DABCO-mediated self-assembly of zinc porphyrin-perylene bisimide monodisperse multichromophoric nanoparticles

Article abstract of DOI:10.1002/chem.200902944

Line up! Fourfold zinc porphyrin functionalized perylene bisimide has afforded well-defined supramolecular particles by intramolecular cofacial bridging of zinc porphyrin units through the ditopic ligand DABCO as revealed by UV/Vis and NMR spectroscopy an

Full text of DOI:10.1002/chem.200902944

DOI: 10.1002/chem.200902944
DABCO-Mediated Self-Assembly of Zinc Porphyrin–Perylene Bisimide
Monodisperse Multichromophoric Nanoparticles**
Peter Osswald, Chang-Cheng You, Vladimir Stepanenko, and Frank Wꢀrthner*[a]
Dedicated to Professor Arndt Simon on the occasion of his 70th birthday
The light-harvesting systems (LHS) of photosynthetic
plants and bacteria have inspired the design of a multitude
of artificial dye assemblies to mimic the functional proper-
ties of natural LHS. The primary focus of most of those
studies was on the incorporation of a multitude of dyes in a
defined molecular framework and the elucidation of energy-
transfer efficiencies between the constituent dyes. The den-
drimer concept has been shown to be very successful in this
regard and molecular systems incorporating up to 32 dye
units have been achieved.[1]
Metalloporphyrins and perylene bisimides (PBIs) are
amongst the most frequently applied chromophores for arti-
ficial dye assemblies. While in natural LHS the protein
matrix controls the position and orientation of the dye
manifold in a rather precise manner, most artificial con-
structs, however, have to be considered as conformationally
ill-defined with more or less randomly oriented and fluctuat-
ing transition dipole moments.
Herein, we introduce a small model system composed of
four metalloporphyrins that are tethered to a PBI dye core
by quite a long spacer unit, respectively. Our spectroscopic
and microscopic studies reveal that this conformationally ill-
defined pentachromophoric system can be transformed to a
well-defined three-dimensional nanoobject by noncovalent
binding of a small-molecule additive that, in a sense, mimics
the role of the protein matrix. Our approach is based on the
well-established interaction of zinc porphyrins and the di-
topic 1,4-diazabicycloACHTNUGETRN[UNG 2.2.2]octane (DABCO) ligand, which
exhibits high binding constants.[2]
Thus, we have synthesized the tetra(zinc porphyrin)-func-
tionalized PBI building block 4 according to the route de-
picted in Scheme 1. The esterification of tetra(3-hydroxy-
phenoxy) PBI 1 with free-base porphyrin carboxylic acid 2
(for the synthesis of 1 and 2, see the Supporting Informa-
tion) using dicyclohexylcarbodiimide (DCC) and 4-(N-dime-
thylamino)pyridinium toluene sulfonate (DPTS) as coupling
reagents in dichloromethane at room temperature provided
the free-base porphyrin–PBI conjugate 3 in 33% yield. Sub-
sequent metallation of 3 in dichloromethane with a saturat-
ed solution of zinc acetate in methanol afforded the target
pentachromophoric system 4 in pure form after silica-gel
column chromatography with chloroform as eluent in 57%
1
yield. Compound 4 was properly characterized by H NMR
spectroscopy, MALDI-TOF mass spectrometry, and elemen-
tal analysis (for details, see the Supporting Information).
The optical properties of 4 were investigated by UV/Vis
spectroscopy in chloroform. The absorption spectrum of 4 is
dominated by the intense Soret band of the zinc porphyrin
(420–430 nm), whereas in the Q-band region of the porphy-
rin (500–650 nm) the absorptions of both chromophores
overlap (Figure 1). A comparison of the UV/Vis spectrum
of 4 with that of the parent zinc tetraphenylporphyrin
(ZnTPP)[3] reveals a good agreement for the Soret band
(423 nm), indicating that no ground-state interaction be-
tween the zinc porphyrin units and the PBI of 4 is present.
By contrast, in the Q-band region significant spectral differ-
ences are observed that can be attributed to the PBI chro-
mophore. The difference spectrum (see Figure 1, inset) of 4
and four ZnTPP moieties (the spectrum of ZnTPP is magni-
fied by a factor of four, since 4 contains four ZnTPP unites)
shows the characteristic spectral features of aryloxy-substi-
tuted PBIs with an absorption maximum at 569 nm, which is
in good agreement with the values reported for such PBIs.[4]
Accordingly, the longest wavelength transition in the ab-
sorption spectrum of 4 (Figure 1, inset) can be attributed to
[a] Dr. P. Osswald, Dr. C.-C. You, Dr. V. Stepanenko,
Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie
and Rçntgen Research Center for Complex Material Systems
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax : (+49)931-31-84756
E-mail: wuerthner@chemie.uni-wuerzburg.de
[**] DABCO=1,4-diazabicycloACHTNUTRGNE[NUG 2.2.2]octane
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902944.
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COMMUNICATION
Scheme 1. Synthesis of tetra(zinc porphyrin)-functionalized PBI 4 and schematic representation of 4 and DABCO.
Figure 1. Comparison of the absorption spectra of 4 (solid line) and
ZnTPP (magnified by a factor of four, dashed line) in chloroform. Inset:
An expansion of the Q-band region of absorption spectra of 4 (solid line)
and ZnTPP (dashed line), and difference spectrum of 4 and ZnTPP
(dotted line) in chloroform.
Figure 2. UV/Vis titration of 4 (c=9ꢃ10ꢀ7 m) with DABCO in CHCl3 at
238C. Inset: an expansion of the Q-band region. Arrows indicate spectral
changes with increasing amounts of DABCO (from 0 to 2 molar ratio).
both chromophores (ZnTPP, PBI), which exhibit almost
equal S0!S1 excitation energy.[3]
The self-assembly of 4 with DABCO was investigated by
constant host titration by using UV/Vis spectroscopy.
Figure 2 shows the absorption spectra of 4 in chloroform
upon successive addition of DABCO. During the titration, a
distinct band arises at 601 nm and the broad Soret band gets
narrowed with a concomitant increase in intensity as the
half-width of the Soret band changes from 770 to 436 cmꢀ1.
The appearance of a clearly recognizable absorption band at
601 nm upon ligation of DABCO with the zinc porphyrin
units readjusts the energy levels in such a way that the
lowest excited state can now be clearly attributed to the
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F. Wꢀrthner et al.
ZnTPP units. The position of the Soret band at 424 nm, as
previously observed for ZnTPP–DABCO sandwich com-
plexes,[2g] supports the formation of a sandwich complex.[5]
The spectral changes observed upon ligation of DABCO to
the multichromophoric building block 4 apparently stem
from a more defined arrangement of the chromophore units
that restricts the conformational flexibility of the system.
During the titration, isosbestic points (at 421, 427, 435,
557, 579, 592 nm) were observed up to a 1:1 ratio of 4 and
DABCO, implying that two species are present in equilibri-
um. Upon further addition of DABCO up to a ratio of 1:2
(4/DABCO), a second process emerges that is characterized
by slightly displaced isosbestic points (421, 427, 434, 558,
582, 591 nm). These observations imply that two individual
two-state equilibria are involved in this self-assembly pro-
cess. A schematic representation of the possible equilibria is
shown in Figure 3. The titration data were analyzed by non-
Figure 4. Sections of the 1H NMR spectra (600 MHz) of 4 (6.4ꢃ10ꢀ4 m)
recorded during the titration with DABCO in CDCl3. Aromatic resonan-
ces (left) and the high-field region (right) with DABCO resonances are
depicted. The number of equivalents of DABCO added is indicated on
each spectrum.
8.5 ppm sustained with higher intensity. This observation
supports the 1:2 stoichiometry of the complex formed, and
thus corroborates the results of UV/Vis titration experi-
ments. The characteristic upfield shift of porphyrin resonan-
ces (0.3 ppm) indicates the formation of a double-decker
structure in which the aromatic ring current of the two por-
phyrins influence each other. This is further confirmed by
the very pronounced upfield shift of DABCO methylene
Figure 3. Schematic representation of a two-step formation of self-assem-
bled macrocycles and definition of the respective binding constants. The
PBI is shown in red, the zinc porphyrin in blue, and DABCO ligand in
green.
linear least-squares analysis[6] by using a model consisting of
1:0, 1:1, and 1:2 species (see Figure S4 in the Supporting In-
formation). The binding constant for the formation of the
1:1 species, K11, was determined to be (1.3ꢁ0.3)ꢃ107 mꢀ1
and for the equilibrium between the 1:1 and the 1:2 species
(K12) a value of (5.6ꢁ1.5)ꢃ106 mꢀ1 was obtained.
1
protons (d=ꢀ4.8 ppm), since this is a typical H NMR spec-
troscopy signature for zinc porphyrin–DABCO sandwich
complexes.[2g] At a 1:2 stoichiometry of 4 and DABCO, the
PBI and zinc porphyrin resonances are well resolved and
appear sharp, indicating the formation of a structurally well-
defined assembly, namely, 1:2 (4/DABCO) complex, rather
than polymeric species. Upon the addition of more than two
equivalents of DABCO, the corresponding DABCO signal
gets broadened and shifted downfield, and almost disap-
peared at a ratio of 8:1 (DABCO/4). Such behavior was
often observed in similar systems and attributed to an ex-
change process of the bound and free DABCO molecules
on the NMR timescale.[2g]
The UV/Vis titration experiments described above reveal
a stepwise ligation of DABCO at the zinc coordination site.
In principle, self-assembly of multifunctional compounds
can lead to the formation of polymeric entities or defined
macrocycles. The comparison of the binding constants esti-
mated for the DABCO-mediated self-assembly of 4 with
those of previously reported systems leading to macrocyclic
structures,[2k] however, strongly suggests the formation of
two macrocyclic structures in a “figure-eight”-type arrange-
ment, as schematically shown in Figure 3. Accordingly, well-
positioned transition dipole moments are evoked by
DABCO coordination, while more or less random orienta-
tion of the transition dipole moments of the five chromo-
phore units prevails in the precursor molecule 4.
To provide further evidence that no polymers or higher
cyclic oligomers are created by self-assembly of 4 and
DABCO, two-dimensional DOSY NMR spectroscopy ex-
periments were performed in CDCl3. The DOSY spectra
were recorded first for the host 4 and then after the addition
of two equivalents of DABCO for the complex
1
Subsequently, a H NMR spectroscopic titration was car-
4·ACHTNGUTERNU(NG DABCO)2. The diffusion coefficients were determined as
ried out in deuterated chloroform at 300 K under constant
host conditions with a 0.67 mm solution of 4. Upon succes-
sive addition of DABCO (0 to 2 equiv) to a solution of 4 in
chloroform, the signals of b-pyrrole protons of porphyrin at
d=8.8 ppm (center of multiplet) decreased gradually and a
new resonance arose at around d=8.5 ppm (Figure 4).[7] At
a 1:2 ratio of 4 and DABCO, the initial signal at d=8.8 ppm
disappeared completely and the newly emerged signal at d=
the mean value of ten individual signals. The obtained
values for the free host 4 (2.29ꢃ10ꢀ10 m2 sꢀ1) and a 1:2 (4/
DABCO) sample (2.33ꢃ10ꢀ10 m2 sꢀ1) do not differ signifi-
cantly, revealing that the species formed by self-assembly of
4 with DABCO at a 1:2 ratio should be of similar size as 4,
which is actually in compliance with the fact that the molec-
ular weights of 4 and 1:2 (4/DABCO) complex do not differ
substantially. From the diffusion coefficients, the hydrody-
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Self-Assembly of Monodisperse Multichromophoric Nanoparticles
COMMUNICATION
namic radii of the molecules can be calculated by employing
the Stokes–Einstein Equation (1) under the assumption of
spherical particles:[8]
resolution AFM images of 4·ACTHUGNTERNNU(G DABCO)2 (Figure 6C, D)
clearly show small isolated nanosized particles that are ar-
ranged in distorted hexagonal packing on the surface. It is
noteworthy that, in contrast to DABCO, the more extended
4,4’-dipyridine ligand as an additive did not afford any well-
defined structures (see AFM images in Figure S3 in the Sup-
porting Information). This implies that a proper size of the
additive is decisive for the formation of well-defined nano-
objects of the present multichromophoric system.
kBT
6phr
ð1Þ
D ¼
According to Equation (1), radii of 1.59 and 1.56 nm were
estimated for 4 and 4·(DABCO)2 complex, respectively.
Pleasingly, this value (1.56 nm) for the 4·(DABCO)2 com-
AHCTUNGTRENNUNG
For the observed ellipsoid particles of 4·ACTHNURTGNENG(U DABCO)2, an
AHCTUNGTRENNUNG
average length of (4.7ꢁ0.5) nm and width of (3.4ꢁ0.5) nm
were determined by Fourier transformation. The height of
the particles was elucidated by
plex is in excellent agreement with the radius (1.54 nm) ob-
tained from molecular modeling (Figure 5 and Figure S2 in
cross-section
analysis
as
0.48 nm, indicating the presence
of a monolayer. Based on these
data, the area covered by each
supramolecular particle was es-
timated to be 12.6 nm2. Again,
the particle size determined by
AFM analysis fits well with the
dimensions (10.9 nm2) of the
macrocyclic structure calculated
by molecular modeling, con-
Figure 5. Structure of 4·ACHTUNGTRENNUNG(DABCO)2 obtained from molecular modeling. A laterally (stretched conformation)
bridged structure was calculated with a MM3 force field by using a CaChe Quantum CaChe Workspace 5.0.
View is along the N,N-axis of the PBI. The PBI is highlighted in red, zinc porphyrins in blue, and DABCO in
green.
firming
the
monodisperse
nature of the particles. These
results clearly demonstrate that
the Supporting Information). These results confirm that no
polymers are formed by the self-assembly of 4 and DABCO,
rather discrete structures are created at a 1:2 (4/DABCO)
stoichiometry.[9]
In principle, two isomers are possible for the macrocyclic
structure of 4·ACHTUNGTRENNUNG(DABCO)2, since upon complexation the
DABCO molecules can link two zinc porphyrins either in a
lateral or a diagonal manner (as shown in Figure 5 and Fig-
ure S2 in the Supporting Information).[10,11] Unfortunately, in
contrast to structurally similar covalent macrocycles,[10] the
two possible isomers of the present supramolecular macro-
cycle cannot be distinguished by spectroscopic means. How-
ever, since aryloxy-substituted PBIs prefer a horizontal ori-
entation of the aryloxy residues,[10] and the building block 4
contains such residues, the laterally bridged isomer
(Figure 5) should be more probable structure for the self-as-
sembled macrocycle 4·
ACHTUNGTNER(NUNG DABCO)2.
To assess the self-organization behavior of 4·ACHTNUGTRNE(NUG DABCO)2
on surfaces, AFM analysis was performed on highly ordered
pyrolytic graphite (HOPG) in tapping mode. In AFM
Figure 6. Height AFM images of a 1ꢃ10ꢀ5 m solution of 4 (A) and
4·ACTHNUGTRNE(UNG DABCO)2 (B–D) in chloroform spin-coated on HOPG under
5000 rpm, and cross-section analysis (E, along the yellow line in D).
Image D is a magnified region of image C (yellow frame). The scale bars
are 150 (A, B) and 60 nm (C), and the z scale is 2 nm.
images of self-assembled 4·ACHTNUGRTENUNG(DABCO)2 macrocycles (Fig-
ure 6B–D), a highly ordered pattern was observed over a
wide range of the surface. For comparison, a sample of un-
complexed building block 4 was analyzed under similar con-
ditions (Figure 6A). In contrast to 4·
G
the well-defined self-assembled macrocycles 4·ACTHUNGTERNNU(G DABCO)2
4 showed only ill-defined agglomerates on the surface, indi-
cating that the defined arrangement of the chromophores in
can be deposited on surfaces by spin-coating as single mo-
lecular entities in a highly ordered pattern.
4·
(DABCO)2 complexes is responsible for the observed
In summary, a novel multichromophoric system 4 based
on zinc porphyrin and PBI dyes was synthesized. Taking to-
unique ordering of these complexes on the surface. High-
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F. Wꢀrthner et al.
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scopic investigations, it can be concluded that the DABCO-
mediated self-assembly of 4 occurred with high binding con-
stants under exclusive formation of a singular species with
well-positioned dipole moments. The self-assembled
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surface by spin-coating to afford a regular 2D adlayer of
photofunctional monodisperse nanoparticles. Such multi-
chromophoric nanoobjects incorporating well-oriented chro-
mophore units are still rare in supramolecular chemistry, al-
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Experimental Section
[7] The slight broadening of the signals stems from an intramolecular
dynamic process evoked by the increased barrier for the inversion
process between the (P)- and (M)-atropoenantiomers (see P. Oss-
wald, F. Wꢀrthner, J. Am. Chem. Soc. 2007, 129, 14319–14326) of
the core-tetrasubstituted PBI as the temperature-dependent spectra
For experimental details, see the Supporting Information.
of 4·ACTHUNTRGENNG(U DABCO)2 reveal (Figure S1 in the Supporting Information).
Acknowledgements
Upon cooling from 293 to 273 K, a significant sharpening of the sig-
nals was observed, especially of those related to the aryloxy residues
attached to the perylene core. Whereas the signal for DABCO pro-
tons remained unchanged, indicating that complexation is not influ-
enced by temperature change.
We thank the Alexander von Humboldt Foundation for a postdoctoral
fellowship (C.C.Y.) and the DFG for financial support (grant project:
WU 317/7).
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[9] Unfortunately, all attempts to record mass spectra for the complex
Keywords: dyes/pigments
· multichromophore systems ·
porphyrinoids · scanning probe microscopy · self-assembly
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self-assembled systems and is attributed to the reversible nature of
the self-assemblies and the lability of such self-assembled systems
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Received: October 23, 2009
Published online: January 28, 2010
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