Perylene bisimides with rigid 2,2′-Biphenol bridges at bay area as conjugated chiral platforms

Article abstract of DOI:10.1021/ol1011482

(Figure Presented) Facile nucleophilic substitution of two chlorine atoms by 2,2′-biphenol at one of the two bay areas (1,12- and 6,7-positions) of core-tetrachlorinated perylene bisimide afforded a novel, completely desymmetrized perylene bisimide building block, which could be further functionalized by substitution of the remaining two chlorine atoms. The atropisomers (P- and M-enantiomers) of the core twisted perylene bisimides were resolved by HPLC on a chiral column at room temperature, and the activation parameters for racemization were elucidated.

Full text of DOI:10.1021/ol1011482

ORGANIC
LETTERS
2010
Vol. 12, No. 14
3204-3207
Perylene Bisimides with Rigid
2,2′-Biphenol Bridges at Bay Area as
Conjugated Chiral Platforms
Zengqi Xie and Frank Wu¨rthner*
UniVersita¨t Wu¨rzburg, Institut fu¨r Organische Chemie and Ro¨ntgen Research Center
for Complex Material Systems, Am Hubland, 97074 Wu¨rzburg, Germany
wuerthner@chemie.uni-wuerzburg.de
Received May 19, 2010
ABSTRACT
Facile nucleophilic substitution of two chlorine atoms by 2,2′-biphenol at one of the two bay areas (1,12- and 6,7-positions) of core-tetrachlorinated
perylene bisimide afforded a novel, completely desymmetrized perylene bisimide building block, which could be further functionalized by
substitution of the remaining two chlorine atoms. The atropisomers (P- and M-enantiomers) of the core twisted perylene bisimides were
resolved by HPLC on a chiral column at room temperature, and the activation parameters for racemization were elucidated.
Natural chiral molecules such as nucleic acids, amino acids,
and sugars are of vital importance for biological systems.
Chiral compounds are also of great significance for chemical
processes and materials science as they are widely applied
in numerous fields such as chiral recognition1 and self-
assembly,2 enantioselective catalysis,3 nonlinear optics,4 and
as molecular switches.5 In recent years, conjugated chiral
systems have gained much attention in the field of organic
materials.6 Most prominent examples of chiral compounds
with inherent chirality used in this context are biphenyl7 and
binaphthyl8 derivatives, annelated phenylenes,6 and helicenes
and helicene-like systems.9
Perylene bisimides (PBIs) with functional substituents at the
carbocyclic scaffold in the bay areas have attracted a great deal
of attention in the past years owing to their outstanding optical,
electronic, and light fastness properties and their high solubility
in common organic solvents.10 Recently, Wang et al. signifi-
cantly extended the concept of bay functionalization by
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Bunz, U. H. F., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim,
Germany, 2007; Chapter 15, pp 547-581. (b) Rajca, A.; Safronov, A.;
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Adima, A.; Moreau, J. J. E.; Wong Chi Man, M. J. Mater. Chem. 1997, 7,
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10.1021/ol1011482  2010 American Chemical Society
Published on Web 06/18/2010

synthesizing fully conjugated triply linked di- and tri-PBIs11
and also monobay-dichlorinated PBIs.12
sibility for further functionalization on the other bay area, which
makes it a promising conjugated chiral platform.
PBI 2 with a 2,2′-biphenoxy bridging unit at one bay area
was synthesized directly from tetrachloro-substituted PBI 1
and 2,2′-biphenol (mole ratio 1:1) in 57% yield as outlined
in Scheme 1. The further nucleophilic substitution of the
remaining two chlorine atoms in PBI 2 with excess amount
of 2,2′-biphenol afforded PBI 3 with two biphenoxy bridges.
Different phenoxyl groups with functional substituents may
be introduced to PBI 2 by nucleophilic substitution of the
two chlorine atoms to form PBIs with unsymmetrical
substitution pattern on the two bay areas. The proof-of-the-
concept example of the unsymmetrical bay-substituted PBI
4 was synthesized in order to compare the optical properties
of the PBIs with different rigidity.
The introduction of bay substituents endows the perylene
core with a twisted π-system. It has been recognized already
some years ago that such twisted PBI fluorophores might
possess interesting chiroptical properties.13 However, it is
usually difficult to isolate enantiomerically pure PBIs due
to the low barriers for the interconversion of the two core-
twisted atropisomers in solution. The kinetic racemization
process could be, in principle, prevented by increasing the
size of the sterically demanding bay substituents.14 Another
strategy that was developed in our group to restrict the
dynamic racemization process of tetraphenoxy-substituted
perylene bismides is based on the bridging of phenoxy
substituents at the bay positions with oligo(ethylene glycol)
chains through macrocyclization.15 However, this strategy
suffers from the inherent difficulty of further chemical
functionalization of the chiral PBI unit.
Compared with the tetrachloro-substituted PBI 1, its
monocyclization to PBI 2 with a 2,2′-biphenoxy bridge at
one bay area might introduce more rigidity, which would
make the interconversion of the twisted conformers (P and
M) less facile. Indeed, the enantiomers of PBI 2 are separable
by HPLC on a chiral colunm (Reprosil 100 chiral-NR) at
room temperature using dichloromethane as the eluent (see
Figure S7 in Supporting Information). The configurations
of the first and the second eluted atropisomers (retention
times 4.0 and 5.3 min) are assigned as P and M, respectively,
by comparing their circular dichroism (CD) spectra with
those of previously reported atropisomerically pure macro-
cyclic PBIs (details are given in following sections).16
The molecular geometry of PBI 2 was optimized with
HyperChem using a semiempirical method (AM1), and the
results are shown in Figure 1. As can be seen in the geometry
optimized structures, one phenyl ring of the biphenyl group is
situated above the perylene core, while the other one extends
to the side of the perylene core.17 Accordingly, the molecular
structure loses symmetry elements owing to the different
substituents on two bay areas and the twisted configuration of
the biphenyl unit relative to the perylene core. This structual
Herein we present a new approach to configurationally stable
chiral PBIs, e.g., PBI 2, bearing reactive substituents at one of
the two bay areas for further functionalization. This was
achieved by introduction of a rigid 2,2′-biphenol bridge at one
bay area. The two adjacent phenoxyl groups are connected
directly at ortho positions by a covalent C-C bond, which not
only introduces an asymmetric structure at the bay area but also
imparts rigidity to the perylene core. The rigid and twisted
π-system achieved by this approach facilitates the separation
of the atropo-enantiomers, and thus investigation of their
chiroptical properties and elucidation of activation parameters
for the racemization. The most attractive feature of this
unprecedented chiral monobiphenyl-bridged PBI is the pos-
(7) (a) Omote, M.; Nishimura, Y.; Sato, K.; Ando, A.; Kumadaki, I.
Tetrahedron 2006, 62, 1886. (b) Ceccacci, F.; Mancini, G.; Sferrazza, A.;
Villani, C. J. Am. Chem. Soc. 2005, 127, 13762. (c) Bringmann, G.; Price
Mortimer, A. J.; Keller, P. A.; Gresser, M. J.; Garner, J.; Breuning, M.
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2005, 70, 6164. (b) van Delden, R. A.; Mecca, T.; Rosini, C.; Feringa,
B. L. Chem.sEur. J. 2004, 10, 61. (c) Ishow, E.; Credi, A.; Balzani, V.;
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1
feature is supported by H, HH-COSY and 13C NMR spectra
of PBI 2 (see Supporting Information). The similar structural
information was also obtained for PBI 4. In the case of bis-
biphenoxy-substituted PBI 3, two diastereomers are expected
as the two biphenyl groups may locate at the same face (denoted
as minor-3) or two different faces (denoted as major-3) of the
perylene core. The 1H NMR spectra and semiempirical calcula-
tions indeed support the coexistence of the two diastereomers
of PBI 3 (for details, see Supporting Information).
The atropo-enantiomers of PBI 2 were successfully
resolved by HPLC using a semipreparative chiral column at
room temperature. The CD spectra of the isolated enanti-
omers of PBI 2 show a clear mirror image relation as
depicted in Figure 2 (bottom). In the region of 460-570 nm
of the CD spectra, a broad monosignated peak with a
maximum at 539 nm can be seen that nicely correlates with
the absorption maximum at 545 nm (Figure 2, top). In the
´
Carren˜o, M. C.; Enríquez, A.; Garc´ıa-Cerrada, S.; Sanz-Cuesta, M. J.;
Urbano, A.; Maseras, F.; Nonell-Canals, A. Chem.sEur. J. 2008, 14, 603.
(c) Wigglesworth, T. J.; Sud, D.; Norsten, T. B.; Lekhi, V. S.; Branda,
N. R. J. Am. Chem. Soc. 2005, 127, 7272. (d) Rajca, A.; Miyasaka, M.;
Pink, M.; Wang, H.; Rajca, S. J. Am. Chem. Soc. 2004, 126, 15211. For a
review, see: (e) Amemiya, R.; Yamaguchi, M. Org. Biomol. Chem. 2008,
6, 26.
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A. C.; Mu¨llen, K. Angew. Chem., Int. Ed. 2005, 44, 5592.
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129, 10664. (b) Shi, Y.; Qian, H.; Li, Y.; Yue, W.; Wang, Z. Org. Lett.
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Soc. 2008, 130, 17970. (d) Qian, H.; Yue, W.; Zhen, Y.; Motta, S. D.;
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Germany, 1995.
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Org. Lett., Vol. 12, No. 14, 2010
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Scheme 1.
Synthesis of PBIs 2-4, Chiral Resolution of the Atropisomers of 2, and Structure of Reference PBI 5a
a R ) n-C4H9; R′ ) 2,6-di(i-Pr)Ph; NMP ) 1-methyl-2-pyrrolidone.
Figure 1. Geometry optimized structues of PBI 2 calculated with
HyperChem using a semiempirical method (AM1). The alkyl chains
and the protons are omitted for simplicity. Left: view from the top
of the molecule. Right: view along the N-N direction. Only the
M-enantiomer is shown.
Figure 2. UV-vis absorption (top, identical for P-2 and M-2) and
CD spectra (bottom) of P-2 (solid red line) and M-2 (dotted red
line) enantiomers of PBI 2 in dichloromethane at 20 °C.
CD spectral region of 460-570 nm of the enantiomers, the
S0-S1 transition is located whose transition dipole moment
is polarized along the long axis (N-N axis) of the PBI. On
the basis of previously reported stereochemical assignment
of atropisomerically pure macrocyclic PBI systems,16 the
region of 375-460 nm of the CD spectra, an unsymmetric
bisignated signal was observed with a negative first Cotton
effect at a longer wavelength and a positive second one at a
shorter wavelength for the first eluted enantiomer P-2 and
vice versa for the second eluted enantiomer M-2. In both
cases, the peak maxima of the first Cotton effects are located
at 440 nm and those of the second ones are found at about
405 nm with a crossover point at 425 nm. This zero transition
corresponds well to the absorption band of the S0-S2
transition. Similar Cotton effects corresponding to the S0-S2
transition were also observed for 1,6,7,12-tetrachloro- and
tetrabromo-substituted PBIs previously.14
positive signal for the longest wavelength transition (λmax
)
539 nm) in the CD spectrum of the first eluted fraction of 2
can be attributed to a right-handed helical configuration of
the twisted perylene core, and thus this enantiomer can be
assigned to P-2. Accordingly, the second eluted fraction with
a negative Cotton effect for the longest wavelength transition
can be assigned to M-2.
On the other hand, for the S0-S2 transition (the transition
dipole is polarized along the short axis of the PBI) in the
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Org. Lett., Vol. 12, No. 14, 2010

The optical properties of the PBIs 3 and 4 and the reference
PBI 5 were investigated by UV-vis and fluorescence spec-
troscopy. The UV-vis absorption spectra (Figure 3a) of PBIs
order to determine the activation parameters for the racemization
of PBI 2, time-dependent HPLC measurements were performed
at different temperatures in the range of 318-333 K (see
Supporting Information). The free activation enthalpy ∆Gq323K
for the racemization process of PBI 2 was determined to be
98.5 ( 0.1 kJ mol-1 according to the Eyring equation, which
is higher than that of the previously reported 1,6,7,12-tetra-
chloro-substituted PBI.14 Table 2 collects the ∆Gq values for
Table 2. Free Enthalpy of Activation for the Racemization of
PBIs
perylene bisimide
∆Gq /kJ mol-1
method
1,6,7,12-tetrachloroa
2
97.8 ( 0.1
98.5 ( 0.1
99.1 ( 0.1
98.2 ( 0.1
60 ( 3
by CD at 303 K14
by HPLC at 323 K
by HPLC at 333 K
by HPLC at 323 K
by dynamic NMR14
3
4
1,6,7,12-tetraphenoxya
a For structure, see reference 14.
Figure 3. (a) UV-vis absorption and (b) normalized emission
the racemization of PBIs with different substituents at bay area.
The comparison of the ∆Gq values for five differently bay-
substituted PBIs reveals a dependence of the free enthalpy of
activation for the racemization process on the bay-area func-
tionalization, which is quite evident in the case of PBI 4 and a
previously reported 1,6,7,12-tetraphenoxy-substituted PBI.14 The
main structural difference between these two PBIs is that in
the former two phenoxyl groups at one bay area are intercon-
nected by a covalent C-C bond, which increases the free
activation enthalpy of the racemization dramatically from 60
( 3 kJ mol-1 (for 1,6,7,12-tetraphenoxy PBI)14 to 98.2 ( 0.1
kJ mol-1 (for PBI 4). The influence of the substituents at the
other bay area on the free activation energy of racemization is
rather small (compare values for PBI 2 and 4), which indicates
that indeed the 2,2′-biphenoxy linkage governs the stability of
this chiral platform with regard to racemization.
spectra of PBIs 3 (blue lines), 4 (red lines), and 5 (dotted lines) in
dichloromethane at room temperature.
3 and 4 with mono- or dicyclization at bay area show
hypsochromic shifts of the absorption maxima of the S0-S1
transition (∼550 nm) by 20 and 12 nm, respectively, in
comparison to that of PBI 5, accompanied by pronouncedly
narrowed absorption bands, indicating an increase of the rigidity
of the perylene cores of PBIs 3 and 4. Similar conclusions can
be drawn from the fluorescence spectra. As shown in Figure
3b, fluorescence spectra of PBIs 3 and 4 show hypsochromic
shifts with enhanced vibronic fine structure compared to that
of PBI 5. Moreover, the Stokes shifts of PBIs 3 and 4 in
dichloromethane are much smaller than that of PBI 5 (Table
1), revealing again perylene cores of 3 and 4 more rigid than
In conclusion, we have shown that a simple nucleophilic
substitution reaction of tetrachloroperylene bisimide with
2,2′-biphenol at one of the two bay areas affords a novel
chiral PBI building block 2 with unprecedented features. PBI
2 is unsymmetric, bearing a rigid biphenoxy group at one
and chlorine substituents at the other bay area. The high
configurational stability imparted by the rigid 2,2′-biphenoxy
bridge enabled chiral resolution of the atropo-enantiomers
of this core-twisted PBI. Further functionalization of PBI 2
has been demonstrated by nucleophilic displacement of the
two chlorine substituents to give PBIs 3 and 4. The free
activation enthalpies of these PBIs are sufficiently high to
consider applications of these and related conjugated asym-
metric platforms in chiral recognition and catalysis.
Table 1. Optical Properties of PBIs 3 and 4 and Reference 5 in
Dichloromethane at Room Temperature
λmax (abs)
(nm)
εmax
(M-1 cm-1
λmax (em)
(nm)
Stokes
shift (nm) ΦPL (%)
)
3
4
5
557
565
577
48 800
48 500
49 900
567
586
606
10
21
28
95.4
99.8
96.0
that of PBI 5, which is indicative of smaller change of the
geometry between the ground and the excited states of 3 and
4. Both PBIs 3 and 4 show very high fluorescent quantum yields
of >95%.
Acknowledgment. We thank the Alexander von Humboldt
Foundation for a fellowship for Z.X.
The perfect baseline separation of the atropo-enantiomers of
PBIs 2-4 by HPLC (chromatogram of PBI 2 is shown in Figure
S7 in Supporting Information as an example) enables accurate
determination of the mole ratio of enantiomers of a PBI by
intergration of the respective chromatographic peak areas. In
Supporting Information Available: Experimental details
and characterization of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
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