we
feel is important in the context of crystal engineering. The
successful examples of ten plastically flexible crystals
from the three different types of compounds demonstrate
that the proposed model has potential to be transferred to
other functional materials, while the failed cases provided
further insights into the structural requirement for plastic
flexibility. The study may act as a good starting point to
further improvise the strategies to design future stimuli
responsive dynamic crystalline materials.
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3. CONCLUSIONS
Here we successfully demonstrated a crystal engineer-
ing strategy to achieve plastically flexible single crystals
by introducing active slip planes in the structure by care-
fully placing selected non-interfering van der Waals (such
as ─t-Bu, ─OMe, ─Me, ─Cl/Br etc.), π-stacking and hy-
drogen bonding groups, on three different classes of
compounds. The key factor for achieving the desired crys-
tal packing with low frictional planes, as required by the
bending model, is primarily the non-interfering and self-
sorting nature of the selected functional groups, i.e. (i)
the van der Waals (vdW) groups, (ii) hydrogen bonding
groups that form 0D cyclic hydrogen bonded dimers, and
(iii) the aromatic π-stacking groups. The dispersive nature
of the vdW groups that with spherical shape, have favored
formation of smooth molecular planes with least attach-
ment energy (i.e. active slip planes) in the structure. The
vdW shape synthons act as lubricated planes and allow
molecular motions with minimal friction between molec-
ular sheets. These groups allow sliding of molecules upon
mechanical action while the π-stacking interactions aid
the flexibility of sheets during mechanical bending. The
absence of extended rigid hydrogen bonding networks is
also a key factor for facilitating plastic flexibility in crys-
tals. It appears that the strong self-sorting and non-
interfering nature of the three distinct types of groups
employed here is not only important for driving crystal
packing, but also probably critical for preserving the short
range structural order and promoting coordinated molec-
ular movements in the process of crystal bending. The
three diverse series studied here emphasize the potential
of various weak vdW interactions for achieving plastic
flexibility. Further, these unexploited weak interactions
can also allow tuning the properties of diverse types of
stimuli responsive organic solids. The well coordinated
local molecular movements involved here are comparable
to the supramolecular structural changes that generally
occur in many other dynamic crystalline materials.
Hence, our first design approach potentially has broad
implications for the materials chemistry. Quantification
of the mechanical properties by nanoindentation tech-
nique on the number of crystals studied here is planned
for future.
Figure 6. Formation of shape synthons in unsymmet-
rical Schiff base building blocks. (a) Packing in (E)-2-((4-
chlorophenylimino)methyl)-6-methoxyphenol, 4b, and (b)
(E)-2-((4-bromophenylimino)methyl)-6-methoxyphenol, 4c,
show the formation of slip planes by multiple vdW groups, –
OMe (yellow) and Cl (green)/Br (grey). (c) Plot shows the
first three lowest attachment energies of all the studied mol-
ecules. In all the cases, the first lowest Eatt shows a good
agreement with the visually identified slip plane.
2.6 Calculated Attachment Energies: Identification of
Low Energy Active Slip Planes
The attachment energy, Eatt, is the energy released upon
attachment of a growth slice to the growing face of a crys-
tal. The Eatt = Elatt – Eslice where Elatt is the lattice energy,
Eslice is the energy released on the attachment of a growth
slice of a thickness equal to the inter planar d-spacing for
the crystallographic plane representing the face. Calculat-
ed Eatt data of all the crystals in this study (SI, Table S3)
shows an excellent agreement with the slip planes identi-
fied by visual inspection of respective crystal structures.
Notably, in all the plastic bending crystals, the slip planes
formed by the specifically chosen vdW groups for this
model always possess the lowest Eatt, as evident from the
listed Eatt values of the three weakest planes of each struc-
ture, shown in Figure 6c (also see SI, Table S3). Hence,
this not only justifies the choice of our vdW groups, but
also their reliable transferability to other molecules.
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4. EXPERIMENTAL SECTION
fined with the ShelXL47 refinement package using Least
1
Squares minimization.
4.1 Materials. All the reagents were purchased from Sigma-
Aldrich. Commercially available solvents were used (for syn-
thesis and crystallization) as received without further purifi-
cation.
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4.5 Scanning Electron Microscopy (SEM). High resolution
SEM was performed on a Zeiss microscope; SUPRA 55VP-
Field Emission Scanning Electron Microscope. High perfor-
mance variable pressure FE-SEM with patented GEMINI col-
umn technology. Schottky 5 type field emitter system, single
condenser with crossover-free beam path. Resolution: 1.0 nm
at 15 kV; 1.6 nm at 1 kV high vacuum mode. 2.0 nm at 30 kV
at variable pressure mode.
4.2 Synthesis
4.2.1 NDI derivatives. N,N'-bis(methyl)-1,4,5,8-naphthalene
diimide (1a), N,N'-bis(n-propyl)-1,4,5,8-naphthalene diimide
(1b), N,N'-bis(p-tolyl)-1,4,5,8-naphthalene diimide (1c), and
N,N'-bis(p-tertiarybutylphenyl)-1,4,5,8-naphthalene diimide
(1d) were synthesized by condensation of the carboxylic acid
dianhydride with two equivalents of the respective substitut-
ed amines in DMF at temperatures above 110°C for over-
night.35 All the obtained compounds were purified by recrys-
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4.6 Attachment Energies. The attachment energies of all
the crystals were computed by Crystal Graph software using
Dreiding forcefield and Gesteiger charges. The attachment
energies were computed at medium quality using inputs
from Crystal Graph.
1
tallization and characterized by using H-NMR and X-ray
structure determination.
ASSOCIATED CONTENT
1H-NMR data, crystal packing diagrams with respective crys-
tal images depicting mechanical deformation, videos, table
containing crystallographic information, geometrical param-
eters of hydrogen bonding, ORTEP diagrams and a table of
attachment energy values for each compound are provided as
supplementary information. This material is available free of
charge via the Internet at http://pubs.acs.org.
4.2.2 Amides. 3,4-dichloro benzamide (3a) and 4-
tertiarybutyl benzamide (3b) were prepared from previously
reported procedure.40
4.2.3 Imines. All five imines 4a, 4b, 4c, 4d and 4e were pre-
pared by solid state mechanical grinding of O-vanillin and
respective substituted aniline taken in 1:1 stoichiometric ra-
tio.41
AUTHOR INFORMATION
4.3 Preparation of single crystals. The derivatives of all
NDIs were recrystallized from dichloromethane and all re-
maining compounds were recrystallized from diethyl ether
by slow evaporation at ambient conditions. Crystals suitable
for testing mechanical properties and single crystal data were
obtained in 4 to 5 days in all the cases.
Corresponding Author
* cmreddy@iiserkol.ac.in or cmallareddy@gmail.com
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript. ‡ These authors contributed equally.
4.4 Single crystal X-ray structure determination. Single
crystals of all the compounds were individually mounted on
a glass pip. Intensity data of all the compounds (except 2e, 3b
& 4e) were collected on a Bruker’s KAPPA APEX II CCD Duo
system with graphite-monochromatic Mo Kα radiation (λ =
0.71073 Å). The data for 1a, 1c and 3b were collected at 100 K
and all other compounds were collected at 296 K. Data re-
duction was performed using Bruker SAINT software.42 Crys-
tal structures were solved by direct methods using SHELXL-
97 and refined by full-matrix least-squares on F2 with aniso-
tropic displacement parameters for non-H atoms using
SHELXL-97.43 Hydrogen atoms associated with carbon atoms
were fixed in geometrically constrained positions. Hydrogen
atoms associated with oxygen and nitrogen atoms were in-
cluded in the located positions. Structure graphics shown in
the figures were created using the X-Seed software package
version 2.0.44
ACKNOWLEDGMENT
C. M. R. acknowledges financial support from the CSIR
(02(0156)/13/EMR-II). G. R. K., R. D. and G. L. thank IISER-
Kolkata for fellowship. The authors thank Dr. Tejender S.
Thakur (CSIR-Central Drug Research Institute, Lucknow,
India), for the attachment energy calculations.
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