Mechanical Properties of a Series of Macro- and Nanodimensional Organic Cocrystals Correlate with Atomic Polarizability

Article abstract of DOI:10.1021/jacs.5b07873

A correlation between Youngs modulus, as determined by using nanoindentation atomic force microscopy (AFM), and atomic polarizability is observed for members of a series of cocrystals based on systematic changes to one cocrystal component. Time domain spectroscopy over terahertz frequencies (THz-TDS) is used for the first time to directly measure the polarizability of macro- and nanosized organic solids. Cocrystals of both macro- and nanodimensions with highly polarizable atoms result in softer solids and correspondingly higher polarizabilities.

Full text of DOI:10.1021/jacs.5b07873

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Communication
Mechanical Properties of a Series of Macro- and Nano-
Dimensional Organic Cocrystals Correlate with Atomic Polarizability
Thilini P. Rupasinghe, Kristin M. Hutchins, Bimali S Bandaranayake, Suman Ghorai, Chandana Karunatilake,
Dejan-Krešimir Bu#ar, Dale C. Swenson, Mark A Arnold, Leonard R. MacGillivray, and Alexei V Tivanski
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07873 • Publication Date (Web): 25 Sep 2015
Downloaded from http://pubs.acs.org on September 25, 2015
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Mechanical Properties of a Series of Macro- and Nano-Dimensional
Organic Cocrystals Correlate with Atomic Polarizability
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Thilini P. Rupasinghe, Kristin M. Hutchins, Bimali S. Bandaranayake, Suman Ghorai, Chandana
Karunatilake, Dejan-Krešimir Bučar, Dale C. Swenson, Mark A. Arnold, Leonard R.
MacGillivray*, and Alexei V. Tivanski*
Department of Chemistry, University of Iowa, Iowa City, IA. 52242-1294, United States
Supporting Information Placeholder
and compressibility, which is inversely proportional to
ABSTRACT: A correlation between Young’s modulus, as
Young’s modulus, has been also demonstrated in
halomethanes.11 A linear relationship has also been noted
between Young’s modulus and binding energy within
graphene nanoribbon materials, which was attributed to the
decrease in molecular polarazability.12 No such relationship
has been discussed in the context of organic crystalline
solids. Moreover, given that elastic properties of nano-sized
materials may differ considerably from larger and extended
particles,18 it will be of paramount importance to determine
relationships between polarizability and mechanical
properties to facilitate the rational design, or crystal
engineering,7,8 of novel multi-component solids with
controllable and useful physical-chemical properties.
determined by using nanoindentation atomic force
microscopy (AFM), and atomic polarizability is observed for
members of a series of cocrystals based on systematic
changes to one cocrystal component.
Time domain
spectroscopy over terahertz frequencies (THz-TDS) is used
for the first time to directly measure the polarizability of
macro- and nano-sized organic solids. Cocrystals of both
macro and nano-dimensions with highly-polarizable atoms
result in softer solids and correspondingly higher
polarizabilities.
Studies on the mechanical properties of organic crystalline
solids have gained significant attention owing to potential
applications in fields such as pharmaceutics, electronics, gas
storage, biophysics, explosives, and device fabrication.1-5
Mechanical property characterization is essential, for
example, to achieve materials with increased tabletability in
pharmaceutics and to understand structural changes (e.g.,
phase transitions).6 For device fabrication, especially in
flexible electronics, knowledge of mechanical properties is
important to establish and optimize operational limits.7
Understandings of mechanical properties can also provide
insights into relative strengths of intermolecular interactions
in solids (e.g., hydrogen bonds), which can serve to link size-
dependent structural properties (e.g., elasticity) that span
atomic to macroscopic levels.8,9,10
Scheme I. Mechanical properties of cocrystals
(res)·(4,4’-bpe) and (4,6-di-X-res)·(4,4’-bpe) (X = Cl,
Br, I).
In this context, polarizability is a property of a chemical
system that describes the tendency of charge distribution to
be distorted in response to an external electric field. At the
atomic level, polarizability increases as volume occupied by
electrons increases, although much less is known regarding
polarizabilities of molecules, assemblies of molecules, and
those of corresponding bulk material solids. There is
emerging evidence that polarizability can be inversely related
to the stiffness of a material as measured by its Young’s
modulus.11-145 Specifically, an inverse relationship between
stiffness and atomic or molecular polarizability has been
observed for metals, oxides, covalent crystals and polymers.9,
Herein, we present Young’s modulus and polarizability
measurements on a series of macro- and nano-dimensional
organic cocrystals composed of either resorcinol (res) or 4,6-
di-X-res (X = Cl, Br, I) and trans-1,2-bis(4-pyridyl)ethylene
(4,4’-bpe) (Scheme I). From atomic force microscopy (AFM)
nanoindentation measurements,18-23 we show that both
macro- and nano-sized cocrystals display a decrease in
Young’s modulus when the size of the substituent is
increased from parent res (H) to Cl to Br to I. A correlation
between the Young’s modulus and polarizability is
demonstrated through both measurements and DFT
calculations. Terahertz (THz) time domain spectroscopy
(TDS) is also used to directly measure the polarizabilities of
14-17
A linear relationship between molecular polarizability
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Figure 1. X-ray structures: (a) 1, (b) 2, (c) 3, and (d) 4 highlighting
layered packing of assemblies. AFM planes probed for macro-sized
crystals highlighted in red. Offset layers highlighted in gray.
the solids and, in doing so, verify the AFM measurements.
Our work, thus, also establishes THz-TDS as a convenient
and practical method to gain insight into relationships
between atom-to-bulk properties of organic solids.
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Single-crystal X-ray experiments reveal each solid to
exhibit a closed hydrogen-bonded tetramer of molecules
sustained by four O-H···N hydrogen bonds (O···N separations
(Å): O(1)···N(1) 2.71(1), O(2)···N(2) 2.76(1) 3; O(1)···N(1) 2.77(1),
O(2)···N(2) 2.69(1) 4) (Fig. 1). Cocrystals 1, 3, and 4 are
isostructural, crystallizing in the triclinic space group Pī,
while 2 lies in the monoclinic space group P21/n.
Our study involves macro- and nano-sized cocrystals of
(res)·(4,4’-bpe) 1, (4,6-di-Cl-res)·(4,4’-bpe) 2, (4,6-di-Br-
res)·(4,4’-bpe) 3 and (4,6-di-I-res)·(4,4’-bpe) 4. Macro-sized
crystals of 1 and 2 were generated as reported.24,25 Macro-
sized crystals of 3 and 4 were formed via slow solvent
evaporation of 4,4’-bpe and the appropriate res (1:1 ratio) in
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EtOH. Formulations of 3 and 4 were confirmed using H
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The tetramers in each cocrystal self-assemble to form
NMR spectroscopy and single-crystal X-ray diffraction.
Nano-sized cocrystals of 1-4 were readily generated using
sonochemistry12 (See SI).
offset layers. The tetramers of
1
interact in the
crystallographic (042) plane via face-to-face π-π forces (C···C:
3.44 Å) of the olefins (Fig. 1a). The layers stack offset along
the b-axis and interact via C-H(pyridine)···O(res) (C···O: 3.33,
3.56 Å) and C-H(pyridine)···π(res) forces (C···C: 3.68 Å).
Tetramers of 2 interact in the crystallographic (103) plane via
Cl···Cl forces (3.55 Å). The Cl···Cl interactions are classified
as Type II (|θ1-θ2| = 36o),26 which define halogen bonds
between the res molecules (Fig. 1b). The layers also stack
along the c-axis via C-H(pyridine)···π(res) forces (C···C: 3.63
Å). For 3 and 4, the tetramers interact in a layer via O···Br
(3.16 Å, 3) and O···I (3.22, 4) forces, respectively (Figs. 1c,d).
While the O···Br and O···I distances are shorter than the van
der Waals distances, the interactions are Type I (|θ1-θ2| 2o (3)
and 2o (4)), which is consistent with strong effects of close
packing.26-28 The layers stack offset, interacting via C-
H(pyridine)···π(res) forces (C···C: 3.63 Å 3, 3.61 Å 4).
Characterizations of Young’s moduli for both macro- and
nano-sized samples of 1-4 were carried out using AFM
nanoindentation.7,18 The macro-sized cocrystals of 1-4
exhibited prism morphologies with bases ca. 0.30 × 0.05 mm
and heights of ca. 0.05 mm. Top and bottom crystal faces
that correspond to the crystallographic (001) and (00-1)
planes of 1, (10-1) and (-101) planes of 2, and (0-10) and (010)
planes for 3 and 4 were directly probed by AFM. The planes
probed for 1 correspond to the long-axis of a hydrogen-
bonded tetramer within a layer, extending along the c-axis
(Fig. 1a). The planes for 2 are also within a layer, extending
along the a-axis (Fig. 1b). The planes for 3 and 4 bisect
neighboring layers that sit along the b-axis (Figs. 1c,d).
Repeated force-displacement curves were recorded on the
macro-sized cocrystals at ca. 100 positions of both top and
bottom faces. Top planes correspond to the initial phase of
crystal growth while bottom planes towards the end. Two
conclusions are drawn from the histogram data of the
Young’s moduli measurements (Fig. 2, left column and Table
1).29 First, the top faces for 2-4 show 10-20% higher Young's
modulus values relative to the bottom, while the Young's
modulus for the top face of 1 is ~20% lower than that for the
bottom. The differences may reflect the presence of either
hydrogen-bond-donor or -acceptor groups at the individual
crystal surfaces (i.e. either res or 4,4’-bpe at face).18,30 More
importantly, the AFM measurements of the cocrystals show a
general dependence of the Young’s modulus on the nature of
the res substituent (Fig. 3a). Specifically, we observe an
increase in crystal softness that is correlated with an increase
in atomic polarizability11 (H = 0.67, Cl = 2.18, Br = 3.05, I = 4.7
Å3).
The relationship is particularly noteworthy for
isostructural 1, 3 and 4, where the change across the series
can be directly attributed to the identity of the H and
halogen atoms.
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We next studied nano-sized cocrystals (bases ca. 0.8 × 0.8
measure of polarizability as related to dielectric properties.
THz-TDS is rapidly-developing technique that uses
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µm and heights of ca 50-200 nm) where the measurements
are expected to yield an orientation averaged response and,
thus, are considered more reflective of bulk properties18 (Fig.
2, Table 1). Specifically, similar to the macro-sized solids, the
nano-dimensional cocrystals of 1 exhibited the highest
Young’s modulus values, followed by 2, 3, and 4. Thus, a
systematic decrease in Young’s modulus was also observed
with increasing atomic polarizability from H to Cl to Br to I
(Fig. 3a). The decrease in stiffness was on the order of 55%
from H to Cl, 25% from Cl to Br and 42 % from Br to I. The
nano-sized X-substituted cocrystals also displayed a size-
dependent increase in stiffness compared to the macro-sized
solids (33% for 2, 120% for 3, 95% for 4).11,18
a
electromagnetic radiation (0.3 to 4.0 THz) to provide
information related to vibrations in crystalline solids,
intermolecular interactions in liquids, and rotational
transitions in gases.31-33 Dielectric constants were, thus,
determined for 1-4 from 10 to 20 cm-1. The constants were
determined within 4-5 min by measuring the delay in a THz
pulse transmitted across a pressed pellet composed of 5%
cocrystal
embedded
within
a
matrix
of
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polytetrafluoroethylene. Average values from measurements
over several pellet preparations were used to calculate
polarizability using the Clausius-Mossotti relationship (see
SI).34,35 Given the nature of the dielectric measurements, the
resulting polarizability values correspond to ensemble
measurements over a population of crystals randomly
oriented with respect to the optical axis of the THz pulse,
thereby, representing an average property of 1-4.
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Table 1: Young’s modulus (YM) and polarizability (α)
measurements
(crystallographic
plane
in
SAMPLE
1-macro
YM (MPa)
α (Å3)
1600 ± 350 (00-1)
1250 ± 350 (001)
270 ± 25 (10-1)
235 ± 25 (-101)
120 ± 15 (0-10)
95± 10 (010)
54 ± 3 (0-10)
40 ± 2 (010)
570 ± 200
88.5 ± 1.6
2-macro
3-macro
4-macro
93.1 ± 0.7
98.3 ± 0.6
104.1 ± 1.4
1-nano
2-nano
87.6 ± 1.4
89.6 ± 1.0
97.5 ± 1.7
102.2 ± 1.3
370 ± 140
3-nano
275± 140
4-nano
160 ± 50
parentheses).
Figure 3. Plots: (a) inverse Young’s modulus vs atomic
polarizability for 1-4 (weighted fit macro R2=0.85, nano R2=0.93)
and (b) inverse Young’s modulus vs polarizability from THz-TDS for
1-4 (weighted fit macro R2=0.93, nano R2=0.93). Data for macro-
sized crystals for top plane (see SI for plot using bottom plane
data).
Figure 2. Histograms of Young’s modulus for macro- (left) and
nano-sized (right) 1-4 (Gaussian fits as red lines). For macro
crystals, top plane data in blue bars, bottom plane in green bars.
To probe the observed correlation between Young’s
modulus and polarizability, we turned to THz-TDS. The
coherent nature of THz spectroscopy permits a direct
In general, the measured polarizability values were
comparable to benzoic acid, sucrose, and thymine as
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determined from THz-TDS measurements on single crystals
(Fig. 3 and Table 1).36 Moreover, the polarizabilities from the
THz-TDS measurements display clear correlation
obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/conts/retrieving.html.
1
2
a
AUTHOR INFORMATION
Corresponding Author
3
4
5
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(weighted fit macro R2=0.93, nano R2=0.93) with inverse
Young’s modulus obtained using AFM measurements (95%
confidence limit) (Fig. 3b). These results also agree
qualitatively with DFT calculations using the RB3LYP
method (see SI).37,38 Hence, the polarizabilities of the solids
increase with increasing size of the res substituent, which
corresponds to the decrease in stiffness consistent with the
expected inverse relationship.
alexei-tivanski@uiowa.edu
len-macgillivray@uiowa.edu
ACKNOWLEDGMENT
T.P.R. and A.V.T. gratefully acknowledge financial support
from the National Oceanic and Atmospheric Administration
(NOAA) Climate Program Office, Earth System Science
Program, award NA11OAR4310187. L.R.M. gratefully
acknowledges partial financial support from the National
Science Foundation (DMR-1408834).
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While the correlation between Young’s modulus and
atomic polarizability in 1-4 may seem surprising given the
highly-anisotropic nature of organic crystals, two features of
the crystal structures of 1-4 are noteworthy in relation to the
mechanical data. First, the tetramers in each cocrystal
assemble to form layers. That the components maintain
layers across the series is likely a consequence of the isosteric
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comparable volumes are considered isosteric, while
molecules that differ only in substitution of isosteres at a
specific position are generally expected to form similar
crystal structures.39 Even the anomalous behavior of 2 in
forming Type II halogen bonds26,40 is unable to circumvent a
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In this report, we have demonstrated that
a bulk
mechanical property in the form of Young’s modulus for a
series of organic cocrystals is correlated to atomic
polarizability. The inverse relationship has been verified
using THz-TDS, which establishes the analytical technique as
a rapid and convenient method to obtain polarizability data
related to atomic, molecular, and supramolecular structure.
Given the now demonstrated relationship between chemical
structure and physical properties, we expect present and
future findings to establish atomic-to-bulk correlations that
enable the rational design of a variety of multicomponent
materials with desired mechanical and chemical properties.
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Experimental details, AFM height images of macro- and
nano-sized cocrystals, nanoindentation analysis, single
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charge via the Internet at http://pubs.acs.org. Supplementary
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Table of Contents
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49
50
51
52
53
54
55
56
57
58
59
60
6
ACS Paragon Plus Environment