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Perylene Bisimide Based Macrocycles Effective Probes for the Assessment of Conformational Effects on Optical Properties.

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Communications
Macrocyclic Perylene Bisimides
Perylene Bisimide Based Macrocycles: Effective
Probes for the Assessment of Conformational
Effects on Optical Properties**
Peter Osswald, Dirk Leusser, Dietmar Stalke, and
Frank Wrthner*
Scheme 1. Schematic presentation of diagonally bridged (A) and laterally bridged (B) macrocyclic perylene bisimides.
Dedicated to Professor Johann Weis
on the occasion of his 60th birthday
During the last few years, perylene bisimides bearing
phenoxy substituents in the bay positions have been
developed to become one of the most useful classes of
fluorophores. They can be diversely functionalized at
the imide and the bay phenoxy units to achieve
desired optical and electrochemical properties.[1]
Indeed, in multichromophoric dendritic perylene
bisimides[2] as well as in self-organized metallosupramolecular squares[3] or mesoscopic assemblies[4] highly
efficient energy- and electron-transfer processes have
been demonstrated—such supramolecular arrays
based on phenoxy-substituted perylene bisimides
(PPBIs) may serve as artificial light-harvesting systems.[5] Furthermore, PPBIs show potential for application in solar cells,[4b, 6] polymeric light-emitting
diodes,[7] and as molecular probes in biological systems.[8] Despite this remarkable versatility of PPBIs,
the effect of different conformations on the optical
properties of this class of fluorophores has been barely
explored. Single-molecule spectroscopic studies
revealed that different conformations—that is, the
twist of the perylene bisimide chromophore and the
orientation of the phenoxy groups—have an effect on
the photophysical properties of immobilized PPBIs.[9]
Nevertheless, conformational effects on the optical
properties of perylene bisimides in solution have not
been reported yet.[1b]
To identify the conformational effect on the
optical properties of PPBI chromophores, conformationally restricted macrocyclic perylene bisimides,
which are diagonally bridged through 1,7- and 6,12linkages (Scheme 1, A) seem to be properly suited
Scheme 2. a) H3C-(OCH2CH2)3-O-p-Tos, Cs2CO3, DMSO, argon, 80 8C, 5 h;
b) p-Tos-O-CH2CH2O-(CH2CH2O)n-p-Tos (n = 0–2), Cs2CO3, DMSO, argon,
100 8C, 5 h. Tos = tosyl, DMSO = dimethyl sulfoxide.
[*] P. Osswald, Prof. Dr. F. Wrthner
Institut fr Organische Chemie
Bayerische Julius-Maximilians-Universitt Wrzburg
Am Hubland, 97074 Wrzburg (Germany)
Fax: (+ 49) 931-888-4756
E-mail: wuerthner@chemie.uni-wuerzburg.de
Dr. D. Leusser, Prof. Dr. D. Stalke
Institut fr Anorganische Chemie
Bayerische Julius-Maximilians-Universitt Wrzburg
Am Hubland, 97074 Wrzburg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
250
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
since the conformational freedom of the perylene core should
be restricted in these systems and a particular conformation of
the phenoxy substituents can be achieved through variation of
the length of the bridging unit. Oligo(ethylene glycol) units
were chosen as the bridging unit because, on the one hand,
this functionality can be introduced in perylene bisimide by
etherification of the resorcin groups in the bay positions and,
on the other hand, the chain length can be varied through the
appropriate choice of ditosylates. Herein we present the
synthesis (Scheme 2) of diagonally bridged macrocyclic
perylene bisimides 3 a–c as well as the corresponding laterally
bridged isomers 4 a–c, and report on their optical properties in
comparison to those of the open-chain reference compound 2,
which confirm a pronounced conformational effect on the
optical properties of perylene bisimides.
DOI: 10.1002/anie.200461585
Angew. Chem. Int. Ed. 2005, 44, 250 –253
Angewandte
Chemie
As shown in Scheme 2, compound 2 was synthesized by
etherification of 1 with 2-[2-(2-methoxyethoxy)ethoxy]ethyl
tosylate and cesium carbonate in DMSO at 80 8C in 28 %
yield. The macrocyclic perylene bisimides 3 a–c were synthesized by employing batch cyclization as a convenient one-pot
macrocyclization method with a perylene bisimide concentration of 6 103 m. [10] For this purpose 1 was treated with the
corresponding oligo(ethylene glycol) ditosylates and cesium
carbonate in DMSO at 100 8C. After purification by column
chromatography the diagonally bridged macrocyclic perylene
bisimides 3 were isolated in 1–5 % yield and the laterally
bridged isomers 4 were obtained in 5–15 % yield for mono-,
di-, and tri(ethylene glycol) chains.[11]
The presence of four resorcin units in the precursor
bisimide 1 causes formation of isomeric bis(macrocycles) by
etherification. Only based on spectroscopic data, in particular
1
H NMR data, reliable assignment of the isolated macrocycles
to a particular isomer is not possible. Therefore, crystallization of the macrocycles has been carried out and single
crystals suitable for X-ray diffraction were obtained in the
case of 3 b and 4 a by dissolving the compounds in dichloromethane, subsequent addition of equal amounts of methanol,
and slow evaporation of dichloromethane at room temperature.
Figure 1 shows the molecular structures of 3 b and 4 a in
the crystal.[12] Compound 3 b possesses the envisioned macrocyclic structure with the bridging units lying above and below
Figure 1. Molecular structures of 3 b (top, P enantiomer) and 4 a
(bottom, P enantiomer). View along the N–N axis of the perylene bisimide showing the twisted nature and the cyclic structure of both isomers. Note that both centrosymmetric crystal structures contain P and
M enantiomers (for crystal packing see the Supporting Information).
Angew. Chem. Int. Ed. 2005, 44, 250 –253
the perylene core. A further characteristic feature of this
diagonally bridged isomer is an almost perpendicular orientation of the phenoxy substituents relative to the perylene
core which resembles a conformation suggested based on
molecular modeling studies of related noncyclic compounds.[9]
For the macrocyclic systems 3 an interconversion between the
conformational enantiomers (MQP) as observed for nonrigid
twisted perylene bisimides[13] is not possible because in the
case of 3 a bond cleavage is required. Thus, these systems
possess favorable structural features to assess the conformational effect of the bay phenoxy substituents on the optical
properties of this class of chromophores. The twist angle
between the two naphthalene units of the perylene core of 3 b
was determined to 338 and is thus higher than that known for
a phenoxy-substituted diazadibenzoperylene derivative,[14]
which underlines the rigid character of these isomers.
The molecular structure of 4 a shows that the phenoxy
groups are connected laterally through the ethylene glycol
groups and that the twist angle between the two naphthalene
units is 258. Whereas the phenoxy groups in 3 b are
orthogonally orientated, in 4 a, they are horizontally orientated to the perylene core.
Although single crystals could not be obtained for all
compounds, the structures of 3 a,c and 4 b,c could be
unequivocally assigned by comparison of their 1H NMR
data with those of 3 b and 4 a. For this purpose, the resonance
of the proton situated between the two oxygen atoms of the
resorcin substituents is particularly informative, because this
proton is differently shielded by the aromatic ring current in
lateral and diagonal isomers (see the Supporting Information).
The optical properties of the macrocyclic perylene
bisimides were investigated by UV/Vis, steady-state, and
time-resolved fluorescence spectroscopy. The absorption
spectra of laterally bridged macrocycles 4 (Figure 2) show
typical patterns for PPBIs with a maximum around 580 nm
assigned to the S0 !S1 transition, a shoulder of the second
vibronic transition, and a broad maximum at 450 nm belonging to the S0 !S2 transition. For the derivatives 4 b and 4 c, the
absorption maxima as well as the molar absorption coefficients are in good agreement with that of the reference
compound 2; thus, 2 may possess a similar conformation to
that of the lateral isomers. Only the smallest macrocycle 4 a
Figure 2. Absorption spectra of 4 a (dashed line), 4 c (dotted line), and
the reference compound 2 (solid line) in dichloromethane.
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
251
Communications
adopts an exceptional position because the maximum of the
S0 !S1 transition is hypsochromically shifted by 20 nm
compared to that of 4 b and a decrease of the molar
absorption coefficient is observed. This shift is presumably
due to the conformational restriction imposed by the short
spacer units.
For the diagonally bridged macrocycles 3 significant
spectral differences were observed compared to those of the
reference compound 2. The absorption maxima of the S0 !S1
transition are shifted up to 41 nm towards shorter wavelength
from 575 nm (2) to 545 nm (3 a,c) and 534 nm (3 b) (Figure 3).
3 a), whereas the radiative rate constants kr are almost identical
for all compounds presented here.
Based on the fact that the spectroscopic data of the
diagonally bridged isomers 3 differ significantly from those of
the reference compound 2, it can be concluded that nonmacrocyclic PPBIs do not exhibit a perpendicular orientation
of the phenoxy residues in solution. Instead, based on the
observed optical properties of the macrocyclic perylene
bisimides 4, which are similar to the non-macrocyclic
reference 2, a conformation with horizontal orientation of
the phenoxy groups is most likely for non-macrocyclic PPBI
dyes such as 2 in solution. These results establish for the first
time the conformational effect on the optical properties of
these chromphores in solution. The question to what extent
the charge-density distribution and, hence, the optical properties of the perylene core correlate to the various conformations will further be investigated. The results of the current
study will be of major impact to the development of improved
molecular probes for biological systems,[8] applications of
these dyes in single-molecule spectroscopy,[9] and the development of PPBI based fluorescence sensors.[16] In this context
the isolation of enantiopure macrocyclic dyes 3 a–c is one of
our particular goals.
Received: August 9, 2004
Figure 3. Absorption spectra of 3 a (dotted line), 3 c (dashed line), and
the reference compound 2 (solid line) in dichloromethane.
This hypsochromic shift and the observed pronounced
hypochromicity may be attributed to the formation of a
conformer with less conjugation between the phenoxy groups
(+M effect) and the electron-deficient perylene bisimide.[15]
The fluorescence properties of the reference compound 2,
diagonal isomers 3, and lateral isomers 4 are given in Table 1
(emission spectra are shown in the Supporting Information).
As the data reveal, fluorescence quantum yields and lifetimes
of the lateral isomers 4 a–c are indistinguishable with that of
the reference compound 2, which again indicates that the
behavior of these isomers is similar to that of the open-chain
PPBI derivatives. In contrast to the situation with the lateral
isomers 4, the fluorescence quantum yields of the diagonally
bridged macrocycles 3 are much lower than that of the
reference compound 2. This effect is clearly related to an
increase of the nonradiative rate constants knr (2 < 3 c < 3 b <
Table 1: Emission properties of macrocyclic perylene bisimides in
dichloromethane.
lmax [nm]
ffl[a]
tfl [ns][b]
kr [s1][c]
knr [s1][d]
2
609
0.89
5.6
1.6 108
2.0 107
3a
3b
3c
562
573
582
< 0.02
0.09
0.43
< 0.5
0.8
2.7
> 4.0 107
1.1 108
1.6 108
> 2.0 109
1.1 109
2.1 108
4a
4b
4c
588
605
606
0.78
0.82
0.84
5.4
5.3
5.6
1.4 108
1.6 108
1.5 108
4.1 107
3.4 107
2.9 107
[a] ffl 0.03. [b] t 0.5. [c] kr = fflt1. [d] knr = (1ffl)t1.
252
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
.
Keywords: conformational effects · dyes/pigments ·
fluorescent probes · macrocycles
[1] For reviews on perylene bisimides see: a) H. Langhals, Heterocycles 1995, 40, 477 – 500; b) F. Wrthner, Chem. Commun. 2004,
1564 – 1579.
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De Schryver, K. Mllen, Chem. Eur. J. 2004, 10, 528 – 537.
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Eur. J. 2001, 7, 894 – 902; b) F. Wrthner, A. Sautter, Org.
Biomol. Chem. 2003, 1, 240 – 243; c) C.-C. You, F. Wrthner, J.
Am. Chem. Soc. 2003, 125, 9716 – 9725; d) B. K. Kaletas, R.
Dobrawa, A. Sautter, F. Wrthner, M. Zimine, L. De Cola,
R. M. Williams, J. Phys. Chem. A 2004, 108, 1900 – 1909.
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[5] M. J. Ahrens, L. E. Sinks, B. Rybtchinski, W. Liu, B. A. Jones,
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b) A. P. H. J. Schenning, J. van Herrikhuyzen, P. Jonkheijm, Z.
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[7] C. Ego, D. Marsitzky, S. Becker, J. Zhang, A. C. Grimsdale, K.
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Angew. Chem. Int. Ed. 2005, 44, 250 –253
Angewandte
Chemie
[8] a) J. Qu, C. Kohl, M. Pottek, K. Mllen, Angew. Chem. 2004, 116,
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[9] J. Hofkens, T. Vosch, M. Maus, F. Khn, M. Cotlet, T. Weil, A.
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[10] U. Lning, M. Mller, M. Gelbert, K. Peters, H. G. von Schnering, M. Keller, Chem. Ber. 1994, 127, 2297 – 2306.
[11] The compounds have been characterized by NMR spectroscopy
and MALDI-TOF mass spectrometry. Synthetic details, spectroscopic and analytical data will be published elsewhere.
[12] Details of the X-ray structure determination of compounds 3 b
and 4 a are given in the Supporting Information. CCDC-246746
and CCDC-246747 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336-033; or deposit@
ccdc.cam.ac.uk)..
[13] Z. Chen, M. G. Debije, T. Debaerdemaeker, P. Osswald, F.
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[14] F. Wrthner, A. Sautter, J. Schilling, J. Org. Chem. 2002, 67,
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[15] Electron-donating substituents cause a bathochromic shift of the
S0 !S1 transition in perylene bisimide dyes (see Table 1 in
ref. [1 b]).
[16] L. Zang, R. Liu, M. W. Holman, K. T. Nguyen, D. M. Adams, J.
Am. Chem. Soc. 2002, 124, 10 640 – 10 641.
Angew. Chem. Int. Ed. 2005, 44, 250 –253
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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