Perylene bisimide atropisomers: Synthesis, resolution, and stereochemical assignment

Article abstract of DOI:10.1021/jo070056c

(Chemical Equation Presented) The macrocyclization of the tetra-hydroxyphenoxy-substituted perylene bisimide 4 bearing two (R)-configured 2-octyl substituents in the imide positions by etherification with diethylene glycol ditosylate afforded both the dia

Full text of DOI:10.1021/jo070056c

Perylene Bisimide Atropisomers: Synthesis, Resolution, and
Stereochemical Assignment
Peter Osswald, Matthias Reichert, Gerhard Bringmann, and Frank Wu¨rthner*
UniVersita¨t Wu¨rzburg, Institut fu¨r Organische Chemie, Am Hubland, 97074 Wu¨rzburg, Germany
wuerthner@chemie.uni-wuerzburg.de
ReceiVed January 10, 2007
The macrocyclization of the tetra-hydroxyphenoxy-substituted perylene bisimide 4 bearing two (R)-
configured 2-octyl substituents in the imide positions by etherification with diethylene glycol ditosylate
afforded both the diagonally bridged (1,7- and 6,12-linkage) and laterally bridged (1,12- and 6,7-linkage)
regioisomers 6 and 7. The atropo-diastereomers of the diagonally bridged macrocycle 6 were separated
by semipreparative HPLC on a chiral column, and their absolute configurations were determined by
circular dichroism (CD) spectroscopy in combination with quantum chemical CD calculations. The isolated
epimers (P,R,R)-6 and (M,R,R)-6 represent the first examples of diasteriomerically pure perylene bisimide
atropisomers. The optical and chiroptical properties of these epimers were investigated by UV/vis,
fluorescence, and CD spectroscopy, and their conformational properties have been explored by temperature-
1
dependent H NMR studies.
Introduction
polarized emission, which should enable stereoimaging.5 Most
prominent examples of chiral compounds with inherent chirality
used in this context are biphenyl6 and binaphthyl7 derivatives,
as well as helicenes and helicene-like systems.8 In the latter
case, the twisting of the π-conjugated molecule endows the
Apart from the academic interest in chirality as a central topic
of chemistry and biology, chiral molecules are widely applied
in numerous fields such as chiral recognition (with particular
importance in drug research)1 and enantioselective catalysis,2
as well as in nonlinear optics3 and as molecular switches.4
Furthermore, chiral polymeric emitters or chiral dopants in
nematic liquid crystals show potential for application in photo
and electroluminescence devices owing to their circularly
(5) (a) Lemieux, R. P. Acc. Chem. Res. 2001, 34, 845-853. (b) Peeters,
E.; Christiaans, M. P. T.; Janssen, R. A. J.; Schoo, H. F. M.; Dekkers, H.
P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1997, 119, 9909-9910. (c)
Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.; Meskers,
S. C. J.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118,
4908-4909. (d) Riehl, J. P.; Richardson, F. S. Chem. ReV. 1986, 86, 1-16.
(6) Recent examples: (a) Omote, M.; Nishimura, Y.; Sato, K.; Ando,
A.; Kumadaki, I. Tetrahedron 2006, 62, 1886-1894. (b) Ceccacci, F.;
Mancini, G.; Sferrazza, A.; Villani, C. J. Am. Chem. Soc. 2005, 127, 13762-
13763. For a recent review, see: Bringmann, G.; Price Mortimer, A. J.;
Keller, P. A.; Gresser, M. J.; Garner, J.; Breuning, M. Angew. Chem., Int.
Ed. 2005, 44, 5384-5427.
(7) Recent examples: (a) Zhou, Y.; Zhang, D.; Tang, Y.; Zhu, D. J.
Org. Chem. 2005, 70, 6164-6170. (b) Li, Z.-B.; Lin, J.; Qin, Y.-C.; Pu, L.
Org. Lett. 2005, 7, 3441-3444. (c) van Delden, R. A.; Mecca, T.; Rosini,
C.; Feringa, B. L. Chem. Eur. J. 2004, 10, 61-70. (d) Ishow, E.; Credi,
A.; Balzani, V.; Spadola, F.; Mandolini, L. Chem. Eur. J. 1999, 5, 984-
989. For a review, see: Pu, L. Chem. ReV. 1998, 98, 2405-2494.
* To whom correspondence should be addressed. Tel: +49-931-8885340.
Fax: +49-931-8884756.
(1) Recent reviews: (a) Bell, T. W.; Hext, N. M. Chem. Soc. ReV. 2004,
33, 589-598. (b) Pu, L. Chem. ReV. 2004, 104, 1687-1716.
(2) Recent reviews: (a) Muniz, K. Chem. Unserer Zeit 2006, 40, 122-
124. (b) Wessig, P. Angew. Chem., Int. Ed. 2006, 45, 2168-2171. (c) Ding,
J.; Armstrong, D. W. Chirality 2005, 17, 281-292.
(3) For reviews, see: Fischer, P.; Hache´, F. Chirality 2005, 17, 421-
437. Meijer, E. W.; Ferringa, B. L. Mol. Cryst. Liq. Cryst. 1993, 235, 169-
180.
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Geertsma, E. M. Chem. ReV. 2000, 100, 1789-1816.
10.1021/jo070056c CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/28/2007
J. Org. Chem. 2007, 72, 3403-3411
3403

Osswald et al.
chirality. It has been shown that twisting of π-systems can be
enforced by attaching sterically demanding substituents in
appropriate positions. Thus, by applying this principle, a large
variety of functional chiral π-conjugated systems have been
synthesized, among them, e.g., cyanines, phenanthrenes, tri-
phenylenes, and also acene derivatives.9 In general, such twisted
aromatic systems are not conformationally stable, as in most
cases dynamic racemization was observed at elevated temper-
ature depending on the energy barriers for this process, which
can vary significantly depending on the steric demand of the
substituents.10
We are particularly interested in bay-substituted perylene
bisimides, a class of fluorophores that has attracted a great deal
of attention in the past years owing to their outstanding optical
and light fastness properties and their high solubility in common
organic solvents.11 Perylene bisimides (PBIs) bearing substit-
uents in the bay area (1,6,7,12-positions) possess a twisted
π-system with a torsion angle between 4° and 36° depending
on the number and size of the substituents.11c,12 It has been
recognized already some years ago that such twisted fluoro-
phores might possess interesting chiroptical properties.13 How-
ever, due to the low barriers for the interconversion of the two
twisted enantiomeric conformers in solution, atropoisomerically
pure perylene bisimides could not be isolated so far, and racemic
material prevails in the crystalline state. In hydrogen-bonded
co-aggregates of tetraphenoxy-substituted PBIs with oligo-
(phenylenevinylenes) (OPVs) bearing chiral side chains, we have
previously observed a negative (non-bisignated) CD signal for
the perylene bisimide chromophore indicative of an enrichment
of one enantiomer of PBI in the aggregate.14 In this case, the
chirality is transferred from the optically active OPV units to
PBI molecules to afford helical assemblies showing notable
Cotton effects for the optical transition of PBI.
Recently, we have reported a strategy to restrict the dynamic
racemization process of tetraphenoxy-substituted perylene bis-
imides by bridging the substituents at the bay positions with
oligo(ethylene glycol) chains through macrocyclization.15 By
applying this approach, we herein present the synthesis of chiral
macrocyclic PBIs containing two (R)-configured 2-octyl sub-
stituents in the imide positions and report on the resolution of
pure PBI epimers that have allowed the elucidation of the
chiroptical properties of twisted PBI fluorophores. The absolute
configurations of the epimers have been determined by applying
CD spectroscopy in combination with quantum chemical CD
calculations.
Results and Discussion
Synthesis and Resolution of Atropisomerically Pure Mac-
rocyclic Perylene Bisimides. The macrocyclic perylene bis-
imides 6 and 7 were synthesized in five steps starting with the
tetrachloro-substituted perylene bisimide 116 as outlined in
Scheme 1. The nucleophilic substitution of the chlorine atoms
in 1 by 3-methoxyphenol afforded the tetraphenoxy-substituted
perylene bisimide 2 in 36% yield. Subsequent saponification
of 2 with potassium hydroxide in isopropanol under reflux
provided perylene bisanhydride 3 in 90% yield. The chiral
substituents were then introduced by imidization of 3 with 2-(R)-
octylamine in quinoline, employing zinc acetate as a catalyst.
After column chromatography of the crude product on silica
gel with dichloromethane as eluent, the chiral perylene bisimide
4 was isolated in 58% yield. Cleavage of the methyl ether groups
in 4 with boron tribromide in anhydrous dichloromethane
provided the resorcinol-functionalized PBI 5 as a synthetic key
intermediate.
For the macrocyclization of the chiral perylene bisimide 5, a
similar procedure was applied as previously described for achiral
derivatives.15 A diethylene glycol linker was chosen as the
bridging unit because it provides conformationally rigid mac-
rocycles with reasonable yields.15 Thus, the tetra-hydroxyphe-
noxy-substituted PBI 5 was treated with diethylene glycol
ditosylate and cesium carbonate in dimethylsulfoxide at 120 °C
under argon, and after column chromatographic purification of
the crude product, both the diagonally bridged (1,7- and 6,12-
linkage) macrocycle 6 and its laterally bridged (1,12- and 6,7-
linkage) isomer 7 were obtained in 4% and 30% yields,
respectively. In comparison to the previously reported macro-
cyclization of perylene bisimides bearing achiral imide groups,15
a significantly higher yield for the laterally bridged isomer was
observed in the present case.
(8) Recent examples: (a) Fasel, R.; Parschau, M.; Ernst, K.-H. Nature
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(11) (a) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051-5066. (b)
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5629. (c) Wu¨rthner, F. Chem. Commun. 2004, 1564-1579.
(12) (a) Wu¨rthner, F.; Osswald, P.; Schmidt, R.; Kaiser, T. E.; Man-
sikkama¨ki, H.; Ko¨nemann, M. Org. Lett. 2006, 8, 3765-3768. (b) Leroy-
Lhez, S.; Baffreau, J.; Perrin, L.; Levillain, E.; Allain, M.; Blesa, M.-J.;
Hudhomme, P. J. Org. Chem. 2005, 70, 6313-6320. (c) Wu¨rthner, F.;
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(13) Hien, S. Ph.D. Thesis, University of Regensburg, Regensburg, 1995.
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Schenning, A. P. H. J.; Li, X.-Q.; Osswald, P.; Wu¨rthner, F.; Janssen, R.
A. J. J. Phys. Chem. B 2006, 110, 16967-16978. (b) Wu¨rthner, F.; Chen,
Z.; Hoeben, F. J. M.; Osswald, P.; You, C.-C.; Jonkheijm, P.; von
Herrikhuyzen, J.; Schenning, A. P. H. J.; van der Schoot, P. P. A. M.; Meijer,
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Since the macrocyclic perylene bisimides 6 and 7 contain
two (R)-configured chiral imide substituents and the perylene
core possesses a chirality plane, the occurrence of diastereomers
(more precisely, epimers) was to be expected for these
compounds. Indeed, the epimers of the diagonally bridged
macrocycle 6, for which an interconversion of the twisted
conformers (P and M) is not possible, proved to be separable
by semipreparative HPLC on a chiral column (Reprosil 100
chiral-NR) using isopropanol/n-hexane (1:1) as the eluent
(Figure 1). The absolute configurations of the first (6′) and the
(15) Osswald, P.; Leusser, D.; Stalke, D.; Wu¨rthner, F. Angew. Chem.,
Int. Ed. 2005, 44, 250-253.
(16) Wu¨rthner, F.; Sautter, A.; Schilling, J. J. Org. Chem. 2002, 67,
3037-3044.
3404 J. Org. Chem., Vol. 72, No. 9, 2007

Perylene Bisimide Atropisomers
SCHEME 1. Synthesis of Regioisomeric Macrocyclic Perylene Bisimides 6 and 7 and Resolution of the Atropo-diastereomers of 6
second eluted (6′′) epimers (retention times 25 and 30 min) were
determined as (P,R,R) and (M,R,R), respectively, by CD
spectroscopy in combination with quantum chemical CD
calculations (details are given in following sections). The
epimers of the laterally bridged compound 7, by contrast, could
not be resolved, apparently as a result of its low isomerization
(M a P) barriers (vide infra).
similar macrocycles, whose structures were unequivocally
determined by X-ray analysis.15 Thus, for the diagonally bridged
macrocycle 6 a characteristic upfield shift of the signal for the
aromatic protons (δ ) 5.6 ppm) between the two oxygen atoms
of the resorcin residues was observed (Figure 2), indicating that
these protons are located above and below the aromatic perylene
core. The respective protons of the laterally bridged isomer 7
were found to be shifted to lower field (δ ) 6.7 ppm), implying
that in this regioisomer the phenoxy residues are oriented more
in the plane of the perylene core.
The constitutions of the regioisomeric macrocyclic perylene
bisimides 6 and 7 were unambiguously assigned by comparison
1
of their H NMR data with those of the previously reported
J. Org. Chem, Vol. 72, No. 9, 2007 3405

Osswald et al.
FIGURE 3. UV/vis absorption and emission spectra of 6 (dotted line)
and 7 (solid line) in dichloromethane at 25 °C.
TABLE 1. Fluorescence Properties of Perylene Bisimides 6 and 7
a
c
d
solvent
λ
max (nm) Φfl τfl (ns)b kr (s-1
)
k )
nr (s-1
6
7
dichloromethane
diethylether
dichloromethane
570
560
598
0.22
0.73
0.95
2.2
6.2
6.9
1.0 × 108 3.6 × 108
1.2 × 108 4.4 × 107
1.4 × 108 7.3 × 106
FIGURE 1. HPLC chromatograms before (bottom) and after separation
of the epimers of 6 on a Trentec Reprosil 100 chiral-NR column at
ambient conditions using isopropanol/n-hexane (1:1) as eluent (flow
rate 2 mL/min).
b
d
a Φfl ( 0.03. τfl ( 0.5 ns. c kr ) Φfl/τfl. knr ) (1 - Φfl)/τfl.
(Figure 3) reveal the characteristic transitions for PBIs, i.e., a
S0-S1 transition at ∼550 nm and a S0-S2 transition at 450 nm.
The UV/vis absorption spectrum of the diagonally bridged
isomer 6 shows a hypsochromic shift of the absorption
maximum of the S0-S1 transition by 38 nm in comparison to
that of 7, accompanied by a pronounced hypochromicity.
Moreover, 6 exhibits a significantly less intense S0-S2 transition
compared to that of 7. These spectral features are typical of
diagonally bridged PBI macrocycles15 and thus further confirm
the structural assignment based on characteristic 1H NMR data.
The fluorescence properties of 6 and 7 in dichloromethane,
as summarized in Table 1, reveal that the diagonally bridged
isomer 6 is only weakly fluorescent in dichloromethane with a
fluorescent quantum yield of 22%. The corresponding fluores-
cence lifetime was determined as 2.2 ns. These results indicate
that the rate constant for the nonradiative processes is increased
in comparison to laterally bridged macrocycle 7 (see Table 1).15
Moreover, for the diagonally bridged isomer 6 a Stokes shift
of 38 nm was observed, which is 10 nm larger than that of the
laterally bridged macrocycle 7, implying a more pronounced
change of the geometry between the ground and the excited
states of 6. By decreasing the solvent polarity, an increase of
the quantum yield was observed. Thus, 6 exhibits a fluorescence
quantum yield of 73% in diethylether, accompanied by an
increase of the fluorescence lifetime to 6.2 ns. In contrast to 6,
the fluorescence properties of the laterally bridged perylene
bisimide 7 properly corroborate those observed for conforma-
tionally flexible open-chained, phenoxy-substituted perylene
bisimides such as 4 (Φfl ) 94%), as for regioisomer 7 a
fluorescence quantum yield of 95% was observed in dichlo-
romethane. It is noteworthy that the fluorescence quantum yields
of the macrocycles 6 and 7 are slightly higher (by 10-15%)
than those of the previously reported macrocycles bearing aryl
substituents in the imide position.15
FIGURE 2. Aromatic region of the 1H NMR spectra (400 MHz) of 6
(top) and 7 (bottom) in CD2Cl2 at 300 K. The signals of the protons
situated between the two oxygen atoms of the resorcinol residues are
highlighted in red. EG denotes ethylene glycol.
Characteristic aromatic shielding effects were also observed
for protons of the bridging ethylene glycol units. For the laterally
bridged regioisomer 7, the resonances of these protons appear
at 3.8-4.8 ppm, whereas the corresponding protons of the
diagonally bridged analog 6 show resonances at considerably
higher field (3.5-3.9 ppm). Thus, the bridging units in 6
experience a deshielding due to the aromatic perylene core that
is only given for this diagonally bridged macrocycle.
Optical Properties. The optical properties of the epimeric
perylene bisimides 6 and 7 were investigated by UV/vis and
fluorescence (steady-state and time-resolved) spectroscopy. The
absorption and emission spectra of 6 and 7 in dichloromethane
Stereochemical Assignment and Chiroptical Properties of
Atropisomerically Pure Perylene Bisimides. The absolute
configurations of the isolated epimers (6′ and 6′′) of the
3406 J. Org. Chem., Vol. 72, No. 9, 2007

Perylene Bisimide Atropisomers
FIGURE 4. UV/vis absorption (top) and CD spectra (bottom) of the first (6′, solid line) and the second eluted (6′′, dotted line) epimers of 6 in
dichloromethane at 20 °C.
macrocyclic perylene bisimide 6 were determined by CD
spectroscopy in combination with quantum chemical calcula-
tions. For this purpose, the chiroptical properties of these
epimers were explored beforehand.
of the second ones are found at about 250 nm with a crossover
point at 276 nm. This zero transition corresponds very well to
the maximum at 278 nm observed in the UV/vis absorption
spectra (Figure 4, top). The observation of an exciton couplet
at about 280 nm can be attributed to the excitonic interaction
of the two twisted naphthalene imide units of the perylene
bisimide core (phenoxy residues absorb at lower wavelengths).
Therefore, the exciton chirality method18 was applied for a first
tentative assignment of the absolute configurations of the
epimers of 6. Since the first eluted stereoisomer 6′ shows a first
positive Cotton effect (at 278 nm), it should be (P)-configured
and, accordingly, the second eluted epimer 6′′ should have the
(M)-configuration, leading to the full absolute stereostructure
of (P,R,R) for 6′ and of (M,R,R) for 6′′. Since both imide
substituents of 6 are (R)-configured, the absolute configuration
of the first eluted epimer, 6′, can be assigned as (P,R,R) and
that of the second one 6′′ as (M,R,R). The mirror-image relation
of the two CD spectra indicates the pseudo-enantiomeric
character of these diastereomers, i.e., the chiral alkyl substituents
do not contribute to the optical activity in the visible wavelength
The CD spectra of the isolated epimers 6′ and 6′′ show a
clear mirror image relation as depicted in Figure 4 (bottom). In
the visible region (400-650 nm) of the CD spectra a broad
monosignated peak with a maximum at 536 nm can be seen,
which properly correlates with the absorption maximum at 532
nm (Figure 4, top). The absolute ∆ꢀ values for the longest
wavelength transition (536 nm) of the two epimers of 6 are
+67 (6′) and -63 M-1 cm-1 (6′′), giving an absorption
dissymmetry factor g ) ∆ꢀ/ꢀ of 0.0018. This is in accordance
with the values observed for related chiral twisted π-systems,
e.g., hypericin derivatives, which in their 1,6-dioxo form show
a g-factor of 0.0014 for the longest wavelength transition.17
Moreover, in the visible part of the CD spectra (400-650 nm)
of the epimers, two optical transitions are located, i.e., the S0-
S1 (536 and 495 nm) and the S0-S2 transitions (450 nm), whose
transition dipole moments are polarized along the long and the
short axes of the perylene bisimide, respectively. Since these
transitions can be attributed to the whole chromophore, the
monosignated CD signal represents the chirality of the whole
molecule. These observations are characteristic for molecular
chirality and differ from those arising from excitonic coupling.18
On the other hand, for the naphthalene-related optical
transitions in the UV region (250-400 nm) of the CD spectra,
an asymmetric bisignated signal was observed, with a positive
first Cotton effect and a negative second one for the first eluted
epimer 6′ and vice versa for the second eluted diastereomer 6′′.
In both cases the peak maxima of the first Cotton effects are
located at 292 nm (∆ꢀ values are +113 and -100) and those
range. Additionally, the optical rotation values [R]20 for
D
(P,R,R)-6 and (M,R,R)-6 were determined at 20 °C in dichlo-
romethane to be +6600 (c ) 0.007) and -6600 (c ) 0.006),
respectively. Optical rotations of such large sizes had been
previously observed only for twisted π-systems such as helicenes.8i
However, in our case a pronounced dispersion enhancement is
present, since the detection wavelength (λ ) 589 nm) is close
to the absorption maximum of 6. The identical absolute values
of the optical rotations, yet with opposite signs, confirm the
pseudo-enantiomeric behavior of these perylene bisimide dyes.
Such a pseudo-enantiomeric behavior had been previously
demonstrated for related hypericin derivatives, for which the
observed chirality emanates from the inherently chiral chro-
mophore.17
(17) Altmann, R.; Etzlstorfer, C.; Falk, H. Monatsh. Chem. 1997, 128,
785-793.
(18) Berova, N.; Nakanishi, K. Exciton Chirality Method: Principles and
Applications. In Circular Dichroism, 2nd ed.; Berova, N., Nakanishi, K.,
Woody, R. W., Eds.; Wiley-VCH: Berlin, 2000; pp 337-383.
As mentioned above, the absolute configurations of the
epimers of 6 can be tentatively attributed by applying the exciton
chirality method. For a more reliable configurational assignment
J. Org. Chem, Vol. 72, No. 9, 2007 3407

Osswald et al.
of the epimers, quantum chemical calculations of the CD spectra
were performed, with subsequent comparison of the simulated
spectra with the measured ones.19
In a first step, the conformational space of (P,R,R)-6 was
investigated by means of PM3,20 resulting in 24 minimum
geometries within an energy range of 3 kcal/mol.21 For each
single geometry thus obtained, a CD spectrum was calculated
using the semiempirical OM222 Hamiltonian. These individual
spectra were then added up weighted following the Boltzmann
statistics, i.e., according to the corresponding heats of formation,
to give the overall simulated CD curve, which was then UV-
corrected.23 Comparison with the experimental CD spectrum
of the fast eluted epimer 6′ revealed an acceptable agreement
for (P,R,R)-6 and a reversed behavior for (M,S,S)-6 (Figure 5a),24
suggesting that the first eluted epimer is (P)-configured, and
consequently, the second eluted one, 6′′, is (M).
In order to confirm these stereochemical assignments and in
view of the molecular flexibility of 6, further CD calculations
for (P,R,R)-6 were performed, now based on a molecular
dynamics (MD) simulation using the TRIPOS25 force field at a
virtual temperature of 800 K. For the geometries extracted from
the trajectory of motion, single CD spectra were computed again
by means of OM2.21 Subsequent summation of these curves
and UV correction23 delivered the overall simulated CD
spectrum, which now corresponded very well with the experi-
mental spectrum of the first eluted epimer, 6′, while the spectrum
predicted for (M,S,S)-6 was found to be perfectly opposite
(Figure 5b). Consequently, the first eluted epimer is unambigu-
ously confirmed to have the (P)-configuration, whereas the
second one is (M)-configured.
Conformational Properties of Perylene Bisimides 4, 6, and
7. In contrast to the diagonally bridged regioisomer 6, an
interconversion of the epimers (P,R,R and M,R,R) of the laterally
bridged regioisomer 7 and of the chiral, non-macrocyclic PBI
4 is possible. To gain insight into the equilibrium process (M
a P) for the given pseudo-enantiomeric PBIs, 4 and 7, and to
assess whether any additional dynamic processes are involved,
temperature-dependent 1H NMR spectroscopic studies were
performed for the open-chained tetraphenoxy-substituted PBI
4 and for the macrocycles 6 and 7. Figure 6 shows the
resonances for the perylene core protons in temperature-
FIGURE 5. Assignment of the absolute configuration of the first eluted
epimer of 6 by comparison of its experimental CD curve (solid lines)
with the spectra calculated for (P,R,R)-6 and (M,S,S)-624 using (a) the
PM3-Boltzmann approach and (b) the TRIPOS-MD method, followed
in both cases by OM2-CIS-CD calculations (dotted lines).
epimerization occurs in the same time scale as the proton spin
relaxation (NMR time scale). At even lower temperatures, where
the spin relaxation is faster than the epimerization process, two
sets of signals are expected due to the presence of two epimers
(P,R,R and M,R,R). However, as can be seen from Figure 6b
(middle), eight signals are observed for the epimeric mixture
of 6 at low temperature (213 K). At 295 K, these signals merge
into a singlet, again indicating the pseudo-enantiomeric character
of the diastereomers. Since for the pure epimer (M,R,R)-6
(Figure 6b, bottom) only four signals are observed at low
temperature (213 K), the eight signals that appear in the
respective low-temperature 1H NMR spectrum of 6 can be
unambiguously assigned to the two respective epimers. The
occurrence of four signals for each epimer, instead of the
expected one singlet, can be explained either by a self-
dimerization26 or by an intramolecular dynamic process that is
frozen on the NMR time scale at lower temperature. Since both
π-faces of the perylene core in 6 are surrounded and thus
shielded by the bridging units, a dimerization of this PBI
derivative through π-π interactions is not feasible. Thus, the
1
dependent H NMR spectra of 4 and 6.
The perylene core protons were chosen to monitor the
atropisomeric interconversion, which in this case is not a
racemization but an epimerization, since the atropisomeric PBIs
4, 6, and 7 contain two (R)-configured substituents. The process
was investigated upon cooling to a temperature at which the
(19) (a) Bringmann, G.; Gulder, T.; Reichert, M.; Meyer, F. Org. Lett.
2006, 8, 1037-1040. (b) Bringmann, G.; Mu¨hlbacher, J.; Reichert, M.;
Dreyer, M.; Kolz, J.; Speicher, A. J. Am. Chem. Soc. 2004, 126, 9283-
9290. (c) Muranaka, A.; Asano, Y.; Tsuda, A.; Osuka, A.; Kobayashi, N.
ChemPhysChem 2006, 7, 1235-1240.
(20) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209-220.
(21) The contribution of the CD spectra of those structures that lie
energetically higher than 3 kcal/mol above the found global minimum to
the overall CD curve is negligible.
(22) Weber, W.; Thiel, W. Theor. Chem. Acc. 2000, 103, 495-506.
(23) Bringmann, G.; Busemann, S. In Natural Product Analysis; Schreier,
P., Herderich, M., Humpf, H. U., Schwab, W., Eds.; Vieweg: Wiesbaden,
1998; pp 195-212.
(24) Given the extreme dominance of the chiroptical contributions of
the chiral axis vs the two stereogenic centers, as proven by the inverse
experimental CD spectra of the two epimers of 6 (Figure 4), additional
calculations for (M,R,R)-6 appeared unnecessary.
(25) SYBYL; Tripos, Inc.: 1699 South Hanley Road, St. Louis, MO
63144, U.S.A.
(26) Chen, Z.; Baumeister, U.; Tschierske, C.; Wu¨rthner, F. Chem. Eur.
J. 2007, 13, 450-465.
3408 J. Org. Chem., Vol. 72, No. 9, 2007

Perylene Bisimide Atropisomers
FIGURE 6. (a) Sections of the temperature-dependent (295-207 K) 1H NMR (600.13 MHz) spectra of phenoxy-substituted PBI 4 in CDCl3 and
1
(b) sections of the H NMR spectra (500.13 MHz) of diagonally bridged PBI 6 at 295 K (top) and 213 K (middle) in CDCl3 and of epimerically
pure (M,R,R)-6 in CD2Cl2 at 213 K (bottom). Only the resonances of the perylene core protons (H2, H5, H8, and H11) are shown.
FIGURE 7. (a) Assignment of the NMR signal pattern of 4 (top) and of 6 (bottom) at low temperature and (b) anti (top) and syn (bottom)
conformations of the (M,R,R)- and (P,R,R)-epimers. The blue symbols represent the resonances of the perylene core protons of the (P,R,R)-epimer
and the red ones represent those of (M,R,R) (for 4, the assignment is analogous to 6). The circles and squares symbolize the resonances of two
rotamers (tentatively assigned), and the filled and empty symbols denote the chemically equivalent core protons.
appearance of eight signals for 6 at 213 K can be ascribed to
an intramolecular dynamic process. The observed four signals
most probably result from a hindered rotation about the imide
C-N bond, because rotational barriers for C-N bonds of
structurally similar compounds were reported to be about 51
kJ/mol at 228 K.27
For a hindered rotation about the C-N bonds of the perylene
bisimides 4 and 6, the preferred conformations at low temper-
atures are syn and anti, as shown in Figure 7b.28 Such
conformational features have been reported by Shimizu et al.
for unsubstituted perylene bisimide derivatives bearing aryl
substituents in the imide positions.28d In both conformations only
a C2 axis remains as a symmetry element, resulting in two
singlets for the perylene core protons for each of the two
rotamers, syn and anti, Figure 7b. Therefore, the observed four
signals for each epimer of 6 can be ascribed to the presence of
both rotamers at low temperature. The signal pattern for the
rotamers and epimers of 6 are marked in Figure 7a (bottom).
The two singlets of one rotamer (marked with red or blue circles)
are separated by 24 Hz, whereas for the other rotamer (red or
blue squares) a lower distance of 21 Hz was found. Therefore,
1
(27) Kohmoto, S.; Sakayori, K.; Kishikawa, K.; Yamamoto, M. J. Chem.
Soc., Perkin Trans. 2 1999, 833-836.
the signal pattern observed in the H NMR spectrum of 6 at
low temperature (213 K) is due to the presence of two epimers,
whose signals are further split into four signals owing to the
presence of syn and anti rotamers.
(28) (a) Degenhardt, C. F., III; Lavin, J. M.; Smith, M. D.; Shimizu, K.
D. Org. Lett. 2005, 7, 4079-4081. (b) Degenhard, C. F., III; Smith, M. D.;
Shimizu, K. D. Org. Lett. 2002, 4, 723-726. (c) Degenhardt, C. F., III;
Shortell, D. B.; Adams, R. D.; Shimizu, K. D. Chem. Commun. 2000, 929-
930. (d) Shimizu, K. D.; Dewey, T. M.; Rebek, Jr., J. J. Am. Chem. Soc.
1994, 116, 5145-5149.
The temperature-dependent spectral changes for the phenoxy-
substituted non-macrocyclic PBI 4 are shown in Figure 6a. Upon
J. Org. Chem, Vol. 72, No. 9, 2007 3409

Osswald et al.
these dyes. Furthermore, the epimers are present in equal
amounts as the integrated intensities of their NMR signals are
identical, implying that the chiral substituents do not have any
effect on the stability of (P)- and (M)-atropo-diastereomers.
Conclusion
Our results have shown that pure stereoisomers of perylene
bisimides with a (P) or (M) helically twisted perylene core can
be isolated if the interconversion process (M a P) is prevented
by appropriate bridging of the substituents in the 1,7- and 6,12-
positions as demonstrated here for PBI 6. Since the racemization
process for non-macrocyclic 1,6,7,12-tetraphenoxy-substituted
PBIs is fairly fast at ambient temperature, owing to an activation
energy of only about 54 kJ/mol (as estimated for open-chained
PBI 4 and laterally bridged macrocyclic PBI 7), diagonal
bridging through macrocyclization has been the method of
choice for the synthesis of atropisomerically pure perylene
bisimides. The availability of chiral, conformationally stable
perylene bisimide dyes could expand the possible application
of this very important class of fluorophores to enantioselective
recognition and other processes requiring chiral information.
1
FIGURE 8. Sections of the temperature-dependent H NMR spectra
of 7 (500.13 MHz) in CDCl3. Only the resonances of the perylene core
protons are shown.
cooling, the singlet observed for the perylene core protons at
room temperature (295 K) gets broadened and at temperatures
below 230 K it splits into eight signals, as in the case of 6.
Therefore, the signal pattern of 4 at low temperatures can again
be ascribed to the presence of two epimers combined with the
occurrence of two rotamers each as in the case of 6 (Figure
7a). In the signal pattern, two corresponding singlets for each
rotamer are found that are separated by 24 and 12 Hz, and the
chemical shift of each rotamer results from the mean value of
these two singlets. The chemical shifts of the epimers can be
assessed as the center of the calculated chemical shift of the
two respective rotamers (Figure 7) and are located at 8.22 and
8.17 ppm. Thus, the difference ∆υ in chemical shift of the
epimers, which is necessary for the calculation of ∆Gq, was
determined as 31.7 Hz (0.0528 ppm at 600 MHz). The
coalescence temperature for the epimerization process was
estimated to be 255 K from the line broadening. By using the
coalescence method,29 the barrier for the epimerization of 4 was
estimated to be 53 ( 3 kJ/mol. Additionally, based on the
resonances of the methoxy groups of 4 at 3.5 ppm (not shown
in spectra), the coalescence temperature was determined as 244
K, and with a ∆υ value of 7.7 Hz an epimerization barrier of
54 ( 3 kJ/mol was found.
Experimental Section
Synthesis and Characterization of Macrocyclic Perylene
Bisimides.
N,N′-Di(n-butyl)-1,6,7,12-tetra(3-methoxyphenoxy)perylene-
3,4:9,10-tetracarboxylic Acid Bisimide (2). N,N′-Di(n-butyl)-
1,6,7,12-tetrachloroperylene-3,4:9,10-tetracarboxylic acid bisimide
(1)16 (2.50 g, 3.90 mmol), 3-methoxyphenol (2.42 g, 19.5 mmol),
and dry potassium carbonate (1.35 g, 9.76 mmol) were suspended
under argon in 35 mL of NMP and heated for 42 h at 120 °C.
After being cooled to room temperature, the reaction mixture was
added dropwise to 150 mL of 0.66 M hydrochloric acid and stirred
for additional 2 h. The resulting precipitate was collected by
filtration and dried in vacuo. The crude product was chromato-
graphed on silica gel with dichloromethane/n-pentane (3/2) as the
eluent, dissolved in dichloromethane, precipitated by addition of
methanol, and dried in vacuo (10-3 mbar) at 60 °C to afford 1.40
g (1.41 mmol, 36%) of 2 as a red solid: mp 309-311 °C. 1H NMR
(400 MHz, CDCl3, TMS): δ 8.20 (s, 4H), 7.17 (t, J ) 8.2 Hz,
4H), 6.68 (ddd, J ) 8.3 , J ) 2.4, and J ) 0.9 Hz, 4H), 6.56 (ddd,
J ) 8.1, J ) 2.3, and J ) 0.8 Hz, 4H), 6.50 (t, J ) 2.3 Hz, 4H),
4.12 (t, J ) 7.5 Hz, 4H), 3.67 (s, 12H), 1.65 (m, 4H), 1.38 (m,
4H), 0.94 (t, J ) 7.4 Hz, 6H). MS (FAB, 3-nitrophenyl-
octylether) m/z: 990.3 [M]+ (calcd 990.3). Anal. Calcd for
C60H50N2O12: C, 72.72; H, 5.09; N, 2.83. Found: C, 72.32; H,
5.17; N, 2.75. UV/vis (CH2Cl2): λmax/nm (ꢀmax/M-1cm-1) ) 573
(43500), 536 (27600), 450 (16200), 289 (55900). Fluorescence
(CH2Cl2): λmax ) 599 nm, fluorescence quantum yield Φfl ) 0.89.
1,6,7,12-Tetra(3-methoxyphenoxy)perylene-3,4:9,10-tetracar-
boxylic Acid Bisanhydride (3). Perylene bisimide 2 (500 mg, 0.50
mmol) and potassium hydroxide (3.00 g, 53.5 mmol) were
suspended under argon in 25 mL of isopropanol and heated to reflux
for 2 h. After being cooled to room temperature, the reaction mixture
was dropped into 90 mL of glacial acetic acid and heated to 50 °C
for 30 min. The resulting red precipitate was collected by filtration,
washed with water (100 mL), and dried in vacuo (10-3 mbar) at
60 °C to give 396 mg (0.45 mmol) 3 in 90% yield. The product
was used without further purification. 1H NMR (400 MHz, CDCl3,
TMS): δ 8.20 (s, 4H), 7.20 (t, J ) 8.2 Hz, 4H), 6.73 (ddd, J )
8.4, J ) 2.4, and J ) 0.8 Hz, 4H), 6.57 (ddd, J ) 8.1, J ) 2.3, and
J ) 0.9 Hz, 4H), 6.49 (t, J ) 2.3 Hz, 4H), 3.68 (s, 12H). MS
(FAB, 3-nitrobenzylalcohol) m/z: 880.1 [M + H]+ (calcd 879.2).
Anal. Calcd for C52H32014: C, 70.91; H, 3.66. Found: C, 70.44;
H, 4.09.
Figure 8 displays the temperature-dependent 1H NMR spectral
changes for the core protons of the laterally bridged macrocycle
7 in CDCl3. Similar changes for the resonances of the perylene
core protons as in the case of 4 are observed upon cooling to
233 K, although eight individual signals could not be clearly
seen for 7, because upon cooling below 233 K further broaden-
ing of the signals occurred, which might be ascribed to an
additional dynamic process. In this case, the coalescence
temperature was estimated to be 256 K. Thus, compound 7 may
have a similar activation barrier as 4, although absolute values
cannot be given because the low-temperature spectra are not
properly resolved. The results of the temperature-dependent
NMR studies reveal that the barrier for the M a P racemization
in 4 and 7 is not significantly influenced by the presence of a
diethylene glycol linker, while the chiral centers in the imide
positions of perylene bisimide do not affect the dynamics of
(29) Hesse, M.; Meier, H.; Zeeh, B. Spectroscopic Methods in Organic
Chemistry; Thieme: Stuttgart, 1997; pp 88-100.
3410 J. Org. Chem., Vol. 72, No. 9, 2007

Perylene Bisimide Atropisomers
N,N′-Di(2-(R)-octyl)-1,6,7,12-tetra(3-methoxyphenoxy)perylene-
3,4:9,10-tetracarboxylic Acid Bisimide (4). Perylene bisanhydride
3 (0.90 g, 1.02 mmol), 2-(R)-octylamine (0.69 mL, 0.53 g, 4.09
mmol), and catalyic amounts of zinc acetate were dissolved under
argon in 25 mL of quinoline and heated for 8 h at 180 °C. The
reaction mixture was cooled to ambient conditions and slowly
dropped into 50 mL of 0.4 M hydrochloric acid. After addition of
30 mL of methanol, the resulting precipitate was collected by
vacuum filtration and dried in vacuo. Purification by silica gel
column chromatography with dichloromethane/n-hexane (4/1),
precipitation with methanol from dichloromethane solution, and
drying in vacuo (10-3 mbar) at 60 °C afforded 660 mg (0.60 mmol,
column chromatographic purification with dichloromethane as
eluent to yield 15.1 mg (12.7 µmol, 4%) of 6 as orange crystals
and 106 mg (0.09 mmol, 30%) of 7 as red solid.
1
Data for 6: mp 249-251 °C. H NMR (500 MHz, CDCl3): δ
8.24 (s, 4H), 7.25 (m, 4H), 7.15 (m 4H), 6.42 (m, 4H), 5.58 (m,
4H), 5.21 (m, 2H), 3.84-3.76 (m, 4H), 3.67-3.58 (m, 8H), 3.56-
3.49 (m, 4H), 2.23-2.11 (m, 2H), 1.91-1.79 (m, 2H), 1.54 (m,
6H), 1.37-1.18 (m, 16H), 0.83 (m, 6H). HRMS (ESI, dichlo-
romethane/acetonitrile ) 1:1): m/z calcd for C72H70N2O14Na [M
+ Na]+ 1209.4719, found 1209.4713 [M + Na]+. UV/vis (CH2-
Cl2): λmax/nm (ꢀmax/M-1cm-1) 532 (37600), 498 (25600), 460 (sh),
438 (sh), 279 (45600). Fluorescence: (CH2Cl2) λmax ) 570 nm,
fluorescence quantum yield Φfl ) 0.22, fluorescence lifetime τfl )
2.2 ns; (Et2O) λmax ) 560 nm, fluorescence quantum yield Φfl )
0.73, fluorescence lifetime τfl ) 6.2 ns.
1
58%) of 4 as a red solid: mp 279-281 °C. H NMR (400 MHz,
CDCl3, TMS): δ 8.18 (s, 4H), 7.17 (t, J ) 8.2 Hz, 4H), 6.67 (ddd,
J ) 8.3, J ) 2.5, and J ) 0.8 Hz, 4H), 6.57 (ddd, J ) 8.1, J ) 2.3,
and J ) 0.9 Hz, 4H), 6.50 (t, J ) 2.4 Hz, 4H), 5.18 (m, 2H), 3.67
(s, 12H), 2.11 (m, 2H), 1.82 (m, 2H), 1.49 (d, J ) 6.8 Hz, 6H),
1.23 (m, 16H), 0.82 (t, J ) 7.0 Hz, 6H). MS (MALDI-TOF, DCTB)
m/z: 1102.5 [M]+ (calcd 1102.5). Anal. Calcd for C68H66N2O12:
C, 74.03; H, 6.03; N, 2.54. Found: C, 73.99; H, 6.06; N, 2.50.
UV/vis (CH2Cl2): λmax/nm (ꢀmax/M-1cm-1) ) 568 (45100), 530
(28500), 445 (16500), 285 (57100). Fluorescence (CH2Cl2): λmax
) 603 nm, fluorescence quantum yield Φfl ) 0.94.
Data for 7: mp 317-320 °C. 1H NMR (400 MHz, CDCl3,
TMS): δ 8.29 (s, 4H), 7.07 (t, J ) 8.2 Hz, 4H), 6.68 (t, J ) 2.3
Hz, 4H), 6.59 (dd, J ) 8.2, and J ) 1.9 Hz, 4H), 6.54 (dd, J )
8.2, J ) 1.7 Hz, 4H), 5.19 (m, 2H), 4.20 (bs, 8H), 4.00-3.70 (m,
8H), 2.20-2.05 (m, 2H), 1.90-1.70 (m, 2H), 1.50 (d, J ) 6.8 Hz,
6H), 1.35-1.15 (m, 16H), 0.82 (t, J ) 6.8 Hz, 6H). HRMS (ESI,
dichloromethane/acetonitrile ) 1:1) m/z: calcd for C72H71N2O14
[M + H]+ 1187.4900, found 1187.4876. Anal. Calcd for C72H70-
N2O14: C, 72.83; H, 5.94; N, 2.36. Found: C, 72.77; H, 5.94; N,
2.36. UV/vis (CH2Cl2): λmax/nm (ꢀmax/M-1cm-1) 570 (47800), 530
(29100), 450 (16700), 286 (59600). Fluorescence (CH2Cl2): λmax
) 598 nm, fluorescence quantum yield Φfl ) 0.95, fluorescence
lifetime τfl ) 6.9 ns.
N,N′-Di(2-(R)-octyl)-1,6,7,12-tetra(3-hydroxyphenoxy)perylene-
3,4:9,10-tetracarboxylic Acid Bisimide (5). Perylene bisimide 4
(0.60 g, 0.54 mmol) was dissolved in 20 mL of dry dichloromethane
and cooled to 0 °C. Over a period of 30 min a solution of 0.83 mL
(2.18 g, 8.70 mmol) of boron tribromide in 40 mL of dry
dichloromethane was added dropwise and stirred at 0 °C for 1 h
and for an additional 3 h at room temperature. The solvent was
removed by distillation, and a mixture of water (48 mL) and
methanol (12 mL) was added to the residue under ice cooling and
treated for 30 min in a ultrasonic bath. The resulting precipitate
was collected by filtration, washed with water, and dried in vacuo.
Column chromatographic purification on silica gel with dichlo-
romethane/methanol (97/3) yielded 314 mg (0.30 mmol, 56%) of
5 as a wine-red solid: mp 311-315 °C. 1H NMR (400 MHz,
DMSO-d6): δ 9.68 (s, 4H), 7.88 (s, 4H), 7.16 (t, J ) 8.0 Hz, 4H),
6.62 (ddd, J ) 8.2, J ) 2.1, and J ) 0.8 Hz, 4H), 6.60-6.42 (m,
8H), 5.03 (m, 2H), 2.03 (m, 2H), 1.73 (m, 2H), 1.40 (d, J ) 7.0
Hz, 6H), 1.16 (m, 16H), 0.77 (t, J ) 6.9 Hz, 6H). MS (MALDI-
TOF, DCTB) m/z: 1046.3 [M]+ (calcd 1046.4). Anal. Calcd for
C64H58N2O12: C, 73.41; H, 5.36; N, 2.68. Found: C, 72.76; H,
5.58; N, 2.52.
Separation of Epimers (P,R,R)-(+)-6 and (M,R,R)-(-)-6).
Separation of the epimers of 6 was accomplished by HPLC on a
semipreparative chiral column (Trentec Reprosil 100 chiral-NR, Ø
) 2 cm) with isopropanol/n-hexane (1/1) as eluent (flow rate 10
mL/min). Owing to the low solubility of 6 in the eluent, a
dichloromethane solution of 6 was used for injection. The separation
was performed on a scale of 8 mg. (P,R,R)-(+)-6: retention time
(Trentec Reprosil 100 chiral-NR, Ø ) 0.8 cm, isopropanol/n-hexane
(1/1), flow 2 mL/min): 24.7 min; [R]20 (CH2Cl2, c ) 0.007)
D
+6600. CD (CH2Cl2) λmax/nm (∆ꢀ/M-1 cm-1) ) 536 (67), 499 (49),
425 (13), 337 (-13), 292 (113). (M,R,R)-(-)-6: retention time
(Trentec Reprosil 100 chiral-NR, Ø ) 0.8 cm, isopropanol/n-hexane
(1/1), flow 2 mL/min): 29.6 min; [R]20D (CH2Cl2, c ) 0.006) -6600.
CD (CH2Cl2) λmax/nm (∆ꢀ/M-1 cm-1) ) 536 (-63), 499 (-45),
425 (-6), 337 (21), 292 (-97).
Macrocyclic Perylene Bisimides 6 and 7. Perylene bisimide 5
(300 mg, 0.29 mmol), diethylene glycol ditosylate (712 mg, 1.72
mmol), and cesium carbonate (1.23 g, 3.44 mmol) were suspended
under argon in 45 mL of dry DMSO and heated for 6 h at 120 °C.
The reaction mixture was cooled to room temperature and dropped
slowly into 75 mL of 0.33 M hydrochloric acid under stirring. The
resulting precipitate was collected by vacuum filtration and dried
in vacuo. The two regioisomeric macrocycles formed in the reaction
were separated by column chromatography with dichloromethane
as the eluent, and each of the fractions was subjected to an additional
Acknowledgment. Financial support of our work by the
Fonds der Chemischen Industrie is greatly appreciated. We thank
Mrs. M.-L. Scha¨fer and Mrs. E. Ruckdeschel for their help in
1
measuring temperature-dependent H NMR spectra.
Supporting Information Available: General experimental
1
methods and H NMR spectra of 2-6. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO070056C
J. Org. Chem, Vol. 72, No. 9, 2007 3411