Chiral self-recognition and self-discrimination of strapped perylene bisimides by π-stacking dimerization

Article abstract of DOI:10.1002/chem.201001137

(Chemical equation presented) Narcissism of molecules! Configurationally restricted chiral perylene bisimide dyes show a very strong preference for the formation of homochiral dimers (self-recognition) (red pair) upon π stacking in solution. Minor amounts of heterochiral dimers (red-yellow pair) enable the determination of P/M enantiomeric ratios by ¹H NMR spectroscopy without using any chiral auxiliary.

Full text of DOI:10.1002/chem.201001137

DOI: 10.1002/chem.201001137
Chiral Self-Recognition and Self-Discrimination of Strapped Perylene
Bisimides by p-Stacking Dimerization
Marina M. Safont-Sempere, Peter Osswald, Krzysztof Radacki, and Frank Wꢀrthner*[a]
The importance of self-assembly and self-organization for
the creation of higher order functional structures is evident
in natural systems.[1] Both in biological processes and in or-
ganic synthesis, chiral recognition,[2] that is, the ability of a
chiral molecule to differentiate between two enantiomers, is
of great significance as demonstrated in asymmetric cataly-
sis[3] and enantioselective recognition by enzymes and pro-
tein-receptor sites.[4] Chiral recognition between enantiomer-
ic pairs can lead to self-recognition[5] or self-discrimination,[6]
depending on whether an enantiomer preferentially recog-
nizes itself or its mirror image to generate homochiral or
heterochiral self-assemblies, respectively. Such stereoselec-
tive recognition events may have intriguing consequences
such as the formation of homochiral compounds from race-
mates in autocatalytic reactions.[7] Accordingly, a better un-
derstanding of chiral recognition phenomena will ultimately
enable a more rational catalyst design for asymmetric syn-
thesis and may even contribute to a better understanding of
the origin of homochirality in biological molecules (e.g., l-
amino acids and d-sugars and the biopolymers derived
thereof). Despite such important prospects, little attention
has been paid to the elucidation of self-recognition versus
self-discrimination phenomena of chiral compounds in
supramolecular systems. Most of the known examples for
chiral recognition are based on the rather strong “inorganic”
metal–ligand[5a–c,6a–f] and cation–anion interactions,[8] whereas
examples based on weak “organic” interactions like hydro-
gen bonding and p–p interactions remain relatively
rare.[5d,e,6g,h] Notably, some interesting examples of self-asso-
ciation by metal-ion coordination or hydrogen bonding have
been reported for chiral helicates,[9] calix[4]arenes,[10] and
peptides.[11]
Perylene bisimide (PBI) dyes have attracted considerable
attention during the past years as fluorophores and organic
semiconductor materials due to their unique optical and
electrochemical properties.[12a] Moreover, the concurrence of
a large p-surface and a pronounced quadrupole moment of
PBIs enables exceptionally strong p–p-stacking interactions
between these dyes.[12b] PBIs containing substituents in the
bay positions (1, 6, 7, 12-positions) are of particular interest
as they possess chirality due to the twisting of the perylene
core leading to atropo-enantiomers (P and M enantio-
mers).[13]
Very recently, Li and co-workers elegantly explored the
influence of core-twisting on dynamic self-assembly (DSA)
of congigurationally flexible PBIs and their covalent cyclodi-
merization.[14] However, for the investigation of chiral recog-
nition properties of this important class of functional dyes,
enantiopure PBIs with stable configuration are required.
Due to the fast interconversion process between atropo-
enantiomers of PBIs, such enantiopure derivatives are
rather scarce.[13a] Recently, we obtained epimerically pure
tetra-aryloxy-substituted PBIs by restricting the interconver-
sion of atropisomers through bridging the aryloxy bay sub-
stituents.[13b] However, these chiral PBIs are not suitable for
self-assembly studies to explore the chiral recognition prop-
erties since both p-faces of the perylene core are blocked by
bridging units. Therefore, to enable such studies, we have
synthesized the chiral, at the bay position 1,7-disubstituted
macrocyclic PBIs 2a,b and resolved the atropo-enantiomers
(P)-2a,b and (M)-2a,b (Figure 1). The bridging unit on one
p-face of these strapped PBIs 2 allows only dimerization by
interaction of free p-faces and restricts the interconversion
between M and P enantiomers. Self-assembly studies with
these new chiral PBIs revealed that self-recognition, that is,
formation of homochiral dimers, prevails over self-discrimi-
nation, that is, formation of heterochiral dimers, by p–p
stacking. One unexpected finding of our present studies is
that the formation of minor amounts of heterochiral dimers
enables the determination of P/M enantiomeric ratios of
[a] M. M. Safont-Sempere, Dr. P. Osswald, Dr. K. Radacki,
Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie and
Institut fꢀr Anorganische Chemie and Rçntgen Research Center
for Complex Material Systems
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax : (+49)931-31-84756
E-mail: wuerthner@chemie.uni-wuerzburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001137.
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COMMUNICATION
Figure 2. Molecular structure of racemic PBI 2a, as observed by single-
crystal X-ray analysis. (P)-enantiomer is depicted, view along the short
axis of the PBI showing the bowl-like distortion of the perylene core.
Figure 1. Structures of atropo-enantiomeric (P)-2a,b and (M)-2a,b and
schematic representation of homo- and heterochiral dimer aggregates de-
rived thereof.
The atropo-enantiomers (P and M) of PBIs 2a,b were re-
solved by semi-preparative HPLC on a chiral column (Tren-
tec, Reprosil 100 chiral-NR) using a mixture of dichlorome-
thane and n-hexane as eluent (for details, see the Supporting
Information). The stereochemical assignment of the isolated
enantiomers was achieved by comparison of their circular
dichroism (CD) spectra with those of the previously report-
ed structurally similar epimerically pure P- and M-config-
ured macrocyclic PBIs.[13b]
1
chiral PBIs by H NMR spectroscopy without using any ex-
ternal chiral auxiliary.[15]
The strapped PBIs 2a,b were synthesized by macrocycli-
zation of the corresponding 1,7-di(3-hydroxyphenoxy)-sub-
stituted PBIs 1a,b through etherification with di(ethylene
glycol) ditosylate in the presence of cesium carbonate in
DMSO (for 1a) or THF (for 1b) (Scheme 1).[13b,c] The de-
tails of the synthesis and characterization of (rac)-2a,b and
the precursors 1a,b are given in the Supporting Informa-
tion.
Single crystals of (rac)-2a suitable for X-ray diffraction
were obtained by dissolving the compound in dichlorome-
thane, followed by addition of equal amounts of methanol,
and subsequent slow evaporation of the solvent. The molec-
ular structure of (rac)-2a determined by single-crystal X-ray
analysis (Figure 2) unequivocally confirms the macrocyclic
structure of these PBIs with the diethylene glycol bridge
shielding one p-face of the perylene core. The twist angle
between the two naphthalene units of this molecule (which
leads to atropisomers) is about 178 (for further structural
and crystallographic data, see the Supporting Information).
For the investigation of chiral recognition process of these
PBIs in solution, PBI 2b containing six long alkyl chains in
imide substituents was used. This derivative is soluble in
nonpolar solvents such as n-hexane or n-heptane, in which
aggregation of PBIs can be observed at rather low concen-
trations.[16] Chiral recognition in the self-assembly of PBIs
1
2b was studied by CD, UV/Vis, and H NMR spectroscopy.
The CD spectra of monomeric (P)-2b and (M)-2b enantio-
mers in dichloromethane (c=5ꢃ10ꢀ6 m) show monosignated
spectra in the visible region (400–600 nm) in a mirror image
relation (Figure 3, top panel, blue lines) due to intrinsic mo-
lecular chirality.[13b] In nonpolar n-hexane a bisignate Cotton
effect (zero crossing at 542 nm) is observed for both enan-
tiomers at a concentration of 5ꢃ10ꢀ4 m (Figure 3, upper
panel, red lines), which can be attributed to chiral exciton
coupling[17] of the transition dipole moments of the chromo-
phores and thus the formation
of dimer aggregates with a heli-
cal p–p-stacking arrangement.
The negative sign of the exciton
couplet (at 555 nm) for (P)-2b
indicates a left-handed helicity
(the opposite for (M)-2b) of
the respective dimer aggregate
according to exciton chirality
method.[17,18]
The binding constants KD for
dimerization of (rac)-2b, (P)-
2b, and (M)-2b were deter-
mined by concentration-depen-
dent UV/Vis absorption studies
Scheme 1. Synthesis of racemic PBIs 2a,b and resolution of atropo-enantiomers (P)-2a,b and (M)-2a,b.
at 331 K in n-heptane. The con-
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F. Wꢀrthner et al.
Quantitative information about the ratio of homochiral
and heterochiral dimers can be obtained from the KD values
of PBI 2b. The KD value obtained for enantiopure (P)-2b
(or (M)-2b) is obviously related to the formation of homo-
chiral dimers (i.e., KD =KD(homo)), whereas in the case of the
racemate (rac)-2b, the obtained KD(rac) is the average value
of homo- and hetero-dimerization processes. According to
Kol and co-workers,[22] for racemates the binding constant
can be expressed as given in Equations (1) and (2),
½Dꢁ
KDðracÞ
¼
ð1Þ
ð2Þ
2
½Mꢁ
K
DðheteroÞ þ 2KDðhomoÞ
KDðracÞ
¼
4
where [D] is the total dimer concentration (homo- and
heterochiral dimers) and [M] is the free monomer concen-
tration. By applying the respective KD values obtained for
the enantiopure (KD(homo)) and racemic (KD(rac)) PBI samples
to Equation (2), KD(hetero) can be calculated as 400mꢀ1. From
these binding constants it can also be concluded that hetero-
dimers account for less than 7% of the total dimer concen-
tration (for details, see the Supporting Information), con-
firming the prevalence of chiral self-recognition (homodime-
rization) over self-discrimination (heterodimerization) of
these PBIs by p-stacking. Moreover, with KD(homo) and
KD(hetero), the equilibrium constant (K’) and related Gibbs
energy (DG’) for the formation of heterodimers from a race-
mic mixture of homodimers can be calculated as 5ꢃ10ꢀ3 and
14.5 kJmolꢀ1, respectively (details are given in the Support-
ing Information). Note that the obtained Gibbs energy has a
positive sign as the formation of heterodimers from homo-
dimers is an energetically disfavored process.
Figure 3. Top: CD spectra of (P)-2b (solid lines) and (M)-2b (dashed
lines) in CH2Cl2 (5ꢃ10ꢀ6 m) (blue lines) and in n-hexane (1ꢃ10ꢀ3 m) (red
lines) at 298 K. Bottom: concentration-dependent UV/Vis absorption
spectra of (rac)-2b in n-heptane at 331 K; arrows indicate spectral
changes with increasing concentration from 5ꢃ10ꢀ6 m to 1ꢃ10ꢀ3 m.
centration-dependent UV/Vis absorption spectra (all experi-
ments were repeated twice) of these three samples are virtu-
ally identical, indicating similar aggregation behavior (see
Figure S3–S8 in the Supporting Information). In all cases,
isosbestic points are observed at 526 and 466 nm, suggesting
the presence of thermodynamic equilibrium between two
species.[19] The experimental data obtained from the concen-
tration-dependent UV/Vis absorption studies can be ade-
quately fitted by nonlinear regression analysis using the di-
merization model (see Figure S3–S8 in the Supporting Infor-
mation).[20,21] The dimerization constants KD of (rac)-2b and
enantiomers (P)-2b and (M)-2b at 331 K are collected in
Table 1 (and Table S1 in the Supporting Information). It is
remarkable that the KD values determined for the pure
enantiomers are significantly larger than that for the race-
mate, implying that the formation of homochiral dimers is
thermodynamically more favored.
To get more insights into the chiral recognition behavior
1
of PBIs 2b, H NMR investigations in deuterated n-hexane
at 331 K were performed. As expected, identical NMR spec-
tra were obtained for (P)-2b and (M)-2b enantiomers (c=
10ꢀ3 m). However, significant differences were observed in
spectra of (rac)-2b (c=2ꢃ10ꢀ3 m).[23] As can be seen in
Figure 4, the superposed spectra of racemate (rac)-2b and
enantiopure (P)-2b (or (M)-2b) show significant differences
in chemical shift, particularly of the protons in the aromatic
region. For the concentration and temperature used in the
1H NMR experiments, at least 60% of dimeric species
should be present as estimated from UV/Vis studies (Fig-
ure S3–S8 in the Supporting Information). Thus, the ob-
served differences between the spectra of (rac)-2b and
enantiopure (P)-2b (or (M)-2b) should be related to the
contemporaneous presence of homo- and heterochiral
dimers in the case of racemate.
Table 1. Dimerization constants KD determined for (rac)-2b, (P)-2b and
(M)-2b in n-heptane at 331 K by multilinear fit of the concentration-de-
pendent UV/Vis absorption data to the dimerization model.
To clarify the observed differences in chemical shifts of
racemate and pure enantiomers, a series of solutions in
[D14]n-hexane with varying enantiomeric excess (ee) ranging
from 18 to 90% ee at c=2ꢃ10ꢀ3 m were investigated by
1H NMR. The spectra in Figure 4 show two sets of signals
for the aromatic protons of perylene core (denoted as
Sample
(P)-2b
(M)-2b
ACHTUNGTRENNUNG
[a]KD [mꢀ1
]
2800
2800
[a] Details for the determination of these binding constants are given in
the Supporting Information. The error is ꢂ15%.
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Strapped Perylene Bisimides
COMMUNICATION
initiate activities on p-stacking dimerization of other chiral
dyes towards a better understanding of self-sorting phenom-
ena, which is of general interest.
Acknowledgements
This work was financially supported by the DFG (GRK 1221).
Keywords: chiral
recognition
·
dyes/pigments
·
homochirality · self-assembly · pi interactions
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1
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Received: April 29, 2010
Published online: June 11, 2010
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