Templated polar order of a guest in a quasiracemic organic host

Article abstract of DOI:10.1039/c0cc02695f

A quasiracemic mixture of Dianin's compound and its thiol derivative enforces additional anisotropy of the guest-accessible space, thus facilitating a net polar arrangement of guest molecules; guest alignment is rationalized in terms of van der Waals volume considerations.

Full text of DOI:10.1039/c0cc02695f

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Templated polar order of a guest in a quasiracemic organic hostw
Tia Jacobs,a Martin W. Bredenkamp,a Pieter H. Neethling,b Erich G. Rohwerb and
Leonard J. Barbour*a
Received 21st July 2010, Accepted 19th August 2010
DOI: 10.1039/c0cc02695f
A quasiracemic mixture of Dianin’s compound and its thiol
derivative enforces additional anisotropy of the guest-accessible
space, thus facilitating a net polar arrangement of guest
molecules; guest alignment is rationalized in terms of van der
Waals volume considerations.
guest polar ordering in the context of second harmonic
generation. Here we describe a crystal engineering strategy
that exploits the well-known tendency for racemic Dianin’s
compound and its close analogs to form centrosymmetric and
isoskeletal4 columnar inclusion compounds. We have modified
the host synthetically in order to produce a new quasiracemic
mixture,3 thus enforcing anisotropy of the guest-accessible
space to yield SHG activity.
Even-order non-linear optical (NLO) materials are capable of
second harmonic generation (SHG) and can thus be used to
double the frequency of laser light. Net polar alignment of
molecules is an important phenomenon for NLO properties1
and significant effort has thus been devoted to engineer polar
order in crystals. Although several studies have expanded
upon serendipitous observations of polar-ordered guest molecules
in host systems with persistent packing arrangements,2 very
few of these architectures have been specifically tailored with a
view to enforce guest alignment. In such systems, guest
molecules with high molecular hyperpolarizabilities would be
preferred, but these often form side-by-side dimers in a
centrosymmetric arrangement. This can be counteracted by
utilizing narrow channels that facilitate in-line head-to-tail
dipolar arrangements, but this favorable mode of packing
could still be negated if neighboring channels, though
individually polar, would align in an antiparallel fashion to
afford an overall non-polar material.
Dianin’s compound was first isolated in 1914 and is one of
the most recognized organic hosts in inclusion chemistry. In its
pure form, or in the presence of a wide variety of guests
(usually solvent molecules), the as-synthesized racemic
mixture assembles such that six host molecules position
ꢀ
themselves about a site of 3 symmetry. A cyclic up–down
arrangement of alternating R and S isomers is formed, and
successive molecules are linked to each other by OHꢀ ꢀ ꢀO
hydrogen bonds between their phenolic moieties. The hexamers
stack in columns such that successive units interdigitate
to enclose hourglass-shaped guest-accessible cavities of
approximately 240 A.3 Owing to the trigonal symmetry of
the host packing motif, it is usually not possible to model guest
molecules that do not also possess threefold symmetry.
In the absence of suitable guest molecules, the thiol
derivative (2) of Dianin’s compound spontaneously resolves
upon crystallization, separately forming crystals of both R and
The well-known Dianin’s compound (1, Scheme 1) host
system has been adapted3 to produce a chiral guest-accessible
space for enantiomeric separation of guest molecules. However,
to date this robust framework type has not been targeted for
S
enantiomerically pure host. However, when forming
inclusion compound crystals, 2 packs in a manner analogous
to 1, with SHꢀ ꢀ ꢀS hydrogen bonds tethering the enantiomers
together. We therefore postulated that an equimolar mixture
of (R)-1 and (S)-2 would also assemble in an analogous
manner by forming a quasiracemic hexa-adduct held together
by an (ꢀ ꢀ ꢀOHꢀ ꢀ ꢀSH)3 hydrogen bonded ring.
The resolution of racemic 1 was achieved by esterification with
(1S)-(ꢁ)-camphanic chloride, fractional crystallization of the
diastereomeric mixture from 2-methoxyethanol and subsequent
hydrolysis of the appropriate diastereomer to afford (S)-1.5 The
opposite enantiomer of 1 was obtained in an analogous manner
using (1R)-(+)-camphanic chloride as the resolving agent.
Enantiomerically pure (S)-1 was then converted to the thiol-
derivative (S)-2 via a thiocarbamate intermediate.6
Scheme 1 Dianin’s compound.
Equimolar amounts of (R)-1 and (S)-2 (hereafter this
mixture is designated as 3) were dissolved in CCl4 and
clathrate crystals suitable for single-crystal diffraction studies
were obtained by slow evaporation of the solvent. The
corresponding clathrates of compounds 1 and 2 were also
grown from CCl4 and the three different inclusion compounds
will be referred to as 1a, 2a and 3a.z The structure–property
relationships of these related but subtly different crystals are
rationalized below. However, it is important to note that the
discussion assumes each crystal to be defect-free.
a Department of Chemistry, University of Stellenbosch, 7602,
South Africa. E-mail: ljb@sun.ac.za; Fax: +27 21 808 3849;
Tel: +27 21 808 3335
b Department of Physics, University of Stellenbosch, 7602,
South Africa
w Electronic supplementary information (ESI) available: Synthetic
and crystallographic procedures, descriptions of desolvation
experiments and second harmonic generation measurements. CCDC
785097–785099. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc02695f
c
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Chem. Commun., 2010, 46, 8341–8343 8341

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Structure 1a crystallizes in the centrosymmetric trigonal
ꢀ
space group R3 with lattice parameters a = b = 26.8078(12)
along the crystallographic threefold axis. Furthermore, the
larger aperture between adjacent cavities in 2a allows one of
the chlorine atoms to protrude slightly into the adjacent
cavity. This can only occur if the two guest molecules in the
adjacent cavity are also aligned in the same direction along
[001]. Thus, from purely spatial considerations it can be
deduced that each column contains guest molecules that are
all aligned in a polar fashion along the crystallographic c axis.
Although the orientation of any given guest molecule enforces
the same orientation on its cavity partner, and indeed also on
the rest of the guest molecules within the same column, there is
most likely no communication between neighboring columns
with regard to the direction of the polar axis along c.
Therefore the disorder of the crystallographic model implies
that, while individual columns of guest molecules might
be polar, the bulk material consists of a random 50 : 50
distribution in terms of the alignment of these columns along
either [001] or [00ꢁ1].
and c = 10.8706(10) A. Each guest-accessible cavity is
occupied by only one CCl4 guest molecule, which is disordered
over two crystallographically equivalent positions (Fig. 1a)
ꢀ
across a site of 3 symmetry. From a space-filling model, it can
be inferred that the orientation of the guest in one cavity does
not influence that in the adjacent cavity along [001]. Therefore
the guest molecules are most likely statistically (i.e. 50 : 50)
disordered throughout the structure such that no localized
polar alignment exists.
The structure of 2a is analogous to that of 1a, with lattice
parameters a = b = 26.632(2) and c = 12.0349(17) A. In
contrast to 1a, elongation of the cavity by ca. 1.2 A along [001]
allows enclathration of two guest molecules per void in 2a;
both guests are 50 : 50 disordered over two positions in
ꢀ
accordance with the 3 site symmetry of the guest-accessible
space (Fig. 1b). Based on space-filling models, it follows that
the two CCl4 molecules situated within any cavity must both
be oriented with a C–Cl bond aligned in the same direction
Structure 3a crystallizes in the polar and chiral space group
R3 (note the reduction in symmetry from the centrosymmetric
ꢀ
space group R3 obtained for 1a and 2a) with lattice parameters
a = b = 26.6670(16) and c = 11.7710(14) A. The anticipated
quasiracemic crystal structure (Fig. 1c) is a hybrid of 1a and
2a. In addition to being chiral, the arrangement of the host
molecules is also polar—throughout the structure, the hydroxyl
moieties of 1 are all directed towards one end of the crystal
along its trigonal axis, and the thiol extremities of 2 are facing
the opposite end. Each cavity is bounded by two
(ꢀ ꢀ ꢀO–Hꢀ ꢀ ꢀS–H)3 hydrogen bonded rings (Odonorꢀ ꢀ ꢀSacceptor
and Sdonorꢀ ꢀ ꢀOacceptor = 3.428(3) and 3.522(3) A, respectively)
Fig. 1 Columnar stacking of CCl4 clathrates of Dianin’s compound
and its analogs. (a) 1a, (b) 2a and (c) 3a. All projections are
perpendicular to [001]. Molecules of 1 and 2 are shown in red and
purple, respectively (capped-stick representation). In each case the
CCl4 guest molecules are disordered over two possible positions
(yellow and green, ball-and-stick representation). Guest-accessible
volume is shown as semitransparent grey surfaces. All hydrogen
atoms, except those of the hydroxyl- and thiol-moieties have been
omitted for clarity.
Fig. 2 Perspective view of 3 along [00ꢁ1] illustrating the trigonal
symmetry of the host–guest adduct. Only hydrogen atoms belonging
to the hydroxyl and thiol groups are shown. Host carbon atoms are
represented as capped-sticks while oxygen and sulfur atoms are shown
in ball-and-stick mode. The two CCl4 guest molecules within the guest-
accessible cavity that are present in the 85% occupancy state are
shown in space filling representation (dark green in the foreground and
light green in the background). The alternating (ꢀ ꢀ ꢀO–Hꢀ ꢀ ꢀS–H)3 ring
is indicated using dashed red cylinders to represent hydrogen bonds.
c
8342 Chem. Commun., 2010, 46, 8341–8343
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hybrid system 3a was specifically constructed to increase the
anisotropy of the crystal, resulting in a dramatic alteration of
the optical properties of a well-known type of material. The
presence of the guest molecules is essential for ‘‘gluing’’ the
host molecules together and the overall acentricity of the
system is derived from multiple sources—i.e. both components
of the host framework, as well as the guest molecules exhibit
polar alignment. The guest templates the formation of the host
framework, which in turn templates the polar alignment of
the guest.
Notes and references
Fig. 3 Plot of the relationship between the input power of the
fundamental wave (800 nm) and the measured intensity of the second
harmonic wave (400 nm) for 1a to 3a.
z Crystal data for 1a: C109H120Cl4O12, M = 1763.96, colorless block,
3
ꢀ
0.38 ꢂ 0.23 ꢂ 0.11 mm , trigonal, space group R3 (No. 148), a = b =
26.8078(12), c = 10.8706(10) A, V = 6765.6(8) A3, Z = 18,
Dc = 1.299 g cmꢁ3, F000 = 2814, MoKa radiation, l = 0.71073 A,
T = 100(2) K, 2ymax = 56.51, 14 393 reflections collected, 3588 unique
(Rint = 0.0569). Final GooF = 1.037, R1 = 0.0515, wR2 = 0.1288, R
indices based on 2853 reflections with I 4 2s(I) (refinement on F2),
200 parameters, 2 restraints. Lp and absorption corrections applied,
m = 0.196 mmꢁ1. Crystal data for 2a: C110H120Cl8O6S6, M = 2014.08,
and contains a total of two disordered CCl4 guest molecules
(see Fig. 2). In this case, the two components of the disordered
model are not related to each other by the crystallographic
symmetry, although the rationalization of the alignment of the
guest molecules in 3a is similar to that for 2a. That is, owing to
spatial constraints, all of the guest molecules in any given
column are aligned such that they have the same orientation
along [001]. However, the enforced asymmetry of the cavity
introduces a bias such that the disorder no longer represents a
50 : 50 distribution of the overall direction of the polar
alignment of the guest molecules. Refinement against intensity
data yielded a model for CCl4 that is biased 85 : 15 in favor of
alignment of the crystallographically trigonal C–Cl bond in
the direction of the host phenolic moiety (the green guest
molecules in Fig. 1c).
3
ꢀ
light yellow block, 0.30 ꢂ 0.27 ꢂ 0.21 mm , trigonal, space group R3
(No. 148), a = b = 26.6320(19), c = 12.0349(17) A, V = 7392.3(13)
A3, Z = 3, Dc = 1.357 g cmꢁ3, F000 = 3180, MoKa radiation,
l = 0.71073 A, T = 100(2) K, 2ymax = 56.61, 15 611 reflections
collected, 3863 unique (Rint = 0.0602). Final GooF = 1.063,
R1 = 0.0651, wR2 = 0.1855, R indices based on 3220 reflections with
I 4 2s(I) (refinement on F2), 215 parameters, 6 restraints. Lp and
absorption corrections applied, m = 0.412 mmꢁ1. Crystal data for 3a:
C110H120Cl8O9S3, M = 1965.87, colorless block, 0.25 ꢂ 0.21 ꢂ
0.18 mm3, trigonal, space group R3 (No. 146),
a
=
b
=
=
26.6670(16), c = 11.7710(14) A, V = 7249.2(11) A3, Z = 3, Dc
1.351 g cmꢁ3, F000 = 3108, MoKa radiation, l = 0.71073 A,
T = 100(2) K, 2ymax = 56.51, 15 661 reflections collected, 7407 unique
(Rint = 0.0346). Final GooF = 0.976, R1 = 0.0593, wR2 = 0.1375, R
indices based on 5469 reflections with I 4 2s(I) (refinement on F2),
410 parameters, 35 restraints. Lp and absorption corrections applied,
m = 0.358 mmꢁ1. Absolute structure parameter8 = 0.06(7).
It is interesting to note that desolvation of 2a results in the
formation of a polycrystalline powder comprising a mixture of
resolved (R)-2 and (S)-2 (see ESIw). Similarly, 3a desolvates to
yield resolved (R)-1 and (S)-2. These experiments imply that
the presence of guest is essential for the formation of the
racemic or quasiracemic host framework.
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intense SHG for incident light of wavelength 800 nm (red
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harmonic response (Fig. 3), even for incident intensities up
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to 88 GW cmꢁ2
.
In summary, we have expanded upon the concept of
utilizing quasiracemates7 in order to sculpt the topology of
the guest-accessible space of a noncentrosymmetric inclusion
compound. Our approach has been to exploit the notion that,
although quasiracemates mimic the structural patterns of their
racemic counterparts, they are devoid of inversion symmetry
in a rigorous sense, owing to stereochemical restrictions. Thus
a rational design strategy has been devised and implemented to
control guest alignment within the well-known Dianin’s host
system by introducing asymmetry. In this contribution we
have rationalized the lack of polar order in the two related
racemic host frameworks 1a and 2a, and have shown that
these materials exhibit no NLO activity. The quasiracemic
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c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8341–8343 8343