Inclusion compound based approach to forming arrays of artificial dipolar molecular rotors: A search for optimal rotor structures

Article abstract of DOI:10.1002/adma.201203294

Hexagonal tris(o-phenylenedioxy)cyclotriphosphazene (TPP) is used as ahost for organizing dipolar molecular rotor guests into regular trigonal arrays. Inclusion of molecular rotors with transversely dipolar rotators into TPP channels is followed by solid-state nuclear magnetic resonance, diifferential scanning calorimetry, X-ray diffraction, and dielectric spectroscopy. The more polar of the two rotors does not form an inclusion. The second rotor forms two different inclusions differing in crystallite size and the rotational barriers. Copyright

Full text of DOI:10.1002/adma.201203294

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Inclusion Compound Based Approach to Forming Arrays
of Artificial Dipolar Molecular Rotors: A Search for Optimal
Rotor Structures
Lukáš Kobr, Ke Zhao, Yongqiang Shen, Richard K. Shoemaker, Charles T. Rogers,
and Josef Michl*
A series of recent review articles[1–7] reveals considerable cur-
rent activity in the field of artificial molecular rotors, motivated
by an intrinsic interest and their possible future applications as
ultrafast organic dielectrics. The term “molecular rotor” refers
to the whole molecule, and “rotator” refers to its rotating part
that has the smaller moment of inertia. When needed, “stator”
is the part that has the larger moment of inertia relative to a
common axle. Surface-mounted rotors[1] have been the subject
of many investigations as isolated molecules[8–14] or as collec-
tions of nearly independent noninteracting molecules.[15–21]
Our interest is in collective motion in regular two-dimensional
(2D) arrays of highly dipolar rotors with very low rotational bar-
riers, especially if arrays with a ferroelectric ground state could
be fabricated, for example, in a triangular lattice.[22]
We are examining two approaches toward fabricating peri-
Figure 1. Surface inclusion of a molecular rotor. A) rotator outside;
odic arrays of dipolar rotors: (i) allowing dipolar rotator-carrying
B) rotator inside; C) Top view of TPP crystal surface. Left: the first two
units to assemble into regular grids on the surface of a liquid
layers of TPP molecules. Right: the trigonal array of hexagonal channels
resulting from layer superposition.
followed by transfer to a solid substrate (the preparation of such
grids is currently the subject of considerable attention[23–28]),
and (ii) the approach adopted presently, which is to allow such
units to assemble as guests in empty channels offered by a suit-
able host crystal. The guests could be inside a thin crystalline
layer (bulk inclusion compound), or located on a surface of a
suitable facet of the host crystal, if their entry into the bulk is
prevented by a bulky “stopper” (surface inclusion compound,
Figure 1) .
The hexagonal form of tris(o-phenylenedioxy)cyclotriphosp-
hazene (TPP)[29–32] has been suggested[33,34] as a suitable host.
It contains parallel hollow channels oriented perpendicular to a
favored surface plane, which they intersect in a triangular array
(Figure 1). The lattice constant is 11.5–12 Å, depending on the
guest. The internal channel diameter is 4.5–5 Å, permitting
easy entry of linear alkanes, benzene, etc.[35–44] The crystal con-
sists of layers ≈5 Å thick. The shape of the channel is triangular
within each layer, and the triangles rotate by 60° from one layer
to the next. The ring currents of TPP benzene rings shield the
included guest nuclei by several ppm, making an inclusion easy
to recognize by solid-state NMR spectroscopy.
TPP also has a monoclinic modification,[36,39] which does not
form inclusion compounds and has a very distinct solid-state
NMR spectrum and X-ray diffraction pattern. It actually is more
stable than the empty hexagonal form, but when hexagonal TPP
is filled with a nonvolatile guest, it is stabilized against transfor-
mation into the monoclinic form, up to at least 250 °C.[38]
Having selected TPP as a host, we need to design and synthe-
size suitable rotor guests, rod-shaped molecules carrying a free
rotator with a large transverse dipole moment and a size that
avoids mechanical interference with neighbors. They also have
to contain a stopper large enough to prevent total insertion into
the TPP channel. It is preferable to locate the rotator outside
the TPP surface (Figure 1A) rather than just inside (Figure 1B)
to avoid attenuation of inter-rotator electrostatic interactions by
the dielectric constant of TPP and to minimize barriers to rota-
tion. The use of bulk inclusion compounds would suffer from
a similar disadvantage but would make it easy to work with a
much larger number of dipolar rotors.
Dr. L. Kobr, Dr. R. K. Shoemaker, Prof. J. Michl
Department of Chemistry and Biochemistry
University of Colorado
215 UCB, Boulder, CO 80309-0215, USA
E-mail: michl@eefus.colorado.edu
Dr. K. Zhao, Dr. Y. Shen, Prof. C. Rogers
Department of Physics
University of Colorado
390 UCB, Boulder, CO 80309-0390, USA
Prof. J. Michl
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic
16610 Prague 6, Czech Republic
DOI: 10.1002/adma.201203294
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Scheme 1. Synthesis of 1 and 2.
Presently, we are searching for structural features that
dictate the formation of surface versus bulk inclusion com-
pounds, and have chosen to investigate 1-(1-p-terphenylyl)-
12-(2,3-dichlorophenylethynyl)-p-dicarba-closo-dodecaborane (1,
Scheme 1) and 1-(1-p-terphenylyl)-12-(2,3-dicyano-5,6-methylen-
edioxyphenylethynyl)-p-dicarba-closo-dodecaborane (2). The ter-
phenyl unit is the shaft, the icosahedral carborane the stopper,
and the substituted benzene ring the dipolar rotator. The dipole
moment of o-dichlorobenzene is 2.51 D[45] and its longer in-
plane dimension is ≈7.8 Å,[46] significantly less than the ≈11.5 Å
channel-to-channel distance in TPP.
We note in passing that the use of the term rotator for the
dichlorophenyl ring assumes that the stopper and shaft parts
of the molecule are attached firmly inside a TPP channel and
therefore have an essentially infinite moment of inertia. If they
rotate inside the channel, the dichlorophenyl ring might have
to be called the stator. This is merely a question of nomencla-
ture and has no effect on the interpretation of data.
to TPP-d12. The signal C-B also contains tertiary carbons (signal
C-6) of the p-terphenylyl substituent. The signals of the rotor
carbon atoms in the spectrum of 15%1@TPP-d12 are different,
better resolved, and better separated than those in the spectrum
of neat 1. They display both upfield and downfield changes
in the chemical shift relative to neat 1 (C-1: ≈ +0.2 ppm, C-3:
≈ –1.2, C-6: >–2 ppm, C-8: +0.1 ppm, C-9: –1.8 ppm, C-10:
–4.7 ppm, C-11: +0.4 ppm). The signal C-5 in the spectrum of
15%1@TPP-d12 survives both dipolar dephasing and applica-
tion of a short contact time (0.2 ms), indicating that it contains
both tertiary and quaternary carbon signals. The signal C-B is
enhanced at a short contact time, and thus also contains the
tertiary p-terphenylyl carbons of 1.
Relative to solution spectrum, the chemical shifts of the inclu-
sion compound of 1 show upfield changes in the chemical shift
by 1.0–3.2 ppm for all resolved signals. The 13C NMR signals
in the spectrum of 15%1@TPP-d12 were assigned by compar-
ison to the solution spectrum of 1 (Figure S1 in the Supporting
Information provides a detailed justification of signal assign-
ments in solution spectra using correlation NMR spectroscopy).
As explained in detail elsewhere,[33,34] this comparison is prefer-
able to comparison with the solid spectrum of neat 1. We find
that C-1 corresponds to Cl, Cp and Cq, C-2 to Cm, and C-3 to Ci.
The carbons C-4, C-5, and C-7 belong to the 2,3-dichlorophenyl
rotator. The signal C-5 is partly due to Cc and partly to tertiary
aromatic carbons, C-7 belongs to Ca, and C-8, C-9, C-10, and
C-11 correspond to Cw, Ch, Cx, and Cg, respectively.
The synthesis route is shown in Scheme 1. Deprotonation
of p-carborane followed by transmetallation with CuCl and
Ulmann coupling with 3 afforded 4 in 81% yield. The iodide 3
was prepared by iodination of p-terphenyl according a modified
literature procedure (94% yield).[47] Deprotonation of 4 followed
by transmetallation with CuCl and Ulmann coupling with
2-iodovinyl chloride[48] gave 5 (76% yield). Subsequent dehydro-
halogenation of 5 with methyllithium afforded 6 in 99% yield.
The rotors 1 and 2 were prepared by Sonogashira coupling of 6
with 1,2-dichloro-3-iodobenzene (92% yield) or 7 (83% yield).
The inclusion compound 15%1@TPP-d12-B (15 mol%
1 in TPP-d12 annealed at 200–210 °C) shows 31P solid-state
NMR spectra identical to those of 15%1@TPP-d12-A, but its
solid-state 13C CP MAS NMR spectrum (Figure 2) shows
sharper and better resolved peaks with chemical shifts similar
to those of 15%1@TPP-d12-A. For example, the peak width
measured for signals C-9 and C-11 is 141 Hz for 15%1@
TPP-d12-A and 106 Hz for 15%1@TPP-d12-B. The upfield
shoulder of the C-C carbons is missing in the spectrum of
15%1@TPP-d12 -B.
1
The NMR signals of 1 were assigned using H -1H gCOSY
(Figure S1A, Supporting Information), 1H -13C gHSQC
1
1
(Figure S1B), H NMR, {11B} H -13C gHMBC NMR (J = 8 Hz:
Figure S2A, J = 18 Hz: Figure S2B), 13C NMR, and DEPT-135
spectra, relying on standard procedures (see the Supporting
Information). Both 31P CP MAS and 31P SPE MAS ss-NMR
spectra of the inclusion compound 15%1@TPP-d12-A (15 mol%
1 in TPP-d12 annealed at 70 °C) show a single resonance at
34.1 ppm (Figure S3).
Solid-state 13C CP MAS NMR spectra of neat 1 and of its
inclusion compound 15%1@TPP-d12-A (5 ms contact time)
are shown in Figure 2. The former contains a series of partially
resolved aromatic resonances and well separated resonances
for each p-carborane and acetylenic carbon. The latter contains
three intense aromatic signals (C-A, C-B and C-C) that belong
The solid-state 13C CP MAS NMR spectrum of a mixture
of TPP-d12 with 15% of 2, which clearly failed to produce the
desired inclusion compound 15%2@TPP-d12 even after pro-
longed milling and annealing, shows three very weak resonances
of hexagonal TPP carbons, and a pattern of peaks with chemical
shifts identical to those observed in the solid-state 13C CP MAS
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peaks (black solid symbols) fit a simple hexagonal structure
(black solid lines). For 15%1@TPP-d12-A (15%1@TPP-d12-B),
the hexagonal in-plane lattice parameter is 12.180
0.001
(12.199 0.001) Å, hexagonal layer spacing is 10.045 0.001
(10.031 0.001) Å, and the estimated powder particle size is
260 (2700) Å. The in-plane lattice parameter and layer spacing
show a 6.2% (6.3%) expansion and a 1.1% (1.1%) contraction,
respectively, compared to the 11.454 0.005 Å and 10.160
0.005 Å reported[49] for empty hexagonal TPP. These results
demonstrate that both 15%1@TPP-d12 inclusion compounds
take on a global hexagonal structure, consistent with the local
hexagonal structure shown by the ss-NMR data. The higher
temperature annealing conditions produce larger crystallites
and a slightly increased expansion of the in-plane lattice.
Figure 3C,D shows dielectric losses in 15%1@TPP-d12-A
and 15%1@TPP-d12-B of the same powder samples that
were used for ss-NMR and XRD. The rotation of a dipole
moment makes a contribution to the dielectric signal by
aligning with the applied electric field. For a dipole moving
in a rotational potential energy surface with an energy bar-
rier between wells, the contribution to the dielectric response
is temperature dependent and is negligible at high tem-
peratures, where dipoles are largely free to align with the
applied field, and at low temperatures, where they are trapped
in potential wells. At a temperature where the measure-
ment frequency, f, equals the rate of thermal activated pas-
sage over the rotational barrier, dipole motion lags behind
the electric field and dielectric loss ensues. Loss peaks
are observed at absolute temperatures, T, given by 2πf =
(1/τ0)exp(–Eb/kbT), where Eb is the barrier to rotation, 1/τ0 is
a characteristic “attempt frequency” of the potential, and kb is
the Boltzmann constant. Measurements at different frequen-
cies and temperatures then yield Eb and 1/τ0. Figure 3C shows
the dielectric loss of 15%1@TPP-d12-A. The large dispersing
loss structure near room temperature is associated with the
Figure 2. 13C NMR spectra. A) CP MAS of neat 1 (tC = 5 ms); B) solution
of 1 in CDCl3; C) CP MAS of 15%1@TPP-d12-A (tC = 5 ms); D) CP MAS
of 15%1@TPP-d12-B (tC = 5 ms).
NMR spectrum of neat 2 (Figure S4, Sup-
porting Information). 31P NMR spectra and
the DSC trace of the mixture agreed that no
inclusion compound was present.
Neat 1 displays a single endotherm at
276 °C and the inclusion compound 15%1@
TPP-d12-A shows several endotherms
(252 °C, 263 °C, and 272 °C) and an exotherm
at 203 °C (see Figure S5, Supporting Infor-
mation). The inclusion compound 15%1@
TPP-d12-B shows two endotherms at 258 °C
and 277 °C.
Figure 3A,B shows the results of copper
Kα 1 wavelength X-ray powder diffraction
(XRD) measurements on 15%1@TPP-d12-A
and 15%1@TPP-d12-B samples that were
used to obtain the ss-NMR spectra. The scat-
tered X-ray photon counts are plotted against
the magnitude of the scattering wave vector,
q in Å−1 , related to the scattering angle θ
and the X-ray wavelength λ by q = 4π sin
θ/λ. The observed sequences of diffraction
Figure 3. A,B) X-ray powder patterns from inclusion compounds. Solid black symbols: diffrac-
tion data. Black lines: a fit assuming hexagonal peak positions and a measured background
from the sample capillary tube. A) 15%1@TPP-d12-A (annealed at 70 °C). B) 15%1@TPP-d12-B
(annealed at 210 °C). C,D) Dielectric loss at frequencies of 120 (solid line), 1200 (dashed line),
and 12 000 (dash dot dot line) Hz. C) 15%1@TPP-d12-A); D): 15%1@TPP-d12-B.
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empty capacitor. Near 150 K, the powder sample introduces
a broad dispersing loss shoulder that displays a central bar-
rier to rotation of 8.3 0.5 kcal mol−1 (1 cal = 4.184 J) and an
unexceptional attempt frequency of 2 1 ×1013 Hz. Figure 3D
shows the dielectric loss behavior of 15%1@TPP-d12-B, with
qualitatively similar results and Eb = 11.3 0.5 kcal mol−1 .
Previous results for one molecular rotor[33] suggested that
the placement of a bulky p-carborane cage next to the shaft as
a stopper successfully prevents the complete entry of the rotor
molecule into the TPP channel and promotes the formation of a
surface inclusion compound. This outcome would be expected
based on a simple consideration of the nominal van der Waals
radii of p-carborane (7.6 Å) and of the interior of the channel
(4.5–4.8 Å).[33] However, results for two other molecular rotors
containing the same p-carborane stopper and a 2,3-dichloroph-
enyl rotator (7.8 Å diameter) were the opposite in that bulk
inclusion compounds were formed.[34] It became clear that
there is enough flexibility in small crystallites of hexagonal TPP
to accommodate much thicker molecular rods than are nomi-
nally expected. Such accommodation is reached by an expan-
sion of the layers present in the TPP crystal and an increase in
the in-plane lattice parameter.
The degree of mismatch between the diameter of the rod
and that of the TPP channel appears to be only one of the
parameters that determine whether a surface or a bulk inclu-
sion compound will be formed with any particular rod, and
the affinity of the rod for the walls of the channel is likely to
be another. An additional factor that needs to be considered is
the angle that the symmetry axis of p-carborane makes with the
channel axis. This will dictate the angle that the axle of the out-
side rotator forms with the surface normal. The previous use of
an alkyl group as a shaft[33] may have twisted the p-carborane
axis away from the channel axis and skewed the 2,3-dichloroph-
enyl rotor, allowing it to add to the stopping action and forcing
the formation of a surface inclusion compound.
Sufficient uncertainty surrounds the issue of surface versus
bulk inclusion compound formation that we felt that an exami-
nation of several additional rotor structures with a p-carborane
stopper would be beneficial. The rotors 1 and 2 that we have
chosen to examine presently may answer other questions as
well. Specifically, we asked to what degree the nature of an
inclusion compound formed can be controlled by the condi-
tions of its preparation, and an insertion of a triple bond
between the stopper and the rotator was meant to provide
information on the origin of the observed barriers. It should
reduce barriers intrinsic to the molecular structure and would
make it difficult for the rotator located on the outside to touch
the crystal surface, but it would have little effect on steric hin-
drance to rotation imposed by channel walls when the rotator
is inside a channel. The purpose of experimenting with a
rotor carrying a large dipole moment was our ultimate desire
to form a ferroelectric surface inclusion compound. Since the
Curie temperature goes roughly with the square of the dipole
moment, large values are desirable. However, an increase in
the polarity of the rotor molecule will undoubtedly decrease
the probability that any inclusion compound will be formed at
all, since phase separation into a highly polar phase containing
the rotor molecules and a nonpolar TPP phase will be increas-
ingly favored.
The 31P CP MAS solid-state NMR data of both inclusion
compounds of 1 show a single phosphorus signal, attributed
to the TPP-d12 molecules that surround the guest. This is a
result of cross-polarization between the guest protons and the
phosphorus atoms of the TPP-d12 lattice. In the absence of a
guest with protons, TPP-d12 shows no signal in the 31P CP-MAS
solid-state NMR experiment. The single phosphorus signal is
an indication of the inclusion behavior and of a local hexagonal
structure filled with guest molecules, which is also confirmed
by 13C CP MAS ss-NMR, where cross-polarization of all TPP-
d12 carbons from the guest molecules produces three singlets
for the three sets of equivalent TPP-d12 carbons characteristic of
hexagonal TPP. The single peaks in the 31P SPE MAS solid-state
NMR spectra of the inclusion compound reveal the presence
of the hexagonal lattice throughout the entire sample. We have
provided elsewhere[33] arguments for comparing the chemical
shifts of guests in the inclusion compound to those of the guest
in a solution spectrum in a non-coordinating solvent and not to
those of the neat solid guest when attempting to determine the
extent of insertion of the guest into the channel.
The upfield changes in the chemical shifts of all the carbon
atoms in both inclusion compounds of 1 relative to the solution
spectrum indicate that their rotors are inserted completely into
the channels of TPP. The difference between the more homo-
geneous spectrum of 15%1@TPP-d12-B and the more hetero-
geneous spectrum of 15%1@TPP-d12-A is in line with bulk
inclusion in both cases, as the guest molecules in the interior
of the ten times larger crystallites of the inclusion compound
B would all be in a similar environment, whereas in small
crystallites of the inclusion compound A a large fraction of
guests would necessarily be near the surface and thus in a dif-
ferent environment. The downfield shoulder of the TPP signal
C-C in the solid-state 13C NMR spectrum of 15%1@TPP-d12-A
corresponds to the chemical shift of carbon C-C in guest-free
hexagonal TPP-d12, yet in order to be observed, it must be in
proximity of the guest. This shoulder is absent in the spectrum
of 15%1@TPP-d12-B, and since no evidence of monoclinic TPP
is found in the DSC trace and XRD data, all the TPP is filled in
15%1@TPP-d12-B.
The observation that the ss 13C CP MAS NMR spectra of 2
and of 15%2@TPP-d12 display identical chemical shifts for all
carbons indicates that no inclusion compound was formed in
this case and 15%2@TPP-d12 actually is a misnomer. This is
also supported by the small intensity of the TPP carbons, which
most likely originates from trace amounts of water in the TPP
channels. We did not investigate 2 further.
The low-temperature endotherm in the DSC trace of 15%1@
TPP-d12-A is attributable to the melting of TPP-d12. This is most
likely due to expulsion of the guest from the channel, behavior
usual for a number of inclusion compounds.[50] The remaining
two endotherms are due to the melting of the inclusion com-
pound and of the free rotor. The exotherm observed in the DSC
trace of 15%1@TPP-d12-A is associated with further insertion of
the rotor into TPP-d12, as supported by its absence in the DSC
trace of 15%1@TPP-d12-B, which was prepared by annealing
at the temperature at which this exotherm was observed. The
inclusion compound 15%1@TPP-d12-B only shows a small
endotherm associated with the melting of free TPP-d12, which
was again likely formed by expulsion of the guest from the
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channel, and a high-temperature broad endotherm corre-
sponding to melting of the inclusion compound.
Experimental Section
See the Supporting Information for a detailed description of the
experimental details.
The XRD patterns provide a superior tool for detecting dif-
ferences between 15%1@TPP-d12-A and 15%1@TPP-d12-B,
both with respect to the size of the crystallites and to the in-
plane lattice parameters. The information provided by NMR
is insensitive to variations in the inclusion structure at this
level.
Given the size of the the p-carborane cage and the dichlo-
rophenyl rotator, and the more expanded lattice parameter, the
XRD results are consistent with a picture where the rotor guest
molecules are fully included into a coherent expanded bulk
structure of inclusion compound B, whereas the less expanded
lattice parameter of inclusion compound A indicates that the
large-scale coherent structure must be different. Perhaps the
dichlorophenyl rotators are close to a TPP surface that enjoys
more structural flexibility than the bulk, and do not participate
in the coherent structure of the bulk, yet they still feel the full
NMR screening.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This material is based upon work supported by the European Research
Council (FP7/2007-2013/ERC grant 227756), the U.S. National Science
Foundation (CHE 0848663), and the Institute of Organic Chemistry and
Biochemistry (RVO: 61388963).
Received: August 10, 2012
Published online: October 8, 2012
The dielectric loss structures are not clearly defined peaks
but shoulders that extend from low temperatures until they
merge with the high temperature dissipation peak associated
with the empty capacitors. The 8.3 and 11.3 kcal mol−1 rota-
tional barriers in A and B, respectively, were obtained with an
uncertainty of 0.5 kcal mol−1 from the lower end of the shoul-
ders, where the dispersing character of the losses is clearly
displayed. With respect to rotational barriers experienced by
the rotators, the samples are clearly quite heterogeneous in
that we observe a broad dispersing region, rather than iso-
lated peaks. Such a dispersing region can be understood as
the result of superposition of a broad range of rotator activa-
tion energies, in this case running from at least 8.3 kcal mol−1
to above roughly 15 kcal mol−1 for A and 11.3 kcal mol−1 to
above 15 kcal mol−1 for B. The upper rotational energy barrier
is estimated by the point where the dielectric losses become
dominated by the empty capacitor signature between 200 and
250 K. In accord with the proposed bulk inclusion structure,
these rotational barriers are high. The picture outlined above
explains the generally smaller rotational barrier in A as indica-
tive of the rotator/surface TPP environment as having more
structural flexibility, with TPP softer and better able to adjust
to rotator motion.
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and do not form a surface inclusion compound, which would
apparently require a stopper larger than p-carborane. The fact
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