Influence of supramolecular organization on energy transfer properties in chiral oligo(p-phenylene vinylene) porphyrin assemblies

Article abstract of DOI:10.1021/ja072548c

A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of supramolecular arrangement obtained in organic solvents

Full text of DOI:10.1021/ja072548c

Published on Web 07/13/2007
Influence of Supramolecular Organization on Energy Transfer
Properties in Chiral Oligo(p-phenylene vinylene) Porphyrin
Assemblies
Freek J. M. Hoeben,† Martin Wolffs,† Jian Zhang,‡ Steven De Feyter,*,‡
Philippe Lecle`re,†,§ Albertus P. H. J. Schenning,*,† and E. W. Meijer*,†
Contribution from the Laboratory of Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, the Department
of Chemistry and Institute of Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen,
Celestijnenlaan 200F, B-3001 LeuVen, Belgium, and the SerVice de Chimie des Mate´riaux
NouVeaux, UniVersite´ de Mons-Hainaut, B-7000 Mons, Belgium
Received April 18, 2007; E-mail: stevendefeyter@kuleuven.be; e.w.meijer@tue.nl; a.p.h.j.schenning@tue.nl
Abstract: A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all
trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of
supramolecular arrangement obtained in organic solvents proves to be strongly dependent on the nature
of the covalent connection. In the case of all trans-vinylene linkages, a J-type intermolecular packing is
obtained and the assemblies are only of moderate stability. Conversely, the supramolecular structures
obtained from the amide-linked system display an H-type stacking arrangement of enhanced stability and
chirality as a consequence of intermolecular hydrogen bonding along the stack direction, favorably
interlocking the stacked building blocks. Interestingly, the observed differences in stability and organization
are qualitatively illustrated by monitoring the sequential energy transfer process in both types of assemblies.
Efficient intramolecular energy transfer from the OPVs (donors) to the respective porphyrin cores is followed
by energy transfer from Zn-porphyrin (donor) to free-base porphyrin (acceptor) in both systems. However,
the improved intermolecular organization for the amide-linked system increases the energy transfer efficiency
along the stack direction. In addition, the water-soluble (OPV)-appended porphyrin system forms highly
stable assemblies in an aqueous environment. Nevertheless, the poor energy transfer efficiency along the
stack direction reveals a relative lack of organization in these assemblies.
also self-assembly was accounted for in such studies.6 Although
chemists have accessibility to a rich toolbox of noncovalent in-
Introduction
The design of supramolecular multichromophoric arrays
displaying directional energy and electron transfer is a promising
strategy in the quest for organic, opto-electronic devices.1 For
this purpose, an optimal mesoscopic organization of the
constituent donor and acceptor chromophores is essential for
obtaining efficient energy or charge-transfer processes. Sequen-
tial energy and/or electron-transfer steps, as observed in
photosynthetic systems,2 have been studied in synthetic covalent,
multichromophoric (dendritic) systems.3 The relative ease of
porphyrin synthesis and derivatization, for example, has stimu-
lated innumerable studies dedicated to the covalent function-
alization of porphyrins4 with appended chromophores,5 mostly
for fundamental energy and electron-transfer studies. Recently,
teractions, in particular the synergistic interplay between multiple
weak intermolecular forces is only poorly understood. Aiming
for the simultaneous use of cooperative noncovalent interactions
will not only allow for a fundamental understanding of the re-
quired supramolecular design principles but also be crucial to
the successful implementation of functional assemblies in future
applications like, e.g., artificial photosynthesis.7
The present paper aims for a comparative study between 1,
2 in water8 and similar structures (3, 4) in organic solvents.
(3) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. (b) Miller, M. A.;
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† Eindhoven University of Technology.
‡ Katholieke Universiteit Leuven.
§ Universite´ de Mons-Hainaut.
(1) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491-1546.
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J. AM. CHEM. SOC. 2007, 129, 9819-9828
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A R T I C L E S
Hoeben et al.
Chart 1. (OPVn)4Porphyrins Designed to Self-Assemble into
Functional Supramolecular Architecturesa
Moreover it describes the stabilization of supramolecular
assemblies by combining π-π stacking interactions with
intermolecular hydrogen bonding (5, 6). We compare some
supramolecular interactions often used and demonstrate their
profound influence on the resulting self-assembly characteristics,
the stability of the formed architectures, and the efficiency of
the sequential energy transfer process along the stacking
direction. Comparative studies of energy and/or electron-transfer
efficiencies in synthetic covalent, multichromophoric (dendritic)
systems have often been reported in contrast to supramolecular
systems.9
In order to compare the different structures we first highlight
the previously reported results on OPV-appended porphyrins 1
and 2 (Chart 1).8,10 These systems are equipped with oligo-
(ethylene oxide) chains, thus enabling self-assembly of 1 and 2
into H-type aggregates in water. Heating these assemblies yields
a chiral-to-achiral transition, although the supramolecular
structures remain assembled at high temperatures due to their
enormous stability. The UV/vis spectra of 1 and 2 are indicative
of ground-state electronic communication between the OPVs
and the porphyrin chromophores, resulting in highly modified
absorption spectra with respect to reference OPV and porphyrin
compounds.5e,j Fluorescence spectra of 1 and 2, combined with
quenching studies, display the characteristic features of efficient
intramolecular energy transfer from the peripheral OPVs to both
respective porphyrin cores. Furthermore, mixtures containing
co-assembled 1 and 2 in water suggest cascade energy transfer
from the peripheral OPVs via Zn-porphyrin to free-base
porphyrin. However the energy transfer efficiency is rather low
(5) For examples using various chromophores, see: (a) Vollmer, M. S.;
Wu¨rthner, F.; Effenberger, F.; Emele, P.; Meyer, D. U.; Stu¨mpfig, T.; Port,
H.; Wolf, H. C. Chem.sEur. J. 1998, 4, 260-269. (b) Holten, D.; Bocian,
D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. (c) Loewe, R. S.;
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G.; Burn, P. L.; Samuel, I. D. W. J. Mater. Chem. 2003, 13, 235-242. (e)
Li, B.; Li, J.; Fu, Y.; Bo, Z. J. Am. Chem. Soc. 2004, 126, 3430-3431. (f)
Choi, M.-S.; Yamazaki, T.; Yamazaki, I.; Aida, T. Angew. Chem., Int. Ed.
2004, 43, 150-158. (g) Imahori, H. Org. Biomol. Chem. 2004, 2, 1425-
1433. (h) Duan, X.-F.; Wang, J.-L.; Pei, J. Org. Lett. 2005, 7, 4071-4074.
(i) Loiseau, F.; Campagna, S.; Hameurlaine, A.; Dehaen, W. J. Am. Chem.
Soc. 2005, 127, 11352-11363. (j) Jiu, T.; Li, Y.; Gan, H.; Liu, H.; Wang,
S.; Zhou, W.; Wang, C.; Li, X.; Liu, X.; Zhu, D. Tetrahedron 2007, 63,
232-240.
(6) For example, see: (a) Prokhorenko, V. I.; Holzwarth, A. R.; Mu¨ller, M.
G.; Schaffner, K.; Miyatake, T.; Tamiaki, H. J. Phys. Chem. B 2002, 106,
5761-5768. (b) van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.;
Weiss, E. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124, 9582-
9590. (c) de la Escosura, A.; Martinez-Diaz, M. V.; Thordarson, P.; Rowan,
A. E.; Nolte, R. J. M.; Torres, T. J. Am. Chem. Soc. 2003, 125, 12300-
12308. (d) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B.
A.; Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski,
M. R. J. Am. Chem. Soc. 2004, 126, 8284-8294. (e) Li, X.; Sinks, L. E.;
Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 10810-
10811. (f) Wu¨rthner, F.; Chen, Z.; Hoeben, F. J. M.; Osswald, P.; You,
C.-C.; Jonkheijm, P.; van Herrikhuyzen, J.; Schenning, A. P. H. J.; van
der Schoot, P. P. A. M.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C.
J.; Janssen, R. A. J. J. Am. Chem. Soc. 2004, 126, 10611-10618. (g)
Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43,
1229-1233. (h) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Am.
Chem. Soc. 2004, 126, 12268-12269.
a (Top)Hydrophilic(OPV4)4porphyrins1and2andlipophilic(OPV4)4porphyrins
3 and 4 containing all-trans vinylene linkages. (Bottom) (OPV3)4Porphyrins
5 and 6 containing amide linkages.
(compared to 5 and 6, Vide infra). This inefficiency can probably
be ascribed to the relatively poor organization of these as-
semblies (for both features, see Table 1). The latter may be due
to the presence of kinetically trapped assemblies as a result of
the stability of the supramolecular structures even at high
temperatures.
Results and Discussion
(7) For some recent reviews, see: (a) Wasielewski, M. R. J. Org. Chem. 2006,
71, 5051-5066. (b) Kobuke, Y. Eur. J. Inorg. Chem. 2006, 12, 2333-
2351. (c) Fukuzumi, S. Bull. Chem. Soc. Jpn. 2006, 79, 177-195. (d) Inoue,
H.; Funyu, S.; Shimada, Y.; Takagi, S. Pure Appl. Chem. 2005, 77, 1019-
1033. (e) Konishi, T.; Ikeda, A.; Shinkai, S. Tetrahedron 2005, 61, 4881-
4899. (f) Szacilowski, K.; Macyk, W.; Drzewiecka-Matuszek, A.; Brindell,
M.; Stochel, G. Chem. ReV. 2005, 105, 2647-2694. (f) Imahori, H. Org.
Biomol. Chem. 2004, 2, 1425-1433. (g) Choi, M.-S.; Yamazaki, T.;
Yamazaki, I.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 150-158. (h)
Kobuke, Y.; Ogawa, K. Bull. Chem. Soc. Jpn. 2003, 76, 689-708.
(8) Wolffs, M.; Hoeben, F. J. M.; Beckers, E. H. A.; Schenning, A. P. H. J.;
Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 13484-13485.
Self-Assembly of Apolar Analogues 3 and 4 in MCH. Apart
from previously reported hydrophilic 1 and 2,8 their hydrophobic
analogues 3 and 4 bearing aliphatic chains have been synthesized
according to similar procedures (Chart 1).10 Compounds 3 and
4 exist as dark brownish and dark greenish solids, respectively,
which were fully characterized using 1H NMR, 13C NMR,
COSY-NMR, HMQC-NMR, and IR spectroscopy, mass spec-
trometry, and elemental analysis.
(9) Hoeben, F. J. M.; Schenning, A. P. H. J.; Meijer, E. W. ChemPhysChem
2005, 6, 2337-2342.
The optical properties of 3 and 4 were investigated in
chloroform and methylcyclohexane (MCH), aiming for self-
(10) See the Supporting Information.
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Chiral Oligo(p-phenylene vinylene) Porphyrin Assemblies
A R T I C L E S
Figure 1. UV/vis, PL, and CD spectra for (top) 3 and (bottom) 4 in
chloroform (dashed line) and MCH (solid line) at room temperature (10-5
M, λexc ) 346 nm).
assembly of the molecules in the latter solvent (10-5 M, Figure
1).11 In chloroform, the UV/vis spectrum of 3 displays a broad,
structureless band at λmax ) 444 nm (ꢀ ) 4.6 × 105 L mol-1
cm-1) accompanied by the characteristic porphyrin Q-bands12
at common wavelengths of λ ) 521, 563, 596, and 654 nm.
The presence of the OPV moieties is merely revealed by their
S2rS0 transition around λ ) 340 nm. Similar to the case for 1,
the UV/vis spectrum of 3 reflects ground-state electronic
communication between both chromophores. As a result, the
main absorption band comprising the π-π* transition of the
OPVs and the S2rS0 Soret band of the porphyrin is much
broadened and exhibits an appreciable loss in oscillator strength.5e,j
The fluorescence spectrum of 3 in chloroform (λexc ) 346 nm,
in order to excite the OPVs with maximum probability) shows
characteristic porphyrin luminescence at λem ) 666 nm and λem
) 722 nm. OPV fluorescence is effectively quenched, indicating
intramolecular energy transfer from the peripheral OPVs to the
porphyrin core.5 A quenching factor of Q ≈ 150, determined
in a comparative study on the combined fluorescence of its
building blocks,10 yields a lower limit for the energy transfer
rate of kENT ) 1011 s-1 when a typical OPV4 lifetime of τ )
1.3 ns is taken into account.13,14 Characteristic optical changes
Figure 2. (a) STM image showing an ordered monolayer of 3 on HOPG,
physisorbed from a 1-phenyloctane solution (1.8 × 10-5 M). Image size is
36.7 nm × 36.7 nm, Iset ) 0.6 nA, Vset ) -0.426 V. (b) A high-resolution
STM image also revealing the adsorbed alkyl chains of 3. A molecular
model, without alkyl chains, is superimposed on the STM image. The
dimensions of the unit cell are a ) 5.2 nm, b ) 4.7 nm, and γ ) 70 °.
occur upon Zn2+ insertion into the porphyrin core. In chloro-
form, while the Soret band remains situated at λ ) 442 nm (ꢀ
) 3.9 × 105 L mol-1 cm-1), the increased symmetry of the
resulting chromophore causes the S1rS0 transition to reduce
to two Q-bands at λ ) 552 and 593 nm.12 Due to the increased
energy of the S1rS0 transition the main emission peak is
observed at λem ) 610 nm with a vibrational shoulder at λem
)
654 nm. OPV luminescence remains highly quenched as a result
of energy transfer to the Zn-porphyrin. The 56 nm hypsochromic
shift in porphyrin luminescence with regard to its free-base
analogue makes Zn-porphyrin in 4 an excellent energy donor
for the free-base porphyrin in 3 due to a highly favorable Fo¨rster
overlap integral.15
Scanning tunneling microscopy (STM) measurements were
performed at the liquid-solid interface by physisorbing lipo-
philic 3 from a 1-phenyloctane solution (1.8 × 10-5 M) onto
highly oriented pyrolytic graphite (HOPG). STM images show
an ordered pattern of cross-like features (Figure 2a) identified
as individual molecules of 3 physisorbed parallel onto the
substrate into a regular pattern. Such a regular pattern is not
formed immediately but appears within a few hours after the
(11) As in the case for previously studied pure OPV assemblies, dodecane was
also investigated as a non-solvent, but since 3 to 6 precipitated in this solvent
the results were not reproducible.
(12) Hashimoto, T.; Choe, Y.-K.; Nakano, H.; Hirao, K. J. Phys. Chem. A 1999,
103, 1894-1904.
(13) Neuteboom, E. E.; van Hal, P. A.; Janssen, R. A. J. Chem.sEur. J. 2004,
10, 3907-3918.
(14) Peeters, E.; Marcos, Ramos, A.; Meskers, S. C. J.; Janssen, R. A. J. J.
Chem. Phys. 2000, 112, 9445-9454.
(15) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7.
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A R T I C L E S
Hoeben et al.
Figure 3. Temperature-dependent (a) UV/vis, (b) PL (λexc ) 346 nm), and (c) CD spectra of 3 in MCH (10-5 M). The arrows indicate a temperature
increase from 10 °C to 70 °C. The insets in (a) and (c) show the optical properties of 3 in MCH at low (10-7 M, dashed line) and at high (UV/vis, 10-4 M;
CD, 6 × 10-5 M, solid line) concentrations at room temperature.
solution is applied onto the substrate and reflects relatively slow
monolayer formation dynamics. When probing in more detail,
also the alkyl chains of the (tridodecyloxy)wedges are observed
(Figure 2b). The shape of the individual molecules and the high
uniformity of the pattern indicate an all-trans vinylene confor-
mation of the OPV parts.16 As the OPV units are decorated with
(S)-2-methylbutoxy groups, it is natural to evaluate the influence
of molecular chirality on monolayer formation. The observed
patterns are clearly two-dimensionally chiral, and the enantio-
meric excess is reflected in the presence of one form exclu-
sively.17
Upon dissolving 3 in MCH, subtle optical differences are
observed. Although the position of the absorption maximum
does not shift, the combined main OPV/porphyrin absorption
significantly broadens and develops a shoulder which overlaps
with the first Q-band absorption. Moreover, the Q-bands exhibit
a bathochromic shift of approximately 5 nm. The fluorescence
trace is shaped similarly as in the case of chloroform, but its
main emission peak exhibits a modest 5 nm bathochromic shift
to λem ) 671 nm. These relatively minor effects can be assigned
to self-assembly of the system by π-π stacking of the
conjugated backbones, since concomitantly a bisignate Cotton
effect18 emerges corresponding to the UV/vis absorption. The
observed Cotton effect is positive at long wavelengths (g )
+5.8 × 10-4 at λ ) 468 nm) and negative at shorter
wavelengths (g ) -8.4 × 10-4 at λ ) 427 nm) with the zero-
crossing of the CD effect at λ ) 449 nm. Whereas the
chloroform solution lacked supramolecular chirality, π-π
stacking and solvophobic effects ensure the formation of
functional assemblies in MCH, which remain soluble due to
the surrounding apolar alkyl chain jacket. Similar UV/vis and
PL changes as obtained for 3 are found for 4 when dissolved in
MCH. However, the absence of a Cotton effect indicates that
aggregation occurs in an achiral fashion or without preferential
helicity. Concentration-dependent UV and CD measurements10
indicate that 3 self-assembles in MCH at concentrations higher
than 5 × 10-6 M at room temperature. This indicates that,
despite the enlarged π-surface available for stacking interactions,
relatively weak assemblies are formed with respect to pure OPV
assemblies.19 Figure 3 shows temperature-dependent UV/vis,
PL (λexc ) 346 nm), and CD studies on 3 in MCH (10-5 M).
When the temperature is increased, the UV/vis spectra display
a hypsochromic shift from λmax ) 445 to λmax ) 438 nm. This
is combined with a narrowing of the main absorption peak which
moreover gains in oscillator strength. The disassembly of the
system is corroborated by the disappearance of the Cotton
effect above a transition temperature of 35 °C. Upon disas-
sembly, the emission from the sample exhibits a hypsochromic
shift from λem ) 672 to λem ) 665 nm while remaining at a
similar intensity. The combined results indicate that, at tem-
peratures below its transition temperature, 3 self-assembles into
right-handed helical J-type aggregates. As opposed to pure OPV
assemblies,19 the luminescence of 3 is not lost upon aggre-
gation, which is a direct consequence of its J-type stacking
mode.20 At high temperature, the optical properties of 3 are
similar to those in chloroform at room temperature, indicating
that the chromophores are present as molecularly dissolved
species. Similar temperature-dependent optical measurements
on 4 in MCH10 yield a transition temperature of 30 °C (10-5
M), indicating that 4 self-assembles into structures of similar
stability.
The J-type stacking behavior displayed by 3 and 4 is in sharp
contrast with that of compounds 1 and 2, which assemble into
H-type aggregates in water.8 In addition, the intrinsic magnitude
of the bisignate Cotton effect for 3 is enhanced with respect to
1. Most likely, the ability of the molecules to attain their optimal
organization is overruled in water as a result of the strong
hydrophobicity.21 This forces the π-conjugated surfaces together
in an H-type fashion to minimize the contact with the aqueous
environment and suggests that hydrophobicity is the main
structural determining factor when studying π-conjugated self-
assemblies in water.
In order to obtain images of the supramolecular (OPV)4por-
phyrin assemblies in the solid state, AFM was implemented.
When 3 is dropcast onto highly ordered pyrolytic graphite from
MCH, fiberlike structures are observed, which are 3.5 nm in
height and up to hundreds of nanometers in length. No
morphological difference is observed between starting solutions
containing either aggregated (Figure 4a) or disassembled (Figure
4b) chromophores. The measured height is smaller than the
(16) (a) Gesquie`re, A.; Jonkheijm, P.; Schenning, A. P. H. J.; Mena-Osteritz,
E.; Ba¨uerle, P.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. J. Mater.
Chem. 2003, 13, 2164-2167. (b) Miura, A.; Chen, Z.; De Feyter, S.;
Zdanowska, M.; Uji-i, H.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer,
E. W.; Wu¨rther, F.; De Schryver, F. C. J. Am. Chem. Soc. 2003, 125,
14968-14969. (c) Gesquie`re, A.; Jonkheijm, P.; Hoeben, F. J. M.;
Schenning, A. P. H. J.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W.
Nano Lett. 2004, 4, 1175-1179. (d) Miura, A.; Jonkheijm, P.; De Feyter,
S.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Small 2005,
1, 131-137.
(17) Ernst, K. H. Top. Curr. Chem. 2006, 265, 209-252.
(18) (a) Nakanishi, K.; Berova, N.; Woody, R. W. Circular Dichroism; VCH:
Weinheim, 1994. (b) Berova, N.; Nakanishi, K.; Woody, R. W. Circular
Dichroism, 2nd ed.; VCH: New York, 2000.
(19) Jonkheijm, P.; Hoeben, F. J. M.; Kleppinger, R.; van Herrikhuyzen, J.;
Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2003, 125, 15941-
15949.
(20) Kobayashi, T., Ed. J-Aggregates; World Scientific: Singapore, 1996.
(21) Chandler, D. Nature 2005, 437, 640-647.
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Chiral Oligo(p-phenylene vinylene) Porphyrin Assemblies
A R T I C L E S
Figure 4. AFM images showing fiberlike morphologies of (a, b) 3 and (c) 5 on HOPG, dropcast from (a) 10-5 M, (b) 10-6 M, and (c) 5 × 10-6 M MCH
solutions.
molecular length (L ) 5.5 nm) implying a certain degree of
tilting of the constituent chromophores inside the fibers with
respect to the surface. When similar measurements are per-
formed using either hydrophilic glass or mica surfaces, dewetting
occurs and a height of 5.0 nm is found.
Sequential Energy Transfer in Mixed Assemblies of 3 and
4. The observed efficient intramolecular energy transfer from
the peripheral OPVs to the porphyrin core in 3 and 4 is a
prerequisite for further excitation transfer along the stacking
direction. For this purpose, mixed assemblies of 3 and 4 were
envisaged in which the Zn-porphyrin moiety acts as an energy
donor toward the free-base porphyrin.22 In order to avoid inner
filter effects,23 mixing experiments had to be performed at
optical densities below 0.1. At these concentrations (5 × 10-7
M), however, 3 and 4 are not present as assembled species and
an initial mixing experiment at 20 °C was executed for reference
purposes (Figure 5, top). In order to study mixed assemblies,
consecutive samples were cooled to -100 °C. At this temper-
ature, a considerable bathochromic shift of pure 4 luminescence
from λem ) 615 nm to λem ) 654 nm indicates chromophore
aggregation. In addition, the decreasing temperature causes the
shape of the fluorescence spectrum to change in favor of the
0-0 emission band, in accordance with Boltzmann’s principle.24
Excitation occurs in both cases at λexc ) 346 nm, where
Figure 5. PL spectra for the addition of (top) 0 to 24 mol % 3 to 4 at
20 °C in MCH and (bottom) 0 to 26 mol % 3 to 4 in MCH at -100 °C. In
both cases λexc ) 346 nm, the concentration of 4 is fixed at 5 × 10-7 M.
predominantly the OPV chromophores absorb.
The inset compares donor luminescence quenching I0/I at λem ) 610 nm
The reference experiment conducted at 20 °C clearly shows
the increasing solution content of free-base analogue 3 (0 to 24
mol %), which emits at λem ) 663 and λem ) 726 nm. Performed
in this way, the experiment probes energy transfer from 4 to 3
in the molecularly dissolved state, which is clearly absent since
the fluorescence characteristic of 4 at λem ) 610 nm remains
constant. The results obtained for a similar mixing experiment
at -100 °C (with acceptor addition at room temperature) are
in clear contrast with the behavior at room temperature (Figure
5, bottom). Upon addition of free-base analogue 3 (0 to 26 mol
%), fluorescence characteristic of 4 around λem ) 639 nm is
quenched to a high degree, in favor of acceptor luminescence
(9, 20 °C) and λem ) 630 nm (b, -100 °C).
at λem ) 673 nm and λem ) 739 nm. The relative increase in
acceptor fluorescence seems to exceed that observed for the
disassembled state. The combined data strongly suggest a
sequential energy transfer process from the peripheral OPVs
via Zn-porphyrin to free-base porphyrin. However, we are at
the moment not able to exclude other excitation transfer
pathways, e.g., as a consequence of fast exciton diffusion among
different π-stacked OPV chromophores25 or by direct OPV
(attached to Zn-porphyrin) to free-base porphyrin energy
transfer. However, the poor organization of self-assembled 4
(following from the lack of supramolecular chirality) and the
fast intramolecular energy transfer process to both porphyrins
most likely preclude these respective supplementary pathways.
In addition, it is difficult to ascertain whether the sensitized
(22) (a) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: London,
2000; Volume 6, pp 1-42. (b) Harvey, P. D. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: London,
2003; Volume 18, pp 64-250.
(23) (a) Alan Mode, V.; Sisson, D. H. Anal. Chem. 1974, 46, 200-203. (b)
Demasa, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
(24) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press: Oxford,
1998.
(25) Herz, L. M.; Daniel, C.; Silva, C.; Hoeben, F. J. M.; Schenning, A. P. H.
J.; Meijer, E. W.; Friend, R. H.; Phillips, R. T. Phys. ReV. B 2003, 68,
045203/1-045203/7.
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9823

A R T I C L E S
Hoeben et al.
Figure 6. Top: UV/vis and normalized photoluminescence spectra (λexc ) 394 nm for 5 and λexc ) 395 nm for 6) at 10-6 M for (left) 5 and (right) 6 and
CD spectra for 5 (8.3 × 10-6 M) and 6 (9.5 × 10-6 M). Data in chloroform (dashed line) and MCH (solid line). Bottom: (left) calculated CD spectrum for
5 and (right) schematic picture (porphyrin ) square, OPV ) rectangle) of the supramolecular stacking arrangement of 5.
free-base porphyrins are present as isolated energy traps or as
clusters of acceptor moieties. Previously studied OPV donor/
acceptor co-assemblies displayed a significant change in the
vibrational ratio of sensitized acceptor luminescence, clearly
indicating the clustering of acceptor molecules at higher
incorporation ratio.26 For the present system, however, no such
obvious features are observed.
molecularly dissolved, which is concluded from their sharp,
intense Soret bands at λmax ) 424 nm (ꢀ ) 7.1 × 105 L mol-1
cm-1 for 5 and ꢀ ) 6.5 × 105 L mol-1 cm-1 for 6), which
overlap with the π-π* transition of the OPVs. The optical
transitions of the porphyrin moieties are so intense that the
absorption maximum of OPV3, which is typically found at λ
) 400 nm, is merely visible as a small shoulder for both
molecules. In sharp contrast to 1-4, the UV/vis spectra of 5
can be qualitatively reproduced by a summation of the spectra
of the individual components OPV3-NH2 and 5,10,15,20-
tetrakis(4-carboxyphenyl)porphyrin10 since the amide linkage
electronically isolates the chromophores. The Q-bands of both
porphyrins are located at λ ) 516, 552, 591, and 647 nm for 5
and at λ ) 548 and 587 nm for 6. Photoluminescence spectra
of 5 and 6 in chloroform indicate that OPV fluorescence at λem
) 463 nm is almost completely quenched for both systems, in
favor of porphyrin luminescence. In line with previous results8
and those presented in Figure 1 this indicates efficient intramo-
lecular energy transfer from the OPVs to the porphyrin core,
especially since the quantum yield of OPV314 is generally much
higher than that for porphyrin derivatives. For 5, typical
Hydrogen Bonding in (OPV)4Porphyrin Assemblies 5 and
6 in MCH. To enhance the stability of the self-assembled
lipophilic (OPV)4porphyrin architectures, porphyrin derivatives
5 and 6, connecting the peripheral OPVs to the porphyrin core
via amide bonds, were synthesized. The porphyrin core was
equipped with an OPV3 derivative in order to maintain the total
number of phenyl rings and thus, conceivably, the overall π-π
stacking interactions. The carbonyl group of the amide bond
was placed at the center of the system, i.e., directly attached to
the porphyrin, based on previous experience with C3-sym-
metrical OPV discs.27 (OPV3)4Porphyrins 5 and 6 were
synthesized using standard procedures.10 Both exist as purple
1
solids and were fully characterized using H NMR, 13C NMR,
IR spectroscopy, and mass spectrometry.
The optical properties of 5 and 6 were investigated in various
solvents (Figure 6). In chloroform, both compounds are
porphyrin luminescence is detected at λem ) 654 and λem
)
716 nm with a slightly enhanced quantum yield (φ ) 0.14) with
respect to unsubstituted tetraphenylporphyrin (φ ) 0.11). A
comparative study for 5 and a 4:1 molar mixture of OPV3-
NH2 with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin in THF
(in which 5 is molecularly dissolved) reveals that OPV
fluorescence at λem ) 484 nm is quenched with a factor QOPV
(26) (a) Hoeben, F. J. M.; Herz, L. M.; Daniel, C.; Jonkheijm, P.; Schenning,
A. P. H. J.; Silva, C.; Meskers, S. C. J.; Beljonne, D.; Phillips, R. T.; Friend,
R. H.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 1976-1979. (b)
Ajayaghosh, A.; Vijayakumar, C.; Praveen, V. K.; Babu, S. S.; Varghese,
R. J. Am. Chem. Soc. 2006, 128, 7174-7175.
(27) van Herrikhuyzen, J.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.
Org. Biomol. Chem. 2006, 4, 1539-1545.
9
9824 J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007

Chiral Oligo(p-phenylene vinylene) Porphyrin Assemblies
A R T I C L E S
≈ 100 (λexc ) 395 nm).10 Combined with a typical lifetime of
τ ) 1.5 ns for OPV3,14 this means that the lower limit for the
bisignate behavior, as observed for the H-type assemblies formed
by 18 or the J-type architectures formed by 3, but seem to consist
of two overlapping bisignate Cotton effects. To gain more
insight into this specific feature we used a mathematical
program32 to calculate the CD spectrum of a supramolecular
dimer consisting of two molecules of 5. The hypsochromic shift
in UV/vis indicates that the molecules are stacked on top of
each other where the lateral distance is assumed to be 3.5 Å
corresponding to the π-π stacking interaction.10 The measured
shape of the Cotton effect could be simulated by varying the
angle of rotation (æ) and the displacements in the x (dx) and y
(dy) directions between two molecules of 5, without loosing
the H-type stacking arrangment. The transition-dipole moments
were determined from the reference compounds OPV4-NH2 and
5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin in THF and the
bandwidths of the OPV S1rS0 transition and the Soret-band
of the porphyrin were assumed to be equal.10 In this way a
spectrum can be simulated (æ ) 9.5°, dx ) 3.6 Å, dy ) 3.9 Å)
that nicely resembles the shape of the measured spectrum. This
indicates that the existence of two overlapping bisignate Cotton
effects (of equal sign) is a plausible interpretation for the
observed behavior.
Concentration and temperature-dependent UV/vis measure-
ments were performed on both (OPV3)4porphyrins in MCH in
order to establish aggregate stability.10 The absence of any
significant changes in the UV/vis spectra with decreasing
concentration for self-assembled 5 and 6 indicates stability up
to the point where the optical density is too low to be measured,
i.e., ∼5 × 10-8 M. This is an excellent improvement in
supramolecular stability with respect to all trans-vinylene-linked
3 and 4. Temperature-dependent UV/vis spectra of 5 in MCH
at 3 × 10-7 M indicate a transition from the assembled to the
dissolved phase around ∼85 °C (without ever becoming
completely molecularly dissolved), again stressing the enormous
assembly stability. An isosbestic point at λ ) 414 nm confirms
the presence of two coexistent species in solution. Also self-
assembled 6 (3 × 10-7 M) remains stable at high temperatures
indicating that, despite the change in stacking configuration,
the stability of its assemblies is similar to 5.
The proposed intermolecular hydrogen bonding via the amide
functionalities was investigated using infrared spectroscopy on
5 and 6 in chloroform (5 × 10-4 M) and MCH (10-3 M for 5
and 7 × 10-4 M for 6).10 In chloroform, the absorption in the
amide I region (which is mainly composed of the carbonyl
stretching vibration) is detected at ν ) 1677 and 1674 cm-1
for 5 and 6, respectively. A distinct shift to lower wavenumbers
in MCH (ν ) 1649 and 1650 cm-1, respectively) clearly
indicates intermolecular hydrogen bonding in the assembled
state,33 and the narrowing of the absorption band for 5 suggests
a uniform hydrogen-bonding strength. The broader signal found
for 6 suggests a less uniform hydrogen-bonding strength and
moreover indicates that not all chromophores are hydrogen-
bonded. Combined with the UV/vis and CD spectral features,
this is another indication that 6 displays more than one stacking
arrangement, of which only the chiral H-type configuration is
intermolecularly hydrogen bonded. On the other hand, an
energy transfer rate is kENT ) 7 × 1010 s-1 13
.
Fluorescence of
6 is detected at λem ) 600 nm and λem ) 647 nm, again
indicating the large hypsochromic shift as Zn2+ is inserted into
the porphyrin core, whereas OPV luminescence is almost
completely quenched due to intramolecular energy transfer.
Spin-orbit coupling with the zinc atom results in an enhanced
intersystem crossing rate thereby decreasing the fluorescence
quantum yield of 6 to φ ) 0.09.28 The shape of its luminescence
indicates that the transitions to the first and the zero-vibrational
level of the ground state occur with almost equal probability,
which has been observed before for Zn-porphyrins.29
In MCH,11 the Soret band of both systems strongly decreases
in intensity and broadens, indicative of aggregation. The
absorption maximum of 5 shifts hypsochromically to λmax
)
399 nm (ꢀ ) 3.7 × 105 L mol-1 cm-1) with a long shoulder
extending beyond λ ) 450 nm, while the Q-bands diminish in
intensity and shift bathochromically to λ ) 520, 556, 598, and
654 nm. This indicates the formation of H-type aggregates as a
consequence of the interlocking of stacked π-surfaces by
hydrogen bonding. The Soret band of 6, however, is split into
two even further diminished absorption bands at λmax ) 402
nm (hypsochromically shifted, ꢀ ) 2.0 × 105 L mol-1 cm-1
)
and λ ) 432 nm (bathochromically shifted, ꢀ ) 1.9 × 105 L
mol-1 cm-1), indicative of an oblique slipped stacking morphol-
ogy for the porphyrin moiety.30 Its Q-bands are diminished in
intensity and bathochromically shifted to λ ) 560 and 604 nm.
Photoluminescence spectra (λexc ) 387 nm, selected by compar-
ing the UV/vis spectra of OPV3-NH2 and 5,10,15,20-tetrakis-
(4-carboxyphenyl)porphyrin) show that porphyrin luminescence
in 5 in MCH is highly quenched (φ ) 0.07) and bathochromi-
cally shifted 6 nm to λem ) 660 and λem ) 726 nm.31 The
luminescence of self-assembled 6 displays a larger, 18 nm
bathochromic shift to λem ) 618 and λem ) 662 nm (λexc
)
401 nm) while its fluorescence is hardly quenched (φ ) 0.06)
as a consequence of its enhanced J-type stacking behavior. Both
systems display supramolecular chirality in the assembled state
as observed from their similar CD spectra, whereas no optical
activity is observed in the Q-band region (nor for elevated
concentrations). The intrinsic magnitude of the effect is larger
for 5 (g ) +2.5 × 10-4 at λ ) 403 nm and g ) -1.3 × 10-3
at λ ) 391 nm) than for 6 (g ) +1.5 × 10-4 at λ ) 404 nm
and g ) -4.5 × 10-4 at λ ) 394 nm) which stems most likely
from the difference in stacking arrangement as a consequence
of zinc insertion. Since the shape of the CD effect and the zero-
crossing wavelengths are identical for both molecules, an
alternative hypothesis suggests that 6 displays more than one
stacking arrangement, of which only a potential H-type con-
figuration is chiral. In this proposition, we believe that the latter
cofacial arrangement is enforced upon the system by intermo-
lecular hydrogen bonding (Vide infra). Remarkably, the Cotton
effects observed for both (OPV3)4porphyrins do not comprise
(28) Doyle, R. J.; Da Campo, R.; Taylor, P. R.; Mackenzie, S. R. J. Chem.
Phys. 2004, 121, 835-840.
(29) (a) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier: Amsterdam,
1975. (b) Prodi, A.; Teresa Indelli, M.; Kleverlaan, C. J.; Scandola, F.;
Alessio, E.; Gianferrara, T.; Marzilli, L. G. Chem.sEur. J. 1999, 5, 2668-
2679.
(30) Yamaguchi, T.; Kimura, T.; Matsuda, H.; Aida, T. Angew. Chem., Int. Ed.
2004, 43, 6350-6355.
(31) It should be noted that the hypsochromic shifts for the porphyrin absorption
maxima in 5 and 6 hamper selective excitation of the OPV moieties.
(32) Beckers, E. H. A.; Chen, Z.; Meskers, S. C. J.; Jonkheijm, P.; 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.
(33) (a) Schroeder, L. R.; Cooper, S. L. J. Appl. Phys. 1976, 47, 4310-4317.
(b) Skrovanek, D. J.; Howe, S. E.; Painter, P. C.; Coleman, M. M.
Macromolecules 1985, 18, 1676-1683.
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9825

A R T I C L E S
Hoeben et al.
having to resort to experimentally more intricate cooling
conditions. However, since the (OPV3)4porphyrin assemblies
are never completely dissolved at high temperatures, a premixing
method was implemented to obtain mixed assemblies. After
mixing concentrated chloroform solutions of 5 and 6, evapora-
tion of the chloroform was followed by MCH addition which
yielded stable solutions and reproducible results. A considerable
degree of donor/acceptor mixing could be achieved by inter-
mediate heating to 80 °C, but the eventual result was shown to
be inferior to the premixing method. The high aggregate stability
enables the realization of a reference experiment in MCH, in
which self-assembled 5 is added to self-assembled 6 (constant
at 3 × 10-7 M) at room temperature (Figure 7, top). The increase
in free-base porphyrin concentration is apparent from its
increased luminescence at λem ) 661 nm by direct excitation.
Luminescence characteristic of Zn-porphyrin at λem ) 617 nm,
which is now well-separated from free-base porphyrin fluores-
cence, is hardly affected by the acceptor addition due to the
presence of separate stacks of both compounds. The premixing
method indicates a completely different behavior (Figure 7,
bottom). Starting with pure 6 (constant at 3 × 10-7 M in MCH),
mixtures containing up to 36 mol % of free-base acceptor 5
were prepared. Excitation occurs at λexc ) 378 nm in order to
predominantly excite the OPV moieties. This excitation wave-
length was obtained by comparing the absorption spectrum of
5 with that of aggregated 6 (directly exciting the Zn-porphyrin
at λexc ) 433 nm yields similar results). In the actual mixing
experiment, OPV luminescence is strongly quenched due to
intramolecular energy transfer to both porphyrin cores. Fur-
thermore, eventually the luminescence of the Zn-porphyrin
donor at λem ) 617 nm is almost completely quenched which
must imply that it further funnels the delivered excitation energy
to its free-base analogue. The additional increase in free-base
porphyrin luminescence at λem ) 661 nm, as compared to the
reference experiment, seems to confirm this hypothesis. Coin-
ciding with the PL experiments in Figure 7, the UV/vis spectra
of the mixtures of 5 and 6 confirm the addition of free-base
(OPV3)4porphyrin by showing a gradual increase in its typical
absorption characteristics.10 No significant differences were
observed in the UV/vis data for the reference and mixing
experiment of the subsequent samples.
Figure 7. Photoluminescence spectra for mixtures of 5 and 6 in MCH at
room temperature. The concentration of 6 is fixed at 3 × 10-7 M. (Top)
Reference experiment performed by the addition of 0 to 31 mol % self-
assembled 5, λexc ) 433 nm. (Bottom) Premixed samples containing 0 to
36 mol % 5, λexc ) 378 nm. The inset compares donor luminescence
quenching I0/I at λem ) 616 nm of the reference (9) and the actual mixing
experiment (b).
oblique slipped stacking morphology may simply prevent some
carbonyl groups from participating in hydrogen bonding at all.
The latter is corroborated by the temperature-dependent UV/
vis experiments, which do not show any sign of disassembly at
elevated temperatures, suggesting that all chromophores are
hydrogen-bonded at least to some extent. At this time, we cannot
clearly differentiate between these two options. These data imply
that, in both systems, pre-organization via hydrogen bonding
and π-π stacking ensures the formation of strongly bound
assemblies in MCH. Combined with the increased organization
of self-assembled 5 and 6, as expressed in their enhanced
intrinsic Cotton effects when compared to 3 and 4, this confirms
that intermolecular hydrogen bonding is an excellent tool for
improving stability and chirality in synthetic dye assemblies.
AFM measurements were performed on 5 by dropcasting a
dilute MCH solution (3 × 10-7 M) onto HOPG. Elongated
single fibers of hundreds of nanometers in length were observed
with a uniform height of 2.0 nm.10 When dropcasting a more
concentrated solution (5 × 10-6 M, Figure 4c), the surface is
totally covered with fibers that are highly intertwined. The
height which is found for a single fiber poses a problem, since
this is expected to yield the molecular length19 (determined to
be 5.6 nm for fully extended 3). This implies a strong degree
of tilting of the chromophores inside the fibers with respect to
the surface.
Time-correlated single photon counting (TCPSC) was per-
formed on pure 5 and 6 and their consecutive mixtures
(premixed samples containing 0 to 36 mol % 5, λexc ) 400 nm,
λem ) 620 nm, 20 °C).10 For the mixed samples, a distinct
decrease in the lifetime of 6 (observed in the initial part of the
spectra, t ∼1 ns) is accompanied by an increase in overall
lifetime with rising 5 content, due to the longer lifetime of the
latter. Although the energy transfer process can be qualitatively
monitored in this way, the overlapping fluorescence signals
impede the measurement of any quantitative data. Also, the short
time scale at which these characteristic changes occur requires
more sophisticated time-resolved techniques like photolumi-
nescence up-conversion.26a
Comparison of the Different Systems under Investigation.
Judged from donor luminescence quenching, the energy transfer
efficiency (based on acceptor incorporation) of the hydrogen-
bonded donor/acceptor system 6/5 surpasses that of the all-trans
vinylene linked system 4/3, which in its turn is higher than that
for the system 2/1.8 It is appealing to correlate the observed
Sequential Energy Transfer in Mixed Assemblies of 5 and
6. As a next step, energy transfer experiments using 6 and 5 as
energy donor and acceptor, respectively, were conducted at
donor concentrations of 3 × 10-7 M in MCH (Figure 7). The
enhanced aggregate stability allows for mixing experiments to
be conducted at favorable optical densities below 0.1, without
9
9826 J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007

Chiral Oligo(p-phenylene vinylene) Porphyrin Assemblies
A R T I C L E S
differences in energy transfer efficiency to the measured
differences in order parameter (g-value) for the respective co-
assemblies, which follow a similar trend (Table 1 summarizes
the properties of the (OPV)4porphyrin systems studied in this
paper). It has been demonstrated before that an increased degree
of intermolecular order has a beneficial effect on exciton
migration dynamics along stacked chromophores.9 However,
these conclusions should be handled with care, since for all three
systems the degree of donor/acceptor luminescence overlap
differs and the type of supramolecular packing differs. Especially
the supramolecular arrangement is expected to be crucial with
regard to energy-transfer characteristics.34 Nevertheless, the
novel systems 3/4 and 5/6 do seem to display an enhanced
degree of cascade energy transfer, although the process is still
rather inefficient when compared to, e.g., natural chlorosomal
systems, their synthetic analogues,6a and mixed OPV assemblies
previously reported.26a
strategy for obtaining well-defined supramolecular architectures
in water. These data clearly illustrate the strength of a supramo-
lecular approach where various interactions can be simulta-
neously employed to guide self-assembly into desired functional
nanostructures. However, it also demonstrates the delicate
interplay of and the relative lack of control over these interac-
tions resulting in, e.g., non-reversibility for 1 and 5, and their
zinc-incorporated analogues.
Conclusions
From this report, it becomes apparent that the successful
design of supramolecular architectures displaying efficient and
directional energy transfer will rely on an intricate combination
of noncovalent interactions. These combined interactions will
have to be carefully optimized in order to achieve the desired
degree of organization and the guarantee that subsequent
excitation transfer steps occur with high efficiency. In this
contribution, we intended the implementation of intermolecular
hydrogen bonding to significantly increase the intermolecular
energy flux along the stacking direction. The desired assembly
stabilization and organization clearly occurred; however, only
a modest increase in energy transfer efficiency was observed.
In addition, upon going from an apolar environment to water,
the stacking mode of identical chromophores (the solubilizing
chains put aside) unpredictably changed from a J- to an H-type
arrangement. Concomitantly, the desired, internal organization
of the π-stacked architectures was partly lost which was reflected
in a further diminished energy transfer efficiency through the
assemblies. This indicates that the strategy for solubilizing
π-conjugated systems in water by attaching oligo(ethylene
oxide) wedges needs reassessment in order to obtain improved
assembly characteristics.
On a more general level this implies that, even though the
knowledge of successfully implementing noncovalent interac-
tions has not ceased to grow, especially their synergistic
interplay will depend on the development of a comprehensible
set of design rules. The synthesis of the covalent π-conjugated
donor/acceptor systems presented here, their subsequent chiral
self-assembly, and sequential energy transfer processes have
effectively illustrated the significant differences in their resulting
supramolecular architectures. Although a relative lack of control
over supramolecular structure is evident (especially upon going
to water), the different supramolecular design strategies used
here serve as a starting point for future progress.
Table 1. Comparison of the Systems under Investigation
system
1
3
5
electronic communication yes
yes
no
a
(ꢀ/L mol-1 cm-1
)
(3.0 × 105)
(4.6 × 105)
(7.1 × 105)
solvent for self-assembly
most important
water
π-π stacking
hydrophobicity
MCH
MCH
π-π stacking π-π stacking
supramolecular
H-bonding
interactions
stacking arrangement
disassembly transition
temp (concentration)
minimal g-value
H-type
J-type
H-type
no disassemblyb 35 °C
∼85 °C
(3 × 10-7 M)
-1.3 × 10-3
(20 °C)
5.0
(20 °C)
(5 × 10-6 M)
-2.8 × 10-4
(10 °C)
1.4
(-100 °C)
(10-5 M)
-8.4 × 10-4
(20 °C)
3.1
PL quenching (I0/I) at
25 mol % acceptorc
(20 °C)
a Measured in chloroform. b A chiral-to-achiral transition is observed at
43 °C. c As a qualitative indication of the energy transfer efficiency from
their donor zinc analogues.
Apart from strongly stabilizing the co-assemblies, the pres-
ence of intermolecular hydrogen bonding in MCH improves the
organization of the π-stacked architectures, which consequently
induces an improved energy transfer efficiency. The initial J-type
configuration displayed by 3 is lost in favor of the H-type
stacking arrangement of 5, brought about by interlocking of the
constituent building blocks by hydrogen bonding. Conversely,
replacing the dodecyl chains of 3 with ethylene oxide chains in
1 enables self-assembly in water, where strong hydrophobic
forces impose an H-type stacking mode on the system.
Tentatively, this unexpected configuration is not the most stable
arrangement for the system, as reflected in its diminished degree
of order and cascade energy transfer efficiency. Moreover, the
lack of reversible assembly demands additional optimized
preparation techniques, as shown before in related literature
studies.35 This imposes the question whether simply replacing
the solubilizing dodecyl chains for ethylene oxide ones is a good
Acknowledgment. The authors in Eindhoven thank the
Council for Chemical Sciences of The Netherlands Organization
for Scientific Research (NWO-CW). We thank Dr. Pascal
Jonkheijm for additional AFM measurements, Dr. Stefan C. J.
Meskers for help with modeling, and Prof. Rene´ A. J. Janssen
for stimulating discussions. The authors in Leuven thank the
Federal Science Policy through IUAP-V-03. Support from the
Fund for Scientific Research-Flanders (FWO) and K.U. Leuven
is also acknowledged. Ph.L. is Chercheur Qualifie´ of the Fonds
National de la Recherche Scientifique (FNRS-Belgium).
(34) Beckers, E. H. A.; Meskers, S. C. J.; Schenning, A. P. H. J.; Chen, Z.;
Wu¨rthner, F.; Marsal, P.; Beljonne, D.; Cornil, J.; Janssen, R. A. J. J. Am.
Chem. Soc. 2006, 128, 649-657.
(35) (a) Brunsveld, L.; Lohmeijer, B. G. G.; Vekemans, J. A. J. M.; Meijer, E.
W. Chem. Commun. 2000, 2305-2306. (b) Hoeben, F. J. M.; Shklyarevskiy,
I. O.; Pouderoijen, M. J.; Engelkamp, H.; Schenning, A. P. H. J.;
Christianen, P. C. M.; Maan, J. C.; Meijer, E. W. Angew. Chem., Int. Ed.
2006, 45, 1232-236.
Supporting Information Available: Synthetic procedures and
full characterization of (OPVn)4porphyrins 3 to 6, precursors
OPV3-NO2 and OPV3-NH2, and reference compound OPV4-
aldehyde. Characteristics of the previously reported (OPV4)4por-
phyrins 1 and 2 (Figure S1). Comparison of optical character-
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9827

A R T I C L E S
Hoeben et al.
istics of 3 with its building blocks (Figure S2); concentration-
dependent UV/vis and CD spectra of 3 in MCH (Figure S3);
temperature-dependent UV/vis spectra of 4 in MCH (Figure S4);
comparison of optical characteristics of 5 with its building blocks
(Figure S5); temperature-dependent UV/vis spectra on 5 and 6
(Figure S6); additional AFM measurements on 5 (Figure S7);
UV/vis spectra for mixtures of 5 and 6 in MCH (Figure S8);
TCSPC data for 5, 6, and mixtures (Figure S9); model of a
supramolecular dimer of 5 and the corresponding calculated UV/
vis and CD spectra (Figure S10); and FT-IR measurements of
5 and 6 in chloroform and MCH including a model for the
hydrogen bonds in 5 (Figure S11).
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