An investigation of π-π packing in a columnar hexabenzocoronene by fast magic-angle spinning and double-quantum 1H solid-state NMR spectroscopy

Article abstract of DOI:10.1021/ja990637m

¹H NMR methods employing very fast magic-angle spinning (MAS) are applied to the investigation of both the structure and dynamics of an alkyl-substituted hexa-peri-hexabenzocoronene (HBC), which is known to form a columnar mesophase with a very

Full text of DOI:10.1021/ja990637m

6712
J. Am. Chem. Soc. 1999, 121, 6712-6718
An Investigation of π-π Packing in a Columnar Hexabenzocoronene
1
by Fast Magic-Angle Spinning and Double-Quantum H Solid-State
NMR Spectroscopy
Steven P. Brown, Ingo Schnell, Johann Diedrich Brand, Klaus Mu1llen, and
Hans Wolfgang Spiess*
Max-Planck-Institut fu¨r Polymerforschung, Postfach 3148, D-55021 Mainz, Germany
ReceiVed March 1, 1999. ReVised Manuscript ReceiVed May 10, 1999
Abstract: 1H NMR methods employing very fast magic-angle spinning (MAS) are applied to the investigation
of both the structure and dynamics of an alkyl-substituted hexa-peri-hexabenzocoronene (HBC), which is
known to form a columnar mesophase with a very high one-dimensional charge carrier mobility. For the
crystalline phase, three distinct aromatic resonances are identified in the single-quantum MAS spectrum. The
observation of these distinct aromatic resonances is explained in terms of the differing degrees to which the
aromatic protons experience the ring current of adjacent layers. Using the rotor-synchronized double-quantum
MAS method, definite proton-proton proximities are identified, which are shown to be in agreement with the
known crystal structure of unsubstituted HBC. In the liquid-crystalline phase, axial motion of the HBC disks
leads to the averaging of the three sites into a single resonance. Furthermore, the reduction of the dipolar
coupling caused by this motion is quantitatively investigated by an analysis of double-quantum MAS spinning
sideband patterns. For all spectra, the resolution is significantly improved for a compound where the R-carbon
positions have been deuterated; the synthesis of this deuterated compound, in particular the final cyclodehy-
drogenation step, is described.
Introduction
spectroscopy7-12 enabled specific information about hydrogen-
bonding arrangements to be deduced. In this way, the DQ MAS
method can be considered to be the analogue of the solution-
state NOESY1 experiment, the latter being heavily exploited in
the structural elucidation of biopolymers.13
1
Solution-state H NMR spectroscopy has established itself
as an indispensable method for the investigation of structure
and dynamics over a very wide range of applications.1 However,
1
for rigid solids, the application of H NMR spectroscopy is
Another important intermolecular aspect which governs the
mutual arrangement of molecules in condensed phases is the
π-π interaction, in particular that between aromatic moieties.14
There is currently much interest in polycyclic aromatic materials,
which form columnar mesophases, on account of, for example,
their potential applications as vectorial charge transport layers
in xerography, electrophotography, or molecular electronic
devices. One such class of materials are the hexaalkyl-substituted
hexa-peri-hexabenzocoronenes, an example of which has re-
cently been shown to possess an exceptionally high one-
dimensional charge carrier mobility.15
complicated by the strong homonuclear proton-proton dipolar
interaction;2-4 this leads to substantial homogeneous broadening
of the resonances, which is only partially narrowed by magic-
angle spinning (MAS), such that chemical shift information is
usually obscured. As a consequence, the routine use of solid-
state NMR has, to date, focused on nuclei for which the dipolar
interactions are weaker, such as 13C and H.
2
Over the past decade, much progress has been made in the
area of fast MAS,5 and probes capable of a rotation frequency,
ωR/2π, of 35 kHz are now commercially available. Recently,
we have shown that the resolution afforded by such a fast ωR
is sufficient to allow specific structural information to be
In this paper, we then apply 1H MAS and 1H DQ MAS NMR
to the investigation of the structure and dynamics of hexa-n-
obtained for organic solids by H MAS NMR.6 In particular,
the information about the through-space proximity of dipolar-
coupled nuclei provided by H double-quantum (DQ) MAS
1
(7) Geen, H.; Titman, J. J.; Gottwald, J.; Spiess, H. W. Chem. Phys.
Lett. 1994, 227, 79.
(8) Sommer, W.; Gottwald, J.; Demco, D. E.; Spiess, H. W. J. Magn.
Reson. 1995, A113, 131.
1
* To whom correspondence should be addressed.
(1) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear
Magnetic Resonance in One and Two Dimensions; Clarendon: Oxford,
1987.
(2) Abragam, A. The Principles of Nuclear Magnetism; Clarendon:
Oxford, 1961.
(9) Geen, H.; Titman, J. J.; Gottwald, J.; Spiess, H. W. J. Magn. Reson.
1995, A 114, 264.
(10) Gottwald, J.; Demco, D. E.; Graf, R.; Spiess, H. W. Chem. Phys.
Lett. 1995, 243, 314.
(11) Feike, M.; Graf, R.; Schnell, I.; Ja¨ger, C.; Spiess, H. W. J. Am.
Chem. Soc. 1996, 118, 9631.
(3) Mehring, M. Principles of High Resolution NMR in Solids;
Springer: Berlin, 1983.
(4) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid State NMR
and Polymers; Academic Press: New York, 1994.
(5) Jakobson, H. J. Encyclopedia of Nuclear Magnetic Resonance;
Wiley: Chichester, 1996; Vol. 1, p 398.
(12) Graf, R.; Demco, D. E.; Gottwald, J.; Hafner, S.; Spiess, H. W. J.
Chem. Phys. 1997, 106, 885.
(13) Wu¨thrich, K. NMR of Proteins and Nucleic Acids; Wiley: New
York, 1986.
(14) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Elsevier: Amsterdam, 1989.
(6) Schnell, I.; Brown, S. P.; Low, H. Y.; Ishida, H.; Spiess, H. W. J.
Am. Chem. Soc. 1998, 120, 11784.
(15) Van de Craats, A. M.; Warman, J. M.; Mu¨llen, K.; Geerts, Y.; Brand,
J. D. AdV. Mater. 1998, 10, 36.
10.1021/ja990637m CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/25/1999

π-π Packing in a Columnar Hexabenzocoronene
J. Am. Chem. Soc., Vol. 121, No. 28, 1999 6713
dodecyl-hexa-peri-hexabenzocoronene (henceforth referred to
as HBC-C12). This sample is known to form a remarkably stable
columnar mesophase,16 indicating strong π-π packing, and has
been previously studied by variable-temperature 2H NMR
spectra.16 In addition, a STM image of a HBC-C12 monolayer
adsorbed onto graphite,17 as well as solution-state absorption
and fluorescence spectra,18 has been presented.
HBC-C12 samples with the R-carbons of the alkyl side chains
either deuterated (87%, 1; henceforth referred to as R-deuterated)
or simply protonated (2) were prepared and investigated. In
addition to providing structural information, high-resolution 1H
MAS NMR can also be used to study molecular dynamics. This
is demonstrated by recording one- and two-dimensional spectra
of HBC-C12 in both its solid and liquid-crystalline (LC) phases.
The dynamical information so obtained is compared with the
results of the previous investigation, where variable-temperature
2H NMR spectra were recorded for a sample in which the
aromatic hydrogens were deuterated.16
1H NMR Investigation
One-Dimensional MAS Spectra. Figure 1 presents 1H MAS
spectra of the crystalline phases of (a) R-deuterated and (b) fully
protonated HBC-C12, as well as (c) the LC phase of R-deuterated
HBC-C12, recorded (in all cases) at an ωR/2π of 35 kHz. For
comparison, a solution-state spectrum of R-deuterated HBC-
C12 is shown in Figure 1d. The six-fold symmetry of the
individual molecules means that only one distinct aromatic
proton resonance is expected, as is indeed observed to be the
case in the solution-state spectrum. However, in Figure 1a, three
aromatic resonances are clearly resolved; the effect of additional
dipolar couplings to the R-carbon protons means that the three
peaks are less well resolved in Figure 1b. Using data processing
methods known from solution-state NMR, it is possible to
enhance the resolution in the aromatic region. As an example
of what can be achieved, the dashed line in Figure 1b shows
the result of applying a method reported by Traficante and
Nemeth.19 We would, however, like to emphasize that such
methods should be used with extreme care in solid-state NMR,
since artifacts are often introduced, which cannot be distin-
guished from the genuine peaks.
Figure 1. One-dimensional 1H NMR spectra of (a, c, d) R-deuterated
and (b) fully protonated HBC-C12 in different physical states, namely
(a, b) the crystalline phase at 333 K, (c) the LC phase at 383 K, and
(d) dissolved in deuterated chloroform. The spectra a-c were recorded
under MAS at 35 kHz. The dashed line in spectrum b shows the
enhanced resolution achieved by a data processing method reported
by Traficante and Nemeth.19 The asterisk in spectrum d indicates the
signal due to residual CHCl3.
of the single aromatic peak in Figure 1d is significantly shifted
to low field compared to that in Figure 1c (8.2 compared to 6.2
ppm). In this context, it is further interesting to note that the
solution-state aromatic chemical shift was observed to shift to
low field on decreasing the concentration, ranging from 8.0 ppm
at a concentration of 0.017 M to 8.9 ppm at 1.7 × 10-6 M.
On heating of the sample above the melting point, the three
peaks transform into a single peak, with a line width which is
significantly greater than that observed for an unordered melt
(cf. Figure 3 of ref 6). It is also apparent that the chemical shift
1
In principle, the observed changes in the H MAS spectra
can be ascribed to two different scenarios: either there exist
three different types of HBC crystallographic environment,
where the six-fold symmetry of each individual molecule is
maintained, with the three environments then becoming equiva-
lent in the LC phase; or there is a reduction of the six-fold
symmetry of each molecule due to the packing arrangement in
(16) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater.
1996, 8, 510.
(17) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1609.
(18) Biasutti, M. A.; Rommens, J.; Vaes, A.; De Feyter, S.; De Schryver,
F. D.; Herwig, P.; Mu¨llen, K. Bull. Soc. Chim. Belg. 1997, 106, 659.
(19) Traficante, D. D.; Nemeth, G. A. J. Magn Reson. 1987, 71, 237.

6714 J. Am. Chem. Soc., Vol. 121, No. 28, 1999
Brown et al.
Figure 2. Rotor-synchronized 1H DQ MAS NMR spectra, together with skyline SQ and DQ projections, of (a) R-deuterated and (b) fully protonated
HBC-C12, recorded at 35 kHz using one cycle of the BABA recoupling sequence for the excitation and reconversion of DQCs. The assignment of
the peaks is discussed in the text.
the crystal, leading to inequivalent aromatic proton sites within
each HBC disk, with the axial motion in the LC phase then
making the sites equivalent on the NMR time scale. These
scenarios can be distinguished by DQ MAS NMR, which we
now turn to.
DQCs between the aromatic protons and undeuterated alkyl
protons can also be seen.)
For unsubstituted HBC, an X-ray single-crystal study21
showed that the molecules pack in the so-called herringbone
pattern, which optimizes the π-π interactions between adjacent
disks. We hypothesize here, on the basis of the observed spectral
features, that HBC-C12 adopts the same columnar packing of
the aromatic cores as in unsubstituted HBC, with the presence
of the long alkyl chains now meaning that the individual
columns of aromatic cores are much more separated from each
other. Figure 3 shows how such a stacking of the HBC cores
leads to three different aromatic proton environments. Three
aromatic cores are shown; the molecules above and below the
central aromatic core are indicated by dashed and dotted lines,
respectively, with the aromatic protons of the central layer
highlighted.
Two-Dimensional Rotor-Synchronized DQ MAS Spectra.
Information about the relative proximities of the three resolved
aromatic protons is provided by the rotor-synchronized 1H DQ
MAS two-dimensional spectrum of R-deuterated HBC-C12 in
Figure 2a, recorded using one cycle (of duration one rotor
period, τR) of the BABA8 recoupling technique for the excitation
and reconversion of double-quantum coherences (DQCs). A
detailed account of this method and the semiquantitative
interpretation of such DQ MAS spectra is given in ref 6.
Here, we focus on the peaks in the bottom left-hand corner
of the spectrum, due to DQCs involving only aromatic protons
(the top right-hand corner is dominated by the intense signal
due to the alkyl chain protons). As is evident from the molecular
structure of HBC-C12, the six aromatic protons are grouped in
pairs of “bay protons” with a H-H distance of approximately
0.20 nm, while the closest interpair distance is 0.41 nm.
Therefore, the aromatic region of the DQ MAS spectrum will
be dominated, for the short excitation period12 used here, by
the dipole-dipole couplings within the three pairs. In Figure
2a, it is clear that the extension to a second frequency dimension
has yielded a significant resolution improvement over the one-
dimensional spectrum (Figure 1a), and the three separate
resonances can be clearly distinguished without having to resort
to potentially unreliable data processing methods. A strong CC
auto peak and AB cross-peaks are observed, which imply the
presence of only two types of pairs of aromatic protons, HA-
HB and HC-HC, in a ratio, as determined from the peak
intensities, of 2:1. Note that, in such a DQ spectrum, an auto
peak arises from a DQC between two dipolar-coupled-like
(isochronous) nuclei; in contrast, two J-coupled-like nuclei do
not give rise to an auto peak in the analogous solution-state
INADEQUATE experiment.1,20 (In Figure 2a, in addition to the
CC auto peak and AB cross-peaks, weak cross-peaks due to
The interplanar distance in unsubstituted HBC is 0.342 nm,21
and thus the aromatic protons of one layer will additionally
experience the ring currents of the extended π-electron systems
of adjacent layers. As was demonstrated by Waugh and
Fessenden as early as 1956,22 when a proton sits above an
aromatic ring system as in, for example, [10]paracyclophane,
it experiences a large shielding effect and hence a marked high-
field shift (in this solution-state NMR example, the closed loop
structure fixes the position of the central methylene protons
above the aromatic ring).
In HBC-C12, three different aromatic proton environments
can thus be identified with respect to the degree to which the
proton experiences the ring current of the adjacent layers. The
unshaded circles represent protons which lie neither above nor
below the π orbitals of an adjacent layer and therefore
correspond to the least shielded resonance (highest ppm). Fully
shaded and hatched circles then represent protons which lie over
or below an inner and outer part, respectively, of an adjacent
ring system; the fully shaded protons would be expected to be
the most shielded. Assigning the unshaded, hatched, and fully
(21) Goddard, R.; Haenel, M. W.; Herndon, W. C.; Kru¨ger, C.; Zander,
M. J. Am. Chem. Soc. 1995, 117, 30.
(20) Buddrus, J.; Bauer, H. Angew. Chem., Int. Ed. Engl. 1987, 26, 625.
(22) Waugh, J. S.; Fessenden, R. W. J. Am. Chem. Soc. 1956, 79, 846.

π-π Packing in a Columnar Hexabenzocoronene
J. Am. Chem. Soc., Vol. 121, No. 28, 1999 6715
being resolved.23 Thus, we observe that both the aromatic and
the nearby alkyl protons of HBC disks are subjected to the
aromatic ring current of adjacent layers, and the extent of this
effect reflects the packing of the molecules.
DQ MAS Spinning Sideband Patterns. For such a system
as HBC-C12, which contains well-isolated spin pairs, a quantita-
tive evaluation of the dipolar couplings can be achieved by
extending the spectral width in the DQ dimension and then
analyzing the observed DQ spinning sideband patterns.9,10,12,24
The origin of such patterns is of interest, since for an isolated
spin pair there is no anisotropic evolution of the DQCs during
1
t1 (any H chemical shift anisotropy effects will be negligible
at the very fast ωR used). Thus, DQ spinning sidebands cannot
arise by the normal single-quantum mechanism, whereby the
observed sideband pattern can be considered to map out the
anisotropy of the spin interaction which is active during the
evolution period. Instead, it has been shown that the origin of
such spinning sidebands lies in the rotor-encoded change
between the Hamiltonian active during the reconversion period
and that active during the excitation of DQC.
Figure 3. Representation of the proposed stacking of the aromatic
cores in HBC-C12, based on the structure of unsubstituted HBC, which
is known, from an X-ray single-crystal study,21 to crystallize in the
so-called herringbone pattern. Three HBC-C12 molecules are shown;
the molecules above and below the central HBC-C12 molecule are
indicated by dashed and dotted lines, respectively. The labeling of the
aromatic protons of the central layer is discussed in the text.
For an isolated spin pair and using N cycles of the BABA
recoupling method for both the excitation and reconversion of
DQCs, the DQ time domain signal is given by12
s(t1,t2 ) 0) )
x
sin[3/(π 2)D sin(2â) cos(γ + ωRt1)NτR]
x
shaded protons to the A, B, and C resonances in Figure 2,
respectively, the observed presence of only AB and CC pairs
in the ratio 2:1, with the C resonance having the smallest
chemical shift value (most shielded), is then explained. Thus,
it is the second of the two scenarios, i.e., the reduction of the
six-fold to a two-fold symmetry for each individual molecule,
identified at the end of the previous section which is borne out
by experiment.
× sin[3/(π 2)D sin(2â) cos(γ)NτR] (1)
where â and γ are Euler angles relating the principal axes system
of the dipolar coupling tensor to the rotor-fixed reference frame,
and the brackets denote a powder average. The dipolar coupling
constant, D, is defined as
2
(µ0/4π)pγH
D )
(2)
For comparison, Figure 2b shows the analogous DQ MAS
spectrum for the fully protonated HBC-C12. In this case,
although the three aromatic resonances are less well resolved,
the presence of only AB and CC pairs in the ratio 2:1 is still
clearly apparent in the spectrum. It should also be noted that,
in Figure 2b, cross-peaks corresponding to DQCs between
aromatic and alkyl protons, which are largely suppressed in
Figure 2a on account of the deuteration of the R-carbons, are
now of comparable intensity to the aromatic-aromatic peaks.
Although care should be taken with the interpretation of these
aromatic-alkyl peaks on account of their existence as only weak
shoulders of the intense alkyl auto peak, the following features
are noteworthy. First, the cross-peaks involving HA, which are
the best resolved, correspond to DQCs involving alkyl protons
with a chemical shift of 2.3 ppm, whereas the intense alkyl auto
peak is centered at 1.1 ppm. This observation is, of course, in
agreement with what is known from solution-state NMR, namely
that the R-methylene protons are shifted to low field relative to
the other alkyl protons, which are more removed from the
aromatic core. What is more interesting is that the resonances
involving the R-methylene protons close to HB and HC are
apparently shifted to about 1.8 and even 1.0 ppm, respectively.
Such an observation is consistent with the fact, as revealed by
Figure 3, that the R-methylene protons close to HB and HC
experience to a greater extent the ring current of the adjacent
layers than do the R-methylene protons close to HA. In this
respect, we would like to add that we have observed a clearer
demonstration of this effect for a related compound, in which
the long alkyl chains are replaced by tert-butyl groups, with in
this case two distinct methyl resonances (at +1.3 and -0.6 ppm)
r3
where r is the internuclear distance.
Figure 4a and b presents experimental DQ MAS spinning
sideband patterns, together with the best-fit spectra (dotted lines),
for the aromatic protons in the crystalline and LC phases of
R-deuterated HBC-C12, respectively. The MAS frequency was
35 and 10 kHz in (a) and (b), respectively, with two rotor periods
being used for excitation/reconversion in both cases, such that
the product NτR equals 57 and 200 µs in the two cases. The
chosen excitation/reconversion times ensured the presence of
at least significant third-order sidebands, thus improving the
sensitivity of the observed pattern to D. The best-fit spectra
correspond to the Fourier transformation of DQ time domain
data sets generated using eq 1, with the powder average being
performed numerically. As is evident from the insets on the
right of Figure 4, the DQ spinning sideband patterns are very
sensitive to the dipolar coupling constant, D, and the excitation/
reconversion time, NτR.
For the crystalline phase, the displayed spectrum corresponds
to proton HA (at 8.3 ppm), though essentially the same pattern
is obtained for protons HB and HC. In addition to the peaks of
interest due to the aromatic-aromatic DQCs in (a), further peaks
(marked by *) corresponding to DQCs between aromatic and
residual undeuterated R-carbon protons are observed. (Note the
change in the relative intensity of the peaks due to the two types
(23) Brown, S. P.; Schnell, I.; Brand, J. D.; Mu¨llen, K.; Spiess, H. W.
J. Mol. Struct. Accepted for publication.
(24) Friedrich, U.; Schnell, I.; Brown, S. P.; Lupulescu, A.; Demco, D.
E.; Spiess, H. W. Mol. Phys. 1998, 95, 1209.

6716 J. Am. Chem. Soc., Vol. 121, No. 28, 1999
Brown et al.
Figure 4. Extracted columns from 1H DQ MAS spectra of R-deuterated HBC-C12, showing the DQ spinning sideband patterns for (a) the aromatic
protons at 8.3 ppm and (c) the alkyl protons at 1.2 ppm in the crystalline phase (T ) 333 K), and (b) the aromatic protons at 6.2 ppm in the LC
phase (T ) 386 K). In each case, best-fit spectra, generated according to the spin-pair expression in eq 1, are shown (shifted to the left to allow
a better comparison) as dotted lines. A spinning frequency, ωR/2π, equal to 35 and 10 kHz was used for the crystalline and LC phases, respectively,
with the two-rotor-period compensated BABA recoupling sequence (see Experimental Section) being used for the excitation and reconversion of
DQCs in both cases. In (a), additional peaks corresponding to DQCs between aromatic and residual undeuterated R-carbon protons are marked by
asterisks. The insets to the right of the experimental spectra display simulated spinning sideband patterns for various cases discussed in the text.
of DQCs on increasing the excitation/reconversion time from
one (Figure 2a) to two rotor periods (Figure 4a).12)
motion, the dipolar coupling constant is reduced by a factor of
1/2(1 - 3 cos2 θ), where θ is the angle between the principal
axes system (here the internuclear vector) and the molecular
rotation axis.2 Thus, for the case where the internuclear vector
is perpendicular to the rotation axis (θ ) 90°), a reduction by
a factor of 0.5 is expected. If there were small out-of-plane
distortions of the C-H bonds, this would then lead to a deviation
of θ from 90°, and thus a larger reduction; for example, θ equal
to 75° would give the experimentally observed value. Alterna-
tively, the value of 0.40 could be explained by postulating the
presence of out-of-plane motion in addition to the axial rotation.
For an isolated spin pair, only odd-order DQ spinning
sidebands are expected; in real systems, the presence of
perturbing spins is often evidenced by the observation of both
a centerband and even-order sidebands (the analogous observa-
tion of odd-order sidebands in triple-quantum MAS spectra has
been discussed in refs 24 and 25). The effective absence of both
a centerband and even-order sidebands in Figure 4a,b means
that the interpretation of the experimental spectra using the
analytical spin-pair theory is appropriate here. The best-fit
spectra for the crystalline and LC phases then correspond to
dipolar coupling constants, D/(2π), equal to 15.0 ( 0.9 and 6.0
( 0.5 kHz, respectively.
For the crystalline case, the measured D equates to an
internuclear distance, r, between the two aromatic protons of
0.200 ( 0.004 nm. The evaluated distance is thus in good
agreement with the X-ray single-crystal structure, which predicts
such r to be between 0.190 and 0.196 nm.21 The slightly larger
value obtained in this NMR study could provide evidence for
small deviations of the C-C-H bond angles from the ideal
120° and/or slight out-of-plane distortions of the C-H bonds.
Comparing the evaluated D values for the crystalline and LC
phases, a reduction of D by a factor of 0.40 ( 0.04 is observed.
In the LC phase, fast axial rotation of the molecule about an
axis perpendicular to the ring (passing through the center of
symmetry) is expected.26 For a molecule undergoing such a
It is interesting to compare the observed result with a previous
study16 where, using the same molecule but with the aromatic
protons instead of the R-carbon protons deuterated, a reduction
2
of 0.42 in the H quadrupolar splitting was observed for the
LC phase. Although the small discrepancy of the two results is
within the experimental error, it should be remembered that the
principal axes systems (PAS) are different in the two cases; for
2H NMR it is the C-H bond direction (in some cases, the
principal axes system of the electric field gradient tensor for
the 2H quadrupolar coupling may slightly deviate from the C-H
bond direction4) rather than the H-H internuclear direction
which is of interest. Thus, the effect of a motional averaging
process will not necessarily be the same.
The effect of the axial motion in the LC phase was also
apparent in the one-dimensional spectra (Figure 1), where, as
noted earlier, the spectrum changes, upon the phase transition,
from containing three distinct resonances to a single line, with
a frequency close to the average of the three resonances in the
crystalline phase. This means that the axial motion interchanges
positions HA, HB, and HC, but importantly the π-π packing
(25) Friedrich, U.; Schnell, I.; Demco, D. E.; Spiess, H. W. Chem. Phys.
Lett. 1998, 285, 49.
(26) Demus, D., Goodby, J. W., Gray, G. W., Spiess, H. W., Vill, V.,
Eds.; Handbook of Liquid Crystals; Wiley-VCH: Weinheim, 1998.

π-π Packing in a Columnar Hexabenzocoronene
J. Am. Chem. Soc., Vol. 121, No. 28, 1999 6717
arrangement in the columnar phase must be very similar to that
in the crystalline phase. Indeed, support for this statement is
provided by the significant shift to high field of the frequency
of the aromatic resonance in the 1H solution-state NMR
spectrum on increasing the concentration.
conformational distortion, and instead all the evidence strongly
supports the ring current hypothesis. Furthermore, in a follow-
up study,23 we have investigated the crystalline phase of hexa-
tert-butyl-substituted HBC, where a single crystal suitable for
an X-ray structure determination could be prepared. In this case,
1
the more complicated H DQ MAS spectrum was found to be
The DQ MAS spinning sideband pattern shown in Figure 4c
corresponds to the alkyl protons of R-deuterated HBC in the
crystalline phase (the column was extracted from the same two-
dimensional spectrum as that from which Figure 4a was
very well explained on the basis of the known structure,
invoking the same arguments involving the exposure of protons
to ring currents hypothesised here.
1
obtained). A previous H DQ MAS investigation of malonic
The suitability of NMR to the investigation of variable-
temperature phenomena meant that the changes which occur in
the 1H NMR spectra upon the transformation from the crystalline
to LC phase could be easily followed. In the one-dimensional
spectrum, the axial motion in the LC phase leads to an averaging
of the three aromatic proton resonances into a single resonance,
but with a frequency which indicates that the π-π packing
largely persists. Moreover, the analysis of DQ MAS spinning
sideband patterns meant that it was possible to quantify the
reduction of the dipolar coupling due to motion in the LC phase.
By comparison to the value which was determined for the
crystalline phase, where the corresponding internuclear distance
between the aromatic protons was shown to be in good
agreement with the X-ray structure of unsubstituted HBC, a
reduction by a factor of 0.40 was observed. Additionally, again
on the basis of an analysis of the DQ MAS spinning-sideband
pattern, significant mobility of the alkyl chains in the crystalline
phase could be identified.
acid yielded a proton-proton internuclear distance of 0.180 nm
for the rigid CH2 group in a crystalline sample.10 As shown in
the inset, under the excitation conditions used, such a rigid CH2
group would then give rise to a DQ MAS spinning sideband
pattern containing third- and first-order spinning sidebands of
approximately equal height. Clearly, the experimental spectrum
bears no similarity to such a spectrum and rather corresponds
to a dipolar coupling constant, D/(2π), equal to 10.3 kHz
(compared to 20.6 kHz, which would be expected for a rigid
pair with an internuclear distance of 0.18 nm). Thus, the
presence of significant motional narrowing for the alkyl chains
in the crystalline phase is clearly demonstrated. Such alkyl chain
motion was hinted at by the previously presented STM image
of a monolayer of HBC-C12 adsorbed onto graphite.17
Discussion
1
In this paper, the suitability of very fast MAS H NMR
In a previous study,6 we have shown the applicability of the
1H DQ MAS method to the investigation of hydrogen bonding,
where it was emphasized that the method has the advantages
of requiring neither the preparation of a single crystal nor
isotopic labeling, as well as being quick in terms of the required
experimental time, on account of the excellent sensitivity of
1H NMR. For the HBC-C12 sample studied in this paper, it
proved impossible to prepare a single crystal, presumably
because of the mobility of the alkyl chains discussed above;
thus, the applicability of NMR to powdered samples meant that
1H DQ MAS spectroscopy could deliver information which was
not available from scattering methods. Moreover, this NMR
method is also applicable to only partially ordered systems, such
as liquid crystals, and amorphous glassy systems.28 The acquisi-
tion of rotor-synchronized 1H DQ MAS spectra, which provide
such valuable information about proton-proton proximities, was
again quick, with the presented spectra requiring an experimental
time of only 26 min (Figure 2a). It should be noted that the
acquisition of DQ MAS spinning sideband patterns required
significantly longer experimental times (8 h).
methods to the investigation of both the structure and dynamics
of an alkyl-substituted hexa-peri-hexabenzocoronene, HBC-C12,
which forms a columnar mesophase, has been clearly demon-
strated. For the crystalline phase, three distinct aromatic
resonances were resolved in the single-quantum MAS spectrum.
The internuclear proximities of these resolved protons identified
by a rotor-synchronized two-dimensional DQ MAS spectrum
then proved a reduction of symmetry for each molecule from a
six-fold to a two-fold rotation axis. It should be noted that the
reliability of this DQ MAS method for such structural investiga-
tions has been previously demonstrated for a range of systems,
where X-ray single-crystal data are available.6,10,11,27
The observed NMR results were rationalized by hypothesiz-
ing, for crystalline HBC-C12, a herringbone structure of aromatic
cores as in unsubstituted HBC. In this way, the π-π interactions
are particularly pronounced, with the packing resembling that
formed by the carbon layers in graphite, and the observation of
the distinct aromatic resonances could then be explained in terms
of the differing degrees to which the aromatic protons experience
the ring current of adjacent layers. Although it is well known
in solution-state NMR, to the best of our knowledge this
represents the first clear demonstration in the solid state of this
phenomenon.
An alternative explanation for the observation of three
aromatic resonances would be a reduction from six- to two-
fold symmetry arising as a consequence of a conformational
distortion of either the aromatic core or the alkyl chains. For
example, a reduction to a two-fold symmetry, due to a
preferential orientation of the alkyl chains parallel to one of
the three main axes of the underlying graphite axes, was
observed in the STM image of a monolayer of HBC-C12
adsorbed onto graphite.17 However, it is most unlikely that the
very wide range of aromatic shifts (5.6-8.3 ppm) and, in
particular, the pronounced high-field shift of the C resonance
(at 5.6 ppm) observed in the NMR spectra could arise from a
We have also successfully applied the DQ MAS method to
a range of other (fully protonated) alkyl-substituted HBC
samples, in which some or all of the long alkyl chains are
replaced by, for example, isopropyl or tert-butyl groups.23 Thus,
high-resolution solid-state 1H single- and double-quantum MAS
NMR offers itself as a highly promising tool for the study of,
e.g., packing effects in new materials which exploit π-π
interactions for optical and optoelectronic applications.
Experimental Section
Synthesis of r-Deuterated HBC-C12 (1). The synthesis is outlined
in Scheme 1. As the first step, 4-bromododecanophenone (3) was
coupled with trimethylsilylacetylene to yield 4. After cleavage of the
trimethylsilyl group, the resulting acetylene derivative 5 was coupled
with 3 to obtain the tolane 6. The latter was converted to 7 by a cobalt
(27) Dollase, W. A.; Feike, M.; Fo¨rster, H.; Schaller, T.; Schnell, I.;
Sebald, A.; Steuernagel, S. J. Am. Chem. Soc. 1997, 119, 3807.
(28) Feike, M.; Ja¨ger, C.; Spiess, H. W. J. Non-Cryst. Solids 1998, 223,
200.

6718 J. Am. Chem. Soc., Vol. 121, No. 28, 1999
Brown et al.
The melting point (380 K) was observed to be significantly higher than
that previously reported,16 indicating a greater degree of purity of the
final product for the modified synthetic route described here.
Scheme 1
NMR. 1H MAS NMR experiments were performed on a Bruker ASX
1
500 spectrometer at a H Larmor frequency of 500.1 MHz, using a
double-resonance MAS probe supporting rotors of outer diameter 2.5
mm with a spinning frequency of up to 35 kHz. At these very fast
spinning frequencies, the additional heating effect caused by air friction
becomes significant. Using the 119Sn resonance of Sm2Sn2O7 as a
chemical shift thermometer, the correction term relative to the bearing
gas temperature has been calibrated in a separate study.30 All stated
temperatures have been corrected by this procedure. For all samples,
the 90° pulse length was 2.0 µs, and a recycle delay of 3 s was used.
For one-dimensional MAS experiments, 16 transients were averaged.
1
In addition, a solution-state H NMR spectrum of the R-deuterated
HBC-C12 dissolved in deuterated chloroform was recorded on a Bruker
DMX 500 spectrometer at the same 1H Larmor frequency (500.1 MHz).
DQ MAS experiments were performed using the back-to-back
(BABA) recoupling sequence8 for the excitation (p ) 0 f p ) (2,
where p is the coherence order) and reconversion (p ) (2 f p ) 0)
of DQCs. (For rotor-synchronized experiments, the standard sequence,
Px-τ-P-x-Py-τ-P-y, where τ equals τR/2 minus the pulse durations,
was used. For the experiments recorded with the aim of observing full
DQ MAS spinning sideband patterns, a compensated two-rotor-period
variant of the sequence was used:31 Px-τ-Px-Py-τ-P-y-Px-τ-Px-
P-y-τ-Py.) A final 90° pulse was used to create transverse magnetiza-
tion, with the duration of the p ) 0 “z-filter” period set equal to one
rotor period. The total phase cycle32 used consisted of 16 steps with
four steps to select p ) (2 after the excitation sequence and four steps
to select p ) 0 f p ) -1. Sign discrimination was restored in the F1
dimension by the TPPI33 method of incrementing the phase of the
excitation pulses by 45°. In the rotor-synchronized experiments, the
increment in t1 was set equal to one rotor period, with, for each of 32
increments, 16 (Figure 2a) and 32 (Figure 2b) transients being averaged.
To obtain the DQ MAS spinning sideband patterns, 618 increments of
1.0 and 286 increments of 4.0 µs (corresponding to F1 spectral widths
of 500 and 125 kHz) were used for the crystalline (Figure 4a,c) and
LC phases (Figure 4b), respectively, averaging over 16 and 32 transients
in the two cases. In the contour plots, solid and dashed lines represent
positive and negative contours, respectively, with the bottom contour
corresponding to 2.0 and 1.4% of the maximum intensity in Figure 2a
and b, respectively, and subsequent contours corresponding to a
multiplicative increment of 1.3.
octacarbonyl-catalyzed cyclotrimerization. The introduction of deute-
rium was then achieved by a very efficient reduction of the keto group
using lithium aluminum deuteride in the presence of aluminum
chloride29sin a modification of the original process, it was here found
necessary to treat the refluxing reaction mixture with ultrasound. A
small amount of olefinic side product was removed by a subsequent
hydrogenation using hydrogen in the presence of a palladium/charcoal
catalyst. The cyclodehydrogenation of 8 to yield the title compound 1
was achieved with anhydrous iron(III) chloride/nitromethane in dichlo-
romethane.
Synthesis of r-Protonated HBC-C12 (2). A synthesis of 2 is
described in ref 16; in this work, the same route was followed up to
the nondeuterated analogue of the hexaphenylbenzene derivative 8.
However, it was found that the cyclodehydrogenation to yield the title
compound 2 could be performed more reproducibly by the same
anhydrous iron(III) chloride/nitromethane in dichloromethane method
as was used in the synthesis of 1: To a stirred solution of hexakis(4-
dodecylphen-1-yl)benzene (5.0 g, 3.24 mmol) in dry CH2Cl2 (700 mL)
was added a solution of FeCl3 (anhydrous) (17 g, 104.8 mmol) in dry
nitromethane (200 mL). An argon stream was bubbled through the
reaction mixture throughout the entire reaction. After being stirred for
30 min at room temperature, the reaction was quenched with methanol
(1000 mL), and the precipitate was filtered. After being dried under
vacuum, the residue was chromatographed on silica gel, first with
petroleum ether and toluene to remove side products, and then with
hot toluene to elute the yellow band of the product. Recrystallization
first from toluene and then from heptane yielded 3.8 g (77%). K1 f
K2, 315 K [285 K], 21 kJ/mol; K2 f Colho, 380 K [355 K], 92 kJ/mol.
Acknowledgment. S.P.B. thanks the Alexander von Hum-
boldt-Stiftung for the award of a research fellowship. Financial
support from the Deutsche Forschungsgemeinschaft, SFB 262,
is acknowledged. We thank Dr. Manfred Wagner for recording
the solution-state NMR spectrum of HBC-C12 (Figure 1d) and,
together with Andreas Fechtenko¨tter, for carrying out the
investigation of the concentration dependence of the solution-
state aromatic chemical shift. Helpful discussions with Christoph
Kayser are acknowledged.
Supporting Information Available: Detailed experimental
account of the synthesis of 1, together with characterization
1
information, including the assignment of the solution-state H
NMR spectrum of 1 (Figure 1d) (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA990637M
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(31) Feike, M.; Demco, D. E.; Graf, R.; Gottwald, J.; Hafner, S.; Spiess,
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(29) Nystrom, R. F.; Berger, C. R. A. J. Am. Chem. Soc. 1958, 80, 2896.