Solid-state molecular rotors with perdeuterated stators: Mechanistic insights from biphenylene rotational dynamics in ordered and disordered crystal forms

Article abstract of DOI:10.1021/jo9025176

Samples of 4,4′-bis(3,3,3-tri-d₅-phenylpropynyl)biphenyl 2, 9,10-bis(3,3,3-tri-d₅-phenylpropynyl)anthracene 3, 1,4-bis(3,3,3-tri-d₅-phenylpropynyl)naphthalene 4, and 4,4′-bis(3,3,3-tri-d₅-phenylpropynyl)-1,1′-binaphthyl 5 were prepared via a Sonogashira coupling of 3,3,3-tri-d₅- phenylpropyne 7 and the appropriate aryl dibromide. Single crystal X-ray diffraction structures were obtained for an o-xylene clathrate of 2 and for solvent-free crystals of 3. All four molecular rotors were characterized by CPMAS ¹³C NMR experiments with varying contact times in order to determine whether the carbon signals of the central rotator group could be selectively enhanced and studied without interference or overlap of signals from the deuterated stator, which is insensitive to the {¹H}- ¹³C cross-polarization method. It was shown that the ¹³C signals of the natural abundance rotator group can be selectively observed with short contact times (ca. 50 μs) without interference from other ¹³C signals in the molecule. Variable-temperature CPMAS ¹³C NMR studies with a crystalline o-xylene solvate of biphenylene rotor 2 suggested a 2-fold flipping process in the fast exchange regime, even at temperatures as low as 199 K (?74 °C). Indirect support for this was obtained by studies carried out with a disordered, solvent-free solid, obtained by fast precipitation from hexanes and dichloromethane, which displayed slower dynamics within the same temperature range with an activation energy of 8.7 kcal/mol and a pre-exponential factor of 4.9 - 10⁹ s^?1. Confirmation of an exchange process in the megahertz regime for the crystalline solvate was obtained by variable-temperature quadrupolar echo ²H NMR data acquired with samples prepared with a deuterated biphenylene rotator and a natural abundance stator. Although rotational exchange occurs in the solvated samples with a slightly lower barrier of 7.4 kcal/mol, the main difference with the precipitated solid comes from the pre-exponential factor, which is nearly 3 orders of magnitude greater with a value of 2.5 - 10¹² s ^?1. On the basis of these differences, we speculate that efficient rotational motion in the solvated crystals may take advantage of long-range lattice vibrations that couple with molecular modes and that the lack of long-range order may be responsible for the low pre-exponential factor observed in the disordered crystals.

Full text of DOI:10.1021/jo9025176

pubs.acs.org/joc
Solid-State Molecular Rotors with Perdeuterated Stators: Mechanistic
Insights from Biphenylene Rotational Dynamics in Ordered and
Disordered Crystal Forms
Zachary J. O’Brien, Steven D. Karlen, Saeed Khan, and
Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, University of California, Los Angeles,
California 90095-1569
mgg@chem.ucla.edu
Received November 26, 2009
Samples of 4,40-bis(3,3,3-tri-d5-phenylpropynyl)biphenyl 2, 9,10-bis(3,3,3-tri-d5-phenylpropynyl)-
anthracene 3, 1,4-bis(3,3,3-tri-d5-phenylpropynyl)naphthalene 4, and 4,40-bis(3,3,3-tri-d5-phenyl-
propynyl)-1,10-binaphthyl 5 were prepared via a Sonogashira coupling of 3,3,3-tri-d5-phenylpropyne
7 and the appropriate aryl dibromide. Single crystal X-ray diffraction structures were obtained for an
o-xylene clathrate of 2 and for solvent-free crystals of 3. All four molecular rotors were characterized
by CPMAS 13C NMR experiments with varying contact times in order to determine whether the
carbon signals of the central rotator group could be selectively enhanced and studied without
interference or overlap of signals from the deuterated stator, which is insensitive to the {1H}-13C
cross-polarization method. It was shown that the 13C signals of the natural abundance rotator group
can be selectively observed with short contact times (ca. 50 μs) without interference from other 13
C
signals in the molecule. Variable-temperature CPMAS 13C NMR studies with a crystalline o-xylene
solvate of biphenylene rotor 2 suggested a 2-fold flipping process in the fast exchange regime, even at
temperatures as low as 199 K (-74 °C). Indirect support for this was obtained by studies carried out
with a disordered, solvent-free solid, obtained by fast precipitation from hexanes and dichloro-
methane, which displayed slower dynamics within the same temperature range with an activation
energy of 8.7 kcal/mol and a pre-exponential factor of 4.9 ꢀ 109 s-1. Confirmation of an exchange
process in the megahertz regime for the crystalline solvate was obtained by variable-temperature
2
quadrupolar echo H NMR data acquired with samples prepared with a deuterated biphenylene
rotator and a natural abundance stator. Although rotational exchange occurs in the solvated samples
with a slightly lower barrier of 7.4 kcal/mol, the main difference with the precipitated solid comes
from the pre-exponential factor, which is nearly 3 orders of magnitude greater with a value of
2.5 ꢀ 1012 s-1. On the basis of these differences, we speculate that efficient rotational motion in the
solvated crystals may take advantage of long-range lattice vibrations that couple with molecular
modes and that the lack of long-range order may be responsible for the low pre-exponential factor
observed in the disordered crystals.
2482 J. Org. Chem. 2010, 75, 2482–2491
Published on Web 03/16/2010
DOI: 10.1021/jo9025176
2010 American Chemical Society
r

O’Brien et al.
JOCArticle
Introduction
to help advance the fields of functional materials and
molecular machinery,9 short-term goals include investiga-
tions of the types of structural features and supramolecular
architectures that facilitate fast motion in the solid state and
the development of analytical tools to aid the study of such
phenomena.
Among the new venues of artificial molecular machinery,1
studies have centered recently on the potential of molecular,2
coordination,3 and extended solids,4 built with structures
that form a static lattice linked to elements that experience
rapid conformational motions.5 With components at the two
ends of the dynamic spectrum, we proposed the term
“amphidynamic solids” to describe the contrasting proper-
ties of these materials.2 Studies in our group have been
primarily centered on molecular rotors6 based on 1,4-bis-
(triarylpropynyl)benzene7 and related structures2,8 with the
expectation that the two axially disposed and relatively bulky
triarylpropynyl groups acting as a stator6 will provide a low
density “pocket”, where the central phenylene moiety can
play the role of the rotator.6 With structural similarities to
macroscopic gyroscopes, these molecular rotors exhibit
Brownian rotation in the solid state with ambient tempe-
rature exchange frequencies that range from static to
One of the most powerful techniques for the study and
characterization of internal motions in the solid state is 13C
NMR obtained under cross-polarization and magic angle
spinning (CPMAS).10 The technique allows the acquisition
of high-resolution spectra with polycrystalline powder
samples, which under favorable circumstances can be used
to characterize the molecular motions reported by nuclei
experiencing dynamic exchange between magnetically non-
equivalent sites.11 The CPMAS experiment has three key
components that improve the sensitivity and resolution of
the spectrum. The cross-polarization (CP) part is responsible
for signal enhancement by transferring the magnetization
1
from the abundant and sensitive H nuclei to the far less
ca. 109 s-1 2,7,8
While the long-term goal of these studies is
.
abundant and insensitive 13C nuclei. Rapid sample spinning
(e.g., 10 kHz) at the “magic” angle (54.74° relative to the
external magnetic field) removes the line-broadening that
arises from chemical shift anisotropy. The third component
is a strong broadband decoupling RF field that helps remove
the dipolar interactions between the 13C and 1H nuclei.
The intensity of the 13C signals in the CPMAS experiment
is a time-dependent function of cross-polarization and
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depends on the strength of the H-13C dipolar coupling.
The intensity of a given signal will depend on the number and
distance of nearby hydrogen atoms and on the “contact
time” allowed for cross-polarization. In a previous commu-
nication we showed that the dependence of signal intensity to
the presence of nearby hydrogen atoms may be used to
highlight signals of interest among many interfering ones
by selectively substituting the latter with 2H nuclei.12 Using
samples of 1,4-bis(3,3,3-tri-d5-phenylpropynyl)benzene 1
with a perdeuterated bis(trityl)-stator and a natural abun-
dance 1,4-phenylene rotator, we were able to carry out a
detailed variable-temperature (VT) analysis of the exchange
dynamics of the central phenylene in desolvated crystals of 1.
In this article, we explore the generality of the method with a
set of structures that includes 4,40-biphenylene (2), 9,10-
anthrylidene (3), 1,4-naphthylidene (4), and 4,40-(1,1-bina-
phthylidene) (5) playing the roles of potential rotators
(Figure 1). While the bulk of the central aromatics in com-
pounds 3-5 and the modest shielding of unsubstituted trityl
groups suggest that rotation in the solid state is unlikely, we
selected them as valuable test systems because they have well-
characterized chromophores that we plan to use in the future
with a set of bulkier, more shielding stators. We expected and
confirmed that the biphenyl rotator of 2 experiences rela-
tively fast rotational dynamics in the solid state. After
showing that the rate of exchange in the case of 2 is too large
to be measured with confidence within the temperature limits
of our spectrometer by CPMAS 13C NMR (T g 200 K), we
carried out its dynamic characterization by quadrupolar
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FIGURE 1. Compounds studied using selective cross-polarization to highlight the signals of the rotator.
echo 2H NMR with samples containing a deuterated rotator
SCHEME 1
and a natural abundance stator. While the CPMAS 13C
NMR spectral information obtained with compound 3
suggested that there may be sufficient chemical shift disper-
sion to investigate its rotational dynamics in molecular
rotors with larger stators, the spectral resolution observed
with compounds 4 and 5 is significantly lower, indicating
that a determination of their rotational dynamics by this
method should be more challenging.
Results and Discussion
Synthesis and Characterization. Like samples of 1,4-bis-
(3,3,3-tri-d5-phenylpropynyl)benzene 1 before,12 samples of
4,40-bis(3,3,3-tri-d5-phenylpropynyl)biphenyl 2, 9,10-bis(3,
3,3-tri-d5-phenylpropynyl)anthracene 3, 1,4-bis(3,3,3-tri-d5-
phenylpropynyl)naphthalene 4, and 4,40-bis(3,3,3-tri-d5-
phenylpropynyl)-1,10-binaphthyl 5 were obtained by a
straightforward 3-step synthesis from d6-benzene as illus-
trated in Scheme 1.
corresponding to the deuterated carbons occur as small
triplets in the range of 126-129 ppm. Corresponding weak
signals in the 1H NMR spectra occur at ca. 7.4 and 7.3 ppm
due to the residual ca. 0.5% of hydrogens present in the
starting material.
The characterization and X-ray structure of 1 has been
discussed in the detail in the literature.7a,b,12 The 1H NMR
spectrum of 2 is characteristic of symmetric 4,40-disubsti-
tuted 1,10-biphenyls with a pair of doublets between 7.5 and
7.6 ppm (J = 8.5 Hz) in a 1:1 ratio. Signals at 132.1 and 126.8
ppm in the 13C NMR spectrum correspond to the protonated
carbon signals of the biphenyl (C3 and C2, respectively),
while peaks at 140.0 and 122.9 ppm correspond to the
quaternary aromatic carbon atoms (C1 and C4) of the same
moiety.
The preparation of d15-3,3,3-triphenylpropyne 7 via Friedel-
Crafts alkylation of d6-benzene with carbon tetrachloride13
followed by substitution of the resulting chloride by ethynyl-
magnesium bromide was accomplished in 35.0% overall yield.
The final step in the syntheses of compounds 1-5 was accom-
plished via Sonogashira coupling of 7 and the appropriate aryl
dibromide in ca. 60% yield after column chromatography
(hexanes/C6H6/CH2Cl2 = 90:5:5). The structures of 1-5 were
determined by 13C and 1H NMR, attenuated total reflectance
(ATR) FTIR, and high-resolution MALDI-TOF mass spectro-
metry data.
Compounds 1-5 all display the characteristic C-D
stretch at 2274 cm-1 in the IR spectrum as well as the sp2
C-H stretches at ∼3000 cm-1, indicating the presence of
both hydrogen and deuterium atoms. Signals that are com-
mon to all four samples have a good correspondence in the
13C NMR solution spectra. These include the quaternary
trityl carbon at ca. 56 ppm, the two alkyne signals at ca. 96
and 85 ppm, and the signal of the three dynamically averaged
ipso-carbons in the trityl groups at ca. 145 ppm. Signals
The NMR spectra of compound 3 are also characteristic of
symmetric 9,10-disubstituted anthracenes. The 1H NMR
spectrum consists of two complex multiplets of the AA0XX0
0
system (Jax = Ja x = 9.0 Hz, Jxx = 8.0 Hz, Jaa = 0.8 Hz,
0
0
0
0
0
Jax = Ja x = 2.1 Hz) at 8.57 and 7.53 ppm, which correspond
to hydrogens attached to the anthracene R- and β-positions
(C1 and C2, respectively). Correspondingly, signals at 127.2
and 132.6 ppm in the 13C NMR spectrum correspond to
R- and β-carbons (C1 and C2), while peaks at 126.6 and 118.4
ppm correspond to the quaternary carbons of the fused ring
system (e.g., C4a) and the carbons linked to alkyne substit-
uents (e.g., C9), respectively. The 1H NMR spectrum of
compound 4 is similar to that of 3, but with the addition of a
singlet at 7.27 ppm, corresponding to the spin-isolated
and equivalent protons attached to C2 and C3 of the
(13) Bachmann, W. E. Organic Synthesis; Wiley: New York, 1955;
Collect. Vol. III, p 841.
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SCHEME 2
1,4-substituted naphthyl unit. The 13C NMR spectrum is
also very similar to that of 3, except for the addition of a peak
at 129.6 ppm, which also corresponds to the equivalent C2
and C3 of the naphthalene unit. Finally, the 1H NMR
spectrum of 4,40-disusbtituted 1,10-binaphthyl 5 consists of
two doublets at 7.89 and 7.45 ppm (J = 7.3 Hz) from hydro-
gen atoms linked to C2 and C3 of each naphthyl unit, two
doublets at 8.45 and 7.40 ppm (J = 8.3 Hz) due to hydrogen
atoms linked to C5 and C8, and two doublets of doublets at
7.52 and 7.32 ppm (J = 8.3, 8.3 Hz), respectively, from
hydrogen atoms linked to C6 and C7.
FIGURE 2. Unit cell of the o-xylene clathrate of 2 (hydrogens
omitted for clarity) viewed down the b-axis.
An isotopolog of compound 2 with a deuterated bipheny-
lene rotator and protonated stators (10) was synthesized to
determine its rotational dynamics using variable-tempera-
ture quadrupolar echo 2H NMR in order to compare it with
the VT CPMAS data. The deuterated biphenyl rotator 9 was
constructed by electron transfer oxidation of the correspond-
ing cuprate 8.14 The resulting 4,40-dibromobiphenyl-d8 was
then coupled to 2 equiv of 3,3,3-triphenylpropyne using the
same Sonogashira conditions as before (Scheme 2) to give 10.
The 13C NMR spectrum of 10 is the same as that of 2 except
that now the carbon atoms of the rotator are split into triplets
while the stator carbon atoms give only singlets.
FIGURE 3. o-Xylene molecules disorder in two different forms as a
result of inversion through a point (a) midway between the C1 and
˚
C2 atoms or (b) 0.4 A inside the aromatic ring.
Thermal Analyses. Differential scanning calorimetry
(DSC) analysis of 2-5 showed that all four compounds
decompose prior to melting. This is shown by a sharp
endotherm in the heating curve followed by a color change
from white or pale yellow to dark brown. There was no
crystallization on cooling. The heating curve remains
smooth throughout the measurement until the decomposi-
tion, meaning that there are no phase changes (melting or
solid-to-solid) and no residual solvent is expelled.
X-ray Crystal Structure Analysis of o-Xylene Clathrate 2.
X-ray quality crystals of the o-xylene clathrate of rotor 2
were obtained by cooling an o-xylene solution from its
boiling point down to 25 °C. X-ray diffraction data were
collected at 100 K, and the structure was solved in the space
group P21/c . The structure of 2 with the disordered o-xylene
molecules is shown in Figure 2. As is often seen in rotor
molecules, the two triphenylmethyl groups adopt a confor-
mation where the trityl groups at the two ends of the
structure adopt an anti conformation. The molecules have
no center of symmetry, meaning that each carbon atom
occupies a unique crystallographic site. The alkyne axle
deviates slightly from linearity with the ipso carbon of the
rotator, the center of the alkyne bond and the methane
carbon of the stator forming an angle of 175° instead of
180°. The two phenyl groups in the biphenyl rotator are not
coplanar but display a twist of 31.4°.
The two crystallographically distinct o-xylene molecules
are highly disordered (Figure 3). On average, one appears as
a naphthalene-shaped molecule as a result of the two possible
orientations of the o-xylene molecules, which occupy sites
related by an inversion center at the center of the C1-C2
bond (Figure 3a), each with occupancy factors of 0.5. The
other o-xylene molecule is also disordered, but the sites
occupied by C1 and C2 are not shared by both disordered
molecules. Instead, they overlap each other such that the two
˚
C1-C2 bonds are parallel but separated by 0.8 A. The net
result is that the two possible orientations of the molecule
˚
occupy sites related by an inversion center located 0.4 A
inside the aromatic ring (Figure 3b). Again, the occupancy
factor for each site is 0.5.
(14) Miyake, Y.; Wu, M.; Rahman, M. J.; Kuwatani, Y.; Iyoda, M.
J. Org. Chem. 2006, 71, 6110–6117.
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O’Brien et al.
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FIGURE 6. Close up of anthryl rotator (red) in the “pocket”
created by stators of neighboring molecules.
FIGURE 4. Close up of the rotator in the crystal, shown in red
capped sticks, with neighboring solvent (purple) and rotor (blue)
molecules illustrated in space-filling model.
TABLE 1. Unit Cell Parameters for o-Xylene Clathrate of 2 and
Solvent-Free 3
2
3
formula
C54H8D30, C8H10
822.68
monoclinic
P21/c
15.9910(13)
7.5955(6)
36.383(3)
90.00
91.0640(10)
90.00
C56H8D30
740.48
monoclinic
P21/n
8.5969(12)
14.440(2)
16.362(2)
90.00
102.739(3)
90.00
formula weight
crystal system
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
T (K)
4418.3(6)
4
100
1981.1(5)
2
298
molecules so that the “pocket” fits the rotator tightly
(Figure 6). Unit cell parameters for 2 and 3 are given in
Table 1.
FIGURE 5. Unit cell of 3 (hydrogens omitted for clarity) viewed
down the a-axis.
The two rings of the biphenyl rotator each participate in
staggered π-stacking interactions with the two types of
disordered o-xylene, which occupy alternating channels
through the crystal. Closer examination of the biphenyl rota-
tor, shown in red capped sticks, in a space-filling environ-
ment in Figure 4 shows the aforementioned staggered
π-stacking interactions of the rotator phenylenes with the
solvent molecules (purple). The figure also shows the shape
of the cavity made by the surrounding solvent molecules
(purple) and neighboring rotor molecules (blue) occupied by
the rotator.
X-ray Crystal Structure Analysis of 3. Solvent-free X-ray
quality crystals of 3 were grown by slowly evaporating a
90:5:5 hexanes/toluene/dichloromethane solution at room
temperature. The structure was solved at 298 K in the mono-
clinic space group P21/n with 1/2 molecule per asymmetric
unit and 2 molecules per unit cell. The unit cell is shown in
Figure 5. The alkyne axles in this case also deviate from
linearity but to a larger degree with the ipso carbon of the
rotator, center of the alkyne bond, and methane carbon of
the stator forming an angle of 171°. The triphenylmethyl
groups again adopt a staggered conformation. In this case,
however, the rotor molecules possess a center of inversion, so
that each carbon atom in the structure is crystallographically
equivalent to another related by the inversion symmetry
operation. The molecules pack such that the triphenylmethyl
stators surround the anthryl rotators of neighboring
Solid-State CPMAS 13C NMR. The CPMAS 13C NMR
spectra of compound 1 reported in ref 12 was acquired with a
cross-polarization time of 50 μs, and the resulting spectrum
varied from the slow to the fast exchange regime as the
temperature varied from 214 to 308 K. The low temperature
spectrum consisted of two well-resolved signals at 128.5 and
132 ppm that were assigned to sites related by a 180°
rotation, which were shown to coalesce at 280 K and to
sharpen at higher temperatures. The CPMAS 13C NMR
spectra of compounds 2-5 were systematically explored with
cross-polarization times of 50, 500, 5,000, and 15,000 μs in
order to determine the optimal conditions to highlight the
protonated and nonprotonated carbons of the rotator, as
well as the contact times required to transfer polarization to
the carbon atoms of the dialkyne axle and the deuterated
stator, which are significantly more distant from the
H-donors in the center of the molecules. Representative
CPMAS 13C NMR spectra of compound 2 obtained at
298 K with contact times of 50, 500, and 15,000 μs are shown
in Figure 7 along with the corresponding solution spectrum
(CDCl3). Full spectra obtained with similar contact times for
compounds 2-5 are included in Supporting Information
(Figure S20).
As illustrated in Figure 7, the four signals of the biphenyl
rotator in the spectrum of 2 in CDCl3 occur at 140.0 (C1),
132.1 (C2), 127.2 (C3), and 122.9 ppm (C4) (Figure 7d). As it
is apparent from the spectrum in Figure 7a, a contact time of
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FIGURE 8. Relative intensities of protonated rotator carbon (9),
nonprotonated rotator carbon (2), alkyne carbon (ꢀ), quaternary
carbon (0), and trityl ipso carbon (þ) signals of 2 with respect to
contact time.
FIGURE 7. CPMAS 13C NMR spectra of the o-xylene clathrate of
compound 2 acquired with contact times of (a) 50 μs, (b) 500 μs, and
(c) 15,000 μs with the solution spectrum (d) added for comparison.
50 μs reveals only two relatively sharp signals, which are
assigned to protonated carbons C2 and C3. The spectrum in
Figure 7b illustrates the effect of increasing the contact time
to 500 μs with the two types of ipso carbons in the biphenyl
moiety (C1 and C4) starting to become visible at 138.5 and
122.2 ppm. A further extension of the contact time to 15 ms
reveals all the signals in the structure, including those of the
o-xylene-d10 solvent with methyl groups at 19.8 ppm and
aromatic carbons overlapping with those of compound 2. It
is worth noting that there are more signals in the full CPMAS
spectrum than in the one solution, as expected from the lower
molecular symmetry in the solid state. This is particularly
evident with the signals corresponding to the ipso carbons of
the trityl stator. While only one signal is observed in solution
at 145.2 ppm, three signals corresponding to six crystallo-
graphically nonequivalent ipso carbons are observed in the
solid state at 143.4, 145.2, and 147.7 ppm. However, rela-
tively narrow and unique signals for each of the quaternary
trityl (56.1 ppm), alkyne (95.7 and 84.4 ppm), and biphenyl
carbons (138.5, 132.4, 125.4, and 122.2 ppm) indicate that
the two halves of the molecule are coincidentally isochro-
nous despite being crystallographically nonequivalent.
Single resonances for the protonated carbons of the biphenyl
group are consistent with two scenarios: (a) a static structure
with insufficient chemical shift dispersion to resolve the two
sites related by 180° rotation or (b) a rotational motion takes
place in the fast exchange regime at ambient temperature (see
below).
FIGURE 9. Isolated CPMAS 13C rotator signals of compounds
2-5 at short contact time (50 μs).
depends on molecular motions with frequencies on the
order of the spin locking fields (ca. 10-20 kHz), which
are close to rotary motions observed in other molecular
gyroscopes.2a-d,7,8
The NMR signals of the protonated rotators of all com-
pounds can be isolated in the same manner as illustrated in
Figure 9 such that the only 13C signals visible at short contact
times correspond to the protonated carbons. The CPMAS
13C NMR spectra of compounds 3-5 at intermediate and
long contact times are available in Supporting Information
(Figure S20).
Variable-Temperature CPMAS 13C NMR Measurements
of the o-Xylene Clathrate of 2. As shown in Figure 10, the
311 K spectrum of 2 acquired with a 50 μs contact time
consists of two relatively sharp signals corresponding to
C2/C20 and C3/C30, respectively. A modest broadening of
the C2/C20 signal at ca. 132 ppm was observed when the
temperature was lowered to 199 K (Figure 10). While one
could interpret signal broadening in terms of a very rapid
two-site exchange that barely begins to enter the intermedi-
ate exchange regime, one should also consider a situation
where the two sites are actually static but have temperature-
dependent chemical shifts that change the extent of overlap.
We reasoned that changes in rotational dynamics induced
with the help of simple structural perturbations could help
determine whether the changes in line shape in Figure 10
could be assigned to fast motion of the biphenylene
group. As a simple test we decided to prepare and analyze
In order to determine how the signal intensities are
affected by varying contact times, the intensities of several
signals from the spectra in Figure 7 were plotted with respect
to the cross-polarization, or contact time, in Figure 8. From
this experiment one can see that the 13C signal enhancement
is strongly dependent on the proximity of each carbon to the
1H nuclei. The aromatic 13C attached to the 1H nuclei reach
their maximum intensities within 500 μs, whereas the non-
protonated 13C nuclei of the biphenyl rotator located
1
2 bonds away from the H nuclei reach their maximum
intensity around 5,000 μs. The 13C nuclei located even farther
away do not reach their maximum even after 15,000 μs. It is
also clear from the plot that the rotator signals reach their
maxima and begin to decay with rates determined by the
spin-lattice relaxation in the rotating frame, T1F, which
J. Org. Chem. Vol. 75, No. 8, 2010 2487

O’Brien et al.
JOCArticle
by the 180° rotation. While the signal corresponding to
carbon B at 124.9 ppm must undergo the same site-exchange
process, the lack of splitting at low temperatures indicates
that the two sites must have the same chemical shift.15
Approximate exchange rates obtained by simulation of the
line shapes varied from 1 s-1 at 196 K to 2,990 s-1 at 318 K.
An Arrhenius plot constructed with the frequency and
temperature data revealed an activation energy of 8.7 kcal/
mol and a pre-exponential factor of 4.9 ꢀ 109 s-1
.
Having confirmed a dynamic processes in the kilohertz
regime in solvent-free samples of 2 and with hints that
rotation in the o-xylene structure may be even faster, we
decided to investigate the dynamics of the latter by variable-
temperature wide-line quadrupolar echo 2H NMR. To carry
out those measurements, we prepared compound 10 with
a natural abundance stator and a deuterated rotator
(Scheme 2).
FIGURE 10. Experimental (right) and simulated (left) solid-state
CPMAS 13C NMR of the protonated aromatic signals of 2. The
frequencies of the two sites were assumed to be the same as those in
the solvent-free sample shown in Figure 11 at 196 K, and the rate
constants for rotational exchange used for fitting are based on the
2
results of the VT H NMR experiments shown in Figure 12. The
exchange rates in the simulations are 1.0 ꢀ 103 s-1 at 199 K and
9.0 ꢀ 105 s-1 at 311 K.
Variable-Temperature 2H Quadrupolar Echo Measure-
ments with the o-Xylene Clathrate of 10. It is known that
variable-temperature 2H NMR with static powder samples is
a powerful method to determine internal molecular dy-
namics of solids in a range that covers ca. 103-107 s-1 16
.
Deuterium NMR is largely dominated by the orientation-
dependent interaction between the nuclear spin and electric
quadrupole moment at the nucleus that gives rise to a broad
spectrum known as a Pake or powder pattern. Changes in the
spectra occur when the C-2H bonds experience reorienta-
tions that reduce their magnetic interactions by dynamic
averaging.17 Robust methods have been implemented to
study the rotation of phenylene groups, which are known
to have quadrupolar coupling constants (QCC) of ca. 180
kHz.18 Crystalline samples with no phase transitions display
spectra that change as a function of temperature in a
predictable manner, such that for structurally well-charac-
terized dynamic processes the experimental spectrum can be
simulated with a model that considers the trajectories of
motion and a suitable exchange rate.
2H NMR experiments were carried out with a standard
quadrupolar echo sequence on samples of compound 10
freshly crystallized from o-xylene. As shown in Figure 12,
measurements were carried out over a similar temperature
range as the CPMAS 13C NMR experiments in Figures 10
FIGURE 11. Experimental (right) and simulated (left) solid-state
CPMAS 13C NMR of 2. The rotational rate constants used for
fitting, from bottom to top, are (s-1) 1.2, 50, 173, 253, and 2990.
2
a solvent-free sample with a solid obtained by rapid evapo-
ration from CH2Cl2. We were able to confirm the lack of
solvent in a solid material that appeared amorphous. How-
ever, analysis by X-ray powder diffraction revealed broad
signals with a pattern completely different from that of the
o-xylene clathrate, suggesting the formation of a crystalline
solid (Figure S23 in Supporting Information). As it was
hoped for, analysis of this sample by VT-CPMAS 13C
NMR displayed line shape changes characteristic of a
dynamic process in the intermediate exchange regime
(Figure 11). Comparison of the spectra acquired at 318 and
196 K showed that the signal at 132.4 ppm in the high-
temperature spectrum splits into two signals at 134.4 and
129.3 ppm as the temperature is lowered. In going from lower
to higher temperature, the spectra showed coalescence of the
signals at 134.4 and 129.3 ppm as the temperature increased
between 289 and 318 K. The merging peaks correspond to
the carbons that are ortho to the alkyne linkages, carbon A in
Figure 11, and their coalescence occurs as the biphenyl jumps
between the two crystallographically different sites related
and 11. As the H label represents only 0.97% of the total
mass in the sample, at least ca. 2048 transients with 20 s
between pulses had to be averaged to obtain reasonable
spectra. The spectrum acquired at 188 K approaches the
slow exchange regime, and the simulation suggests a 180°
rotation with an exchange rate of 7.0 ꢀ 103 s-1. At the other
extreme of the temperatures analyzed, the spectrum is also
consistent with a fast flipping motion, but the line shape is
slightly narrower than that expected from a simple 180°
(15) While differences in cross polarization efficiencies and relaxation
properties make uncalibrated integration of CPMAS 13C NMR spectra
quantitatively unreliable, the integration of signals A and B remained
relatively constant as a function of temperature, in agreement with the
expected assignments.
(16) Hoatson, G. L.; Vold, R. L. NMR 1994, 32, 1–67.
(17) (a) Kamihira, M.; Naito, A.; Tuzi, S.; Saito, H. J. Phys. Chem. A
1999, 103, 3356–3363. (b) Hiraoki, T.; Kogame, A.; Norio, N.; Akihiro, T.
J. Mol. Struct. 1998, 441, 243–250. (c) Zhang, H.; Bryant, R. G. Biophys. J.
1997, 72, 372. (d) Naito, A.; Izuka, T.; Tuzi, S.; Price, W. S.; Hayamizu, K.;
Saito, H. J. Mol. Struct. 1995, 355, 55–60.
(18) Mantsch, H. H.; Saito, H; Smith, I. C. P. Prog. NMR Spectrosc.
1977, 11, 211–272.
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account for the different barriers. While computational
studies and additional experiments will be needed, it is
possible that local motion and correlated effects of the
adjacent phenylenes of 2 may be responsible for their lower
barrier.
A comparison of the activation parameters obtained for
the solvent-free solid by VT CPMAS 13C NMR with those of
the crystalline clathrate by VT quadrupolar echo 2H NMR is
very interesting. While the activation energies of 8.7 and
7.4 kcal/mol for the solvent-free and solvated solids, respec-
tively, are within the range of experimental uncertainties, the
magnitudes of their pre-exponential factors, A, are quite
different. Also known as the “frequency factor” or “attempt
frequency,”23 the pre-exponential factor represents the high-
est rotational frequency that would be possible in a hypothe-
tical structure with no energy barrier. Its value depends on
the moment of inertia of the rotator, the torsional mode that
describes the oscillation of the central phenylene, and the
vibrational dynamics of the crystal. A pre-exponential factor
of 2.5 ꢀ 1012 s-1 for the crystalline clathrate is consistent with
the values obtained previously for phenylene rotators in
other crystalline environments2,4,7,8 and is close to the value
calculated for a free rotator on the basis of its moment of
inertia.24 However, a pre-exponential factor of only 4.9 ꢀ
109 s-1 for the desolvated sample is 3 orders of magnitude
lower! Given that it is the same molecular structure, it seems
likely that differences in rotational motion arise from diffe-
rences in their solid-state environments. While conforma-
tional and packing differences are likely, differences result-
ing in steric hindrance should affect primarily the activation
the energy. An important difference between the solvent
containing and desolvated crystals that may contribute to
their different pre-exponential factors is the extent of their
long-range order, which is expected to affect their lattice
dynamics.25
FIGURE 12. Experimental (right) and simulated (left) solid-state
2H quadrupolar echo NMR of 10. The rotation rate constants
used for fitting, from bottom to top, are (s-1) 7.0 ꢀ 103, 7.0 ꢀ 105,
1.7 ꢀ 106, and 1.5 ꢀ 107.
rotation. The model used in the 2H NMR simulation
involves the expected 2-fold flipping motion with an addi-
tional (15° libration in the fast exchange limit, as expected
for a rotator with relatively large amplitude oscillation at the
bottom of its energy well.19 Intermediate spectra were also
simulated with the 180° rotational model with the exchange
frequencies listed in the figure. An Arrhenius plot built with
the temperature and frequency data gave an energy barrier of
7.4 kcal/mol and a pre-exponential factor of 2.5 ꢀ 1012 s-1
.
With two transition states related by 180° per cycle, our
results indicate that the intrinsic gas potential of biphenyl
with four transition states of ca. 1.4 and 1.6 kcal/mol for
coplanar and orthogonal orientations of the two rings,
respectively,20 does not play a significant role in the solid
state.
2H NMR experiments carried out with the solvent-free
sample (Supporting Information, Figure S24) provided spec-
tra in the slow exchange regime, as expected from its lower
pre-exponential factor. It should be noted that the 2H NMR
results in Figure 12 do not have sufficient kinetic resolution
to determine whether the two crystallographically nonequi-
valent phenyl groups have distinguishable dynamics. While
the error in the 2H NMR data has been estimated as high as 2
kcal/mol, it is also notable that the activation energy for 2H
NMR rotational exchange of the biphenylene group in
o-xylene-containing crystals of 10 is ca. 4-5 kcal/mol lower
than that reported for the rotation of a single phenylene in
benzene-containing crystals of 1 in the same temperature
range (Ea = 11.3-12.8 kcal/mol).7a,b,12 Considering that
differences in the amount of free volume may be a possible
explanation for this difference, we analyzed the packing
coefficient of crystals 1 and 2 using the increment approach
proposed by Gavezzotti.21 Packing coefficients were defined
by Kitaigorodski as the volume of the molecules in the unit
cell relative to the total volume of the unit cell.22 Values
calculated for compounds 1 and 2 of 0.729 and 0.730,
respectively, suggest that the average free volume cannot
We had noticed that peaks in the CPMAS spectra of the
solvent-free solid are significantly broader than those of the
crystalline clathrate, suggesting a disordered environment.
To confirm that there are large differences in long-range
order between the two samples, we analyzed their X-ray
˚
powder patterns acquired at a wavelength of 1.542 A (Cu
KR).26 The line-widths of the solvent-free powder samples at
half the maximum intensity (e.g., fwhm ≈ 0.27° at 2Θ =
16.8°, Figure S23) are ca. 2.7 times as large as those of
samples from freshly powdered crystals with o-xylene (e.g.,
fwhm ≈ 0.1° at 2Θ = 17.6°). From the Scherrer equation,26
one can estimate that broadening in the solvent-free sample
corresponds to a domain size of ca. 33 nm. Thus, while the
pre-exponential factors of highly crystalline rotor molecules
with phenylene rotators have values in the terahertz regime
(ca. 1012 s-1), we speculate that changes in lattice vibrations
as a function of disorder may result in less efficient mechan-
isms for rotational motion.
Conclusions. We have shown that the rotator signals of
compounds 2-5, as previously shown for 1, are all visible
and with no interference from the carbons of their trityl
stators at contact times of ca. 50 μs. While the strategy is ideal
(19) (a) Aliev, A. E.; Harris, K. D. M.; Champkin, P. H. J. Phys. Chem. B
2005, 109, 23342–23350. (b) Hallock, K. J.; Lee, D. K.; Ramamoorthy, A.
J. Chem. Phys. 2000, 113, 11187–11193.
(20) (a) Johansson, M. P.; Olsen, J. J. Chem. Theor. Comp. 2008, 4, 1460.
(b) Bastiansen, O.; Samdal, S. J. Mol. Struct. 1985, 128, 155.
(21) Gavezzotti, A. J. Am. Chem. Soc. 1990, 105, 5220–5225.
(22) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic
Press: New York, 1973.
(23) Laidler, K. J. Chemical Kinetics, 3rd ed.; Benjamin-Cummings, 1997
(24) Kawski, A. Crit. Rev. Anal. Chem. 1993, 23, 459–529.
(25) Burns, G. Solid State Physics; Academic Press: San Diego, CA, 1985.
(26) West, A. R. Solid State Chemistry and Its Applications; John Wiley &
Sons: New York, 1984.
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O’Brien et al.
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for the study of dynamic processes in the solid state, the
method suffers the inherent limitations of the VT CPMAS
13C NMR experiment and will be useful when the exchanging
signals have sufficiently different resonance frequencies with
structurally well-determined dynamic processes that are
thermally accessible in the range of ca. 100 Hz to 10 kHz.2c
The rotational dynamics of crystalline samples of the
molecular rotor with a biphenylene rotator obtained with a
combination of VT CPMAS 13C NMR (compound 2) and
static 2H NMR (compound 10) revealed remarkably similar
activation energies and pre-exponential factors that correlate
with the extent of order. Given that many previous reports
with ordered crystalline substrates are in agreement with
ordered crystals of 2 and give pre-exponential values in the
1012 s-1 range, it was speculated that the 3 orders of
magnitude lower value measured in the lower quality crystals
may be due to the changes in lattice vibrations, which may be
needed to couple with rotator torsional modes to provide a
good mechanism for internal rotations. Further experiments
with other crystal and glass forming structures accompanied
by computational studies will be carried out to investigate
this possibility.
(0.138 g, 0.443 mmol) were dissolved in the solution, which
was then sparged with argon for 1 h. Bis(triphenylphosphine)
palladium(II) chloride (0.037 g, 0.052 mmol) and copper(I)
iodide (0.012 g, 0.064 mmol) were added, and the reaction was
heated to reflux for 24 h under argon. Reaction progress was
monitored by thin layer chromatography. Upon completion, the
reaction mixture was washed with saturated ammonium chloride
solution, and the organics were extracted with methylene chloride
and dried over magnesium sulfate. The crude product was purified
by column chromatography (hexanes/C6H6/CH2Cl2 = 90:5:5) to
afford 0.125 g (47.4% isolated yield) of 2 as a white solid, mp dec
>253 °C. 1H NMR (500 MHz, CDCl3): δ 7.56 (dd, 4H, J = 2.0,
8.5 Hz), 7.53 (dd, 4H, J = 2.0, 8.5 Hz). 13C NMR (125 MHz,
CDCl3) δ 145.1, 139.9, 132.1, 128.7 (t, 1JCD = 24.0 Hz), 127.5 (t,
1
1JCD = 23.4 Hz), 126.8, 126.3 (t, JCD = 21.7 Hz), 122.8, 96.6,
84.8, 56.0. FTIR (solid HATR, cm-1): 3079, 3062, 3032, 2274,
1559, 1492, 1362, 1327, 824. HRMS (MALDI-ICR) C54H8D30:
calcd 716.4826; found 716.4879.
9,10-Bis(3,3,3-tri-d5-phenylpropynyl)anthracene (3). Compound
3 was prepared using the same procedure as for compound 1 in
55.9% yield (0.156 g) as a bright yellow solid, mp dec >312 °C. 1H
0
0
NMR (500 MHz, CDCl3): δ 8.56 (m, 4H, Jax = Ja x = 9.0 Hz,
0
0
0
0
Jxx = 8.0 Hz, Jaa = 0.8 Hz, Jax = Ja x = 2.1 Hz), 7.51 (m, 4H,
0 0 0 0 0 0
Jax = Ja x = 9.0 Hz, Jxx = 8.0 Hz, Jaa = 0.8 Hz, Jax = Ja x = 2.1
Hz), 7.38 (s, residual trityl protons), 7.33 (s, residual trityl protons).
13CNMR(125 MHz, CDCl3): δ145.1, 132.6, 128.9 (t, 1JCD = 23.9
Hz), 127.7 (t, 1JCD = 23.9 Hz), 126.6, 126.3 (t, 1JCD = 21.7 Hz),
118.4, 108.4, 82.5, 57.0. FTIR (solid HATR, cm-1): 3059, 2274,
1559, 1435, 1392, 1362, 1326, 826, 770. HRMS (MALDI-ICR)
C56H8D30: calcd 740.4826; found 740.4889.
Experimental Section
IR spectra were obtained with a ATR-FTIR instrument. 1H
and 13C spectra were acquired on a spectrometer at frequencies
of 500 and 125 MHz, respectively, with CDCl3 as the solvent.
Solid-state CPMAS 13C NMR spectra were obtained at 75
MHz. DSC analyses were recorded between room temperature
and the melting/decomposition temperature as determined by a
standard melting point apparatus. Mass spectra were acquired
on an FT-ICR-MS with the MALDI ion source.
1,4-Bis(3,3,3-tri-d5-phenylpropynyl)naphthalene (4). Compound
4 was prepared using the same procedure as for compound 1 in
67.7% yield (0.1832 g) as a white solid, mp dec >247 °C. 1H NMR
0
0
0
=
(500 MHz, CDCl3): δ 8.36 (m, 2H, Jax = Ja x = 8.9 Hz, Jxx
0
8.1 Hz, Jaa = 0.8 Hz, Jax = Ja x = 2.1 Hz), 7.69 (s, 2H), 7.54 (m,
0
0
0
2H, (m, 2H, Jax = Ja x = 8.9 Hz, Jxx = 8.1 Hz, Jaa = 0.8 Hz,
0
0
0
3,3,3-Tri-d5-phenylpropyne (7). A 100-mL three-neck round-
bottomed flask equipped with a reflux condenser was charged
with d6-benzene (10.0 g, 0.119 mmol), carbon tetrachloride
(2.0 mL, 0.022 mmol), and 20 mL of 1,1,2,2-tetrachloroethane.
The solution was sparged with argon for 30 min and then cooled
to 0 °C. Aluminum chloride (3.1 g, 0.023 mmol) was then added
slowly with stirring. The solution was stirred for 15 min before
being allowed to warm to room temperature. The solution was
heated for 90 min and then cooled. Solution was quenched with
1 M HCl, and the organics were extracted with methylene
chloride and dried over magnesium sulfate. The crude pro-
duct was purified by column chromatography (hexanes/ethyl
acetate = 9:1) to afford 3.24 g of 7 (50.1% isolated yield).11 The
trityl chloride was then dissolved in 10 mL of acetyl chloride and
heated to reflux for 1 h. Acetyl chloride solvent was then
removed in vacuo, and 100 mL of dry benzene was added and
subsequently removed in vacuo. An additional 500 mL of dry
benzene was then added followed by 100 mL of ethynylmagne-
sium bromide (0.5 M in THF) via cannula. The solution was
heated to reflux overnight. The cooled solution was then
quenched with a saturated ammonium chloride solution, and
the organics were extracted with methylene chloride and dried
over magnesium sulfate. The crude product was purified by
column chromatography (hexanes) to afford 1.80 g of 6 (69.8%
isolated yield) as a white solid.
0
0
Jax = Ja x = 2.1 Hz), 7.46 (s, residual trityl protons), 7.38 (s,
residual trityl protons), 7.33 (s, residual trityl protons). 13C NMR
1
(125 MHz, CDCl3): δ 145.1, 133.5, 129.6, 128.8 (t, JCD
=
1
23.9 Hz), 127.6 (t, JCD = 24.0 Hz), 127.1, 126.6, 126.4 (t,
1JCD = 23.5 Hz), 102.0, 83.4, 56.4. FTIR (solid HATR, cm-1):
3059, 3047, 2277, 1560, 1384, 1362, 1327, 846, 826, 768. HRMS
(MALDI-ICR) C52H6D30: calcd 690.4760; found 690.4704.
(()-4,40-Bis(3,3,3-tri-d5-phenylpropynyl)-1,10-binaphthyl (5).
Compound 5 was prepared using the same procedure as for
compound 1 from racemic starting material in 92.3% yield
1
(0.260 g) as a white solid, mp dec >268 °C. H NMR (500
MHz, CDCl3): δ 8.46 (d, 2H, J = 8.4 Hz), 7.90 (d, 2H, J = 7.3
Hz), 7.53 (dd, 2H, J = 7.1, 7.1 Hz), 7.47 (d, 2H, J = 7.3 Hz), 7.46
(d, 2H, J = 8.4 Hz), 7.33 (dd, 2H, J = 7.3, 7.3 Hz). 13C NMR
(125 MHz, CDCl3): δ 145.2, 138.6, 133.7, 132.6, 129.8 128.8 (t,
1JCD = 24.3 Hz), 127.6 (t, 1JCD = 24.0 Hz), 127.2, 126.8, 126.7,
126.6, 126.5, 121.3, 101.0, 83.6, 56.5. FTIR (solid HATR,
cm-1): 3089, 3052, 2274, 1575, 1381, 1362, 1326, 825, 758.
LRMS (MALDI-TOF) C62H12D30: calcd 816.52; found 816.52.
4,40-Dibromo-d8-biphenyl (9). A dry 500-mL round-bottom
flask was charged with 1,4-dibromobenzene-d4 (120 mg, 0.507
mmol), and 30 mL of dry tetrahydrofuran. The solution was
cooled to -78°, tert-butyllithium (0.37 mL, 0.56 mmol) was
added, and the solution was stirred for 90 min. Copper(I) cya-
nide (22 mg, 0.25 mmol) was then added, and the solution was
stirred vigorously as it warmed up to room temperature. Upon
reaching room temperature, 2,5-dimethyl-p-benzoquinone
(104 mg, 0.761 mmol) was added, and the resultant dark blue
solution was stirred at room temperature for 3 h. The reaction
was then quenched with 20 mL of 2 M HCl, and the organics
were extracted with diethyl ether and dried over MgSO4. The
crude product was purified by column chromatography
1,4-Bis(3,3,3-tri-d5-phenylpropynyl)benzene (1). The synthesis
and characterization of compound 1 has been described in the
literature.11
4,40-Bis(3,3,3b-tri-d5-phenylpropynyl)biphenyl (2). A 100-mL
three-neck round-bottomed flask equipped with a reflux con-
denser was charged with 40 mL of THF, 20 mL of diisopropyl
amine, and a magnetic stir bar. The 3,3,3-tri-d5-phenyl-1-
propyne (0.209 g, 0.738 mmol) and 4,40-dibromobiphenyl
2490 J. Org. Chem. Vol. 75, No. 8, 2010

O’Brien et al.
JOCArticle
(100% hexanes, silica) to afford 121 mg of 9 (75% isolated yield) as
96.6, 84.8, 56.1. FTIR (solid HATR, cm-1): 3082, 3058, 3031,
2278, 2251, 1596, 1490, 1446, 1398, 1327, 1033, 755. LRMS
(MALDI-TOF) C54H30D8: calcd 695.35; found 695.33.
a white solid, mp 162 °C. H NMR (500 MHz, CDCl3): δ. 13C
1
NMR (125 MHz, CDCl3): δ 145.2, 138.6, 133.7, 132.6, 129.8 128.8
(t, 1JCD = 24.3 Hz), 127.6 (t, 1JCD = 24.0 Hz), 127.2, 126.8, 126.7,
126.6, 126.5, 121.3, 101.0, 83.6, 56.5. FTIR (solid HATR, cm-1):
2274, 1569, 1375, 1362, 1326, 825, 758. LRMS (MALDI-TOF)
C12D8Br2: calcd 317.95; found 317.96.
Acknowledgment. The National Science Foundation sup-
ported this work through grant DMR-0605688.
4,40-Bis(3,3,3-triphenylpropynyl)biphenyl-d8 (10). Compound
10 was prepared using the same procedure as for compound 1 in
46% yield (47 mg) as a white solid, mp dec >253 °C. 1H NMR
(300 MHz, CDCl3): δ 7.29-7.45 (m, 30H), 13C NMR (75 MHz,
CDCl3): δ 145.2, 139.7, 131.6 (t, 1JCD = 25.0 Hz), 129.1, 128.0,
Supporting Information Available: Synthetic procedures,
spectroscopic data for all compounds, and crystallographic
information files in CIF format for compounds 2 and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
1
127.0, (t, JCD = 24.0 Hz), 126.5 (t, JCD = 25.0 Hz), 122.6,
J. Org. Chem. Vol. 75, No. 8, 2010 2491