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Direct Observation of Enantiomorphous Monolayer Crystals from Enantiomers by Scanning Tunneling Microscopy.

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Direct Observation of Enantiomorphous
Monolayer Crystals from Enantiomers by
Scanning Tunneling Microscopy
Forrest Stevens, Daniel J. Dyer, and David M. Walba*
Pasteur's brilliant observation, using optical microscopy, that
crystals of a salt of "racemic acid" actually existed in two enantiomorphous forms['] not only uncovered the existence of enantiomers, but indeed stimulated the development of modern
structural theory. The direct (without resorting to diffraction
techniques) observation of supermolecular chirality and the relationship between supermolecular chirality and molecular
structure have fascinated chemists ever since. Starting with early
studies of the morphology of chiral crystals by optical microscopy, the scale for the observation of chiral supermolecular
assemblies has been approaching that of the molecules themselves. McConneli and Weis first recorded the formation of chiral crystalline monolayer domains in Langmuir films of enantiomerically pure lipids by epifluorescence optical microscopy.[zl More recently, chiral monolayers have been observed at molecular resolution by scanning probe microscopy.
Thus, chiral symmetry breaking from achiralc31 and racemicf4I
molecules has been detected in Langmuir films by atomic force
microscopy (AFM), and enantiomorphous monolayer domains
from achiral liquid crystal (LC) molecules on graphite have been
observed by scanning tunneling microscopy (STM) .Is1
In related
work, amino acids have been spontaneously resolved by crystallization in two dimensions at the air-water interface.16]
We report here on the molecular resolution STM observation
of enantiomorphous images of crystal monolayers grown from
pure enantiomers. Furthermore, a racemic mixture of the same
molecules produced images of co-existing enantiomorphic domains indistinguishable from those obtained from the enantiomerically pure materials, providing strong evidence for chiral
symmetry breaking to give a two-dimensional (2-D) conglomerate. These results suggest a complete 2-D analog of Pasteur's
famous system.
The experiments were performed on enantiomerically enriched and racemic biphenylbenzoates 1. These have one tetra-
hedral stereogenic center (indicated by an asterisk). Both enantiomers were prepared in approximately 99 YOee; the racemate
was also prepared."] All three compounds were liquid crystalline with a smectic A phase at room temperature.
Each STM sample was prepared by placing a small amount
(< 1 mg) of the liquid crystal on a freshly cleaved surface of
highly oriented pyrolytic graphite. Scanning tips were prepared
from mechanically cut 0.025 cm platinum/iridium (80: 20) wire.
Images were recorded in air at room temperature using a commercially available STM (Nanoscope 11, Digital Instruments,
Inc.) with tip positive bias.
[*] Prof. D. M . Walha. F Stevens. Dr. D. J. Dyer
Department of Chemistry and Biochemistry and
Optoelectronic Cornputtiig Systems Center
Campus Box 225. University
of Colorado
Boulder. CO 80309-0215 (USA)
F a x . Int. code +(303) 492-5894
e-mail. walbaco colorado edu
This work was supported by the National Science Foundation (Grants DMR9202312 and DMR-92241681
The images obtained for monolayers of either enantiomer
varied somewhat in appearance, probably due to variations in
the STM tip. However, by using autocorrelation analysis (see
below)[81 the measured unit cell dimensions for all the images
obtained were the same within experiniental error. Thus, while
polymorphism has been observed with similar molecules,L91
enantiomerically pure 1 apparently produces only one monolayer crystal structure.
When the images were of sufficient quality to resolve the
molecules, similar structures were always observed, as illustrated in Figure 1 for samples of (S)-1 (Fig. 1A) and (R)-1 (Fig. 1B).
Fig. 1. Enantiomorphous images obtained from (Sj-1 (A) and (Rj-1 (B). The dimensions of the image are 8 n m x 11.5 nm: the gray bars representing the bright
regions in the images are 16 A in length on this scale. These images are yaw data.
rotated and cropped from the original 15 x 15 nm (400 x 400 data points) scans to
aid in the comparison. The scanning parameters were: A : 400 mV,1 n A , B: 300 mV.
I nA.
The unit cell dimensions are the same within experimental error,
but examination of the images shows that they are clearly enantiomorphous. Images of monolayer crystals grown from both
enantiomers exhibit well-defined rows of tilted, rod-shaped
bright regions (i.e. areas of enhanced tunneling efficiency), but
the bright rods are tilted in opposite directions for the enantiomers.
While the precise mechanisms involved in imaging of organic
monolayers overlying conducting surfaces is not known, it is
well established that aromatic molecular units image brightly,
while aliphatic units are darker."', ' ' I This is the case whether
the surface is biased negatively or positively relative to the tip.
In the present case the bright regions are approximately 16 8, in
length, consistent with the size and shape of the biphenylbenzoate aromatic units. In the monolayers formed from the ( S )
liquid crystal, the cores are tilted about 30" clockwise from the
row normal (Fig. I A ) , while the monolayers formed from the
( R ) liquid crystal show the cores tilted counterclockwise by a
similar amount (Fig. IB). For the purposes of the present discussion, the image type in Figure lA, obtained from (S)-l,will
be denoted ( + ) for clockwise, while that obtained from (R)-1
will be denoted ( - ) for counterclockwise.
In all cases where monolayer chirality could be resolved, the
handedness of the images was coupled to the configuration of
the molecules. Out of nine attempts to image (S)-1 six gave
monolayer crystals providing images with clearly distinguishable ( + ) chirality, while in six separate sessions (R)-1 gave discernible ( - ) images. Hundreds of images were recorded in these
sessions, and in no case was observable ( + ) chirality associated
with (R)-1 or (-)chirality associated with (S)-1. Thus, it can be
stated with a high degree of certainty that molecular chirality is
transferred to a chiral supermolecular packing mode, which in
turn gives rise to chiral images by STM.
When monolayer crystals are grown from racemic 1, the formation of a racemic monolayer might be expected by analogy
with three-dimensional (3-D) organic crystals, which show some
tendency to form racemic crystals rather than conglomerates.
However, in ;I total of eight imaging sessions with racemic 1,
( + ) images were observed six times, ( - ) images were observed
seven times. and in four separate cases domain walls between
regions of opposite chirality were observed, as illustrated in
Figure 2. While Eckhardt et all4] have used AFM to observe
Fig Z C i i - w \ t i i i g cnantioinoi-phous images obtained from racemic 1. Image A
\hou\ an dreii of'iliincnaioiis 27 nm x 38 nin. and image B shows a 15 nm x 15 nm
These imago .ire raw data. cropped from originals of 50 x 50 nm and
20 x 10 nm. re\pecri\cly The scanning parameters were: A: 300 mV. I nA: B 600 in\'. I nA
taken and their unit cells averaged. Only these "pair averaged"
values were used in the analysis.
Over one hundred pairs of images were analyzed for each
material (the two enantiomers and the racemate), and the unit
cells from both the pure enantiomers and from the racemate
were found to be the same within experimental
strongly suggests that the racemate forms a 2-D conglomerate
of enantiomerically pure monolayer crystals upon adsorption
on graphite-these enantiomerically pure crystals are identical
to those obtained from the pure enantiomers. This is similar to
the results obtained by Nassoy et a1['31 who used grazing incidence X-ray diffraction to show that the unit cells in monolayers
of D,L, and racemic myristoyl-alanine were identical. However,
they were unable to resolve domains within the racemate or to
obtain images of the structures with molecular resolution.
We have used STM to observe monolayers formed by both
enantiomers of a chiral material, and the corresponding racemate. The pure enantiomers gave monolayer crystals that produced chiral 2-D images by STM, and enantiomeric molecules
delivered enantiomorphic images reproducibly. The racemic
material produced monolayer crystals showing both chiralities
in two dimensions, and the domains were indistinguishable from
images obtained from enantiomerically pure material. This
strongly suggests that the racemate forms a 2-D conglomerate
of enantiomerically pure monolayer crystals on the graphite
Received. Novembei 2. 1995 [28521 IE]
German version : Angrit.. Cliow 1996. / O X , 955-957
enantiomorphous domains in a racemic material, they did not
have access to the corresponding enantiomerically pure material
and so could not compare the structures formed by the racemate
to structures formed by pure enantiomers, as we have done.
Observation of enantiomorphous domains from the racemic
material by STM could result from two different mechanisms:
1 ) The racemic material could spontaneously resolve into monolayer crystals of pure ( R ) and pure (S) material forming a 2-D
conglomerate. 2) Both enantiomorphous domains could be
composed of racemate-an example of formation of chiral supermolecular structures from "achiral" pairs of enantiomeric
molecules. In both cases the enantiomorphous domains represent enantiomeric monolayer crystal structures; in the latter
case the enantiomeric domains are composed of mixtures of ( R )
and ( S ) molecules. while in the former case the domains are
composed of homochiral molecules (it is not possible to physically separate the domains to measure optical activity in solution, as Pasteur did, to distinguish between these two possibilities). Of course, the crystals formed by these two mechanisms
must be diastereomeric.
If resolution occurs and the monolayer crystal domains are
enantiomerically pure, the chiral images observed from the
racemic material must be identical to those obtained from one
or the other of the pure enantiomers. On the other hand, if the
monolayer crystals are racemic, the observed images should be
different ft-om those of the pure enantiomers, though due to the
limits of the STM technique the racemic crystal could fortuitously producc images indistinguishable from those derived
from the pure enantiomers.
Autocorrelation was used to calculate unit cells for monolayer crystals grown from enantiomerically pure and racemic 1. Tip
drift often caused "up" scans to be stretched, while "down"
scans were compressed, or vice versa. To compensate for this
effect, whenever- possible consecutive up and down scans were
Keywords: chirality . liquid crystals
croscopy . symmetry
scanning tunneling mi-
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R M. Weis. H. M McConnell. Ncituri, 1984, 310. 47-49
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Leizerowitz. poster presented at 7th International Syinpoaium on Chiral Discrimination (ISCD 7) Jerusalem: J A m Clioii. So(.. submitted.
The synthesis and novel ferroelectric liquid crystalline properties of compounds 1 w i l l he described elsewhere. The enantiomericall! enriched aamples
were prepared from ( R ) - and (S)-?-octanol (A!drich.>99"<,w).
W. 0. Saxton. J. Frank, C'/rroiiii~~ro.scopi1977. 2. 219 277
F. Stevens. D J. Dyer. D. M Walba. Lmi,qmiii-, 1996. /?. 136 440.
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D M .Walbd. F. Stevens. D. C Parks. N A Clark. M D. \V,iiid. S u m w 1995.
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Average unit cell metrics observed for compounds 1 by autocorrelation analysis
were as follows. (S)-l.[O] =1.61 k0.06 nin. [ l ] = 3.16+0.14 nm. angle =
XX+6 . ( R ) - I . [ 0 ] = 1 . 6 5 ~ 0 0 8 n m .[ 1 ] = 3 . 1 7 ~ 0 . 1 3 n n i .angle=XX+6 ;
( r o r ) - l - [ O =] I 60&0.06nm. [ I ] = 1 . 0 8 ~ 0 . l Z n m . a n g l e= X7+6
P. Nassoy, M Goldmann. 0 . Bouloussa. F. Rondelez. Phi 5 Rev. Lrrr 1995.
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crystals, enantiomers, tunneling, observations, microscopy, monolayer, direct, enantiomorphism, scanning
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