Energy transfer in multichromophoric self-assembled molecular squares

Article abstract of DOI:10.1039/b208582h

Metal-ion directed self-assembly afforded well-defined molecular squares with a mass of 12 kDa incorporating four perylene bisimide and sixteen pyrene chromophores organized in a circular way. Energy transfer from the pyrene to the perylene dye manifold is investigated by emission and excitation spectroscopy.

Full text of DOI:10.1039/b208582h

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Energy transfer in multichromophoric self-assembled molecular
squares†
Frank Würthner*‡ and Armin Sautter
Abteilung Organische Chemie II, Universität Ulm, Albert-Einstein-Allee 11, D-89081 Ulm,
Germany
Received 3rd September 2002, Accepted 5th November 2002
First published as an Advance Article on the web 9th December 2002
Metal-ion directed self-assembly afforded well-defined
molecular squares with a mass of 12 kDa incorporating
four perylene bisimide and sixteen pyrene chromophores
organized in a circular way. Energy transfer from the
pyrene to the perylene dye manifold is investigated by
emission and excitation spectroscopy.
One of the most fascinating scientific topics is the conversion
of sunlight into chemical energy in natural photosynthesis.1
Chemistry can contribute to this field of research in two ways,
one is by synthesis and spectroscopic characterization of model
systems for the natural counterparts,2 another is by application
of the acquired knowledge to derive ‘biomimetic’ photovoltaic
devices.3 In nature, two subsystems are required to achieve
efficient photosynthesis which consist of the well-characterized
reaction centers and the much larger light harvesting systems
(LHS) that incorporate up to hundreds of dyes. Depending on
the respective species and their natural inhabitation different
kinds of chromophores and principles of organization (e.g.
rods, tubes, rings) are found in the LHS but a precise position-
ing of the dyes in space is always provided by non-covalent
interactions to proteins or between the dyes. In the last years
chemists could realize extended dye aggregates4 that are con-
sidered to be reasonable models for the rod-like LHS of green
bacteria as well as multichromophoric dendrimers5 which
exhibit a spatial distribution of the dyes that resembles the LHS
of green plants. On the other hand, synthetic analogues to the
structurally and functionally most intriguing cyclic LHS of
purple bacteria are still lacking.6 These LHS consist of two
types of cyclic arrays of bacteriochlorophyll (BChl) dyes which
are organized in space through non-covalent interactions with
proteins. The larger LHS I contain a single ring of ∼30 strongly
coupled BChls and accommodate the reaction center of photo-
synthesis in its interior. The smaller peripheral LHS II contain a
total of 24 BChls that are distributed in two rings, a strongly
coupled one of aggregated dyes and a weakly coupled one of
isolated dyes around an eightfold symmetry axis.7
Fig. 1 Formation of the natural circular dye arrays in LHS II of
purple bacteria (top)1 and of artificial square-type dye arrays by self-
assembly (bottom). The yellow and red colors indicate dye molecules.
The blue color indicates proteins (top) or metal ions (bottom) that
direct the self-assembly process and define the spatial arrangement of
the dye molecules.
required which exhibit high fluorescence quantum yields, a
good spectral overlap of the absorption spectrum of the energy
acceptor dye with the fluorescence band of the energy donor
dye and specific absorption bands in the two dyes that can be
excited individually. The first two properties are a prerequisite
for an efficient through-space resonance energy transfer
(Förster mechanism)14 and the latter condition is important for
an easy evaluation of the energy transfer efficiency. Taking
these facts into consideration our first multichromophoric
molecular square was synthesized according to Scheme 1 from
tetra-p-butoxyphenoxyperylene bisanhydride 115 by imidization
with 4-aminopyridine,12 subsequent BBr3-promoted cleavage of
the butyl ether groups, and esterification of the phenolic groups
with pyrene butyric acid through activation of the carboxylic
acid group by dicyclohexylcarbodiimide (DCC) in the presence
of dimethylaminopyridinium toluenesulfonate (DPTS).16 All
reactions proceeded smoothly yielding analytically pure 4 in an
overall yield of 21% based on 1.§ Finally, equimolar mixing of
4 and [Pt(dppp)][(OTf )2] in CH2Cl2 at room temperature led to
a quantitative conversion to molecular square 5 which could be
isolated in 67% yield as a fine powder after precipitation by
addition of diethylether.
In our recent work we applied the concept of metal-directed
self-assembly8–11 to obtain highly fluorescent and electroactive
perylene bisimide dye squares.12,13 In the given work we will
equip this nanosize scaffold with sixteen additional antenna
dyes and demonstrate efficient energy transfer from the outer to
the inner chromophores. Notably, as shown in Fig. 1, not only
the structural features of these multichromophoric assemblies
are reminiscent of those of the natural LHS II (circular dye
arrays) but also their formation is likewise directed by non-
covalent forces.
For an efficient energy transfer from the outer to the inner
dye arrays (like in the natural LHS) a pair of chromophores is
1
The structure of 5 was confirmed by 500 MHz H and 202
MHz 31P{1H} NMR spectroscopy and Electrospray Fourier
Transform Ion Cyclotron Mass Spectrometry (ESI-FTICR-
MS). The 31P{1H} NMR spectrum displays only one singlet
at δ = Ϫ14.4 ppm (vs. 85% aq. H3PO4), shifted by about ∆δ =
Ϫ10 ppm with respect to the precursor complex [Pt(dppp)]-
[(OTf )2], which is in accordance with coordination of two
† Dedicated to Professor Waldemar Adam on the occasion of his 65th
birthday
‡ New address: F. Würthner, Institut für Organische Chemie,
Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
E-mail: wuerthner@chemie.uni-wuerzburg.de
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T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Scheme 1 Synthesis of pyrene-substituted perylene bisimide 4 and metal-ion directed self-assembly to multichromophoric square 5: i) 4-amino-
pyridine, Zn(OAc)2, quinoline, 180 ЊC, 16 h, Ar, 58% yield; ii) BBr3, CH2Cl2, 25 ЊC, 2 h, 92% yield; iii) 4-pyrene-1-yl butyric acid, DCC, DPTS, CH2Cl2,
25 ЊC, 3 d, 40% yield; iv) [Pt(dppp)][(OTf )2], CH2Cl2, 25 ЊC, 24 h, Ar, 67% yield.
pyridine ligands at the Pt() metal ion and the high symmetry
four and five triflate anions, respectively (Fig. 2). The calculated
average masses of the two species, (2814.85 and 2222.07, resp.)
are in excellent agreement with the experimental results, and
deconvolution of the two signals amounts to a molecular
mass of 11854.65 Da (calculated average mass: 11855.65 Da)
for the neutral molecular square. Several fragments could also
be identified, e.g. loss of one corner (C) and one diaza ligand
(L) [5 Ϫ 1C Ϫ 1L]4ϩ, m/z 2073.77; loss of two corners and two
diaza ligands [5 Ϫ 2C Ϫ 2L]3ϩ, m/z 1826.80, and loss of three
corners and two diaza ligands [5 Ϫ 3C Ϫ 2L]2ϩ, m/z 2361.63.
The optical properties of molecular square 5 have been
studied by UV/Vis absorption, excitation and emission spectro-
1
of the square structure.12,17 In the H NMR spectrum typical
changes of the pyridine Hα protons (∆δ = 0.4 ppm) are observed
and only one set of signals for the perylene bisimide ligand
and one set for the dppp moiety are present. Several signals
are significantly broadened which indicates restricted conform-
ational flexibility as already found and discussed for more
simple dpppPt() squares.12,17
ESI-FTICR-MS unambiguously proved the existence of
the square structure of 5 and its considerable stability. The
spectrum of 5 displays the intact square species [5 Ϫ 4OTf]4ϩ at
m/z 2814.54, and [5 Ϫ 5OTf]5ϩ at m/z 2221.87 after the loss of
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Fig. 2 ESI-FTICR-MS of light-harvesting molecular square 5 (from a
solution in acetone–CH2Cl2 1 : 1).
scopy. For a proper assignment of the absorption and emission
bands the two dyes 6 and 7 have been synthesized and included
in our study.§ In the top graph of Fig. 3 the absorption spectra
Fig. 3 UV/Vis absorption spectra in CH2Cl2 for a) the tetrapyrene–
perylene ligand 4 (black line), pyrene 6 (blue line) and perylene 7 (red
line) and b) the self-assembled molecular square 5 (black line). In
addition in b) the calculated spectrum ε = 4 × ε(7) ϩ 16 × ε(6) is shown
(red line).
dyes. The absorption spectrum of this compound is shown in
Fig. 3b with distinct bands for the pyrene and the perylene
subunits. Compared to the calculated spectrum for a super-
molecule containing sixteen pyrene dyes 6 and four perylene
bisimide dyes 7 we note two small changes in the absorption
features. For the perylene bisimide transitions slight
bathochromic shifts are observed upon self-assembly. This
effect is well-known from our earlier studies12 and is attributed
to the coordination of the pyridine receptor unit to the Pt()
metal ion. The second effect is a slight broadening of the pyrene
bands that lead to reduced peak intensities in the absorption
maxima. However, integration shows that the oscillator
strength remains unchanged. Therefore also for the self-
assembled square two distinct chromophoric systems are
given which exhibit no interaction in the ground state. Taking
structural aspects into account we can conclude that square 5
exhibits a cyclic tetrameric scaffold of weakly coupled perylene
dyes absorbing in the visible which is surrounded by another
chromophoric system containing sixteen independent pyrene
dyes which absorb in the UV range.¶
Upon photoexcitation of square 5 an intense red fluorescence
is observed whose spectral position and shape is independent
of the excitation wavelength (Fig. 4). Thus, even for selective
excitation of the pyrenes at 344 nm the observed emission arises
mainly from the S0–S1 transition of the perylene bisimides and
only a very weak emission of the pyrenes is seen upon magnifi-
cation (Inset Fig. 4) suggesting an almost quantitative energy
transfer process from the pyrene to the perylene chromophores.
This observation was surprising to us because the overlap of the
fluorescence band of the pyrene chromophores at 360–440 nm
with the absorption band of the perylene bisimide is not at all
ideal|| for a Förster-type resonance energy transfer, that is,
however, the only feasible mechanism in the case of weakly
coupled chromophores.14 Therefore we carried out another
experiment in which the fluorescence intensity of the perylene
emission is recorded upon excitation over the wavelength range
from 320 to 620 nm.18 As seen from Fig. 4, the thus obtained
fluorescence excitation spectrum of 5 exhibits a good match to
of these two dyes are shown together with the spectrum of the
ligand 4. From this comparison we see that the individual bands
of the pyrene and the perylene chromophores can be clearly
assigned. Three sharp and intense transitions are observed for
the pyrene chromophore at 314, 328 and 344 nm and two broad
absorption bands are given at ∼450 nm and ∼575 nm for the
perylene bisimide. The superposition of the perylene-related
bands of dyes 4 and 7 is perfect and the intensity of the pyrene-
related peaks is four times higher in 4 than in 6 as suggested by
the molecular structures. Accordingly we can conclude that the
pyrene and perylene chromophores of 4 exhibit no interaction
in the ground state and can be excited with high selectivity at
344 nm for pyrene and at 400–600 nm for perylene. Remarkably
this situation remains unchanged for the fully assembled square
5 which now incorporates sixteen pyrene and four perylene
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2001, 34, 40–48; F. Würthner, Nachrichten aus der Chemie, 2001, 49,
1284–1290.
3 A. Hagfeldt and M. Grätzel, Acc. Chem. Res., 2000, 33, 269–277.
4 H. Engelkamp, S. Middelbeck and R. J. M. Nolte, Science, 1999,
284, 785–788; A. Jesorka, T. S. Balaban, A. R. Holzwarth and
K. Schaffner, Angew. Chem., Int. Ed., 1996, 35, 2861–2863;
N. Nagata, S. Kugimiya and Y. Kobuke, Chem. Commun., 2000,
1389–1390; F. Würthner, C. Thalacker, A. Sautter, W. Schärtl,
W. Ibach and O. Hollricher, Chem. Eur. J., 2000, 6, 3871–3885.
5 V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and
M. Venturi, Acc. Chem. Res., 1998, 31, 26–34; C. Devadoss,
P. Bharathi and J. S. Moore, J. Am. Chem. Soc., 1996, 118,
9635–9644; M. Plevoets, F. Vögtle, L. De Cola and V. Balzani,
New J. Chem., 1999, 63–69; A. Adronov and J. M. J. Fréchet, Chem.
Commun., 2000, 1701–1710; M. Sakamoto, A. Ueno and H. Mihara,
Chem. Commun., 2000, 1741–1742; X. Zhou, D. Tyson and
F. N. Castellano, Angew. Chem., Int. Ed., 2000, 39, 4301–4305;
T. Weil, E. Reuther and K. Müllen, Angew. Chem., Int. Ed., 2002,
41, 1900–1904; M.-S. Choi, T. Aida, T. Yamazaki and I. Yamazaki,
Chem. Eur. J., 2002, 8, 2669–2678.
6 We note some beautiful examples of cyclic arrays of porphyrins by
the groups of Hunter and Nolte and a dye-modified cyclodextrin
by Lehn and Valeur: R. A. Haycock, C. A. Hunter, D. A. James,
U. Michelsen and S. R. Sutton, Org. Lett., 2000, 2435–2438;
A. P. H. J. Schenning, F. B. G. Benneker, H. P. M. Geurts, X. Y. Liu
and R. J. M. Nolte, J. Am. Chem. Soc., 1996, 118, 8549–8552;
L. Jullien, J. Canceill, B. Valeur, E. Bardez, J.-P. Lefèvre, J.-M. Lehn,
V. Marchi-Artzner and R. Pansu, J. Am. Chem. Soc., 1996, 118,
5432–5442.
Fig. 4 UV/Vis absorption (red line), fluorescence excitation (blue line,
λem = 630 nm) and emission (black line, λex = 344 nm) spectra for square
5 in CH2Cl2. The inset shows a magnification of the residual pyrene
emission. The absorption spectra were recorded at
a square
concentration of 1.7 × 10Ϫ6 M . For the fluorescence spectra this
solution was diluted until an absorbance < 0.05 was reached.
the corresponding UV/Vis absorption spectrum in the area of
the perylene absorption bands between 400 and 600 nm but has
a significantly reduced intensity for the pyrene bands. Based on
the integrated intensities from 320 to 360 nm only about 50% of
the excited pyrene dyes transfer their excitation energy to the
perylene manifold.
7 X. Hu, A. Damjanovic, T. Ritz and K. Schulten, Proc. Natl. Acad.
Sci. U. S. A., 1998, 95, 5935–5941; For another purple bacterial LHS
II 27 BChl dyes are self-assembled around a ninefold symmetry axis:
G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-
Lawless, M. Z. Papiz, R. J. Cogdell and N. W. Isaacs, Nature, 1995,
374, 517–521.
8 J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995.
9 M. Fujita, Chem. Soc. Rev., 1998, 27, 417–425.
10 S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100,
853–908.
11 R. V. Slone, K. D. Benkstein, S. Bélanger, J. T. Hupp, I. A. Guzei and
A. L. Rheingold, Coord. Chem. Rev., 1998, 171, 221–243.
12 F. Würthner and A. Sautter, Chem. Commun., 2000, 445–446;
F. Würthner, A. Sautter, D. Schmid and P. J. A. Weber, Chem. Eur.
J., 2001, 7, 894–902.
13 A. Sautter, D. Schmid, G. Jung and F. Würthner, J. Am. Chem. Soc.,
2001, 123, 5424–5430.
14 T. Förster, Discuss. Faraday Soc., 1959, 27, 7–17; N. J. Turro,
Modern Molecular Photochemistry, University Science Books,
Sausalito, California, 1991.
15 Compound 1 was obtained by the same procedure as described
recently for related dyes: F. Würthner, A. Sautter and J. Schilling,
J. Org. Chem., 2002, 67, 3037–3044.
Taken together these results pinpoint a more complicated
photophysical behavior of such multichromophoric assemblies
which cannot be revealed by simple steady-state spectroscopy.
Time-resolved and single molecule spectroscopic studies are
already in progress whose suitability for a deeper understanding
of the electronic interactions between dyes in dendrimers has
recently been demonstrated by De Schryver and Müllen.19 In
addition a larger variety of well-defined multichromophoric
superstructures is desirable to reveal structure–functionality
relationships for light-harvesting systems and to understand
why nature has chosen special types of chromophores and
principles of organization.
We are grateful to Professor Günther Jung and Dietmar
G. Schmid (University of Tübingen, Germany) for the ESI-
FTICR mass spectrometric characterization of square 5 and to
the Volkswagen Foundation for supporting our research within
the framework “Physics, Chemistry and Biology with Single
Molecules”.
16 J. S. Moore and S. I. Stupp, Macromolecules, 1990, 23, 65–70;
C. J. Hawker and J. M. J. Fréchet, J. Chem. Soc., Perkin Trans. 1,
1992, 2459–2469.
17 P. J. Stang, D. H. Cao, S. Saito and A. M. Arif, J. Am. Chem.
Soc., 1995, 117, 6273–6283; J. Fan, J. A. Whiteford, B. Olenyuk,
M. D. Levin, P. J. Stang and E. B. Fleischer, J. Am. Chem. Soc., 1999,
121, 2741–2752.
Notes and References
§ All new compounds are characterized by 1H NMR, MS, UV/Vis
spectroscopy and elemental analysis.
¶ Molecular modeling indicates that eight pyrenes are located above
and eight pyrenes are located below the rigid perylene bisimide scaffold
as suggested in Fig. 1b.
|| The fluorescence spectrum of pyrene 6 has a mirror-symmetry to the
absorption spectrum and maxima at 378 and 397 nm. As the weak
emission observed for square 5 (Fig. 4, inset) exhibits the same position
and vibronic structure it unambiguously originates from the pyrene
manifold.
18 Taking into account the problem of the nonlinear relationship
between luminescence intensity and absorbance at higher concen-
tration these experiments have been carried out on very dilute solu-
tions (A ≈ 0.04), see: J. N. Demas and G. A. Crosby, J. Phys. Chem.,
1971, 75, 991–1024; A. Credi and L. Prodi, Spectrochim. Acta, Part
A, 1998, 54, 159–170.
1 Photosynthetic Light Harvesting Systems, Eds. H. Scheer and
S. Schneider, Walter de Gruyter, Berlin, 1988; R. Huber, Angew.
Chem., Int. Ed. Engl., 1989, 28, 848; T. Pullerits and V. Sundstöm,
Acc. Chem. Res., 1999, 29, 381–389.
2 Reviews: A. Harriman and J.-P. Sauvage, Chem. Soc. Rev., 1996,
41–48; D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res.,
19 R. Gronheid, J. Hofkens, F. Köhn, T. Weil, E. Reuther, K. Müllen
and F. C. De Schryver, J. Am. Chem. Soc., 2002, 124, 2418–2419;
G. De Belder, G. Schweitzer, S. Jordens, M. Lor, S. Mitra,
J. Hofkens, S. De Feyter, M. Van der Auweraer, A. Herrmann,
T. Weil, K. Müllen and F. C. De Schryver, Chem. Phys. Chem., 2001,
1, 49–55.
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