Angewandte
Chemie
The right- and left-handed helical block segments sepa-
rated by helical reversals in individual polymer chains could
also be visualized in the 2D crystals (Figure 2b). The cross-
section height profile denoted by a white dashed line in
Figure 2b clearly shows a unidentifiable plateau section with
virtually the same height along with the block segments of the
helices of opposite sense, which is assumed to be a helical
reversal in the polymer chain. Force-field calculations suggest
that a poly-Aib chain having a helical reversal can be
constructed so as to maintain its rodlike structure (Figure 2b;
right). In contrast, in the height profile indicated in Figure 2a,
there is a gap between helical blocks of opposite sense
(Figure 2a, white arrows) which is different from the helical
reversal; in this case, two poly-Aib chains align with a distance
between them of 3.4 nm.[17]
Statistical analysis of a series of high-resolution AFM
images of poly-Aib helix bundles and height profiles along
with each polymer chain may provide the average lengths of
right- and left-handed helical blocks and the population of
helical reversals. According to Lifson, Green, and co-work-
ers,[1a,18] the length of the helical-sense blocks (L), in other
words, the number of monomer units between the helical
reversals, can be calculated as a function of the free-energy
difference between the helical (DGh) and the helical reversal
states (DGr), the most important thermodynamic stability
parameters for representing a unique feature of dynamic
helical polymers, L = exp(DGr/RT) (R and T are the gas
constant and the absolute temperature, respectively). Poly-
Aib is an equal mixture of right- and left-handed helices, and
therefore, DGh = 0. This fact suggests that if the helical sense
blocks of poly-Aib could be measured with certainty by the
AFM, the DGr value for poly-Aib in the 2D crystal state could
be calculated. However, the average molecular length of poly-
Aib determined by single-molecular AFM measurements
(216 nm which corresponds to 1034 monomer units) (Sup-
porting Information, Figure S7)[10d,19] is longer than each side
of the high-resolution AFM images (ca. 60 nm; Figure 2a),
and it was difficult to determine the average lengths of helical
blocks (L) between the helical reversals in individual polymer
chains in such a limited area. We measured the sequential
high-resolution AFM images of poly-Aib deposited on
HOPG such that each AFM image partially overlapped
with the scan area (100 100 nm) in which the helical
structures of poly-Aib could be clearly observed. The four
or five sequential high-resolution AFM images thus obtained
were then overlapped to reconstruct the whole images
covering a wider area with maintaining the high-resolution
(Supporting Information, Figure S3 and S4).
about 2.5–3.7 kcalmolÀ1 on a monomer-unit basis for helical
poly(phenylacetylene)s with different chiral or achiral pen-
dant groups in bulkiness on the basis of the temperature-
dependent changes in their CD (circular dichroism) inten-
sities in solution.[20] The estimated DGr value of poly-Aib in
the 2D crystalline state is in fair agreement with those of
helical poly(phenylacetylene)s determined in solution. As a
consequence, the results revealed a rarely occurring helical
reversal along with the long helical blocks with the opposite
sense which appeared only once in every 287 monomer units
on average in dynamic helical poly-Aib.
We investigated a possibility of spontaneous resolution of
dynamically racemic poly-Aib helices into homochiral
domains with the same helical sense in the 2D crystals
prepared at different temperatures.[21] Careful observations of
the AFM images (Figure 2a; Supporting Information S3, S4,
and S6) reveal that poly-Aib tends to self-assemble into small
domains composed of several helical blocks of the same sense
under benzene vapor exposure. However, the average
homochiral domain size appears to be small at the macro-
scopic level.
In conclusion, we have successfully and directly observed
the unique structure of a dynamically racemic helical
poly(phenylacetylene) on HOPG upon exposure to organic
solvent vapors. In particular, the helical reversals along with
the long helical blocks with the opposite sense as well as the
helical pitch and handedness were clearly visualized for the
first time based on the 2D self-assembled helix-bundle
formation using AFM with molecular resolution. Poly-Aib
forms a lyotropic nematic LC phase in concentrated benzene
solution. We anticipate that the nematic LC poly-Aib phase
will convert into the cholesteric counterpart by doping with
optically active small molecules or helical poly(phenylacety-
lene)s, such as cholesteric liquid-crystalline poly-l-Ala in
solution. The dynamically racemic helical poly-Aib may be
transformed into an excess of one helical sense in the
cholesteric state, which can be visible and quantified by the
change in its helical pitch in the cholesteric liquid crystal and
further visualized by high-resolution AFM.[22] The work along
this line is now in progress.
Received: April 9, 2007
Published online: August 31, 2007
Keywords: chirality · helical structures ·
.
scanning probe microscopy · self-assembly · surface analysis
[1] a) M. M. Green, N. C. Peterson, T. Sato, A. Teramoto, R. Cook,
f) J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte,
197 – 205; h) M. Fujiki, J. R. Koe, K. Terao, T. Sato, A. Teramoto,
2006, 265, 47 – 88.
On the basis of an evaluation of 375 helical-sense blocks
between the helical reversals and those between the polymer
end and the helical reversal in individual polymer chains in
the AFM images including the images in the Supporting
Information, Figure S3 and S4, the number-average length of
helical blocks (Ln) was estimated to be 60 nm, which
corresponds to 287 monomer units (Supporting Information,
Figure S5). Based on the Ln value, the free energy difference
between the helical reversal states (DGr) of poly-Aib was
calculated to be approximately 3.4 kcalmolÀ1. Previously, this
key free energy (DGr) was experimentally determined to be
Angew. Chem. Int. Ed. 2007, 46, 7605 –7608
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim