Towards enantioselective adsorption in surface-confined nanoporous systems

Article abstract of DOI:10.1039/c4cc08826c

The adsorption of chiral molecules in surface-confined chiral porous networks shows pronounced selectivity, as a result of complementary host-guest interactions. This journal is

Full text of DOI:10.1039/c4cc08826c

Volume 51 Number 23 21 March 2015 Pages 4723–4920
ChemComm
Chemical Communications
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COMMUNICATION
Kazukuni Tahara, Yoshito Tobe, Steven De Feyter et al.
Towards enantioselective adsorption in surface-confined nanoporous
systems

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Towards enantioselective adsorption in
surface-confined nanoporous systems†
Cite this: Chem. Commun., 2015,
1, 4766
5
a
a
b
a
a
Elke Ghijsens,‡ Hai Cao,‡ Aya Noguchi, Oleksandr Ivasenko, Yuan Fang,
b
b
a
Received 5th November 2014,
Accepted 5th December 2014
Kazukuni Tahara,* Yoshito Tobe* and Steven De Feyter*
DOI: 10.1039/c4cc08826c
www.rsc.org/chemcomm
The adsorption of chiral molecules in surface-confined chiral porous
networks shows pronounced selectivity, as a result of complementary
host–guest interactions.
Mirror image chiral molecules are called enantiomers. The most
popular techniques to separate enantiomers are crystallisation
1
–4
and chromatography. In both cases the separation is based on
the formation of diastereomeric complexes with the chiral agent
or the stationary ligand. Due to the different physical properties
of diastereomers, the resolution of the complexes is straight-
forward. Diastereomeric segregation on solid surfaces is also
possible and few studies report such a phenomenon with nano-
5–7
Fig. 1 (A) Chemical structures of achiral and chiral DBAs. (B) Molecular
models for the counterclockwise (CCW) and clockwise (CW) nanowells,
based on the interdigitation of alkyl chains (Æ type) and the rotation of the
vectors pointing from the DBA core to the end of the alkyl chain at the rim
of a nanowell.
scale resolution using scanning tunneling microscopy (STM).
The resolving agent selectively crystallizes with one of the enantio-
mers on the surface, leaving the mirror-enantiomer behind in the
solution phase.
2
D porous networks show great potential in chiral recognition
8,9
processes. The regular spaced nanowells provide a confined
environment together with great accessibility of the pores. There- system at the liquid/solid interfaces has been investigated
fore, host–guest interactions can easily be studied. Currently, extensively.
these systems only show selective interactions towards guest a nanoporous network at the interface between a liquid and highly
adsorption based on size and/or shape complementarity,
and the chemical environment. Here we show a certain control concentration conditions, highly ordered networks are formed,
in the occupation of the 2D porous network based on the chiral leaving a regular pattern of hexagonal nanowells. The diameter
complementarity between the guest and the host. of these nanowells depends on the length of the alkyl chains of the
The molecular system we investigated is based on dehydrobenzo- DBA molecules. Note that the nanowells are chiral, as a result of
1
4–21
These alkylated molecules may self-assemble into
10–16
oriented pyrolytic graphite (HOPG). Under appropriate solute
1
7
19
[12]annulene (DBA) derivatives (Fig. 1A). The self-assembly of this the alkyl chain interdigitation pattern. Depending on the orienta-
tion of the alkyl chains at the pore periphery, we label the nano-
wells as clockwise (CW) or counterclockwise (CCW), as illustrated
a
KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium.
E-mail: steven.defeyter@chem.kuleuven.be; Fax: +32 1632 7990;
Tel: +32 1632 7921
in Fig. 1B. Achiral DBA molecules form domains with nanowells of
opposite handedness, each domain being composed of supra-
molecular nanowells of the same handedness. In contrast, in
the case of chiral DBA (cDBA) enantiomers, only supramolecular
nanowells of one particular handedness, not the opposite, are
formed. Specifically, R-type enantiomers and S-type enantiomers
b
Division of Frontier Materials Science, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan.
E-mail: tahara@chem.es.osaka-u.ac.jp, tobe@chem.es.osaka-u.ac.jp;
Tel: +81-(0)6-6850-6225
†
Electronic supplementary information (ESI) available: Materials and methods,
18
form CCW and CW porous patterns on the surface, respectively.
additional experimental data, details of molecular modelling and synthesis of
Recently, we showed that mixing of achiral and chiral DBA
molecules may lead to the exclusive formation of nanowells of
cDBA-OC12(R). See DOI: 10.1039/c4cc08826c
‡
These authors contributed equally.
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18
one given handedness. This phenomenon, where a few percent of Fig. 2A shows an STM image of the co-adsorption of chiral and
chiral molecules dictates the chirality of the supramolecular com- achiral molecules as part of the porous CW network. The
22
plexes, is called the sergeants-and-soldiers principle, which has co-adsorption of these chiral molecules directs the overall chirality
23,24
been well established for supramolecular systems in solution.
During the course of sergeants-and-soldiers experiments, we dark rim around the bright central triangular core.
identified another interesting phenomenon, which we label as While most of the pores in the network are ‘‘empty’’, i.e. they
of the network. We identify the chiral molecules as those having a
18
auto-enantioselective adsorption, the topic of this communica- do not show a particularly pronounced contrast, some pores
tion. As expected, a premixed solution of 30 mol% cDBA-OC12(S) differ in contrast, i.e. they appear (in part) brighter than the rest.
À6
and 70 mol% DBA-OC12 at a total concentration of 2.5 Â 10
M
This suggests that such pores are filled by molecular species that
self-assembles at the interface between 1-phenyloctane and differ from the ones without particular contrast. The latter ones
HOPG, which results in a porous adlayer with amplified clock- are most probably filled by solvent molecules in a transient way,
wise (CW) hexagons (95 Æ 3% CW hexagons, Table S1, ESI†). i.e. the solvent molecules are dynamic and cannot be captured at
the time scale of STM imaging.
Remarkably, there is a clear difference in the occupation of
the pores of CW and CCW domains: the domains in which the
porous network consists only of achiral molecules, i.e. the CCW
domains, show a higher degree of occupied pores. The mirror
image situation is observed for monolayers formed from mix-
tures of cDBA-OC12(R) and achiral DBA-OC12 (Fig. S1, ESI† and
Table 1), confirming that this phenomenon is a genuine one.
No such kind of preference of nanowell occupation is observed
for pure samples of DBA-OC12 (Fig. S2 and Table S2, ESI†).
This brings us to the issue of identification of the trapped
species. STM imaging is not very helpful in this. However, one
important element is the difference in guest occupation based
on the chirality of the nanowells. In addition, opposite prefer-
ences for occupation in the solutions containing cDBA-OC12(S)
and cDBA-OC12(R) leave no other option than that in the case
of the absence of chiral impurities, the chiral DBA molecules
themselves occupy the nanowells.
A closer inspection of the occupied pores reveals some
differences in STM appearance (Fig. 2B). We identify three
different occupation levels: incorporated (I), fuzzy (F) and half-
fuzzy (H). Guest molecules appearing as a bright round blob
Fig. 2 (A) STM image of a premixed solution containing 30 mol% cDBA-
surrounded with some kind of fine structure inside the pore are
OC12(S) and 70 mol% DBA-OC12. Iset = 200 pA and Vbias = À250 mV. The
defined as incorporated, which is barely observed in the mono-
white line marks the boundary between a CCW domain (upper left) and a
layers formed by pure DBAs (Fig. S2, ESI†). This bright rounded
CW domain (lower right). The black arrows indicate the major symmetry
axes of the HOPG surface underneath. The scale bar measures 5 nm. The blob is assigned to the conjugated DBA-core having some degree
red arrows indicate cDBA-OC12(S) sergeant molecules. (B) Examples of
four different types of pores: incorporated (I), fuzzy (F), half-fuzzy (H) and
bright blob can be attributed to the presence of the cDBA alkyl
empty (E). Alkyl chains of the guest molecule are marked with red arrows.
of mobility inside the pore. The fine structure surrounding the
chains with windmill-like conformation. Pores that are defined
(C) Modelling of completely confined (‘incorporated’) cDBA-OC12(S/R) guest
as fuzzy appear as diffuse bright clouds that cover the complete
hexagonal void. This appearance may be related to a higher
molecules in CCW and CW DBA-OC12 nanowells, respectively. For clarity,
only the guest molecules are displayed in ball-type.
Table 1 Statistics of the guest adsorption in the adlayers of premixed solutions of cDBA-OC12(S/R) and DBA-OC12 and sequential deposition of
DBA-OC12 and cDBA-OC12(S/R). The total number of pores analysed (complete dataset) per void handedness for a specific solution mixture is also
specified. Non-C6 refers to irregular hexagonal nanowells
Premixed solution
0 mol% cDBA-OC12(S)
Sequential deposition
3
30 mol% cDBA-OC12(R)
30 mol% cDBA-OC12(S)
30 mol% cDBA-OC12(R)
CCW
%)
Non-C6
(%)
CW
(%)
CCW
(%)
Non-C6
(%)
CW
(%)
CCW
(%)
Non-C6
(%)
CW
(%)
CCW
(%)
Non-C6
(%)
CW
(%)
(
Total pores
Incorporated
Fuzzy
Half fuzzy
Empty
145
2
1616
4291
6
775
960
10
1397
1758
5
1713
60 Æ 4
3 Æ 1
3 Æ 3
34 Æ 1
0 Æ 0
0 Æ 0
0 Æ 0
100 Æ /
0 Æ 0
7 Æ 2
8 Æ 4
85 Æ 7
0 Æ 0
9 Æ 3
8 Æ 2
83 Æ 3
0 Æ 0
0 Æ 0
0 Æ 0
100 Æ /
18 Æ 9
7 Æ 2
12 Æ 3
63 Æ 11
65 Æ 1
12 Æ 1
5 Æ 1
18 Æ 4
0 Æ 0
10 Æ /
0 Æ 0
90 Æ /
6 Æ 2
5 Æ 1
7 Æ 1
82 Æ 2
1 Æ 1
6 Æ 4
4 Æ 2
88 Æ 6
0 Æ 0
0 Æ 0
0 Æ 0
100 Æ /
38 Æ 4
2 Æ 4
4 Æ 1
55 Æ 3
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degree of mobility. Finally ‘‘half-filled’’ pores are defined as half- we aimed at creating an equal surface coverage of CW and CCW
fuzzy. In line with the previous reasoning, these guests are highly pores. To equalize the number of CW and CCW nanowells on
À6
mobile. In addition, empty pores (E), with a more homogeneous the surface, firstly 12 mL achiral DBA-OC12 (2.5 Â 10 M) is
dark contrast appearance inside the void, are also defined. For deposited (which yields equal amounts of CW and CCW pores).
À6
sure, this type of classification leaves some room for interpreta- After 30 min 6 mL of cDBA-OC12(S/R) (2.5 Â 10 M) is added.
tion. The proposed assignment is to a certain extent arbitrary, We call this procedure sequential deposition. Upon ideal mixing
however, evaluation of the total occupation level of the nanowells and in the absence of competitive adsorption, this preparation
is more robust. Nevertheless, there is a relationship between the method should yield a solution mixture with 33 mol% of cDBA
classification and the mobility of the guest molecules. Fuzzy and and can as such be compared directly with the previous results.
half-fuzzy guests move easily between consecutive images. All the The data are also summarized in Table 1. In the case of cDBA-
pores that appear fuzzy and half-fuzzy (blue circles in Fig. S3, ESI†) OC12(S), 18 Æ 4% CCW pores are empty versus 82 Æ 2% CW
are observed as empty in the consecutive STM images. This is valid pores. If cDBA-OC12(R) is deposited in addition to DBA-OC12,
for guests adsorbed in CW and CCW pores. In contrast 8 out of 55 Æ 3% CW pores are empty versus 88 Æ 6% CCW empty pores.
19 incorporated guests (highlighted by the green circles in Fig. S3, Upon comparing the results from the premixed solutions with
ESI†) remained incorporated in the same host-pore between those from the sequentially deposited solutions, one can con-
consecutive STM images. This lesser degree of dynamics is a clude that there is indeed a more favourable interaction with
clear sign of better interaction between the host and the guest. CCW pores in case the solution mixture contains cDBA-OC12(S)
We have performed complementary modelling of these host– and with CW pores in the case of cDBA-OC12(R). It should be
guest complexes with molecular mechanics force field, recently noted that the occupancy in Non-C6 nanowells, so those that
2
5
demonstrated to properly describe an intricate relationship are formed by a combination of + and À alkyl chain interdigita-
between various supramolecular interactions in DBA assemblies. tion patterns, is pretty low in all cases.
Using a developed protocol we have modelled ‘incorporated’, i.e.
We conclude that the deposition method (pre-mixing vs.
fully confined (Fig. 2B) DBAs with windmill-like conformation, sequential deposition) does not change the qualitative outcome
and got the optimized structures of cDBA-OC12(S/R) in CCW and of the experiments. Moreover, studies with different ratios of
CW nanowells (Fig. 2C), respectively, which is well consistent chiral and achiral molecules, having an impact on the ratio of
with the experimental observations. And we also found that CCW and CW domains, show the same qualitative trend: the
indeed, for similar molecular conformations, CW/CCW pores occupation of nanowells that have less favoured handedness
À1
are more favourable, by 6 kcal mol , for cDBA-OC12(S)/cDBA- induced by a chiral DBA is always larger (Fig. S6 and Table S4,
OC12(R) guests, respectively (Fig. S4 and S5, ESI†). The major ESI†). We tested if there is any significant evolution of guest
contribution to this energy difference comes from fewer van der occupancy in time (dynamics). No significant time effects were
Waals contacts between the methylene units when the chirality observed (Table S3, ESI†).
of the host does not match the direction of the windmill-like
In a further attempt, we performed similar experiments for a
conformation of the guest, which in turn, is totally defined by DBA derivative with a different alkyl chain length: cDBA-OC13(R)
the methyl groups at the stereogenic centers of cDBAs. Thus, it and DBA-OC13, in a different solvent, 1-octanoic acid, to show the
appears that chiral recognition in these systems occurs via the generality of this approach. cDBA-OC13(R) molecules form exclu-
alkyl chain interactions.
sively CCW hexagonal nanowells (Fig. S7, ESI†). cDBA-OC13(R)
The degree of occupation of the different pores for the acts as a sergeant molecule too. A premixed solution of 30%
premixed solutions, containing 30 mol% cDBA and a total cDBA-OC13(R) and 70% DBA-OC13, with a total concentration
À6
À6
concentration of 2.5 Â 10 M, was quantified according to of 1 Â 10 M, gives rise to CCW dominated porous networks
the former listing. The results are summarised in Table 1. The (Table S1, ESI†), akin the sergeants-and-soldiers effect observed
details of the analysis and the calculation of the error bars are for cDBA-OC12(S)/(R) and DBA-OC12.
specified in the ESI.† The results of the cDBA-OC12(S) mixture
Porous networks with an equal amount of CW and CCW
show indeed some higher level of interaction with the CCW domains were generated through depositing a droplet of DBA-
À6
pores (only 34 Æ 1% CCW pores are empty versus 85 Æ 7% CW OC13 solution with a concentration of 1 Â 10 M (Fig. S7,
empty pores) and the cDBA-OC12(R) mixture has better inter- ESI†). Upon further adding one droplet of cDBA-OC13(R) solution
À6
action with the CW pores (only 63 Æ 11% CW pores are empty (5 Â 10 M) to the preformed DBA-OC13 network, a clear
versus 83 Æ 3% CCW empty pores). There is an unexpected difference in pore occupation was observed: up to 90% of the
difference in guest occupancy in these two experiments, which CW pores are filled, while only around 20% of the CCW pores
we cannot explain. We reason that kinetic factors might be are filled (Fig. 3A). A domain boundary can be easily identified
at play. Most importantly, the trend is robust: cDBA-OC12(S) from the STM image. Moreover, adsorbed cDBA-OC13(R)s are
prefers CCW pores while cDBA-OC12(R) prefers to adsorb in more mobile in CCW than in CW pores, which is reflected by
CW pores. This is also confirmed by the control experiments their appearance in the STM images. The selectivity is also
described below.
supported by the statistics of occupation types (Fig. 3B). It should
To examine if the ratio between CW and CCW pores on the be noted that the amount of cDBA-OC13(R) in the solution phase
surface has any influence on the interaction with the guest is five times higher than DBA-OC13, but still a lot of CCW
molecules, the deposition method was varied. In particular, nanowells are empty.
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Policy Office (IAP-7/05), and JSPS KAKENHI Grant Number 10252628
and 26620063. This research has also received funding from the
European Research Council under the European Union’s Seventh
Framework Programme (FP7/2007-2013)/ERC Grant Agreement
No. 340324. H.C. is an FWO Pegasus Marie Curie Fellow.
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enantioselective adsorption in surface-confined nanoporous
networks. Enantiopure DBAs have a preference to adsorb in
nanowells of a given handedness. These results obtained using
different molecules, solvents, and sample preparation condi-
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This journal is ©The Royal Society of Chemistry 2015
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